COEFFICIENT OF CONSOLIDATION FROM PIEZOCONE DISSIPATION TESTS

Transcription

1 Poceedings of the Intenational Symposium on Lowland Technology, Septembe 4 COEFFICIENT OF CONSOLIDATION FROM PIEZOCONE DISSIPATION TESTS J.-C. CHAI Institute of Lowland Technology, Saga Univesity, Japan J. P. CARTER Depatment of Civil Engineeing, The Univesity of Sydney, Austalia N. MIURA Institute of Soft Gound Engineeing, Japan T. HINO Institute of Lowland Technology, Saga Univesity, Japan ABSTRACT. A method fo intepeting the hoizontal coefficient of consolidation (C h ) fom non-standad piezocone dissipation cuves is poposed. The cuves obtained fom these tests ae non-standad because the measued excess poe pessue does not decease monotonically with time. They ae mainly obseved in tests whee the poe pessue tansduce is located at the shoulde o on the shaft of a cone penetomete. It is suggested that possible mechanisms esponsible fo the non-standad dissipation cuves include: () shea-induced dilatancy obseved in ove-consolidated clays and sandy soils, and () the unloading effect fo soil elements undegoing a elative movement fom the tip to the shoulde of a cone, which will esult in a lowe excess poe pessue at the measuing point than elsewhee aound the cone. Radial plane stain consolidation analyses wee conducted to investigate the effect of the initial excess poe pessue distibution on the dissipation pocess. Based on the esults of this numeical analysis, an empiical equation is poposed to coect t 5, the time coesponding to 5% dissipation of the measued maximum excess poe pessue. This time is then used in the standad intepetation of the value of C h. The poposed method is applied to the intepetation of field test data fom Saga, Japan. It is shown that the values of C h evaluated by the method ae in the ange of back-calculated field values. INTRODUCTION One of the advantages of the piezocone test is that it can be used to evaluate the coefficient of consolidation of the gound. Filte elements fo measuing poe pessue can be located on the middle face of the tip, the shoulde o the shaft of a cone (Fig. ). Howeve, the shoulde position is now consideed to be the standad location (Lunne et al. 986; Buns and Mayne 998). With a shoulde filte element and fo tests in ove-consolidated clays and sandy soils, some field data show a dissipation cuve with the poe pessue fist inceasing fom an initial value to a maximum, and then deceasing to the hydostatic value (Buns and Mayne 998; Sully et al. 999), as illustated schematically in Fig.. This kind of dissipation cuve will be efeed to as non-standad. Most methods fo evaluating the coefficient of consolidation fom piezocone dissipation tests ae based on the assumption of monotonic excess poe pessue decease with time (e.g., Teh and Houlsby 99) and can not be used diectly to intepet tests like those shown in Fig.. Buns and Mayne (998) poposed a method to back fit the Piezocone 3On shaft On shoulde On middle face Figue Locations of filte Excess poe pessue u max u=u max / t umax t 5 Δu max Time, t Figue Non-standad dissipation cuve field dissipation cuves and evaluate the coefficients of consolidation. Thei method combines cavity expansion and citical state soil mechanics theoies and the solution pocess equies a compute pogam and iteation to obtain a good fit of the measued dissipation cuve. Duing the fitting pocess both the hoizontal coefficient of consolidation (C h ) and the igidity index (I ) ae vaied, which may be poblematic and lacking a physical basis. Sully et al. (999) poposed two methods to coect the non-standad dissipation cuves and then to use these coected cuves with existing methods of intepetation (e.g., Teh and Houlsby 99) to evaluate C h. One of the coection methods involves shifting the oigin of time to that point whee the measued poe pessue is a maximum. Anothe fits a squae oot of time plot to the post-maximum poe pessue dissipation cuve in ode to back-extapolate the value of the initial poe pessue. The methods poposed by Sully et al. (999) ae simple, but they do not popely conside the mechanism causing the non-standad phenomenon. Fo example, shifting the oigin of time ignoes the effect of poe