Relationship between CPT parameters and properties of saturated ultrasoft clays in the Pearl River delta

Transcription

1 3 rd International Sympoium on Cone Penetration Teting, La Vega, Nevada, USA Relationhip between CPT parameter and propertie of aturated ultraoft clay in the Pearl River delta Z.M. Li Geotechnical Engineering Technology Center,Guangdong Univerity of Technology, Guangzhou, China ABSTRACT: Theoretical relationhip between CPT parameter, elatic parameter and undrained trength of oil are preented baed on an elatic-platic olution for cylindrical cavity expanion. The relation between CPT parameter and hear trength from vane tet i alo preented from the theoretical reult. Thu, the CPT parameter can be determined directly by elatic parameter and hear trength or vane hear parameter; and vice vera. That make poible to ave the high tet cot and provide theoretical formula to avoid ome tet limited due to the ite and/or other condition. Reult are compared between the theoretical reult and in itu data from CPT and VST, at a large-cale ultra-oft ground treatment project in the Pearl River Delta The reult howed conitency of the two kind of reult, a well a the feaibility and practicality of the theoretical formula. 1 INTRODUCTION The Cone Penetration Tet (CPT) i uually ued to determine the geotechnical engineering propertie of oil and to delineate oil tratigraphy, and i one of the mot widely ued and accepted method for oil invetigation worldwide, including in China. A large-cale baic contruction i conducted in China, epecially in the Pearl River delta, they inevitably encounter a lot of oft oil ground, even ultra-oft oil whoe natural void ratio i greater than 1.5, and water content greater than the liquid limit. The CPT i a method to determine the baic parameter of oil characteritic, including tip reitance q c, ide friction f, friction ratio R f (= f /q c x 100%)and pecific penetration reitance P (the quotient of total penetration force and projected area of the probe. Thee parameter can be utilized to determine the bearing capacity for foundation, but (according to the Building Code in China (Code DBJ ) the CPT can not be ued to etimated the undrained hear trength S u and oil enitivity S t, which are uually obtained from In-Situ Vane Shear Tet (VST). However, both CPT and VST reflect, to ome extent, the oil characteritic of the reitance to hear force, and there i reaon to believe that certain correponding relationhip exit between CPT and VST parameter. In fact, many reearcher have made the link (Chui & Ding 004, Lin 1994, Li 011, Roberton & Campanella 19, Roberton & Cabal 01, Yan et al. 009, Yang & Xiong 010). However, for aturated ultra-oft clay, related work i very rare. Baed on elatic-platic analyi of the cavity expanion proce and the penetration characteritic of aturated oft clay, thi paper i aimed at etablihing theoretical relation among VST, CPT and other often ued mechanical parameter, and comparing in ite meaured data to evaluate the relationhip to provide a bai for further undertanding of the nature of ultra oft oil, and alo to reduce ome tet o a to ave tet cot and improve tet efficiency in practice. 393

