Interpretation of Subgrade Reaction from Lateral Load Tests on Spun Piles in Soft Ground

Transcription

1 Interpretation of Subgrade Reaction from Lateral Load Tests on Spun Piles in Soft Ground Ir. Tan Yean Cin, Ir. Dr. Gue See Sew & Ir. Fong Cew Cung G&P Geotecnics Sdn Bd, Kuala Lumpur, Malaysia ABSTRACT: Tis paper presents te results and interpretation of a lateral load test on a fully instrumented spun pile in soft ground for te land viaduct section of a ig speed train project in Alor Pongsu area. Te lateral load test of te spun piles was carried out on a pile wit fixed pile penetration to measure te lateral response of single pile and was tested by jacking against a reaction pile in accordance to ASTM D9 99. Palmer and Tompson s (9) formula for subgrade reaction (k ) was used to interpret te instrumented results of te lateral load test. Comparison wit Davisson s proposed k wit undrained sear strengt (s u ) was also made. Te results sow a correlation wit te coefficient of subgrade reaction (n ) wic removes te effect of subsoil dept and pile diameter. At depts up to m, Davisson s prediction is conservative but actual n is muc iger due to overconsolidation of subsoil at sallow depts. For deeper subsoil, it sligtly overpredicts due to passive resistance of te subsoil not yet fully mobilised. Te back analysed Davisson s constant, is found to be and is proposed for local soft ground conditions.. INTRODUCTION Te site is part of a project for a ig speed railway traversing te nortern region of Malaysia. Figure sows te location of te site. A portion of te railway of about km lengt is supported by land viaducts due to it s cost effectiveness and sorter construction duration as compared to piled embankment. As part of te pile verification tests, a fully instrumented preliminary test pile was installed and tested to validate te lateral pile performance. Tis paper presents te interpretation of te subgrade modulus from te results of te fully instrumented lateral pile test. Subgrade reaction approac was used as te design of te land viaduct is based on pile bent approac were te foundation design is based on integral bridge design concept witout bearings and minimum joints. Tis is a variation on te column bent approac were te supporting columns and foundation are replaced wit individual supporting piles (Tonias, 99). Hence, te bridge superstructure designer uses te subgrade reaction values as design input to analyse and design te forces in te land viaduct superstructure.

2 Site Location Figure : Location of te Site. LATERAL PILE RESPONSE MODELS. General Tere are many metods of analysing te response of a laterally loaded pile. Tese metods can be categorised into subgrade reaction approac and elastic continuum approac. Subgrade reaction approac as been initially proposed by Palmer & Tompson (9) and subsequently furter developed by Reese and Matlock (9). Furter advancements lead to te development of p-y curves and are commonly used to model te non-linear pile and soil beaviour. Tese ave been described by McClelland and Foct (9) and Davisson and Gill (9). Furter details and descriptions of p-y curves are summarised by Reese & Van Impe (). In tis paper, te subgrade reaction approac is used to analyse te results of te instrumented test pile. Tis metod is commonly used by bridge engineers to model and analyse pile bent structures. Te istorical development of subgrade reaction metod begins wit Winkler in modelling a beam on soil and subsequently adopting it to model embedded piles by oters. It caracterises te soil as a series of unconnected linearly-elastic springs in response to loading on te pile. In te model, te orizontal pressure (p) and te corresponding deflection at a point (y) is related by a orizontal modulus of subgrade reaction (k ): p = k Were: p = soil reaction per unit lengt of pile y = pile deflection k = subgrade reaction in units of force/lengt Palmer & Tompson (9) subsequently expressed te above equation in te form of: y