pessue edistibution in the vicinity of the cone befoe the measued poe pessue eaches its maximum, and this tends to ove-estimate the time fo 5% dissipation of the measued maximum value (t 5 ) which esults in an unde-estimate of the value of C h. At pesent, thee is no well established and easy to use method fo intepeting C h values of the gound fom tests displaying the phenomenon illustated in Fig.. In this pape the mechanisms which possibly give ise to dissipation cuves of the fom shown in Fig. ae fist discussed. The influence of the initial excess poe pessue distibution on dissipation pocess is then investigated by an uncoupled plane --

2 stain adial consolidation analysis (solved using the finite diffeence method). Based on the esults of this numeical analysis, an empiical equation is poposed to coect t 5, the time coesponding to 5% dissipation of the measued maximum excess poe pessue. This time is used subsequently in the intepetation of the value of C h. Finally, the poposed method is applied to the intepetation of field test esults obtained in Saga, Japan. POSSIBLE MECHANISMS Seveal possible easons ae suggested fo the occuence of non-standad excess poe pessue dissipation cuves. () Mechanism : Shea induced dilatancy effect. Shea induced dilatancy will cause the excess poe pessue in a zone nea the shoulde and shaft of a cone to be lowe than that in the adjacent soil (Coop and Woth 989). Duing consolidation, the highe excess poe pessues in the adjacent aeas will be edistibuted causing a local ise in poe pessue at the suface of the cone whee the poe pessue filte element is located. Sully et al. (999) agued that if this is the pincipal mechanism, then fo all locations on the shaft, iespective of the distance behind the tip, the measued poe pessues would all attain peak values at the same time. This agument ignoes the fact that the poe pessue distibution in the soil aound a cone is non-unifom. () Mechanism : Unloading effect. Fig. 3 (afte Teh and Houlsby 99) shows the pedicted stain distibutions aound a 6 cone fom stain path analysis. In the figue, a negative value indicates compession. Point A is just below the shoulde and point B is just above the shoulde (at which the filte element is located). With the pocess of penetation, the soil at point A will move to point B. Fom A to B, the adial stain ε does not change much (pehaps about 5%), but the stain in the vetical diection (ε zz ) changes sign fom compession to tension with a maximum tensile stain of about % (as estimated fom the contou plot). The stain path analysis is unde the condition of constant volumetic stain. Howeve, fo a eal soil, changing the stain fom compession to a lage value of extension may cause cetain volumetic stain (hee expansion). Fo soil elements away fom the cone but at the level of the shoulde, the vetical stain is also tensile but the magnitude educes apidly with adial distance fom the cone. In addition, the compessive adial stain is geneally inceased at these locations and thus thee will be no unloading effect. These defomation phenomena ae likely to esult in a lowe excess poe pessue at the shoulde than in the soil moe emote fom the shoulde. This mechanism applies to both ove-consolidated and nomally consolidated clays, but it may be moe ponounced in heavily ove-consolidated soils Futhemoe, fo stiff soils, thee may be mico cacks caused by the tensile staining, which will contibute to the geneal eduction in excess poe pessue close to the cone shoulde. Figue 3 Stains aound a 6 cone (afte Teh and Houlsby 99) A B / ε % A B 5 ε zz % / (3) Mechanism 3: Poo satuation of filte element. Patially satuated filte elements can exhibit sluggish esponse to changes in poe pessue (Buns and Mayne 998; Sully et al. 999), and so in some instances may account fo measuements that ae below the actual values. Howeve, poo satuation is unlikely to be a geneal eason fo this phenomenon. (4) Mechanism 4: Redistibution of poe pessue. Duing piezocone penetation, the excess poe pessue geneated at the tip is geneally highe than at the shoulde and shaft of a cone, especially in ove-consolidated soils. When penetation ceases, the highe poe pessue at the tip will edistibute