Characterization of weathered volcanic rocks in Hong Kong T. Y. Irfan

Transcription

1 Quarterly Jurnal f Engineering Gelgy, 32, 31% /99 $ The Gelgical Sciety f Lndn Characterizatin f weathered vlcanic rcks in Hng Kng T. Y. Irfan WSP Envirnmental, Newcastle Business Park, Newcastle upn Tyne, UK (frmerly f Getechnical Engineering Office, Hng Kng) Abstract The mineralgical and fabric characteristics f weathered vlcanic rcks, which are widespread in Hng Kng, are prly dcumented. Cnsequently, the significant variatins cmmnly bserved, even at the same lcality, in engineering prperties and in situ behaviur f these rcks and the resulting sils cannt be adequately related t their basic mineralgical and fabric characteristics. A mdel fr weathering f vlcanic rcks, particularly thse f pyrclastic rigin, is presented in this paper. Mdificatins, similar t thse recently prpsed fr granites by Irfan, are prpsed t the material and mass weathering schemes cmmnly adpted in Hng Kng fr the characterizatin f weathered vlcanic rcks in engineering use. The apprpriateness f varius prpsed petrgraphic and chemical weathering indices t generally fine-grained vlcanic rcks is examined. A number f mdificatins are prpsed t the standard sil preparatin and testing methds fr the sils frmed frm trpical weathering f vlcanic rcks in rder t btain meaningful and mre repeatable results. Keywrds." clay minerals, igneus rcks, index tests, weathering Intrductin The rcks in Hng Kng are deeply weathered, typically t depths f up t 6 m in the granites and up t 2 m in vlcanic rcks, which are the predminant rcks in the Territry. Althugh the mineralgical and textural aspects f weathered granites have been described in detail by Irfan (1996a), the prperties f weathered vlcanic rcks f Meszic age are prly dcumented. There is als little mineralgical and chemical data frm ther cuntries n the weathering aspects f vlcanic rcks f similar age. T understand the labratry and field behaviur f weathered rcks and the resulting sils, the basic mineralgical and fabric prperties f these rcks and sils need t be characterized. Hwever, characterizing these prperties fr engineering purpses is nt easy because f their great variability and inhmgeneity, particularly in the case f vlcanic rcks, as a result f their gelgical and weathering histry. Althugh many quantitative and semi-quantitative weathering indices have been derived fr quantifying changes in terms f gelgical prperties, few have been used t relate weathering effects t engineering prperties. Many f the weathering indices have been specifically develped fr characterizing carse-grained igneus rcks, particularly granites. Sme quantitative weathering indices have als been applied t fine-grained igneus rcks including vlcanic rcks. Varius weathering indices prpsed t date and their suitability fr granitic rcks in Hng Kng were reviewed by Irfan (1996a). This paper presents the results f a recent study n sme weathered vlcanic rcks in Hng Kng, and dcuments the typical chemical, mineralgical and fabric changes that ccur in these rcks due t weathering and ther alteratin effects. It examines the applicability f varius quantitative weathering indices, in particular thse derived frm the results f chemical and petrgraphical analyses, in characterizing the degree f weathering f fine-grained vlcanic rcks. N attempt has been made t relate, quantitatively, the mineralgical and fabric characteristics f the weathered vlcanic rcks t their physical and mechanical prperties as the index and engineering prperties f these rcks have nt been determined systematically. Hwever, the applicability f standard sil classificatin and index tests t the sils frmed frm weathering f vlcanic rcks is discussed in the light f the mineralgy and micrstructure that are typically develped in these sils. Gelgical setting f vlcanic rcks in Hng Kng Granites, grandirites and the assciated vlcanic rcks f Meszic age underlie a great part f Hng Kng (Fig. 1). The vlcanic rcks f Upper Jurassic t Lwer Cretaceus age, several thusand metres thick, have been intruded by a number f granitic plutns f slightly yunger age. The vlcanic rcks, recently gruped by the Hng Kng Gelgical Survey, cmprise a series f lithstratigraphic frmatins, each with its wn distinctive gelgical characteristics (Table 1). They cnsist mainly f pyrclastic tufts derived frm ash fall and ash flw depsits with sme lavas and intercalated sedimentary

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3 CHARACTERIZATION OF WEATHERED VOLCANIC ROCKS IN HONG KONG 319 Table 1. Lithstratigraphy f Hng Kng (based n Strange et al. 199 and Langfrd et al. 1995), nt all vlcanic frmatins are shwn Age Named rck divisins Principal rck types Meszic Upper t Lwer Cretaceus Upper Jurassic t Lwer Cretaceus Lwer Jurassic Palaezic Permian Carbniferus Devnian Majr Intrusive Rcks Meszic Upper Jurassic-Lwer Cretaceus Minr Intrusive Rcks Tertiary Palaecene Meszic Upper Jurassic-Lwer Cretaceus Kat O, Prt Island, Pat Sin Leng Frmatins Repulse Bay Vlcanic Grup Lai Chi Chng Frmatin High Island Frmatin Clear Water Bay Frmatin Mang Kung Uk Frmatin Silverstrand Frmatin Mt Davis Ap Lei Chau Tsuen Wan Vlcanic Grup Tai M Shan Frmatin Shing Mun Frmatin Yim Tin Tsai Frmatin Tuen Mun Frmatin Tsing Shan Frmatin Tl Channel Frmatin Tl Harbur Frmatin San Tin Grup Lk Ma Chau Frmatin Yuen Lng Frmatin Ma On Shan Bluff Head Frmatin Cnglmerate, sandstne, siltstne Fine ash tuff with tuffite Fine ash vitric tuff Mainly lavas, sme tuff, tuffite Tuff, tuffite, sandstne Eutaxite Carse ash crystal tuff Fine ash vitric tuft Carse ash crystal tuff Lithic and crystal tufts, tuff breccia Carse ash crystal tuff Meta-andesite lava Sandstne, siltstne, mudstne Siltstne, mudstne Siltstne, sandstne Metasandstne, metasiltstne Marble Marble Sandstne Fine- t carse-grained granites, grandirite belnging t Lin Rck and Lamma Suites Andesite, basalt, gabbr, lamprphyre Rhylite, aplite, pegmatite, syenite hrizns. The rcks are generally rhylitic (acidic) in cmpsitin with sme andesitic rcks ccurring lcally. Macrscpic features such as flw-banding, flwflding, aut-brecciatin and clumnar jinting are present in the lavas as well as in the tufts. Fine ash vitric tufts and carse ash crystal tufts are the dminant rck types ccurring in mst frmatins. Other lithlgies include tuff breccia, lapilli tuff, andesitic lavas and tuffites (tuffaceus sedimentary rcks). The rcks may shw effects f cntact metamrphism, metasmatic and thermal alteratin (hydrthermal alteratin) particularly near granitic plutns. The granites and vlcanic rcks are affected by faulting dating back t Meszic times. The mst dminant and persistent fault directins in Hng Kng are ENE-WSW. Strng fliatin r schistsity is present in sme f the vlcanic rcks, in particular in the Nrthwest New Territries, assciated with verthrusting. The stress release related sheeting jints are generally prly develped r nn-existent in vlcanic rcks. The jint spacing is usually much clser than in the granitic rcks, except perhaps in sme carse-grained tufts, being generally.1 t.6 m. Sme large-scale gentle and pen flds ccur in the vlcanic sequences and these are, at least in part, related t the rise f the underlying granitic plutns int a relatively cmpetent vlcanic cver. A number f cllapsed caldera-type structures have been pstulated (Strange et al. 199; Langfrd et al. 1995).

4 32 T.Y. IRFAN Fig. 2. Weathering prfile and crestne develpment in carse-grained vlcanic rck. Weathering and classificatin f weathered vlcanic rcks Weathering prfile in vlcanic rcks The majrity f studies n rck weathering and prfile descriptins have been cnfined t the granitid rcks in Hng Kng. The vlcanic rcks have received less attentin and nly brief references have been made t weathering prfile develpment in these rcks. Irfan (1996a) reviewed the cmmn weathering prcesses respnsible fr deep weathering prfiles, which have develped ver the igneus rcks in Hng Kng. Cataclasis and shearing f vlcanic rcks by tectnic prcesses and granitic intrusins and assciated hydrthermal alteratin are a majr influence in prfile develpment and sil frmatin at many lcatins. Sme vlcanic rcks, particularly the finer grained types, are mre resistant t weathering than thers, resulting in well defined escarpments where resistant rcks cap and prtect the weaker mre easily weathered tufts. The general weathering prfile in lithlgically unifrm vlcanic rcks is similar t that f granitic rcks, grading frm residual sil and saprlite thrugh a mixed rck and sil zne t fresh rck at depth. Hwever, the mixed rck and sil znes (Znes 2, 3 and 4 f Ann 1995, see Table 2) is usually relatively thin at many lcalities in Hng Kng, except in carse-grained vlcanic rcks which have a similar prfile develpment t medium- and carse-grained granites. In sequences f interbedded vlcanic lithlgies, the rck mass cnsists f relatively unweathered bands r beds alternating with decmpsed units. In sme cases, the sil znes (i.e. the saprlitic and residual sil znes f Getechnical Cntrl Office 1988, r Znes 5 and 6 in hetergeneus rck masses f the Gelgical Sciety Wrking Party Reprt (Ann 1995)) is underlain by fresh rck, with intermediate znes being absent.in general, sil znes f the weathering prfile are cmparatively shallw in vlcanic rcks, usually less than 15 m thick, in cntrast t the deep prfiles that ccur ver granitic rcks. Hwever, thick sil prfiles f up t 35 m have als develped in sme places, particularly in the carser grained vlcanic rcks (Irfan et al. 1987). Deeper sil prfiles have been recrded, but these are generally recgnized as being cnfined t faulted and intensely fractured znes. Crestne frmatin is nt cmmn in weathering prfiles develped ver vlcanic rcks, except in carse ash tufts (Fig. 2). The residual sil zne is nt very well develped ver the vlcanic rcks. In cmmn with the granitic terrain, the weathering prfile frmed under the prevailing subtrpical climate is t immature. This zne is generally transitinal t saprlite, with the lwer prtin cntaining sme rck fabric but n relict structures.