3 Were: k L = value of k at pile toe (z = L) n = a coefficient greater tan zero k = kl z L n For sands and normally consolidated clays under long term loading, n is taken as unity. For overconsolidated clays, n is commonly taken as zero. However, te commonly used form wen n = and adopting a variation of k wit dept: k = n z d Were n is te coefficient of subgrade reaction and d is pile diameter. Tis applies to coesionless soils and normally consolidated clays were tese soils indicate increasing strengt wit dept due to increase in overburden pressure. () () Figure : Subgrade Reaction Model of () Actual Soil Reaction on Pile & () Elastic Spring Model of Soil Reaction - after Prakas& Sarma (99). Coesive Soils A number of empirical correlations for k in coesive soils ave been proposed by Broms (9), Skempton (9) and Baguelin et.al (9). Broms (9): k =. * E / d Skempton (9): Were: k = ( ) * C u / E = secant modulus at alf te ultimate stress in undrained test C u = undrained sear strengt d

4 d = pile diameter However, for preliminary design before any verification test, a conservative approac suggested by Davisson s (9) was used: k = For coesive soils wit k increasing linearly wit dept, k is usually expressed in te form of k = n * z / d. Table summarises te typical values of n for coesive soils by various autors. C d u Soil Type n (kn/m ) Reference Soft NC Clay NC Organic Clay 9-9 Peat 9 Reese & Matlock (9) Davisson & Prakas (9) Peck & Davisson (9) Davisson (9) Davisson (9) Wilson & Hilts (9) Loess Bowles (9) Table : Typical Values of n for Coesive Soils. Coesionless Soils For piles in coesionless soils, Terzagi (9) proposed : γ n = A (tons/ft ). Typical values of dimensionless factor A is sown in Table Relative Density Loose Medium Dense Range of values of A n, Dry moist sand (kn/m ) [ton/ft ] n, Submerged sand (kn/m ) [ton/ft ] [] [] [] [] Table : Typical Values of n for Coesionless Soils - after Terzagi (9) 9 [] 9 []. LAND VIADUCT Te land viaduct consists of multiple spans, eac span is typically m long between piers. Eac.m wide pier is supported by six mm diameter ig strengt

5 circular spun piles. Te spacing between te piles is tree times te pile diameter. In te pile bent design, te spun pile ead is directly cast into te crossead. Figure sows a typical cross section of te land viaduct. Tis pile bent design is limited to m ig between te ground level and te soffit of te crossead. Figure sows part of te completed land viaduct. Figure : Typical Cross Section Figure : Completed Land Viaduct

6 . GEOLOGICAL AND GEOTECHNICAL CHARACTERISATION. General Geology Most of te route of te project traverses near te coastal areas of western Peninsular Malaysia. Tese areas are often underlain by marine alluvium formation. Figure sows te existing railway on te geological map. Te railway alignment traverses troug different materials ranging from soft marine clay to dense residual soil. In te test pile area, te subsoil is generally of soft marine clay from Quaternary and Holocene periods. Table describes te geology of tese two sections. Te test pile is located in Alor Pongsu, near Kamunting, Perak and is also indicated in Figure. Section & Location Formation Age Litology Taiping to Parit Buntar Superficial Deposit Table : General Geology of te Site Quaternary Gravel, Sand, Clay (Alluvium) LEGEND Marine and continental deposits Sale, Sandstone, conglomerate Cross-bedded Sandstone wit sale/mudstone Interbedded Sandstone Siltstone & Sale Pyllite, Slate & Sale wit subordinate sandstone & scist Pyllite, Slate, Sale & Sandstone Pyllite, Scist & Slate wit locally limestone. Scist, Pyllite, Slate & Limestone Sandstone/metasandstone wit subordinate siltstone, sale. Location of Test Pile Figure : General Geology of te Site