to the shoulde location, possibly causing the measued poe pessue at the shoulde to incease fo a peiod (Davidson 985; Buns and Mayne 998). Teh and Houlsby (99) conducted numeical analysis on piezocone penetation and subsequent dissipation of excess poe pessue. The esults indicate that duing penetation, the poe pessue below the shoulde is usually highe than above the shoulde, but dissipation analysis did not show a non-standad dissipation cuve of the type shown in Fig.. Futhe, non-standad dissipation cuves wee also obseved at the shaft filte element (Sully et al. 999). It is unlikely that the highe excess poe pessue geneated at the tip of a cone will be edistibuted to the shaft location duing a dissipation test. Filte element Cone Effect of Shea induced dilatancy Unloading Initial excess poe pessue distibution Figue 4 Effect of dilatancy and unloading on poe pessue distibution Fo a field test, it is possible fo each of the above mentioned mechanism to occu. Howeve, it is consideed that shea-induced dilatancy in heavily ove-consolidated clays o sandy soils and the unloading effect ae pobably the main --

3 easons these non-standad dissipation cuves ae obseved. In this case, the initial excess poe pessue distibution aound a cone will be as illustated in Fig. 4. This kind of initial excess poe pessue distibution can esult in a dissipation cuve of the type shown in Fig.. In the following section, the effect of the initial excess poe pessue distibution on the dissipation pocess, and subsequently the way to evaluate the coefficient of consolidation fom the non-standad dissipation cuve will be investigated. EXCESS PORE PRESSURE INDUCED BY CONE PENETRATION Excess poe pessue due to a cylindical cavity expansion Fo an undained cylindical cavity expansion, which of couse ignoes any effects of vetical sheaing associated with cone penetation, the excess poe pessue u aound the cavity is as follows (Vesic 97): R p u = Su ln () whee R p = adius of plastic zone, = adius fom the cente of the cavity ( < < R p,whee is the adius of a cavity), and S u = the undained shea stength of the soil. The expession fo R p is as follows (Vesic 97). R p = whee I = the igidity index of the soil (= G/S u, whee G is the elastic shea modulus of the soil). Fo > R p (in the zone of elastic soil), the mean stess change induced by the cavity expansion is zeo, and so u =. Initial excess poe pessue distibution The assumed initial poe pessue distibution is illustated in Fig. 5. In the figue, the line maked with is the initial excess poe pessue induced by undained cylindical cavity expansion (Eq. ()), is the excess poe pessue distibution fo a soil expeiencing shea-induced dilatancy and/o unloading effects. The magnitude of the effect can be adjusted by vaying the shea zone thickness, t d, and the paamete m. The diffeence in excess poe pessue between the maximum value and the value at the filte element shown in Fig. 5 is denoted as Δu. The dissipation pocess coesponding to the initial distibution will be compaed with that coesponding to distibution (Fig. 5). Each dissipation pocess has been calculated using an uncoupled consolidation analysis, assuming plane stain adial dainage, to investigate the effect of the initial excess poe pessue distibution on the dissipation pocess. The taget is to develop a method fo intepeting the dissipation cuves given an initial excess poe pessue distibution of type (Fig. 5). In the analysis it is assumed that fo both distibutions and, the soil has the same values of C h, I, I () and S u. Compaed to an actual field situation, the conditions assumed in the analysis pobably suffe fom the following shotcomings. () The dilatancy-induced non-unifomity of the coefficient of consolidation and its vaiation duing the consolidation pocess ae ignoed. () Thee dimensional (3D) poe pessue dissipation is ignoed. (3) The excess poe pessue vaiation is not coupled to the effective stess changes and theefoe, the defomation of soil mass. Because of the above mentioned shotcomings, it can not be expected that a plane stain, adial dainage, uncoupled consolidation analysis can simulate the field dissipation pocess exactly. Howeve, the aim of the analysis