5 CHARACTERIZATION OF WEATHERED VOLCANIC ROCKS IN HONG KONG 321 Engineering classificatin f weathered vlcanic rcks As the weathering prfiles develped ver the vlcanic rcks are similar t thse ver the granitic rcks, the same weathering grade classificatin scheme can be used. The six-fld mass grading scheme recmmended in BS 593 (British Standards Institutin 1981) has fund favur fr classifying varius cmpnents f the weathering prfile develped ver vlcanic and granitic rcks (Irfan 1994, 1996a). The znal scheme prpsed by the Getechnical Cntrl Office (1988) is particularly suitable fr crestne-frming, generally carse-grained vlcanic and granitic rcks (Table 2). Hwever, it is less well suited fr finegrained rcks, which shw gradual strength lss with weathering. The simple descriptive weathering grade scheme f BS 593 has been much criticized as being nt easily applicable t a variety f rck types (Ann 1995). Five different appraches have been prpsed by the mre recent Gelgical Sciety Wrking Party Reprt (Ann 1995) t describe weathered rcks fr engineering purpses. The use f frmal classificatin schemes is discuraged in the general apprach. Fr hetergeneus rck masses, which develp a mixture f relatively strng and weak material in the mass, a six-fld znal classificatin scheme based n Martin & Hencher (1986) is recmmended. This scheme is bradly similar t that recmmended by the Getechnical Cntrl Office (1988) fr granitic and vlcanic rcks. The schemes prpsed by the Getechnical Cntrl Office (1988), British Standards Institutin (1981) and the mst recent classificatin schemes recmmended by the Gelgical Sciety Wrking Party (Ann 1995) fail adequately t characterize sil znes f the weathering prfile fr engineering purpses. A classificatin scheme fr trpical sils has been prpsed by the Gelgical Sciety f Lndn (Ann 199). It is a prcess based (generic) rather than purpse led (engineering) classificatin. The prpsed scheme is cmplex and uses many pedlgical terms unfamiliar t getechnical engineers. In additin, it des nt accunt fr the structural characteristics (bth micr and macr-structure) f saprlitic and residual sils, which distinguish them frm the sedimentary sils and cntribute strngly t their atypical getechnical behaviur in the field and labratry. Wesley & Irfan (1998) prpsed a universal classificatin system fr saprlitic and residual sils, which accunts fr physical and mineralgical cmpsitin as well as the structure f these sils (Table 3). A brief descriptin f this scheme tgether with a critique f varius mass weathering classificatin schemes applicable t igneus rcks in Hng Kng is given in Irfan (1996a) Material weathering grades in vlcanic rcks Traditinally, a six-fld decmpsitin (weathering) grade scheme has been used in Hng Kng t classify the weathered state f rck material (Getechnical Cntrl Office 1988): Residual Sil (RS) Cmpletely Decmpsed (CD) Highly Decmpsed (HD) Mderately Decmpsed (MD) Slightly Decmpsed (SD) Fresh (F) This scheme is generally applicable t mst igneus rcks. Hwever, fr the present purpses there is a need t subdivide the sil grades, as these grades, as defined, can have a very brad range f engineering prperties. Fr example, in situ SPT values, which are cmmnly used fr pile design, can vary frm less than 2 t ver 12 fr the cmpletely decmpsed granite grade. Irfan (1996a) divided the sil grades int subgrades (i.e. decmpsed, disintegrated and altered) in the granitic rcks, based n the dminant type f weathering and alteratin prcess. He als prpsed the term 'transitin sil' fr material that shws partial lss f fabric befre it transfrms int true residual sil. Granitic rcks generally prduce silty sandy saprlitic sils with little clay sized particles. Even in the residual sil grade the clay cntent is generally less than 15% unless the rck has als undergne intense alteratin. Irfan (1996a) further divided each subgrade in granitic sils int lse, dense and very dense based n field assessment f density (relative strength), which is in fact a functin f its degree f weathering. The mst accurate methd f describing the degree f decmpsitin, r weathering in general, is t use a quantitative index. Fr granular saprlitic and residual sils in situ SPT values can be used t characterize each subgrade as has been dne by Irfan (1996a) fr Hng Kng granitic sils. Fr the fine-grained saprlitic and residual sils which have high fines cntents subdivisins can be made in terms f degree f cnsistency (e.g. sft, stiff, firm), with the class limits defined in the field, fr example, by hand penetrmeter values. The subdivisin can als be based n labratry index measurements (e.g. dry density) r field prbing values. Subdivisins can als be made within the Slightly Decmpsed grade based n the degree f disclratin: Slightly Disclured Mderately Disclured Highly Disclured The abve classificatin scheme has als been adpted fr the vlcanic rcks included in this study. Vlcanic rcks generally prduce finer grained sils, with the fines

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7 CHARACTERIZATION OF WEATHERED VOLCANIC ROCKS IN HONG KONG 323 Table 3. Classificatin and characteristics f saprlitic and residual sil grups (after Wesley & Irfan 1998) Majr grup Grup Examples Means f Sub-grup identificatin Cmments n likely engineering prperties and behaviur Grup A Grup B Grup C (a) Strng Highly weathered rcks General appearance macr-structure frm acidic r influence intermediate igneus rcks, and sedimentary rcks (b) Strng Cmpletely weathered General appearance micr-structure rcks frmed frm and evaluatin f influence igneus and sensitivity, liquidity sedimentary rcks index, etc. (c) Little structural influence Sils frmed frm hmgeneus rcks Little r n sensitivity, unifrm appearance (a) Smectite Black cttn sils, Dark clur (grey t (mntmrillnite) many sils frmed in black) and high grup trpical areas in prly plasticity drained cnditins (b) Other minerals (a) Allphane grup (b) Hallysite grup Sils frmed frm vlcanic ash in the wet trpics and in temperate climates Sils largely derived frm lder vlcanic rcks, especially trpical red clays Very high natural water cntents, and irreversible changes n drying Reddish clur, well drained tpgraphy and vlcanic parent rck (c) Sesquixide The sil grup lsely Granular, r ndular grup referred t as 'lateritic', appearance r laterite A very large grup f sils (including the 'saprlites') where behaviur (especially in slpes) is dminated by the influence f discntinuities, fissures etc. Essentially hmgeneus sils which frm a tidy grup much mre amenable t systematic evaluatin and analysis than subgrup (a) abve. Identificatin f nature and rle f bnding (frm relict primary bnds t weak secndary bnds) imprtant t understanding behaviur. A relatively minr sub-grup. Likely t behave similarly t mderately vercnslidated sils. Nrmally prblem sils fund in flat r lw-lying areas, f lw strength, high cmpressibility, and high swelling and shrinkage characteristics. Likely t be a minr sub-grup. Characterized by very high natural water cntents, and high liquid and plastic limits. Engineering prperties are generally gd, thugh in sme cases high sensitivity culd make handling and cmpactin difficult. Generally very fine-grained sils f lw t medium plasticity but lw activity. Engineering prperties generally gd. There is ften sme verlap between allphane and hallysitic sils. A very wide grup, ranging frm silty clay t carse sand and gravel. Behaviur may range frm lw-plasticity silty clay t nn-plastic gravel. (silt and clay) cntent in excess f 2% (Lumb 1962), except perhaps thse sils frmed frm the weathering f sme carse-grained tufts. Previus wrk n clay mineralgy and fabric f vlcanic rcks Published data n the clay mineralgy and fabric f weathered vlcanic rcks are cnfined t few samples cllected frm a limited number f rck types. There is als little mineralgical and chemical data frm ther cuntries n the weathering aspects f lder vlcanic rcks. Previus studies in Hng Kng indicated that the mst abundant clay minerals in the near-surface sils f vlcanic rigin are f the kalin grup. Hallysite is mre cmmn in the saprlitic zne, cexisting with a minr quantity f kalinite. This is t be expected as these sils have been frmed n well-drained sideslpes