7 . Geotecnical Caracterisation Several series of subsurface investigation were carried out around Site A. Tese include boreoles, piezocones, field vane sear, mackintos probes and associated laboratory tests. Figure sows a few of te boreoles near te preliminary test pile wile Figure sows te laboratory test results on te soil samples collected. Location of Test Pile Figure : Boreole Logs From te borelogs, te test pile site is caracterised as igly compressible marine alluvium wit soft clay tickness of m to m. Soil classification tests sow tat te subsoil is generally of ig to extremely ig plasticity clay. In addition, it as ig compressibility as sown by te compression ratio of. to. and recompression ratio of. (Figure ). Stress istory of te subsoil sow tat te upper m is overconsolidated wile te clay stratum is generally sligtly overconsolidated (Figure 9) and very soft as sown by te undrained sear strengt profile (Figure ). In suc difficult subsoil conditions and were ig fills is needed if embankment is used, land viaduct was adopted as a more cost and time effective

8 metod of construction as compared to general ground treatment metods suc as piled embankment. 9 Plasticity Index, PI PI (%) 9 Low Plasticity CL Intermediate Plasticity CI Hig Plasticity CH MH Very Hig Plasticity CV MV Extremely Hig Plasticity CE A Line PI =.(LL-) ME 9 Legend MBH-B MBH-B MBH-B MBH-B MBH-B P9-MABH- P9-MABH- P9-MABH- P9-MAHA- P9-MAHA- P9-MAHA- P9-MAHA- P9-MAHA-9 P9-MAHA- P9-MAHA- P9-MAHA- P9-MAHA-9 P9-MAHA- P9-MAHA- P9-MAHA- P9-MAHA- ML MI 9 Liquid Limit, LL LL (%) (%) Figure : Soil Caracterisation Test Results Dept (m) CR=. Dept (m) RR=. CR= Compression Ratio, CR = cc / ( + eo) Figure : Compressibility of Subsoil Recompression Ratio, RR = cr / (+ec) Dept (m) C vo =. m /yr 9 9 Cvo, m/year Figure 9: Soil Stress History and Consolidation Parameters Dept (m) 9 9 OCR =. OCR =. OCR =. OCR =. OCR =. OCR =. 9 Over Consolidation Ratio 9 9 Legend MBH-B MBH-B MBH-B MBH-B MBH-B P9-MABH- P9-MABH- P9-MABH- P9-MAPZ- P9-MAPZ- P9-MAPZ- MPZ-B MPZ-B MPZ-B MPZ-B MPZ-BA

9 9 Dept (m) 9 9 Mackintos Probes Field Vane Interpreted S u Line 9 Su (kpa) / Mackintos Probe Blow Count per foot Figure : Undrained Sear Strengt Profile. PILE PROPERTIES AND TEST SETUP. Test Setup Te lateral pile test was carried out according to ASTM D9 99 similar to te test setup for testing two piles simultaneously. Te test pile is jacked against a similar or muc stiffer reaction pile at some distance away. For tis test setup, te distance between te two piles is.m centre-to-centre (tree pile diameters). Figure sows te lateral pile test setup. Te preliminary test pile consists of a mm diameter grade circular spun pile wit wall tickness of mm. Te pile is reinforced wit numbers of.mm diameter PC strands wit effective prestress of. MPa. Figure sows te details of te test pile. Te pile was driven to m dept and not to refusal as te aim of te test is to measure te lateral response of te pile wic generally becomes insignificant after about times te pile diameter. Te reaction pile is stiffer due to te ticker wall tickness of mm. Te reaction pile was initially used for te preliminary axial compression test and te manufacturer ad overcast te wall tickness to ensure te minimum mm tick sound concrete (excluding any laitance) was obtained after spinning. Te stiffer spun pile was used as reaction pile as it would provide a better reaction mass due to it s iger stiffness. 9

10 Signal cables of VWSGs, LVDTs & Load Cell to Datalogger. Reference Frame x x UC t VW Load Cell t Hyd. Jack Inclinometer Test Pile Reaction Pile Figure : Lateral Pile Test Setup Figure : Details of Test Pile. Instrumentation Details Instrumentation of te test pile consists of levels of strain gauges at specified levels and inclinometer in te ollow middle of te spun pile as sown in Figure. Te first six levels of strain gauges from ground surface ave four strain gauges in a cross-axis layout in anticipation tat some of te strain gauges may be damaged during pile installation. Te remaining levels are instrumented wit two strain gauges per level as te driving stresses at tese levels are expected to be muc less