is to obtain an undestanding of elative diffeence between the initial excess poe pessue distibutions and (Fig. 5) and how that diffeence affects the intepetation of the field value of C h. Initial excess poe pessue, u Cone m t d t d : mm m: 5 kpa/mm Δu Radial distance, Figue 5 Assumed initial excess poe pessue distibution EFFECT OF INITIAL EXCESS PORE PRESSURE DISTRIBUTION ON DISSIPATION Table Summay of analyzed CASES No. C h (m /min) S u (kpa) I t d (mm) m (kpa/mm) Δu (kpa) Seies , - - Seies Seies Seies The cases analyzed ae listed in Table. Seies- investigates the effect of the amount of excess poe pessue eduction (i.e., the value of m and theefoe Δu), Seies- investigates the effect of the thickness of the shea zone, t d (fo the same Δu), and Seies-3 and Seies-4 investigate the effect of igidity index (I ). Figue 6 shows an example of the esults of these analyses. It can be seen that initial excess poe pessue distibution has a significant effect on the dissipation pocess. The time fo 5% excess poe pessue dissipation (t 5 ) is longe fo distibution (Fig. 6 (a)). The lage the values of t d and -3-

4 m, the longe will be t 5. If the values of t 5 indicated in Fig. 6 (b) ae used diectly to evaluate values of C h, then the initial excess poe pessue distibution will give a lowe C h value than distibution (see Fig. 6 (a)), even though the same value of C h was assumed in both numeical simulations. Initial excess poe pessue (kpa) Excess poe pessue (kpa) I = S u =3 kpa...3 Radial distance (m) (a) Initial excess poe pessue distibution 5 5 t 5 =9.9 min I = S u =3 kpa C h =5-5 m /min t 5 =69.48 min.. Elapsed time (min) (b) Dissipation cuves Figue 6 Effect of initial excess poe pessue distibution on dissipation consolidation is contolled by time facto (T). Fo a given value of T, educing o inceasing C h equies a popotional incease o eduction in time (t). This means the atio t u max / t 5 will not be affected by the value of C h, and theefoe, Eq. (3) is also applicable fo othe values of C h. Among the methods available fo evaluating C h fom piezocone dissipation tests, the one poposed by Teh and Houlsby (99) is pobably most widely used (Robetson et al. 99). Teh and Houlsby s (99) equation is as follows C h C p I = (4) t 5 whee C p is a facto elated to the location of the filte element and the size of the cone. Fo a cone with a coss-sectional aea of cm and with a shoulde filte element, C p =.45 (Teh and Houlsby 99). Equation (4) had been developed fo the condition of monotonic excess poe pessue dissipation (standad dissipation cuves) and stictly can not be applied to non-standad dissipation cuves. In this study it is poposed that the value t 5m should be substituted into Eq. (4) instead of t 5, to intepet non-standad dissipation cuves. t 5 and t 5m (min) I = S u=3 kpa 8 t d=, mm m= - kpa/mm 6 4 t 5 t 5m Tue t 5 3 t 5 and t 5m (min) 5 5 I = S u =3 kpa t d= - 8 mm m= - 5 kpa/mm t 5 t 5m Tue t AN EMPIRICAL EQUATION FOR CORRECTING t 5 (a) Seies- analyses (b) Seies- analyses Based on these numeical esults, the following empiical equation is poposed to coect t 5. t t = (3) 5m t u max I t 5 whee t 5m = coected time fo 5% excess poe pessue dissipation, t umax = time fo the measued excess poe pessue to each its maximum value, t 5 = time diffeence between the maximum and 5% of the maximum excess poe pessue (see Fig. fo meanings of t umax and t 5 ). Fo a given value of I, the magnitude of this coection depends on the atio t u max / t5. Also, the numeical esults indicate that t umax is not much influenced by I, but t 5 inceases with inceasing I. Theefoe, a tem elated to I is included into Eq. (3). Given the value of t 5m, existing methods can then be used to evaluate C h. Fo the conditions listed in Table, compaisons of the values of t 5 and t 5m ae given in Figs. 7 (a) to (d). Figue 7 shows that Eq. (3) is applicable fo a wide ange of conditions. In all cases the value of C h input to the numeical analysis was the same. Fom consolidation theoy, it is known that the degee of t 5 and t 5m (min) 5 I =5 S u =3 kpa 4 t d=, 4 mm m= - 5 kpa/mm 3 t 5 t 5m Tue t 5 3 t 5 and t 5m (min) 5 I =5 S u =3 kpa t d=, 4 mm m= - 5 kpa/mm 5 t 5 t 5m Tue t 5 3 (c) Seies-3 analyses (d) Seies-4 analyses Figue 7 Compaisons of t 5 and t 5m FIELD DISSIPATION TEST IN SAGA, JAPAN Piezocone tests wee pefomed in the Aiake clay deposit at Saga, Japan. Six (6) piezocone penetation tests and 45 poe pessue dissipation tests wee conducted, using a cm cone with a filte element at the shoulde fo poe pessue measuement. The six piezocone penetation tests wee aanged in thee (3) pais, and each pai involved a continuous penetation test and a sepaate test that paused at about. m intevals to measue the dissipation of excess poe pessue geneated duing the peceding penetation (Fig. 8). At the site, the soft Aiake clay deposit is about -5m thick and its index -4-

5 popeties ae given in Fig. 9 (data fom boehole No. 7 as indicated in Fig. 8). The top cust is about. m thick and is in an appaent oveconsolidated state. Below it the soil is nomally to lightly ove-consolidated. The gound wate level fluctuates aound. m below the gound suface. Bidge Embankment Bem Gain Size (%) Sand Silt Clay 4 Boe hole No.7 Rokkaku Rive m 7m No.- m No.- No.- No.- No.3- No.3- m Figue 8 Layout of field tests in Saga, Japan W n Unit Weight W L (%) W P γ t (kn/m 3 ) Void Ratio e 3 Penetation test No. - No. - No. 3- Dissipation test No. - No. - No. 3- Unconfined Yielding Compessive Compesion Vetical Stength qu (kn/m ) Index C c Stess (kn/m ) (Buns and Mayne 998; Sully et al. 999). The test esults in Saga, Japan, indicate that this phenomenon can also occu fo sandy soils and some lightly ove-consolidated soils. To estimate the C h values of the deposit, I values ae needed. It has been epoted that fo the Aiake clay deposit in the Saga aea, the atio E 5 /S u (E 5 is the secant modulus at 5% of peak deviato stess fom unconfined undained compession tests) is between and (Chai et al. 4a). Using a Poisson s atio of.5 and E 5 /S u = 5, an I value of 5 is obtained and this was used in the epoted calculations. Using the field test esults with a non-standad excess poe pessue dissipation esponse, values of t umax, and t 5 wee obtained and the coesponding t 5m values wee calculated by Eq. (3). Values of C h (fom t 5 ) and C hm (fom t 5m ) wee calculated by Eq. (4). Table 3 lists the esults fo the tests denoted as No. -. Fo all dissipation tests with non-standad cuves, the atio C hm /C h is between.6 and 8.8. Geneally, the highe values ae fo locations whee the sand layes and lenses existed, i.e., at elevations above - m, between -5 to -7 m, and below - m. Tip esistanceq t (MPa) Poe-wate pessue u d (kpa) No.- No.- No.3- Depth (m) 6 8 Elevation (m) -5 - No.- No.- No Figue 9 Index popeties of Aiake clay -5-5 Laboatoy consolidation test esults ae available fo elevations -. to -. m, -7. to -8. m and -. to -3. m, and the values of the vetical coefficient of consolidation (C v ) ae listed in Table. Table C v values fom oedomete tests Elevation (m) C v (cm /min) Ove-consolidated Nomally consolidated -. to to to Tip esistances (q t ) and poe pessues (u d ) fom tests No. -, No. - and No. 3- (see Fig. 8 fo test locations) ae given in Fig.. Fo elevations above - m, between -5 to -7 m and below about - m, thee ae zigzag esponses of the penetation poe pessues, indicating the existence of thin sand layes o lenses. Nomalized field excess poe pessue dissipation cuves fo the tests conducted in No. - ae given in Fig.. The best known non-standad excess poe pessue dissipation cuves have been fo heavily ove-consolidated clay deposits Figue Penetation tip esistances and poe pessues As shown in Table, the vetical coefficient of consolidation, C v, fo nomally consolidated soils was in the ange fom.5 to.4 cm /min at elevations above -8. (elevation -. to -3. m is at the bounday between the sand and clay layes). It is well known that laboatoy tests nomally unde-estimate the field coefficient of consolidation. Based on back analysis of test embankments, Chai and Miua (999) and Chai et al. (4b) epoted that fo the Aiake clay deposit in the Saga aea, the atio between the field and laboatoy hydaulic conductivities is to 4, and the hoizontal hydaulic conductivity is about.5 to times the vetical. Assuming that the compession index (C c ) is the same fo the field and the laboatoy, the atio fo hydaulic conductivities can be applied to the coefficient of consolidation. With this infomation, fo elevations above -8. m the field value of the hoizontal coefficient of consolidation at the site is estimated to be fom.5 to.4 cm /min. Excluding locations with thin sand layes o sand lenses, C h values evaluated fom the piezocone dissipation tests using t 5m fall within this ange, and -5-