8 324 T.Y. IRFAN f the muntainus terrain in Hng Kng under a prlnged subtrpical climate. Hallysite and kalinite may give way t gibbsite under advanced weathering cnditins clse t the tp f the prfile (Lumb & Lee 1975). Small amunts f vermiculite and allphane were identified, at least in the initial stages f weathering (Lumb & Lee 1975; Parham 1969). Mre recently, Irfan (1994) published the results f mineralgical and fabric analyses f carse ash tufts frm a landslide site in Hng Kng, which have been included in this study. Study methds The principal aim f the study was t establish the basic mineralgical and fabric characteristics f the mst cmmn vlcanic rcks, as quantitatively as pssible. The fllwing methds were used: detailed descriptin f samples and sampling lcalities ptical micrscpy n thin sectins t determine and quantify the mineralgy (mdal analysis) and fabric characteristics chemical analyses t determine chemical and bulk quantitative nrm mineralgy by x-ray flurescence (XRF), inductively cupled plasma atmic emissin spectrmetry (ICPAES), r atmic absrptin spectrmetry (AAS) x-ray diffractin (XRD) analysis n bulk samples and clay fractins thermgravimetric analysis t prvide a semiquantitative assessment f any kalin phase, gibbsite r gethite present in samples Samples representing different material grades were cllected frm five lcalities (Fig. 1). Brief descriptins f the sampling lcalities and the weathering characteristics f the rcks at each site are given in Table 4. Simplified sketches f sampling lcalities 2 and 3 are presented in Figs 3 & 4. Mineralgy and fabric characteristics f weathered tuffs The mineralgical cmpsitins f the freshest rck specimens frm the five lcalities studied are given in Tables 5 & 6. The selected results f semi-quantitative XRD analyses n bulk and clay fractins are presented in Table 7 fr carse ash tufts frm lcality 1. Typical XRD patterns are given in Fig. 5. A summary f quantitative mineralgical analyses n all rck types is given in Table 8. Schematic representatins f the typical mineralgical changes ccurring with weathering in a fine ash and a carse ash tuff are given in Figs 6 & 7. Micrgraphs in Fig. 8 illustrate the typical mineralgical and fabric features f carse ash tufts weathered t varius degrees. An example f quantitative mineralgical and micrfabric determinatin based n micrscpical mdal analysis is presented in Table 9 fr carse ash tufts frm lcality 1. Results f chemical analyses are nt reprted here but they are used in calculating the chemical indices defined in Table 1 and listed in Tables 11 and 12. Fresh tufts (F) The textural and mineralgical prperties f vlcanic rcks in Hng Kng are very variable depending upn the type f vlcanic activity, the mde and histry f slidificatin, later metamrphism, hydrthermal and ther alteratin prcesses. Cnsequently, wide variatins in texture and mineralgy can even ccur in the same rck type at each lcatin. The fine and carse ash tufts included in the study are all 'acidic' in cmpsitin. Carse ash tufts cnsist f crystals r crystal fragments f feldspar and quartz, with bitite and secndary muscvite set in a fine grained matrix f the same minerals. Fine ash tufts cnsist f a number f crystals dispersed in a very fine vitric (glassy) matrix. The crystal cntent is up t 5% in the carse ash tufts, but in the fine ash tufts it is usually less than 25% f the rck (Table 5). Feldspars are the dminant cnstituents, with plagiclase being mre abundant than alkali feldspar (Table 6). Mst plagiclases and sme alkali feldspars shw slight alteratin t micaceus prducts (sericite) in all the rcks studied. Small amunts f secndary minerals such as muscvite, hydrus mica, chlrite, ccasinally calcite, and in sme cases, kalinite are present in the unweathered rck, in additin t minr amunts f irn xide minerals, mainly magnetite. The majrity f secndary prducts are attributed t epigenetic prcesses during r subsequent t vlcanism and slidificatin. Later cntact metamrphism and hydrthermal alteratin by the intruding granitic magmas might als have cntributed t the frmatin f micaceus minerals in these rcks. Sme crystal alignment and weak t strng welding fabrics with elngate recrystallized siliceus shards and fiamme are present in sme tufts. A welded cmpactin structure arising ut f the alignment f flattened fiamme, stretched pumice and lithic fragments is present in the fine ash tuff (termed eutaxite) frm lcality 2. The fresh rck is unfractured except fr ccasinal shrt and tight micrcracks in quartz and feldspar crystals. Slightly decmpsed (SD) With weathering, nticeable decreases ccur in Na + and Ca 2+ cntents in mst tufts. This indicates the cmmencement f decmpsitin f Na-Ca feldspars

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10 326 T. Y. IRFAN Tp sil I 1 ATw2~ 1 s~l I r.eds,.e 1 1 I Platfrm Shear Zne Fig. 3. Sketch f sampling lcality 2. CWV-FV, Mass weathering grades; [] ATW3, sample lcatin and number. Slpe height is apprximately 4 m.,~... - ~ j!i \\~\ / ~ ~ / \ \ \ - ~ ~ ~ ~ ~ _. ~ / / -'b~-~.: `1 ~/ t nwv m 5m./ I Apprximate Scale Fig. 4. Sketch f sampling lcality 3. CWV-RS, Mass weathering grades; Cil, Clluvium; [] TK1, sample lcatin and number.

11 CHARACTERIZATION OF WEATHERED VOLCANIC ROCKS IN HONG KONG 327 Q O f "i F1 O = 1=2 (a) Slightly Decmpsed (IR1) (c) Cmpletely Decmpsed ] F2I (IR53) ~ K MK K M (b) Highly Decmpsed (IR4) CuI~ radiatin F1 2 F2 K M1 (d) Residual (Transitin) Sil (IR62) O g~ = 4 2 2, degree H? I , degree d-spacing,-~k Fig. 5. Typical bulk sample x-ray diffractin scans, carse ash tuff, lcality 1. F1, Plagiclase feldspar; F2, Alkali feldspar; K, Kalinite/Hallysite; M1, Mica (Bitite); M, Muscvite/Illite; Q, Quartz; Ch, Chlrite; H, Hematite; G, Gethite; IR1, Sample number. (plagiclases) in the slightly decmpsed grade. Hwever, there are variatins in the type and intensity f feldspar weathering amngst the rck types studied. Fr example, all feldspar megacrysts shw slight t mderate degrees f decmpsitin in sme tufts, but the plagiclases are nly slightly decmpsed and the alkali feldspars are nt generally affected by weathering in thers (Fig. 8a). The 'cludy' r 'speckled' znes bserved under the micrscpe are cnsidered t be either prly crystalline alumina-silicate mineral grwths transitinal t kalinite r hallysite r the lcatins f slutin pits (Irfan 1996a). Kwng (1985), using electrn micrscpy, als nticed slight t nticeable slutin f feldspars in the slightly t mderately decmpsed vlcanic rcks frm Hng Kng Island. Reddish brwn staining f the rck fabric r cating alng grain cntacts by irn xides withut any apparent micrfracturing might als be taken as evidence f slutin in the feldspar megacrysts and the grundmass. The majrity f kalin minerals, predminantly kalinite, identified by XRD in the disclured prtins f sme specimens are likely t be by prducts f pre-weathering prcesses. The trace amunts f chlrite and vermiculite r intermixed chlrite-vermiculite identified by XRD in mst tufts (Tables 7 and 8) are cnsidered t be frmed frm the alteratin f bitite. A netwrk f simple-branched and tight micrcracks is develped in the irn xide stained prtins. The frequency f micrfracturing increases twards jint surfaces. The bundaries f quartz and feldspar crystals are tight in the unstained areas but they may be cated with irn xides in the stained uter areas. The staining r cating by irn xides alng grain bundaries may indicate the develpment f intergranular fracturing r the frmatin f disslutin pits and channels arund the exterir f the larger feldspar grains. Mderately decmpsed (NID) There is a significant increase in the decmpsitin f plagiclase, bth the crystals and the grundmass, t kalin and micaceus minerals. The alkali feldspars are nly slightly affected in this grade. The main kalin mineral frmed frm the decmpsitin f feldspars is a

12 328 T.Y. IRFAN WEATHERING GRADE Residual / Transitin Sil Cmpletely Decmpsed I IR6 i IR3 Highly Decmpsed Quartz Mderately Decmpsed IR2 Slightly Decmpsed IRI Fresh! I RELATIVE MINERAL CONTENT (%) I 9 1 Fig. 6. Variatin in mineralgical cmpsitin with weathering, carse ash tuff, lcality 1. Brken line indicates where increase in illite is prbably due t pssible hydrthermal effects. The purpse f figure is t shw general trends, discrepancies are expected due t sample variatin, etc. IRl, Relative apprximate lcatin f sample analysed within the weathering grade. prly crystallized kalinite with a small amunt f hallysite nly detected in sme tufts. A trace amunt f a smectite mineral, r an interstratified clay mineral f pssible illite-smectite cmpsitin was identified in ne f the carse ash tufts (Table 7). The hydrthermal alteratin f the rck at this site might have resulted in the frmatin f metastable smectite in the initial stages f feldspar alteratin, as well as frmatin f chlrite and vermiculite frm alteratin f bitite (Irfan 1994). The mst significant fabric change in the rck, at this grade, is the frmatin f an extensive netwrk f tight t slightly pen micrcracks (Fig. 8b). Cnsequently, a fairly rapid reductin in intact strength ccurs. Mst large feldspars and sme quartz grain bundaries are stained with irn xides, which indicate that the grain bundaries are pening up. Apart frm the pres frmed frm disslved calcite grains in sme specimens, large-scale slutin features have nt been bserved under the micrscpe. Highly decmpsed (HD) Alkali feldspars are nw affected by chemical weathering within this grade in all rcks except the carse ash tuff frm lcality 1, but they are intensely micrfractured (Fig. 8c). Plagiclases are almst cmpletely altered t clay minerals in all tufts, except the carse ash tuff frm lcalities 1 and 5 (Table 8). There are differences between the clay fabrics frmed frm the weathered feldspars. Fr example, the decmpsed feldspar pseudmrphs are very prus in the carse ash tuff frm lcality 1 cmpared t the dense clay fabrics prduced in the carse ash tuff frm lcality 4. The feldspars in the specimens affected by hydrthermal alteratin are, in general, mre clay rich and have denser micrfabrics than thse affected nly by weathering. Kalin minerals becme the dminant clays, with small amunts f mica and trace amunts f chlrite and vermiculite r intermixed chlrite-vermiculite. Small amunts f sesquixides (mstly gethite) ccur in all rcks, generally as prly crystalline aggregatins. The hallysite cntent generally increases in this grade but nt in all tufts. The rck fabric is extensively micrfractured by tight t slightly pen (a few micrns wide) micrcracks (Fig. 8c), which tgether with vids cmprise up t 22% f the rck in sme carse ash tufts (Table 9). The decmpsitin f feldspars in the grundmass is mre advanced in this grade than in mderately decmpsed tuff, with specimens unaffected by hydrthermal alteratin having a mre prus grundmass than thse affected. The intensive slutin f alkali feldspars, which was bserved t be at an advance stage in the highly decmpsed granites (Irfan 1996a),