11 compared to tose near te impact point. All strain gauges were welded to te PC strands of te spun pile from te outer pile perimeter and oused in a protective xx.kg/m C-Cannel ousing. Figure sows te C-Cannel ousing on te test pile prior to pile installation. Te inclinometer is a mm diameter inclinometer tube installed centrally in te ollow centre of te spun pile wit te aid of spacers. Subsequently, te void between te inclinometer tube and inner diameter of spun pile was grouted wit bentonite cement mix in te ratio of : to old te inclinometer tube in place. Sample cylinders of te mix were cast during mixing to ceck te in-situ strengt at days wen te pile test was carried out. Te results on tree cylinders sowed an average unconfined compressive strengt of about kpa, wic approximately corresponds to undrained sear strengt of about kpa and is sufficient for te purposes of te load test. Test Pile Reaction Pile Figure : Elevation View of Testing Setup and Instrumentation Details

12 Figure : C-Cannel Protection of Strain Gauges on Spun Piles (Lee, ) Test Pile Figure : View of Setup during Testing

13 Figure : Top View of Test Pile wit Inclinometer Tube. LATERAL LOAD TEST. General For tis test, te pile was tested to a test load of kn. Altoug te lateral pile working load is kn, te pile was tested beyond several times it s working load to observe te failure beaviour. Te following are te pile design criteria from te bridge designer: Lateral Pile Working Load (LPWL) Minimum Test Load (Lateral) Allowable Pile Head Deflection at x LPWL Allowable Pile Head Deflection at x LPWL Maximum Pile Service Moment kn kn mm mm 9. knm Table : Pile Design Criteria Te lateral load test was generally conducted in load increments of kn to allow te inclinometer to register deflection along te pile lengt. Figure sows te load scedule profile of te lateral pile test.

14 Legend : Applied Load Duration APPLIED LOAD (kn) 9 Duration (Minutes) Figure : Load Scedule Profile. Results of Lateral Load Test Figure sows te results of te pile ead deflection vs applied lateral load of te test pile. Te results generally sow tat at one lateral pile working load (kn), te pile ead deflection was about.mm and at twice LPWL (kn), te pile ead deflection was about.mm. Bot results sow tat te pile design was witin te design deflection criteria. As tis was a sacrificial test pile, te lateral load was furter increased to kn for te first cycle and to kn for te second cycle to observe te lateral deflection beaviour. At kn, te loading portion of te load settlement curve still sows approximate linear beaviour wit no signs of yielding. In addition, te unloading portion of te curve also sows linear rebound altoug te residual deflection is quite large wic is quite common for lateral pile in soft ground. Tis was probably due to passive resistance of te soil still exerting on te pile body after te applied load was removed. Figures 9 and sow te deflection profile along te test pile body for te first and second load cycle respectively. Generally, te test pile under applied lateral load beaves like a free ead pile wic was caracterised by te upper curved deflection profile up to a dept were tere is negligible deflection at about m dept. Altoug te inclinometer still sows some deflection after m dept, tis could be due to te accuracy of te inclinometer wic as accuracy of ±.mm.

15 Legend : Pile Top Deflection APPLIED LOAD (kn) 9 PILE TOP DEFLECTION (mm) Figure : Pile Top Deflection vs Lateral Load Dept (m) 9 9 Lateral Deflection (mm) Legend P=kN P=kN P=kN P=kN P=kN P=kN P=kN P=kN P=kN P=9kN P=kN P=kN P=kN 9 9 Figure 9: Deflection Results from Inclinometer st Load Cycle