2 BASIC EQUATION ESTABLISHMENT FOR PARAMETERS OF SATURATED SOFT CLAY.1 Empiric method A large number of tet data were utilized for tatitic analyi, to etablih certain relation between tip reitance q c, ide friction f and undrained hear trength of coheive oil S u. The typical tatitical fitting equation are hown in Tab. -1 (Li 011, Roberton & Campanella 19, Lin 1994). Table -1. Empiric relation between q c or P with S u No Practical relation Applied condition Source*. 1 S 0.071q 1. q c < 700kPa Tongji univerity 3 u S u 0.039q c c.7 q c < 00kPa Minitry of Railway 3 S u P. 7 P =300~100 Saturated oft clay Wuhan CPT Union q c =100~00kPa 4 S u qc 4. Soft clay in Shanghai and Guangzhou Sichuan intitute of building 5 P S u 0.030PS 4. 0 =100~1500kPa Newport oft clay Navigation deign and reearch intitute 6 S u 0.05PS Newport oft clay The 3rd Int. of Minitry of Railway 7 S u PS 1. 9 P =00~1100kPa Saturated oft clay in Xuzhou Firt indutry deign intitute of Jiangu S u PS 1. P <700kPa The 4 th Int. of Minitry of Railway 9 S u 0.057P lake peat and peat oil of Dianchi Hydropower urvey and deign intitute of S Hunan S q / N ** Lin Zong-yuan 10 u c c c 11 qc Cu Nc *** v Roberton & Campanella, (19) S u 0.063qc kPa< q 1 c <00kPa Zhang-ming Li Reearch group, (011) S u 0.04PS kPa< q c <600kPa * Relation No are from Lin (1994), No.11 and No.1 are from Roberton & Campanella (19) and Li (011). **σ c = Gravity tre at the probe; N c = the empirical coefficient of bearing capacity,n c =9~19 *** N c = cone-tip reitance coefficient; σ vo = the overlying preure The empiric relation in Tab.-1 can be ummed up in a generally linear formula: q A S, or P A Su B c 1 u B 1 Where the tatitical fitting coefficient A 1 and A are poitive contant, while B 1 and B could be poitive or negative. Thee coefficient are regarded a contant in mot tudie except that by Roberton and Campanella (193) the coefficient were thought to be related to the depth of the meauring point. The two coefficient of the empiric expreion, in hort, generally only the fitting coefficient, have no clear phyical-mechanic connotation and cannot be determined by the exiting mechanical parameter.. Formula etablihment baed on the platic theory of patial axil-ymmetrical problem The following aumption were made according to the propertie of aturated oft clay and proce characteritic of CPT: (1) The cone penetration proce can be conidered a an elatic-platic problem in the proce of cavity expanion; () Ue Coulomb trength criterion and aume the value of internal friction angle of aturated oft clay in undrained CPT proce, 0 ; (3) In the cavity expanion proce, elatic volume change of the platic-zone oil i relatively mall and can be ignored. 394

3 Then the radial expanion preure P u of the cavity wall (i.e. lateral urface of probe), after a cylindrical cavity expanion, can be obtained upon the above three aumption (Gong 1999). R E n Pu x c ln I X 1 c ln 1 1 c E I x 1 c Ru Where = the tre at the inide radiu of the cylinder after expanion; C = coheive force; E = the r elatic modulu; and v = Poion ratio. Take the equilibrium condition of penetration probe: D D Lc ( ) qc DLcf ( ) z, namely qc 4 f z D Where q c = Tip reitance; f = Side friction; L c = Effective friction length of penetration probe, D = Probe diameter. Then, f can be got from the radial expanion preure (multiplied by the friction coefficient): R E u f r c[ln 1] (.1) (1 ) c Where = the friction coefficient between the penetration probe and the urrounding oil. The penetration problem i a patial axiymmetric problem in mechanic. The ide and bottom of penetration probe (correponding to the plane, r = R u and z = h, in the cylindrical coordinate) could conidered a principal plane, and according to the balance equation of the axiymmetric problem, that i d z z dz Solve the equation and obtain the contant according to the boundary condition, we got z z / h / Then, the formula on q c can be obtained from the equilibrium condition of penetration probe: 4Lc h 4Lc qc f c(ln I r 1) (.) z D D where = natural gravity denity of oil; h = the depth of penetration probe. In Eq. (.), the right ide i negative, which indicate that the acted direction of q c and σ z are oppoite. Both ingle-bridge (the cone together with the outer leeve, which can only meaure a parameter P, ued widely in China) and double-bridge (the cone eparate with the friction leeve, which can meaure two parameter, i.e. the tip reitance and ide friction) penetration probe have the ame penetration way, then get the pecific penetration reitance P p qc mf (.3) Where m = the ratio of the effective friction leeve urface area of ingle-bridge probe to the doublebridge one. A to the ingle-bridge and double-bridge probe of the ame bottom area, m i the ratio of ide effective contact length. For a tandard probe with a bottom area of 10 cm, m=57/179. For aturated oft clay, the internal friction angle could be aumed, 0. According to the Coulomb trength criterion, get VST hear trength S u = =c, put in the Eq. (.1) and Eq. (.), then the value of q c and f could be calculated. Thee two equation decribe the relation between parameter in CPT and VST of aturated oft clay. Senitivity coefficient, S t = S u / S u ', in which S u i undrained hear trength of unditurbed oil, while S u ' i that of diturbed oil. 395