13 CHARACTERIZATION OF WEATHERED VOLCANIC ROCKS IN HONG KONG 329 WEATHERING GRADE Residual / Transitin Sil Cmpletely Decmpsed TKO8 TKO7 Highly Decmpsed Kalin inerals TKO4 Mderately Decmpsed Quartz TKO3 Slightly Decmpsed Fresh TKO2W TKO2F TKOI RELATIVE MINERAL CONTENT (%) Fig. 7. Variatin in mineralgical cmpsitin with weathering, fine ash tuff, lcality 3. Other minerals include fine-grained micas (up t 1%), irn xides and hydrxides (usually 3 t 5%) and trace amunts f chlrite, vermiculite. TKO1, Relative apprximate lcatin f sample analysed within the weathering grade. Sample TKO6 nt included due t its unusually high quartz cntent. Kalin mineral cntents f fresher samples are attributed t cntact metamrphism and hydrthermal prcesses prir t weathering. Purpse f figure is t shw general trends, variatins likely t ccur due t variatin in fresh rck cmpsitin, weathering histry, etc. generally ccurs later in the cmpletely decmpsed grade in the tufts. Sme disturbances f the fine grained grundmass was bserved in the vicinity f sme micrfractured quartz and feldspar grains, pssibly indicating a slight expansin f the rck at this stage f weathering. Cmpletely decmpsed (CD) The remaining plagiclases are cmpletely decmpsed and alkali feldspars are intensely affected by weathering in this grade (Fig. 8d). Hwever, there are variatins in the way the alkali feldspars decmpse as sme underg slutin whereas thers becme altered t clay minerals. Fr example, the tufts underlying hillslpe envirnments (e.g. lcality 1), where active remval f catins by migrating grundwater ccurs, shw widespread slutin type weathering with advanced hneycmb fabric develpment in the carse ash tufts. The grundmass in these rcks ften assumes a highly prus appearance. Slutin weathering seems t be mre cmmn in the carse-grained rcks than the fine-grained rcks. Sils withut a highly prus fabric may have been subjected Table 5. Cmpsitin f 'fresh' tufts determined frm mdal analysis Rck type Sampling Sample Large crystals (>.5 mm) (%) Matrix minerals (%) lcality n. Feldspars Quartz Bitite Altered Others Fresh Altered Fresh Altered bitite matrix matrix Ttal (%) Crystals Matrix Carse Ash Tuff 1 (IRI) Fine Ash Tuff 2 TK Fine Ash Tuff 3 ATW Eutaxite (Fine) 3 (ATW2) Carse Ash Tuff 4 (SK7) Carse Ash Tuff 5 (YL 1) N/A N/A N/A N/A N/A NIA N/A N/A N/A, nt analysed; (), 'Fresh' specimens btained frm the fresher cres f slightly decmpsed blck samples N/A N/A

14 33 T.Y. IRFAN Table 6. Mineralgical cmpsitin f 'fresh' tufts (weight %, based n chemical analyses and XRD) Rck type Sampling Sample Quartz K-Feldspar Plagiclase Kalin Micas Irn Others Ttal* lcality n. minerals xides Carse Ash Tuff 1 (IR1) (C,V) 99 Fine Ash Tuff 2 TKO Tr 4 Tr (C) 86 Fine Ash Tuff 3 ATW Tr Eutaxite (Fine) 3 (ATW2) Tr Carse Ash Tuff 4 (SK7) Tr 3 Tr (C) 9 Carse Ash Tuff 5 (YL1) The surces f errr are prbably due t the presence f nn-quantified minerals, such as mica, chlrite, carbnates, titanium xides as well as calculatin errrs due t the stichimetries assumed. C, Chlrite; V, Vermiculite; (), 'Fresh' specimens btained frm the fresher cres f slightly decmpsed blck samples; Tr, Trace. t cntact metamrphism r hydrthermai alteratin as these can als result in the frmatin f kalinite and micaceus minerals in the fresh rck. Kalinite, being a relatively stable mineral, wuld nt be subject t slutin type weathering which usually results in the remval f catins particularly frm the feldspars. Weathering under prly drained cnditins, fr example in valley bttms, may als lead t clay rich fabrics. Hallysite is almst as abundant as kalinite in the least weathered specimens in this grade, but in the mre intensely weathered specimens and in the transitin sil grade, kalinite becmes dminant with nly small amunts f hallysite present (Tables 7 and 8). In sme f the cmpletely decmpsed tufts (e.g. lcality 4), nly small amunts f hallysite were detected in the clay fractins. There may be a number f reasns fr the dminance f kalinite in these rcks at this grade, sme f which are given belw: direct alteratin f alkali feldspars t kalinite. The alkali feldspars shw widespread decmpsitin t kalinite rather than slutin type weathering in this grade direct frmatin f kalinite frm pre-weathering (hydrthermal) prcesses pr drainage in a lw-lying castal tpgraphy might inhibit the frmatin f hallysite mst hallysite may be f silt size, as is the case with granites (Irfan 1996a), and hence is nt present in the clay fractins (less than 2 gm) analysed The weathered rck shws extreme fabric variatins in the cmpletely decmpsed grade. In general, a very fractured and prus fabric is develped in the carsegrained tufts in well drained settings. Generally, denser clayey fabrics are frmed in the fine-grained tufts and thse rcks previusly affected by cntact metamrphism r hydrthermal alteratin. The frmatin f mre prus fabrics in the carse-grained tufts may be related t their higher mass permeability due t the develpment f clsely spaced radial macrfractures (ver 1 mm wide) arund less weathered rck (i.e. crestnes), typical f granites, in these rcks. Hwever, envirnmental factrs can als influence the type f fabrics develped in weathered rcks. Prly drained cnditins, fr example, in valley bttms and tpgraphic depressins, may lead t generally clayey fabrics. Als, the cementatin by irn xides may lcally be very intense and ften bliterates the sil fabric. Transitin and residual sil (TS, RS) The final stage f alkali feldspar alteratin is marked by chemical alteratin t clay minerals in the transitin sil stage, subsequent t slutin weathering in the highly and cmpletely decmpsed grades. Chemical analysis results (Irfan 1997) indicate nticeable slutin f quartz within residual sil. The reductin in silica r quartz cntent may als be due t hydrthermal effects in sme rcks. Kalinite is the predminant clay mineral with a trace amunt f hallysite (Table 8). The riginal rck fabric eventually disappears within the transitin sil grade, and the residual sil cmprises sme silt t sand size quartz grains (usually less than 2% f the sil) in a structureless clay mineral and irn xide matrix (Fig. 8e). There may be small aggregatins r 'pebbles' f reddish brwn irn xide minerals thrughut the sil fabric and these may be taken as an indicatin f the nset f laterizatin at sme sites. Irn xide minerals Generally, the irn xide and hydrxide minerals (cllectively termed sesquixides) cmprise less than 1% f the material in the sil grades (Table 8). In thin sectin, irn xide minerals generally appear as prly crystallized aggregatins scattered thrughut the grundmass, and as catings n partially t cmpletely decmpsed minerals, r as micrcrack infillings. Lcally, weak cementatin is als present.

15 CHARACTERIZATION OF WEATHERED VOLCANIC ROCKS IN HONG KONG 331 g Fig. 8. Micrgraphs illustrating mineralgy and fabric characteristics f weathered carse grained vlcanic rcks: (a) slightly decmpsed tuff; (b) mderately decmpsed tuff; (c) highly decmpsed tuff; (d) cmpletely decmpsed tuff (with hneycmbed alkali feldspar, tp and right); (e) transitin sil (with partially cllapsed fabric, left half). Kf, alkali feldspar; P, plagiclase; Q, quartz; bt, bitite; M, fine-grained matrix; c, pen micrcrack; v, vid; CM, cllapsed fabric. Crss-plarized light. Scale : length f phtgraph = 3.5 mm. Detailed studies n granitic rcks indicated that gethite was the main irn mineral present in the granitic sils in Hng Kng and haematite was generally absent (Irfan 1996a). Thermgravimetric analysis carried ut n sme specimens indicated that prly crystalline gethite is als the main irn xide mineral present in the vlcanic sils (Table 8). Hwever, haematite was als detected in sme weathered tuff specimens, pssibly frmed frm direct alteratin f magnetite, which is present in sme tufts. Diffuse grain bundaries bserved in thin sectin in the mderately and highly decmpsed grades may indicate the cmmencement f alteratin f magnetite t haematite. Mre detailed studies are required n the nature f irn xide minerals in vlcanic rcks. Gibbsite was nt identifed in any f the sil specimens analysed, althugh Lumb & Lee (1975) reprted the presence f small quantities f this mineral in near-surface vlcanic sils. The minr ccurrence f gibbsite in Hng Kng sils, in spite f reasnably well drained cnditins, was cnsidered t be due t the relatively immature nature f these sils cmpared t thse fund in the mre trpical regins (Irfan 1996a). Clay veins and ther discntinuity infillings The main types f infilling fund in discntinuities are thse frmed as a result f weathering prcesses and

16 332 T.Y. IRFAN.~ ~. t, ~ t 3 t 3 t '5 as ~ ~ ~ z ~es ~ z ~.= ~ = ~'~.~ ~ ~ ] I I~ I~ I I~ I I I I P,. ~. - ~ ~s ~ ~ r.~ :.~ ~ ~ ''~ - ~.,...~ ~.~ ~ ~=~ O' " III ~'~ = ~ ~ ~~ ~ ~ ~.~.,.~ ~ "~" ~.~ ~,~ ~-~.. ~ ~ ~ ~ ~ ~ ~.~ ~.~_~ ~ ~ ~ ~ ~ ~ ~.~ ~ ~ ~::~ ~'-d ~ t~'~ ~.-~