6 fo most locations the values ae within.3 to.7 cm /min. Nomalized excess poe pessue (u d -u )/(u max -u ) DL-.7m DL-3.7m DL-4.7m DL-5.7m DL-5.77m DL-6.7m DL-7.7m DL-8.7m DL-9.7m DL-.7m DL-.77m DL-.7m DL-.77m DL-.7m DL-.77m. Elapsed time (min) Figue Field dissipation test esults of No. - Table 3 Summay of the field dissipation test esults fom No. - Depth t umax t 5 T 5m C h C hm C hm /C h m min min min cm /min cm /min CONCLUSION A method fo intepeting the hoizontal coefficient of consolidation (C h ) fom non-standad piezocone dissipation cuves was poposed. Fo the non-standad cuve the excess poe pessue does not monotonically decease with time. The non-standad dissipation cuve is mainly obseved fo poe pessue tansduces located at the shoulde o the shaft of a cone. It is consideed that this phenomenon is mainly caused by (a) shea-induced dilatancy of ove-consolidated clays o sandy soils, and (b) unloading effects fo a soil elements moving fom the tip to the shoulde of a cone. Both effects esult in a lowe excess poe pessue at the filte element than beyond it. An empiical equation fo coecting t 5, the time fo 5% dissipation of the measued maximum excess poe pessue has been poposed. Radial plane stain consolidation analyses wee conducted to investigate the effect of the initial excess poe pessue distibution on subsequent dissipation, and the esults povided an empiical equation to coect t 5 in non-standad cases. These coected values of t 5m can be used with existing methods of test intepetation to evaluate the field value of C h. The evised intepetation pocedue was applied to field test data fom Saga, Japan. Fo the non-standad dissipation cuves, C h values evaluated by the poposed method ae.6 to 8.8 times those obtained without the t 5 coection. It was also shown that the C h values evaluated fom the poposed method ae in the ange of independently back-calculated field values. REFERENCES Buns, S.E. and Mayne, P.W. (998). Monotonic and dilatoy poe pessue decay duing piezocone tests in clay. Can. Geotech. J. 35: Chai, J.-C. and Miua, N. (999). Investigation on some factos affecting vetical dain behavio. J. of Geotechnical and Geoenvionmental Engineeing, 5(3): 6-6. Chai, J.-C., Miua, N. and Koga, H. (4a). Lateal defomation of gound caused by soil-cement column installation. Tentatively accepted by J. of Geotechnical and Geoenvionmental Engineeing, ASCE. Chai J.-C., Hayashi, S. and Cate, J.P. (4b). Peloading clayey deposits by combining vacuum pessue with embankment loading. Submitted to J. of Geotechnical and Geoenvionmental Engineeing, ASCE. Coop, M.R. and Woth, C.P. (989). Field studies of an instumented model pile in clay. Géotechnique, 39(4): Davidson, J.L. (985). Poe pessue geneated duing cone penetation testing in heavily oveconsolidated clays. In discussion Session D: Field Instumentation and Field Measuements, Poc. th Inte. Conf. on Soil Mechanics and Found. Engineeing, San Fancisco, 5: 699. Lunne, T., Eidsmoen, T.E., Gillespie, D., and Howland, J. (986). Laboatoy and field calibation of cone penetometes. In Use of In-situ Tests in Geotechnical engineeing. Edited by S. P. Clemence. ASCE, Geotechnical Special Publication 6: Robetson, P.K., Sully, J.P. Woelle, D.J., Lunne, T., Powell, J.J. M. and Gillespie, D. G.(99). Estimating coefficient of consolidation fom piezocone tests. Can. Geotechnical J., 9: Sully, J.P., Robetson, P.K., Campanella, R.G., and Woelle, D.J. (999). An appoach to evaluation of field CPTU dissipation data in oveconsolidated fine-gained soils. Can. Geotech. J., 36: Teh, C.I. and Houlsby, G.T. (99). An analytical study of the cone penetation test in clay. Geotechnique, 4: Vesic, A.S. (97). Expansion of cavities in infinite soil mass. J. of the Soil Mechanics and Foundation Division, ASCE, 98(SM3):