4 Senitivity coefficient of unditurbed oil can alo be etimated by Friction ratio R f of double-bridge. Schmertmann (197) came up with the relation, S t =N /R f, in which R f = f /q c 100%, then S N / R N q / f (.4) t f c N, a a dimenionle coefficient, wa obtained by comparing CPT reult with laboratory reult by Roberton & Campanella (19) and the average value i 6. Studie by Rad & Lunne (196) howed that the N value changed in the range of 5 ~ 10, average 7.5. Lunne et al (1997) argued that the value depended on the mineral, OCR and other function, and no unique value wa uitable for all the clay. The relationhip between enitivity S t from the VST and the parameter of CPT wa decribed in Eq. (.4). A to pecific penetration reitance P, it can get directly from Eq. (.3) upon q c and f. Thi far, four important parameter, including ide friction f, tip reitance q c, pecific penetration reitance P and enitivity coefficient S t can be calculated by Eq.(.1)~(.4)repectively. The three parameter f, q c and P are the theoretical olution; while S t i a emi-empirical olution due to it i related the emi-empirical relationhip of Schmertmann. Note that thee formula are nonlinear in term of trength parameter, rather than linear which are expreed by general empirical formula. Alo, making ue of Eq. (.1) and Eq. (.), the elatic modulu E can be calculated by ide friction f and tip reitance q c, repectively: f ( 1) c E (1 ) c e and D h ( qc 1) 4 clc E (1 ) c e (.5) Moreover, the difficulty in determining bearing capacity of deep oft oil foundation may be effectively olved when uing Eq. (.5). 3 SITE CONDITIONS A ground treatment ite in the Pearl River delta region of China wa ued to collect in-itu data. The ite area i m, the urface layer i aturated ultra-oft clay of 11 m thick, with an average moiture content of 75.0% and average void ratio of.07. A hown in Fig. 3-1, the engineering geological condition within the cope of oft ground treatment wa very poor. The ite, once artificial-urrounded fihing pond of different ize, wa dipoed with hydraulic reclamation. The trata ditribution and phyical indicator are hown in Table 3-1. The tatic -dynamic drainage conolidation method (Li 011) wa utilized in improving thi ultra-oft oil ground; according to the ite condition, 3 to 4 time of point-tamping and one general-tamping were required. Figure 3-1. In itu ituation before the ground improvement 396

5 Table 3-1. Strata ditributing in the tet area Soil name Thickne /m Soil decription Artificial hydraulic fill 0.0~5.5 Ditribution i very uneven with, high water content muck 3.5~0.5 Average1.0m, platic flow tate (water content i greater than liquid limit). The water content i 45.% ~ 114%, average 75.0%; void ratio of ~.99, average.07. Silt clay 0.7~9.5 Alluvial ource, platic tate (water content i greater than platic limit). Sandy clay 1.0~1.7 Completely decompoed granite Strongly decompoed granite Eluvial ource, brown, maroon-baed, hard platic, local platiclike, decompoed granite Gray, maroon-baed, with a hard core of oil column, eaily diintegrating by water.1~ ~13. Purple, gray, oil core were folder or chunky rock, oft rock 4 TEST METHODS AND EQUIPMENTS Before the tet, according to geologic condition and working condition of the ite, chooe the penetration equipment in combination with engineering requirement for the tet depth. Select thrut tonnage of the penetration equipment and prepare a reaction ytem to enure the thrut force. Chooe probe on the bai of the uggetion given by Yilmaz (1991), a hown in Table 4-1, and chooe different capability of probe according to oil condition, een in Table 4-. Table 4-1. Probe election Clay Soil condition q c (MPa) Probe capability (total capacity) (kn) extra oft 5~1.5 10~5 oft ~ medium oft 1.5~5.0 5~50 medium oft ~ hard 5.0~ ~100 Dene and > Table 4-. Probe dimenion Single-bridge Double-bridge probe Probe probe Apex angle diameter Effective friction Friction leeve α/ D/ mm ide wall length urface area L 1 / mm /cm Projected area of the Cone A/ cm Friction leeve length L c /mm Monitoring intrument and preciion: (1) Intrument for monitoring point layout: Leica TC307 Total Station; preciion < 1/4 ; () Single-bridge probe: DQ-10Y, meauring range 0~30KN, preciion 10N; (3) Double-bridge probe: SQ-10Y, meauring range 0~30KN, preciion 10N; (4) DY multi-purpoe digital teter: teting accuracy ±0.5%±1 word. Calibration of the probe including train gauge load cell and preure tranducer were carried out 397