16 Dept (m) 9 9 Lateral Deflection (mm) Legend P=kN P=kN P=kN P=9kN P=kN P=kN P=kN P=kN P=kN P=kN P=kN 9 9 Figure : Deflection Results from Inclinometer nd Load Cycle. INTERPRETED SUBGRADE REACTION From te test results, te subgrade reaction was interpreted assuming a fixity point on te pile at m and a linearly varying soil pressure profile was determined for eac applied load. At eac dept, te soil pressure was interpolated from te linear soil pressure profile and te subgrade reaction (k ) was determined from te division of soil pressure by te orizontal deflection at eac dept respectively. Te results were plotted in Figure alongside te initial calculated subgrade reaction from Davisson s (9). Generally, te subgrade reaction profiles sow a peak at about five to six metres dept except for initial loading up to kn were te profile is dissimilar due to te orizontal soil pressures not fully mobilised. Figure sows te plot of interpreted subgrade reaction wit applied load for several locations along te pile wic indicates te subgrade reaction as mobilised to a peak at applied loads of about kn to kn before softening in response.

17 Dept (m) Legend P=kN P=kN P=kN P=kN P=kN P=kN P=kN P=kN P=kN P=9kN P=kN P=kN P=kN P=kN P=kN P=kN P=kN P=kN P=kN Davisson's Prediction Interpreted Subgrade Reaction, k (kn/m ) Figure : Interpreted Subgrade Reaction Interpreted k, (kn/m ) Grap Dept=.m Dept=.m Dept=.m Dept=.m Dept=.m Dept=.m Applied Load (kn) Figure : Interpreted Subgrade Reaction vs Applied Load From te above plot, tere is no visible trend of te subgrade reaction reacing peak ten strain softening as te load increases. However, te results were plotted in te form of coefficient of subgrade reaction ( n ) wit dept for bot load cycles and tis removes te effect of pile diameter and subsoil dept. Tese were presented in Figure wit te initial predicted k converted from Davisson s (9) plotted alongside.

18 In tis plot, it can be seen tat tere is a general trend between te coefficient of subgrade reaction wit subsoil dept. In addition, most of te plots are witin a narrow range for most of te applied lateral load, suggesting te measured range of n values are closely witin tis range. Tere is also a fair correlation between te predicted (Davisson s -9) and te measured coefficient of subgrade reaction. At sallow depts, n tends to be underestimated significantly. However, tis is probably due to overconsolidation of te subsoil stratum near te ground surface as te n profile is reminiscent of te OCR profile. In te first load cycle, te subgrade reaction starts to be fully mobilised after applied lateral load of about kn. At kn, te profile indicates a partial mobilisation. At depts up to about m, Davisson s prediction is conservative but after m, it is sligtly overpredicted. Overprediction at sallow depts is due to conservativeness in te selection of design line for te undrained sear strengt. However, at deeper depts, te sligt overprediction could be due to passive resistance not fully mobilised at tat dept. Te average n value for tis site was found to be about kn/m. In te second cycle, te general profile is similar to te first cycle but te n values ave reduced indicating a softer response of te soil to loading and correspond wit te expected beaviour in Figure. Tis could be due to te compressed soil after te first cycle and ave not rebound back to it s original state after fully mobilising passive resistance and possible yielding. Dept (m) Interpreted Coefficient of Subgrade Reaction, n (kn/m ) Figure : Interpreted Coefficient of Subgrade Reaction n Legend P=kN P=kN P=kN P=kN P=kN P=kN P=kN P=kN P=kN P=9kN P=kN P=kN P=kN P=kN P=kN P=kN P=kN P=kN P=kN Predicted n (Davisson's)