17 CHARACTERIZATION OF WEATHERED VOLCANIC ROCKS IN HONG KONG 333 Table 8. Results f quantitative bulk and clay mineralgy (based n chemical analyses and XRD) Sample n. Weathering Feldspars Quartz Micas Kalin Others Irn grade Kf Pf % % minerals % xides % % % % Clay prprtin K H M Lc. 1. IR1 IR2 IR4 IR52/53 IR34135 IR61/62 Lc. 2. TKO1 TKO2F TKO2W TKO3 TKO4 Crystal rich TKO6 TKO7 TKO8 Lc. 3: ATW1 ATW2 ATW3 ATW4 ATW5 ATW6 Lc. 4: SK7 SK5 SK4 SK2 SK3 SK1 Lc. 5: YL1 YL6 YL3 YL5 YL4 SD (B) -- 5 (V,C).... MD (B) 4 Tr (V) -- D -- Tr HD (B) CD CD Tr CD (M) 17 Tr (C-V) Tr A D Tr-A CD (M) 31 Tr (C-V) 3 (G) A D Tr-A TS (M) 4 Yr (C-V) Tr (G) D A Yr F Tr 8 < 5 (C) 4 N/A N/A N/A SD (fresher cre) Tr 1 <5 (C) 4 N/A N/A N/A SD (stained rim) Tr 2 <5 (C) 5 D Tr? A-Tr MD Tr 2 <5 (C) 4 (G) D Tr? A-Tr HD (altered) <5 (C) 6 (G) D Tr? A CD D SD Tr CD Tr (V) 4 D -- Tr-A TS Yr 56 Tr (V) 9 (G) D -- Tr F Tr Tr Tr (C) 2 N/A N/A N/A SD (slightly discl.) Tr 11 Tr (C) 4 N/A N/A N/A SD (md. discl.) Tr D -- A HD Tr D A Tr HD (altered) Tr D A-Tr Tr-A CD Tr CD CD Tr SD Tr 15 Tr (C) 3 N/A N/A N/A MD (C) 2 (G) A -- D HD < D Tr Tr-A CD < D -- Tr-A CD < D -- Tr-A RS (G) D -- Tr SD (fresher cre) N/A -- 4 N/A N/A N/A HD CD -- CD HD CD -- CD CD Tr (C) 5 D A A CD D -- A Kf, alkali feldspar; Pf, plagiclase; K, kalinite; H, hallysite; M, micas (undifferentiated fine-grained micaceus minerals, e.g. sericite, hydrus mica, muscvite, illite) unless specified; B, bitite; G, gethite; C, chlrite; V, vermiculite; C-V, interlayered chlrite and vermiculite; N/A, nt analysed r determined. See Table 6 fr dminance scales. Weight percentages d nt always add up t 1%, the surces f errr are prbably due t the presence f nn-quantified minerals as well as calculatin errrs due t the stichimetries assumed. have similar mineralgy t that f the weathered rck (Irfan 1998). The infillings may als be frmed by cntact metamrphism, hydrthermal alteratin r mineralizatin prcesses and are preserved as relict features as the rck weathers. White clayey kalin veins f hydrthermal rigin are cmmn at sme sites in Hng Kng. The XRD analyses f a number f kalin veins taken frm the landslide site in lcality 1 shwed a dminant cmpsitin f kalinite with sme illite and quartz and, in sme cases, a smectite mineral (Irfan 1994). Optical micrscpy indicated that there may be an increase in the micaceus alteratin prducts (mainly frmed within the feldspars) adjacent t these veins and in 'pink feldspar' areas. This may represent the situatin described by Sales & Meyer (1948) wh nted that in the vicinity f hydrthermal veins in igneus rcks, a znal arrangement f clay minerals may ccur, with a sericitic

18 334 T.Y. IRFAN ~ I ~ ~ ~'~.. ~,. "~.b ~ - ~... ~ E C ~2.~ ~ -~.t--.b_ ~ ~.~ ~ ' ~ ~

19 CHARACTERIZATION OF WEATHERED VOLCANIC ROCKS IN HONG KONG 335 mica zne immediately adjacent t the vein, fllwed by kalinite and smectite-chlrite znes. Smectite may als frm frm weathering prcesses in a prly drained envirnment. The infillings derived frm weathering are usually cated r intermingled with irn xides, giving a buff t reddish brwn clur, r mixed with manganese xides, giving a dark grey t black appearance. Bth f these xides are leached frm the upper levels in the weathering prfile as by-prducts f decmpsitin f irn and manganese rich minerals. Humic substances may als ccur mstly in the tp part f the sil zne. Weathering indices A review f the weathering indices prpsed t assess the degree f weathering f rcks and their suitability fr igneus rcks is given in Irfan (1996a). A large number f these weathering indices are derived frm the results f chemical analyses. A number f petrgraphic indices are als determined frm the results f the mdal analyses. Several better knwn chemical and petrgraphical weathering indices prpsed fr igneus rcks were selected and their numerical values were calculated fr the vlcanic rck types studied. As the physical and mechanical prperties f the vlcanic rcks studied were nt available, the indices derived frm these prperties were nt determined. Chemical indices The definitins f the chemical weathering indices used in this study are shwn in Table 1. The results calculated fr carse ash tufts (Table 11) and fine as tufts (Table 12) included in the study indicate that the weathering ptential index (WPI), the weathering prduct index (WP), alumina-t-ptassium-sdium (AKN) rati, silica-t-alumina (SA) rati, the lss-nignitin (LOI) value and the mbiles index (Imb) are generally gd indicatrs f the degree f chemical weathering in vlcanic rcks. These indices were als fund t be gd indicatrs f the degree f chemical weathering f granitic rcks (Irfan 1996a). Hwever, in the case f the vlcanic rcks studied in this paper, which have variable mineralgy and chemical cmpsitin, these indices appear t be mre suitable fr characterizing the weathered materials frmed ver a particular rck type in a particular weathering envirnment. This is illustrated in Fig. 9, which is a plt f silica-t-alumina rati against mbiles index fr tw weathered carse ash tufts. In additin t chemical variatin in the riginal rck, the envirnment and histry f weathering at each site as well as ther alteratin prcesses will affect the mineralgy and chemical cmpsitin f the weathered prduct. Figure 1 shws an almst linear relatinship between WPI and Im b fr ne rck type in cntrast t a wide scatter f values shwn in Fig. 11 when the data frm all vlcanic rcks are included. Nnetheless, an bvius trend is present which suggests the relatinship hlds fr several rck types, except perhaps in advanced stages f weathering. The alumina-t-calcium-sdium (ACN) rati appears t be useful nly fr indexing the rck grades f weathering and is nt sensitive t chemical changes ccurring in the sil grades. The lixiviatin index (13) is nt a gd indicatr f weathering even fr the sil grades, which was the purpse riginally prpsed fr it by Rcha-Filh et al. (1985). The mbiles index prpsed by Irfan (1996a) serves as an index f the degree f decmpsitin f feldspar-bearing rcks, particularly in reasnably well drained (leaching) cnditins, such as thse fund in mst hillsides and hilltps in Hng Kng. The silica-talumina (SA) rati has als been fund t be a gd index f chemical weathering in mst free draining, usually acid, weathering envirnments in humid climates, especially fr acid and intermediate cmpsitin rcks that weather t kalinite r illite grup minerals (Ruxtn 1968). Bth indices are nt cnsidered suitable fr basic and ultrabasic igneus rcks, r fr weathering cnditins that result in smectite r vermiculite minerals. Althugh mst chemical indices are fund t be accurate measures f degree f chemical weathering, the determinatin f these indices requires the use f specialist chemical labratries and sample preparatin techniques. In additin, the chemical indices d nt accunt fr the physical and fabric changes that ccur in the rck with prgressive weathering. The engineering prperties f weathered rcks are greatly dependent n the fabric and bnding characteristics, even in the sil grades (Massey et al. 1989). M icrpetrgraphic indices Petrgraphical methds using an ptical micrscpe and gd quality thin sectins are very useful in studying weathering effects. Nt nly d they allw lw cst and rapid mineral identificatin but als the micrfabric prperties f the materials can be studied and characterized fr the whle range f the weathering spectrum, except in sme sils where thin sectin preparatin is difficult. The pint cunting technique described by Irfan & Dearman (1978) was emplyed t determine the mdal cmpsitins f the mineral cnstituents, vids and micrcracks fr the weathered carse-grained crystal tufts nly (Table 9). Because f the extremely finegrained nature f the matrix minerals, n individual mineral determinatins were made and these were

20 336 T.Y. IRFAN t '3,,.--.,,...~ c~ + Z + -t-..~ e-i + e,l ~ + G ~k e-i + + e,i 6 - H Z Z II E Z ~ y ~,,.-.x.<.~.~..,,,~.g ~1..~.,,~ ~1 c~

21 CHARACTERIZATION OF WEATHERED VOLCANIC ROCKS IN HONG KONG 337 Table 11. Chemical weathering indices fr carse ash tufts Weathering grade Sample Weathering Weathering Silica t Alumina t Alumina t Lixiviatin Mbiles Lss n n. ptential prduct alumina ptassium calcium index index ignitin index index (SA) rati sdium sdium (13) (Imb) (LOI) (WPI) (WP) (AKN) rati (ACN) rati Lc. 1: SD IR SD IR MD IR SD IR HD IR HD IR CD IR52/53/ CD IR34/35/ TS IR61/62/ Lc. 4: SD SK MD SK HD SK CD SK CD SK RS SK cnsidered tgether as the 'fresh matrix' r 'altered matrix' minerals. It was nt pssible t differentiate between varius fine-grained alteratin prducts and small pres in the mre weathered specimens under the nrmal reslutin f an ptical micrscpe. The results are, hwever, still useful as an apprximate means f quantifying the bserved fabric changes ccurring in each material. The technique was fund inapprpriate due t ptical reslutin limitatins, fr the vlcanic rcks with large cryptcrystalline mineral cntents, especially when they were weathered beynd the slightly decmpsed grade. Als, general irn staining r cementatin f the fabric in mderately and mre intensely decmpsed vlcanic rcks made the identificatin f mineral cnstituents very difficult. Table 12. Chemical weathering indices f r fine ash tufts Weathering Sample Weathering Weathering grade n. ptential prduct index index (WPI) (WP) Silica t alumina (SA) rati Alumina t Alumina t Lixiviatin Mbiles Lss n ptassium calcium index index ignitin sdium sdium (13) (Imb) (LOI) (AKN) rati (ACN) rati Lc. 2: F TKO SD TKO MD TKO HD TKO HD TKO CD TKO CD TKO TS TKO Lc. 3." F ATW SD ATW SD ATW HD ATW HD ATW CD ATW

22 338 T.Y. IRFAN Carse Ash Tuff, Aberdeen I I ISAR = Imb [ ~3 < < Carse Ash Tuff, Sai Kung [ X SAR = Imb R 2 =.85 J! Fresh t Slightly decmpsed Highly decmpsed x Transitin sil/residual sil Mderately decmpsed Cmpletely decmpsed - - Least Square Line. I I I I ~ I I I [ MOBILES INDEX (Imeb) Fig. 9. Silica-t-alumina rati versus mbiles index z -I Z el 7, [- < -3-5 Carse Ash Tuff, Aberdeen I R 2WPI=.98= 7, Imb -7 Fresh t Slightly decmpsed Highly decmpsed x Transitin sil/residual sil Mderately decmpsed Cmpletely decmpsed - - Least Square Line -9 I I I ~ I I I I I MOBILES INDEX (Imb) Fig. 1. Weathering ptential index versus mbiles index, carse ash tuff.