6 at regular interval according to relative order, all according to the relevant regulation, to verify and enure the quality of the probe. Initial induce meaure and enibility of enor wa decribed in Table 4-3. Table 4-3. Initial induce meaure and enibility of enor Initial induce level meaure /kpa parameter Level I Maximum penetration reitance.5~5.0mpa Level II Maximum penetration reitance 7.0~1.0MPa Level III Maximum penetration reitance 1.0~0.0MPa p, q c 10~0 30~50 50~100 f 0.1~0. 0.3~ ~1.0 u 5 10 Penetration location were et up roughly-evenly throughout the ite, and the penetration tet of each location wa conducted three (3) time, that i, before tamping, after the 1t point-tamping and after tamping, with a tet depth of ~11m. The monitoring point were et up with the ue of Total Station, to make ure the uniformity of thee point before and after tamping. VST were carried out at the ame plane poition of penetration point. The parameter of vane head: H 100mm,D 50mm,thickne 3mm; vane hear probe: SB-1Y,meauring range 0~30kN, preciion 1Nm. VST were conducted below the urface (the original ground face) with a depth of m or 4 m, 6 m and m. Quite a few meauring point were difficult to tet becaue of the extra-poor ite condition before tamping, therefore, tet data after the 1t pa of point-tamping were utilized in thi article. 5 ANALYSIS AND COMPARISON BETWEEN TEST AND THEORETICAL RESULTS 5.1 In-ite tet reult qc-h curve of CPT (Orifice No.5,elevation:+7.96m) qc/mpa qc-h curve of CPT (Orifice No.5,elevation:+7.96m) qc/mpa Su-h curve of VST (Orifice No.1,elevation: +7.50m) Su/kPa before tamping the nd tamping after tamping 9 before tamping the nd tamping after tamping 7 10 before tamping the 1t tamping 1 after tamping Figure 5-1(a). Relation curve Figure 5-1(b). Amplificatory curve Figure 5-(a).Typical relation curve between q c with h(deepne) between q c with h in the oft oil of VST 39

7 3 Su'-h curve of VST (Orifice No.1,elevation: +7.50m) Su'/kPa qc/mpa f/kpa before tamping after tamping the 1t tamping.5.5 Figure 5-(b). Typical relation curve of VST Figure 5-3. Relation curve of q c Figure 5-4. Relation curve of f Typical CPT and VST curve are hown in Fig. 5-1 and Fig.5- eparately. Fig. 5-1 and Fig. 5- diplayed the change in mechanical trength characteritic of thi ultra-oft clay before, during and after the improvement. It i clear that the improving effect wa obviou in the upper 4m. 5. Comparion between tet and theoretical value of penetration parameter According to oil tet, the relevant parameter were: Su =.4kPa, E = 1.31MPa, ν = 0.35,γ = 1.5kN/m 3, μ= 0.05; Geometric parameter of probe: L c = 179mm, D = 35.7mm. Took N = 6, which i uggeted by Roberton, when calculating enitivity coefficient S t. Calculation depth wa 0.5 ~ m. Becaue of the poor permeability of clay, gravity tre wa obtained by calculating water and earth preure together, and q c or f wa calculated once per 0.1m repectively. Reult of comparion between theoretical and tet data were hown in Fig A hown in Fig. 5-3, the relative law of the tet parameter of CPT were a follow: (1) In proce of penetrating into aturated oft oil, q c grew lowly with h, the lope of the calculated curve wa not a contant, but changed a little; f kept eentially unchanged with the depth h. () The critical depth wa not obviou when the probe wa penetrated in aturated oft oil. The caue might be ultra-oft clay i different with other oil; on account of the mall value of friction angle in aturated oft clay, hear trength and friction kept minor change with the increae of depth. The figure alo indicated that the theoretical value were in good conitency with the tet one. 5.3 Comparion between meaured and theoretical value of S u with other parameter (1) On the relationhip between q c with S u 399