19 Back analyses of te results were also carried out to determine Davisson s constant for te site. Figure sows te results of te back analyses using te interpreted k. For tis site, te proposed Davisson s constant is as compared to proposed by Davisson. Tis constant is more conservative and is proposed for soft ground in local ground conditions. Overall, te initial prediction using Davisson s metod sows fairly good estimate of te coefficient of subgrade reaction and subsequently te subgrade reaction values after taking into consideration te pile size and dept of subgrade. Dept (m) Legend P=kN P=kN P=kN P=kN P=kN P=kN P=kN P=kN P=kN P=9kN P=kN P=kN P=kN P=kN P=kN P=kN P=kN P=kN Average D c Value 9 Interpreted Davisson's Constant wit k, D C Figure : Back Analyses of Davisson s Constant (D c ) wit k. CONCLUSIONS A lateral load test was carried out on a fully instrumented preliminary (sacrificial) test pile to verify te lateral performance of mm diameter spun pile for te land viaduct section of a ig speed train project. Te land viaduct is built on soft marine clay wic is igly compressible and wit low undrained sear strengt. Eac of te pier in te land viaduct is supported by six numbers of mm diameter ig strengt circular spun piles spaced at tree times te pile diameter. Te instrumented lateral pile test was carried out according to ASTM D9 99 to verify te pile performance under lateral loading. Te results of te test are furter analysed to compare te initial prediction using subgrade reaction approac and recorded results. 9

20 Te test pile was tested to maximum test load of kn in two cycles, well beyond te lateral pile working load (LPWL) of kn and proof load of twice te LPWL of kn. At LPWL, te maximum deflection was.mm and at twice LPWL, te pile ead deflection is about.mm. Te test results validate te design of te spun pile under lateral loading and deflection were witin specified limits. Te test results were furter analysed to interpret te subgrade reaction profile ( k ) along te pile dept. Generally, interpreted subgrade reaction profile sows a peak at about five to six metres dept for all loading except for initial loading up to kn. Tis is due to te orizontal soil pressures not fully developed. In addition, te initial interpretation using subgrade reaction does not sow any visible trend wit applied lateral load. However, wen te results are plotted in te form of coefficient of subgrade reaction ( n ) wit dept, it sow a fair correlation wit predicted n using Davisson s metod. At sallow depts, n tends to be underestimated significantly due to overconsolidation of subsoil stratum near te ground surface. At depts up to m, Davisson s prediction is conservative but sligtly overpredicts after m dept due passive resistance not fully mobilised at tose depts. Te average n is kn/m for tis site. Back analyses of Davisson s constant for tis project site sow a proposed value of compared to original value of by Davisson. Tis is more conservative and is proposed for soft ground in local ground conditions. Overall, te initial prediction using Davisson s metod sows fairly good agreement. However, for soft ground in Malaysia, te autors proposed a constant of (ie. k = *S u /d). 9. REFERENCES Amadi, M.M. & Amari, S. (), A Simple Approac to Investigate Laterally Loaded Pile- Soil Interaction, Proceedings of t Souteast Asian Geotecnical Conference, Subang Jaya, Malaysia. Baguelin, F., Jezequel, J.F. & Sields, D.H. (9), Te Pressuremeter and Foundation Engineering, Claustal: Trans Tec Publications. Broms, B. B. (9). Lateral Resistance of Piles in Coesive Soils. Journal of Soil Mecanics and Foundations Div., Vol.9(),. Cen, L.T. & Poulos, H.G. (99) Pile Response due to Unsupported Excavation-Induced Lateral Soil Movement, Canadian Geotecnical Journal, Vol. No., p. -. Cen, L.T. & Poulos, H.G. (99) Piles Subjected to Lateral Soil Movements, Journal of Geotecnical and Geoenvironmental Engineering, Vol. No.9, pp.-. Coutino, R.Q., Horowitz, B., Soares, F.L. & Braga J.M. (), Steel Pile under Lateral Loading in a Very Soft Clay Deposit, t International Conference on Soil Mecanics and Geotecnical Engineering, Vol., Osaka. Davisson, M.T. (9), Lateral Load Capacity of Piles, Higway Researc Record, Transportation Researc Board, Wasington, DC. Kucukarsalan, S., Banerjee, P.K. & Bildik, N. (), Inelastic Analysis of Pile Soil Structure Interaction, Engineering Structure: -9

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