23 CHARACTERIZATION OF WEATHERED VOLCANIC ROCKS IN HONG KONG T ~ 8 z < Z F-! -2 L r,! -4 + N X z < -6-8 Fresh t Slightly decmpsed Mderately decmpsed Highly decmpsed Cmpletely decmpsed x Transitin sil/residual sil + Hydrthermally affected -1 ~- ~ I i ~! I MOBILES INDEX (I,,~) Fig. 11. Weathering ptential index versus mbiles index, all rcks. Classificatin f weathered vlcanic rcks and sils in Hng Kng A classificatin f the material grades in vlcanic rcks f acidic cmpsitin (primarily the tufts) in Hng Kng is given in Table 13 in terms f their mineralgical and fabric characteristics. Althugh nly the mineralgy and fabric characteristics f the fine and carse ash tufts were studied in detail, the general mineralgy and fabric characteristics bserved in these rcks are cnsidered t apply t ther vlcanic rcks in Hng Kng, except perhaps the intermediate (andesitic) types. In sme cases, variatins in fabric and ther prperties are t subtle fr field identificatin f the slightly, mderately and highly disclured subgrades in weathered rck r the decmpsed, disintegrated r altered subgrades in sil. Hwever, in sme cases, a labratry study may be needed, especially fr the sil specimens shwing the effects f mre than ne prcess. Althugh it may suffice t use the six-grade classificatin scheme, r even a simpler scheme, fr general engineering purpses, further subdivisin f sme f the grades, especially the sil grades, may be warranted in sme engineering situatins, fr example, fr slpe stability purpses (Irfan 1994). Further subdivisin f sil grades may be based n simple field assessment f relative strength (density r cnsistency) as carried ut part f a standard sil descriptin. Fr mre detailed wrk, the subgrades wuld need t be defined in terms f a labratry r field measured sil prperty. It is cnsidered that the material classificatin presented in this paper, which is based n the classificatin system cmmnly used in Hng Kng but expanded t include all types f material which can be present in a weathering prfile, is applicable t mst vlcanic and igneus rcks, except perhaps mre recent vlcanic depsits (see Wesley 1988). A classificatin f varius sil types frmed frm vlcanic and granitic rcks is given in Table 14, tgether with a summary f their structural and mineralgical characteristics, in accrdance with the classificatin scheme prpsed by Wesley & Irfan (1996). The saprlitic sils f bth granitic and vlcanic rigin fall int Grup A(a) t (b), as structural influences (e.g. relict discntinuities) are dminant in cntrlling their behaviur. The residual sils wuld generally be classified as Grup A(c) sils. Thin layers r lenses f Grup C(c) sil rich in sesquixides, lsely referred t as lateritic r laterite, may frm near grund surfaces, particularly in prly drained areas. It wuld nt be apprpriate t include the lwer prtin f the highly weathered rck grade f BS 593 r zne 4 f the Gelgical Sciety f Lndn (1995) in any sil classificatin as this subzne generally behaves as a rck in all engineering situatins. It wuld nt be pssible t differentiate between varius sil grades r classes which can frm frm

24 34 T.Y. IRFAN >,.=_ =~ "~,. ~~ ~ :=_ ~.d~,, ~)~,, ~ ~, ~~ :~ ~m 6 ' C~ C ~..., : 6.-~. ~ :::I f~'id._.9. ~ "~._.~._N ~ ~.. OO e- O ~ O ~ "6~ ~',--- ~'~ 8~ "~'~ '~,~ Z,.E: ~,,, ~ ~,..~I r", ~..~,". ~.,,- E,~ "~,. ~..~ r- r, ~.I ~- e.,.- ~, ~.~ ~ ~f~e6,-- ~ u ;:,,., _ ~ -_~.E_~ c~.,.~ ~.~_ m._ ~_~ ~.E,~.~,~,,,,E ~'.,-- ~ ~, ~ ~,~.,~-- ~ -- ~..-~,_- ~,O q,.- ~,~ _~ ~.~i,_~.~ ~2 ~<_~ --..~.~_ ~.~ ~-.-- ~ E "~ ~ -~ ~_~,~ ~'~, ~ ~._ ~. ~ _ ~ ' ~ ~"._ g ~ ~ ~^.::= >, ~ ~.- ~ ~, ~ ~ ~, ~ ~ ~, ~ ~,~, ~, ~.~-', ~ ~ ~, : ~.~ ~-~ ~ --,.~ ~ ~e-..,.~ ~,-, ~ ~.~ ~', ~. N ~ ~"i ~I ~ ~I~.~ "18 ~ ~.-- ~ ~,., ee~ ~ ~ :::,-',. ~.~,,~,~ ~.~ -= r,.~. -~.~,X ~.~,,,.~,_~-. >, - ~-,~,~._..,. ~ 6 ~,,-, ~ ~ ~'.h~ ~ ~ "-.~.~ 6 Z'Z' ~.~ ~ - - ~ - - ~ ~ ~:~. _ ~" ~..~.. "~ ~ u... ~,.~ ~,.~', ~ ~". ~I"~' ~'~,~ ~j::~,-' ~'--,- ~ y.. "N -6 ~,.~ ~-,-- ~ ~-.~ ~.= ~= =.., ~,'~ ~ "~. ~ ~.~,._'~._~,~ ~E~ ~._~.~.. ~ "~' --~-~N E', ~-~, E.~ ~,, N~-',

25 CHARACTERIZATION OF WEATHERED VOLCANIC ROCKS IN HONG KONG 341,.--., ~:= -~,.~= -- ~.=_ ~E.~ ~= ~5r: U-r~= < "d.=. ~-.~ '~ =.~ ~=-'~ -. 8 ~ ~ - ~ ~- ~~ ~.~ ~ ~1~.-~ ~'~ ~ ~'~r. ~.~-~" ~I..,~,j 1-.~ "~E. ~ ~, i = ~.-~ d ~ E "~,.~.- >., r" "~ ~ O. "~ ~.~ I'.~ ~:,.-."{3 = =.=ps =- I===, ~ ~.- I~ ~E ~.. ~ ~ ~ J ~ ~l ~.=,..,J G ~' _ ~ ~- ~ =.~ ~.~ ~ ~ ~ ~ ~ ~ >~ ~ ~, ~..~.-- ~ ~,-I=: --..,, 8~..= w j ~ v, ~.>. ~ "," E ~ ~ ~'~.~,.~..-..el} f ~ = I. ~1 t ~ ~n.--,~,.=,..~ ~ ~. "~ ~. ~-._~ " ~ - E.9 ~1 *-,._.9. ~.._= w = ~...,,-- ~ m.x;~ ~ m,.,4 "-~.~, ;>.~.=.. = t::= =1 I::: "~ E ~'~"~ ='~,., ~.. ~ R =~~ ~-~._,-~. - ~ ~_,.~ ~:.-- >., "~. ~ I= ~,...] ~,.~,.~ "r-,.~.~,~,~,'~ ~ ~, ~ [3 -, "-~ ==..=- '=~.. ~ = ~d ~ z

26 342 T.Y. IRFAN weathering f vlcanic rcks under the classificatin scheme prpsed by the Gelgical Sciety f Lndn (Ann 199). Almst all sil znes including the highly weathered vlcanic rck zne wuld be classified as 'ferruginus (sensu strict) sil', except perhaps the uppermst residual sil zne, which wuld be described as 'ferralitic (kalinitic)'. Effects f fabric and mineralgy n the engineering index and classificatin prperties, and behaviur f vlcanic sils Fabric and bnding The micrstructure (encmpassing bth bnding characteristics and fabric) in saprlitic and residual sils is largely a result f the initial fabric f the parent rck and the histry and lcal envirnment f the weathering and ther alteratin prcesses. Bth weathering and alteratin prcesses invlve frmatin f new minerals, the lss f material by slutin and gain f material by precipitatin. The term fabric is used t mean the gemetric arrangement f particles, including particle size distributin and vid characteristics. Saprlitic vlcanic sils have a relict primary interparticle bnding between individual unaltered, partially altered r cmpletely altered grains. Relict bnding is significant enugh t give a large true chesin t the sil in the least weathered specimens (i.e. highly decmpsed grade). In additin, a weak secndary bnding due t cementatin effects and arising frm the electrical attractive frces between clay minerals may als be present. In the mre intensely weathered sil specimens (i.e. transitin and residual sil grades), the relict primary bnding gradually disappears and secndary bnding may increase. The secndary bnding arising frm the cementatin effects f the sesquixides in Hng Kng is nt as strng as that ccurring in the sils frmed in mre trpical regins. It shuld be nted, hwever, that even a bnding s weak that a sample can scarcely be handled still prvides a cmpnent f strength and stiffness that may have an influence n engineering prperties (Ann 199). A significant amunt f crushable particles (e.g. partially decmpsed feldspar grains) are present, particularly in the highly and cmpletely decmpsed grades. Als, sme clay and fine silt particles are present in the interstices f carse partially decmpsed mineral particles. These finer particles wuld usually nt cntrl the engineering behaviur f the sil in many civil engineering applicatins unless the sil is used in the cmpletely remulded state. The fabric and interparticle bnding are sensitive t disturbance during drilling and sample preparatin. Sme r cmplete lss f relict bnding may ccur even with high-quality air fam drilling, especially in weakly bnded sil. Further destructin f bnding and fabric disturbance is likely t ccur during sample preparatin, particularly f small sized specimens, fr testing in the labratry (Vaughan 1988). M ineralgy Kalinite appears t be mre dminant than hallysite in sils f vlcanic rigin. In cntrast t the granitic sils, the hallysite cntent can be significant, cmprising up t 3% f the sil. These sils als cntain variable quantities f xides, hydrated xides and hydrxides f irn and aluminium (sesquixides). Hallysite and sesquixides have distinctive characteristics, as they are sensitive t the temperature and relative humidity f the in situ envirnment as well as the testing envirnment in the labratry (Irfan 1996b). Sils rich in these minerals d nt nrmally give reprducible results in labratry tests unless strict temperature and humidity cnditins are bserved (Mendza 1985). Hallysite slwly changes t dehydrated hallysite (metahallysite) and then t kalinite in the grund (Lumb & Lee 1975). The change t dehydrated hallysite can als ccur in a lw temperature and humidity envirnment during strage prir t testing (Irfan 1994). Hydrated hallysite cntains water within its crystal structure. Althugh this water has n effect n engineering behaviur, it can be driven ut f the mineral lattice by drying at temperatures ver 5 C r when the humidity falls belw 5 /,, (Ann 199). Therefre, drying f sil at the standard temperature f 15 C can give rise t errneusly high misture cntents in hallysite rich sils. In additin, n drying, the tubular hallysite lattice cllapses frming 'kalinite' plates and metahallysite aggregates, which are f smaller grain size. This may have an imprtant impact n grain size and plasticity changes during the handling and strage f sils rich in hydrated hallysite. Allphane is anther drying-sensitive hydrus alumina silicate mineral that is abundant in sils frmed ver recent vlcanic rcks (Wesley 1988). Hwever, it was identified t be present in vlcanic rcks f Hng Kng nly in the initial stages f feldspar decmpsitin. The main sesquixide minerals present in vlcanic sils are haematite giving the sil a red clur, gethite a brwn r chreus clur and small quantities f gibbsite, which was nt detected in the specimens analysed in this study. Hydrated sesquixides als lse their water f hydratin and becme bth carser in grain size and less plastic when ven-dried at 15 C. This is due t creatin f a strnger bnd between the particles resulting frm the aggregatin