8 qc/kpa f/kpa Su/kPa Figure 5-5. Relation curve between q c with S u Su/kPa Figure 5-6. Relation curve between f with S u Seen from Fig.5-5, the tet data how a poitive correlation between q c and S u. However, the curve deviate from linearity. The reult alo indicate that the value of elatic parameter have an effect on reult from theoretical calculation. In general, it i howed the overall conitency of theoretical and tet value. ()On the relationhip between f with S u From Fig. 5-6, the tet data how a general linear trend of f ~ S u. The theoretical value ha a better conitency with the tet value in a certain range (S u <40kPa), indicating that the theoretical formula i uitable for oft clay. (3)On the relationhip between P with S u P/kPa St Su/kPa Su/kPa Figure 5-7. Relation curve between P with S u Figure 5-. Relation curve between S t with S u Fig.5-7 how that the P ~ S u curve i a combination of the above two curve, f ~ S u and q c ~ S u, and in the ame way, the theoretical value ha a better conitency with the tet value. (1) On the relationhip between S t with S u Fig.5- how that in theory, S t i reduced with the increae of S u. However, the tet data indicate the preence of one kind of oil, whoe S t remain unchanged a oil hear trength increae. It i not hard to ee, emi-empirical formula (.4) uggeted by Schmertmann etc., only how that S t would decreae with oil hear trength. The relation of S t with S u would be further developed. Seen from the above comparion between the tet reult and theoretical value, the two kind of value have good conitency for aturated oft clay. If there i ome deviation, the reaon i the complex mechanical propertie of oft clay, including: (1) the theory aumption are implified, uch a the oil i aumed to be a imple elatic-platic body; () the complex mechanical proce in the penetration proce i not taken into account, like the generation of pore water preure (tip reitance q c can be corrected a pore preure i included), etc.; (3) the variability of oil propertie, like the effect of reidual hell in the oft oil formation, etc. The theoretical formula built the relationhip among thee important mechanical quantitie of in itu tet, which not only make for aving huge amount of tet time and cot for the project deign, but alo provide a way for the further development of relative theory. 400

9 6 CONCLUSIONS (1) The exited empirical relationhip between the parameter of CPT and hear trength can be ummed up a a general linear formula, in which the contant coefficient could be poitive or negative. Phyical-mechanical connotation of thee coefficient i not clear and even conflicting. () A a patial-axiymmetric problem, with platicity theory of cylindrical cavity expanion, theoretical relationhip between the parameter of CPT (including tip reitance q c, ide friction f and pecific penetration reitance P ), the elatic parameter and undrained tength of oil were etablihed. Meanwhile, the relationhip between CPT parameter and hear trength value of vane tet wa alo obtained. (3) The above etablihed theoretical relationhip i nonlinear in term of trength parameter, rather than linear expreed by the general empirical formulae. (4) Comparing reult from theoretical calculation with in itu data of CPT and VST in an actual ultra-oft oil treatment project in the Pearl River Delta, it indicated that the good conitency between theoretical relation (f ~ S u, q c ~ S u and P ~ S u ) and the tet data. And emi-empirical formula on S t of VST, only howed that enitivity coefficient would decreae with oil hear trength, o thi relationhip on enitivity coefficient hould be further developed. (5) Seen from thee derived theoretical relationhip, the penetration parameter can be directly determined by the conventional mechanical parameter (uch a elatic parameter and undrained trength) or vane hear parameter, and vice vera; Elaticity modulu E can alo be calculated by tip reitance q c or ide friction f. All thee make for aving the high cot of teting and provide theoretical method and formula to avoid ome tet limited due to the ite and/or other condition. ACKNOWLEDGEMENTS Thank to Mi Zeng Wenxiu, the graduate of the author, who collate data and drawing for thi paper. Thi work wa upported by National Natural Science Foundation of China (Project No ). REFERENCES Code for teting of building foundation.(dbj ). 00. Beijing: China Architectural Pre. Chui, X.Z. & Ding, H Approximate theoretical and experimental reearch progre on the head reitance of tatic cone penetration. Advance in Mechanic 34():51-6. Gong, X.N Soil Platicity. Hangzhou: Zhejiang Univerity Pre. Lin, Z.Y Tet and monitoring handbook of geotechnical engineering. Shenyang: Liaoning Sci. & Tech. Pre. Li, Z.M Soft oil foundation reinforcement with quality control. Beijing: China Building Indutry Pre. Roberton, P. K. & Campanella, R.G. 19. Guideline for geotechnical deign uing CPT and CPTU. Vancouver: Univerity of Britih Columbia. Roberton, P.k. & Cabal, K.L. 01. Guide to cone penetration teting. California: Gregg Drilling & Teting, Inc. Yan, S.W. & Feng, X.W. & Hou, J.F. & Li, W Deduction and application of trength parameter of oft clay by ue of vane trength. Chinee Journal of Geotechnical Engineering 31(1): Yang, J.H. & Xiong, L Analyi of correlation of itu-tet reult in the Yangtze Delta. Soil Engineering and Foundation 4(1):

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