27 CHARACTERIZATION OF WEATHERED VOLCANIC ROCKS IN HONG KONG 343 k. CO E =.~ =.-~:~ E%.~_ -- ~.~_ =... 9 ~ u.~a._,9. = ~ ~-~ ~-~,.-- "~-g~...~._ - ~ -~ "~- ~ E ~ =~ ~ ~",: e.~,..~.~ ~ E E ~..~:~. :=,~.~ - =.-~ "~ ='-6 ='~ =1 ~ "--.~. ~~_~ ~. - ~, ~ ~ ~, ~ N=.:-- ~= ~ ~E: ~. 1: ~..~ ~ ~ I~.'- b",~'~.- ~ ~.~'~ N ~ ~~.~ ~ ~, ~ ~.=~..., ~, ', =: ~J r~ ~ ~ ~-,~ -- c~ "J *" "~'-- ~J... 3 '~.....: ~, ~ ~% h'v'-g ' '-,,' ~ ~ ~ ~,,,,,~ ~ =._-- ~ >~' ~ ~'E ~'-- E: ~''~, ~-' > -- r -= ~ ~--~ :~ "-~ =~ ~ >~ ~a 2 ~._:'~ -,. ~IX ~ >,>,~:~ ~I," ~,..~.x~,",. ~1~ ~~ ~:~'~.' ~ ~. l.5 ~ ~. ~, {::1~ ~ ~J.~" ~1~ 'a-~ m ~" ~ - ~ ~ ~.-~:~ ~.~. ~ ~ ~ ~ ~' ~ ~- ~ e.- ~1 ~ ~-$~ ~-~ ~ ~" ~. ~-~ ~.- ~ ~ ~ "~ ~ ~ ~,E ~ ~n ~J ~,-~,-. ~' ~"~ E: E~ "~ ~'~: E~6... ~: ~.E. - z ~ -~ ~(J%, Z ~ I 2~IE >~ ~ ~ " ~1.= ~ ~ ~ ~ ~1'~.~ ~~ -- ~3 ~D'-- ': ~ ~lca'- ~ ~, = ~ > " ~ ~1 '.~ "--- ~ :3 ~ =3 " ~ =~. =~. (/3 ~E: ~:~ ca r.~ ~.'J ~...~ r~ I -- ~,.~ C ~. =. b~ = " r.n ~- = ;>,, ~ "E ~jc~ c~ "~ % ~E r..~. = N.=.~=.,~ >,,~ O~ "~ ~.~=~ ~..c: "~ ""~ ~ "~ ---'~ ~ ~ ~',~ ~ *-~ Oca~: ~ ~.- :-: ~ ~'~ ~:.~ - ~ ~j D"- ~E LJ.~ ~,- "~ ~-' ~ ~.~ ~ ~ ~- ~ ~ ~ "F. ~ c~ ~ ~ E- ~-- [.:~

28 344 T.Y. IRFAN f drying particles, which then resist penetratin by water. Sil index and classificatin prperties The vid rati and vid characteristics f saprlitic and residual vlcanic sils may vary widely, independent f riginal rck type and stress histry. Because f the bnded structure, vid ratis can be much larger than is pssible fr de-structured sil f the same grading fr the same stress cnditins. Vid ratis f.3 t 1.2 fr granitic sils have been reprted in Hng Kng (Lumb 1962; Massey et al. 1989). Vlcanic sils als have a similar range f vid ratis (Lumb 1962; in-huse test results). In cmmn with granites, the accurate measurement f particle size pses a majr prblem in vlcanic sils. Due t the presence f crushable particles, dryingsensitive minerals and the weak t relatively strng bnding between aggregated grains, a meaningful grain size cannt be determined using the standard test prcedures. Particle size analysis will be sensitive t the temperature f drying, severity f disaggregatin, duratin and intensity f sieving as well as the type f dispersant used during disaggregatin and prir t sedimentatin (Mendza 1985). Due t varius factrs related t the nature f sil, the prcedures t be fllwed fr particle size tests shuld be decided carefully in the light f experience and the end-use purpse f the data. The results f particle size determinatins shuld nt be used in the cnventinal way fr the identificatin and classificatin f these sils due t the greater influence f relict bnding and structural effects (e.g. relict jints), especially in saprlitic sils. The clay minerals may be lcked in within partially decmpsed primary grains. These minerals wuld nt be dispersed unless such grains are brken dwn by mechanical means, therefre, aviding a significantly lwer clay cntent as btained in standard particle size determinatins. In additin, hallysite may ccur largely in silt sized tubular grains, usually in the range f 4 t 8 pm (Irfan 1996a). Very large variatins in grading have been reprted fr vlcanic sils cllected frm a particular site r even amngst the specimens taken frm the same blck sample (Irfan 1994). In additin t differences due t sample pre-treatment and preparatin prcedures, the main reasn fr this variatin in grain size is the variatin in the mineralgical cmpsitin and fabric f these sils. The mineralgical and fabric variatins result frm variatin in the degrees f physical and chemical weathering, alteratin and leaching cnditins at each site and between the sites. Plasticity characteristics f hallysite rich sils als depend upn sample preparatin techniques. Increased mixing, remulding and drying at high temperatures will result in increased plasticity due t the breakdwn f tubular structures and the release f mre clay sized particles int the sil water mix. Hallysite rich sils ften have index prperties (determined by traditinal labratry techniques) which plt them well belw the A-line chart (Wesley 1988). N test results are available n Hng Kng sils t assess the effects f mineralgy and varius pre-treatment prcedures n plasticity indices. The results f plasticity tests shuld als nt be used in the cnventinal way fr the identificatin and classificatin f such weakly bnded sils. The relatinships develped between plasticity indices and ther sil parameters wuld nt be applicable t these sils, as bnding wuld have a great influence n the engineering prperties. Specific gravity (r particle density) is used in the calculatin f prsity r vid rati. If the specific gravity is nt measured prperly r an incrrect assumptin is made, errrs ccur in the determinatin f vid rati r prsity. The partially decmpsed minerals r whlly decmpsed pseudmrphs, which are present in large quantities in saprlitic sils, cntain numerus internal (i.e. clsed) vids. If these minerals are nt brken dwn cmpletely, errneus results may be btained in particle density determinatins especially when the small pycnmeter methd is used due t difficulty in ensuring cmplete remval f air frm the specimen. Suitability f standard labratry tests t saprlitic and residual sils Until recently, the mst cmmnly adpted sil testing methds in Hng Kng were based n the 1975 versin f the British Standard BS Since 1994, the 199 versin f BS 1377 has been adpted in Hng Kng fr mst sil classificatin and cmpactin tests with sme amendments t suit lcal sil cnditins fllwing a review by the Getechnical Engineering Office (1997). Disaggregatin and mixing The methd adpted fr disaggregatin shuld be suited t the purpse fr which the results f a particular classificatin test wuld be used. In general applicatins, excessive breakage f the particles shuld be avided by adpting a sft apprach, which wuld be n mre severe than using rubber pestle and mrtar. In sme cases, disaggregatin by saking in water fllwed

29 CHARACTERIZATION OF WEATHERED VOLCANIC ROCKS IN HONG KONG 345 by applying gentle finger pressure t crumble r mesh away lumps may be sufficient (Irfan 1988; ASTM D ). ensure hmgeneity and full remulding shuld then be assessed in particular fr thse sils t be used in the remulded state as fills. Misture cntent Since mst trpical sils are affected by drying even at lw temperatures, all classificatin tests fr vlcanic sils shuld be carried ut in their natural state (r field misture cntent) with as little drying as pssible, except fr the determinatin f misture cntent. The Gelgical Sciety f Lndn (Ann 199) recmmends that cmparative testing shuld be carried ut n duplicate samples f each sil type frm each site at a temperature f 15 C and at a temperature belw 5 C. If a significant difference exists between the results, then the misture cntent shuld be reprted fr the lwer temperature. Hwever, it will be difficult t decide n the significance f variatin frm the cmparative tests carried ut n individual specimens due t variable quantities f drying-sensitive minerals in these sils. Therefre, it is recmmended that all misture cntent determinatin f saprlitic and residual sils shuld be carried ut at a temperature belw 5 C, say 45 C. Cmparative tests at rm temperature and at 15 C may still be useful t assess the relative prprtins f hallysite and sesquixides present in the samples. Precautins shuld be taken t limit the misture lsses during transprt t and strage in the labratry. Atterberg limits N test results are available n Hng Kng sils t assess the effects f mineralgy and varius pretreatment prcedures n Atterberg limits. Hwever, as these sils can cntain significant quantities f hallysite, sesquixides and crushable, partly decmpsed grains, the results f Atterberg limits tests are likely t be affected t different degrees depending n the prprtin f any f these cnstituents. Therefre, it is recmmended that all vlcanic saprlitic and residual sils shuld be kept in their natural state withut any pre-treatment by drying, prir t testing fr Atterberg limits. Disaggregatin f thse sils shuld be carried ut by saking in distilled water and crumbling any lumps f sil by hand. In carrying ut plasticity tests fr fine grained sils with all particles passing the 425 gm sieve, the Gelgical Sciety f Lndn (Ann 199) recmmends that trials using a range f mixing times prir t testing shuld be carried ut t determine the sensitivity f the sil t wrking and mixing. The shrtest time t Particle size determinatin The standard methd fr particle size determinatin in British Standard BS 1377:199 requires the sil sample t be ven dried at 15 C t 11 C prir t wet sieving and, fr the material retained n the 63 gm sieve, prir t dry sieving. This is cnsidered inapprpriate fr sils cntaining sesquixides and hallysite because drying at such high temperatures will cause aggregatins f clay particles t frm. The wet sieving methd is nw cnsidered t be the prcedure mst suitable fr Hng Kng sils. The sedimentatin methd can be used t determine the distributin f the fine fractins but the sample shuld als nt be dried in the ven. These prcedures have nw been adpted in the new sil testing standard fr Hng Kng (Getechnical Engineering Office 1997). Specific gravity (particle density) determinatin The British Standard test methd fr the determinatin f particle density has nt yet been revised fr Hng Kng sils. British Standard BS 1377:199 recmmends a temperature f drying f less than 8 C if the sil cntains minerals which are susceptible t lss f water f hydratin at 15 C. The authr recmmends that these sils are tested at their natural misture cntents r as-received state withut drying. Althugh the British Standard recmmends grinding dwn nly particles larger than 2 mm, it is recmmended here that the whle sil be thrughly grund. Extreme care shuld als be taken in ensuring cmplete remval f air frm the test specimen. Als, the presence f a large prprtin f heavier (e.g. sesquixides) and lighter (e.g. hallysite) particles may give erratic results fr the particle density even with greatest care in testing and a number f repeat tests may be needed t btain a gd average. Cmpactin tests Fr the determinatin f dry density/misture cntent relatinship f sils that cntain crushable grains, BS 1377:199 recmmends that a separate subsample shuld be used fr cmpactin at each misture cntent. Care shuld be taken nt t induce excessive breakdwn f the particles and sil structure prir t and during cmpactin as mst Hng Kng sils, especially

30 346 T.Y. IRFAN saprlitic sils, cntain a large prprtin f crushable grains as well as an pen weakly bnded structure. The degree f cmpactin used in the labratry shuld be similar t that likely t ccur in the field. The Gelgical Sciety f Lndn (Ann 199) recmmends the use f the lighter (2.5 kg) rammer methd fr crushable sils. This has als been adpted in the new Hng Kng sil testing standard. Hwever, the heavy (4.5 kg) rammer methd may be mre suitable if the sil will be subjected t heavy in situ cmpactive effrt. Cmplete drying f the sil at a high temperature befre testing is nt recmmended fr the reasns already described. The misture cntent determinatins fllwing cmpactin shuld als be made at temperatures belw 5 C, say at 45 C. Partial drying by air r ven drying at a temperature belw 5 C may be necessary fr sils cntaining misture cntents higher than the desired misture cntent. Cnclusins Althugh chemical weathering has been the dminant prcess, hydrthermal alteratin and tectnic prcesses were als imprtant in prfile develpment and sil frmatin in the vlcanic rcks f Hng Kng. Similar t granites, the residual sil zne (r grade) is prly develped in Hng Kng, usually less than a few metres thick in cntrast t thse fund in the humid trpical regins. This zne may be underlain by a thin transitinal sil zne befre the cmpletely weathered rck mass r saprlitic sil zne is reached. A mineralgy and fabric mdel fr weathering f vlcanic rcks in Hng Kng has been established, particularly applicable t acidic pyrclastic tufts in hilltp and hillside envirnments. Mineralgy and fabric changes ccurring in valley bttms and ther prly drained envirnments and in ther types f vlcanic rcks present in Hng Kng (e.g. andesitic lavas) have nt been dcumented due t the current nn-availability f data. Hwever, it is cnsidered that the mdel is als generally applicable t vlcanic rcks in these envirnments, with the pssible frmatin f sme nnkalinitic minerals and with less slutin effects. In mre basic varieties, which are f limited ccurrence in Hng Kng, a significant amunt f swelling minerals may frm in advanced stages f weathering. In spite f great variatin in their mineralgical and chemical cmpsitin and textures, the acidic vlcanic rcks, particularly the tufts in Hng Kng, shw many similar mineralgy and fabric characteristics in the way they weather. Hwever, significant differences can als ccur, even in the same rck, if the rck has been affected by metamrphism and hydrthermal r ther alteratin prcesses prir t weathering. Variatins in the present-day weathering and tpgraphical envirn- ment can als lead t significant differences in vid characteristics and clay mineralgy f the sils frmed frm the same rck at different lcatins. The mineralgy f sils derived frm intermediate (e.g. andesitic lavas) and basic rcks requires t be investigated fr cmparisn. Althugh it may suffice t use the six-grade classificatin scheme recmmended in Getechnical Cntrl Office (1988) fr general engineering purpses, further subdivisin f sme f the grades, especially the sil grades, may be warranted in sme engineering situatins. Further subdivisin may be based n simple field assessment f the dminant type f weathering prcess and relative strength (density r cnsistency) as carried ut part f a standard sil descriptin. Fr mre detailed wrk, the subgrades wuld need t be defined in terms f a labratry r field measured prperty. It is emphasized that the weathering effects are prgressive and rapid variatins in material grades and subgrades can ccur laterally as well as vertically in many sites. The rck mass weathering classificatin scheme prpsed by Getechnical Cntrl Office (1988) is nt generally adequate t characterize the sil znes in weathered vlcanic rcks. The mineralgically riented residual sil classificatin scheme prpsed by the Gelgical Sciety f Lndn is cmplex and uses many pedlgical terms unfamiliar t getechnical engineers. A mre recent classificatin scheme prpsed by Wesley & Irfan (1998), which grups the sils n the basis f cmmn engineering prperties and n the mineralgical and structural characteristics appears t be mre suitable fr classifying the sils frmed frm weathering f vlcanic as well as the granitic rcks. This scheme can be used as a supplement t the general mass weathering scheme r any ther sil classificatin scheme adpted fr the gruping and subdivisin f the sil znes fr engineering purpses. Mst prpsed chemical indices, in particular the mbiles index, the weathering ptential index and the alumina-t-ptassium rati, are gd indicatrs f the degree f chemical weathering f vlcanic rcks in free-draining, usually acid, weathering envirnments, especially fr acid and intermediate rcks. Hwever, direct cmparisn f index values at a particular weathering grade is nt generally pssible even fr the same vlcanic rck ccurring in different frmatins. Many f these indices may nt be apprpriate fr mre basic vlcanic rcks r fr weathering cnditins that prduce smectite minerals. In additin, the chemical indices d nt accunt fr the physical and fabric changes that ccur in the rck with prgressive weathering. The micrpetrgraphic indices, which are useful fr quantifying the fabric characteristics f carse-grained rcks, are nt apprpriate fr characterizing fine-grained vlcanic rcks, particularly thse with vitrified and cryptcrystalline textures.

31 a b & CHARACTERIZATION OF WEATHERED VOLCANIC ROCKS IN HONG KONG 347 The saprlitic and residual vlcanic sils f Hng Kng d nt have the extreme mineralgies shwn by their equivalents in the mre trpical regins, but they can cntain appreciable amunts f drying and treatment-sensitive hallysite and sesquixide minerals and crushable particles. Therefre, the standard labratry preparatin and testing methds used t determine the engineering classificatin and index prperties f temperate sils need t be mdified in rder t btain mre repeatable and meaningful results fr these sils. In additin t mineralgical cmpsitin, the effects f bnding and ther fabric features shuld be taken int accunt in the labratry testing and interpretatin f test results in these sils. The presence f bnding, which can be strng in deeper immature sils, can make the crrelatins develped fr traditinal sils between the index prperties and engineering behaviur inapplicable fr these sils. Extreme care is required in the sampling and applicatin f apprpriate testing techniques in rder t determine the 'true' engineering prperties f saprlitic and residual sils f igneus rck rigin. Acknwledgements. This paper is published with the permissin f the Directr f Civil Engineering, Hng Kng SAR Gvernment. References ANON 199. Trpical residual sils. Gelgical Sciety Wrking Party Reprt. Quarterly Jurnal f Engineering Gelgy, 23, ANON The descriptin and classificatin f weathered rcks fr engineering purpses. Gelgical Sciety Wrking Party Reprt. Quarterly Jurnal f Engineering Gelgy, 28, BRITISH STANDARDS INSTITUTION Cde f Practice fr Site Investigatins, BS 593." British Standards Institutin, Lndn. BRITISH STANDARDS INSTITUTION 199. Methds f Test fr Sils fr Civil Engineering Purpses, BS 1377.'199, Parts 1, 2 and 4. British Standards Institutin, Lndn. GEOTECHNICAL CONTROL OFFICE Guide t Rck and Sil Descriptins (Geguide 3). Getechnical Cntrl Office. 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