Excess pore pressures and consolidation settlements observed during the construction of a trial embankment placed on four different types of vertical drains are examined with the aim of evaluating: undrained pore pressure response, field coefficient of consolidation and drain performance

Jamiolkowski, M.

Lancellotta, R.

English

Missouri University of Science and Technology (Missouri S&T): Scholars' Mine

Missouri University of Science and Technology Scholars' Mine International Conference on Case Histories in Geotechnical Engineering (1984) - First International Conference on Case Histories in Geotechnical Engineering 07 May 1984, 11:30 am - 6:00 pm Embankment on Vertical Drains - Pore Pressures During Construction M. Jamiolkowski Technical University, Torino, Italy R. Lancellotta Technical University, Torino, Italy Follow this and additional works at: https://scholarsmine.mst.edu/icchge  Part of the Geotechnical Engineering Commons Recommended Citation Jamiolkowski, M. and Lancellotta, R., "Embankment on Vertical Drains - Pore Pressures During Construction" (1984). International Conference on Case Histories in Geotechnical Engineering. 15. https://scholarsmine.mst.edu/icchge/1icchge/1icchge-theme1/15 This Article - Conference proceedings is brought to you for free and open access by Scholars' Mine. It has been accepted for inclusion in International Conference on Case Histories in Geotechnical Engineering by an authorized administrator of Scholars' Mine. This work is protected by U. S. Copyright Law. Unauthorized use including reproduction for redistribution requires the permission of the copyright holder. For more information, please contact scholarsmine@mst.edu. Embankment on Vertical Drains - Pore Pressures During Construction M. Jamiolkowski and R. Lancellotta Department of Structural Engineering - Technical University - Torino, Italy SYNOPSIS Excess pore pressures and consolidation sett~ements observed during the construction of a trial embankment placed on four different types of vertical drains are examined with the aim of eva-luating: undrained pore pressure response, field coefficient of consolidation and drain performan-ce. 1. During the construction of the Porto Tolle Thermal Power Plant on the extreme eastern edge of the delta of the river Po, extensive use of preloading techniques was made for a number of large steel tanks and many secondary structures. Large numbers ("' 1 700 000 m) of vertical drains of various types were installed with the aim of ac-celerating the consolidation of the 22 to 24 m thick layer of soft silty clay shown in Fig. 1. The geotechnical properties of this layer are shown in Fig. 1 and Table I. Further details may be found in the paper of Jamiolkowski et al. ( 1980) • G.L.+O.SABOVE M.S.L. ~ O YELLOW TO BROWN MEDIUM DENSE SAND AND SILTY SAND 10 LIGHT BROWN SOFT SILTY CLAY 30 BROWN DENSE z[ml SILTY SAND TO CASAGRANDE'S PROCEDURE Fig. 1. Soil Profile and Stress History TABLE I. Properties of the Porto Tolle Silty Clay (Hansbo et al., 1981) LIQUID LIMIT =52. 6±6. 9 (%); PLASTIC LIMIT =30.9±6.6{%) wn =36.4±4..:_9; CLAY CONTENT =33.9±4.2 ~u (TX-COMPR.) "' 0.31 a . ~ ~=~=~~=~: ~ "' 0.34 avo vo' <P (TX-COMPR.) "' 29°; - 32° K0 (COEFF. OR EARTH PRESS. AT REST) = 0.5 CR(PRIMARY COMPRESSION RATIO) = 0.20 TX - TRIAXIAL TEST; PS = PLANE STRAIN TEST The available geological and geotechnical infor-mation led to the conclusion that this soft co-hesive stratum is only a few thousand years old and has not yet been subjected to any apprecia-ble secondary compression. The penetration pore pressures measured by means 275 of piezo-cone tests (Battaglia e Maniscalco,1983) indicated that the stratum under consideration possesses a well developed macrofabric which con-sists of thin seems, lenses and pockets of fine sand and silty sand. However, these more permea-ble inclusions are discontinuous and, as a back-analysis of a trial embankment without vertical drains (Bilotta and Viggiani, 1975) showed,the soft silty clay behaved in spite of this macrofa-bric as one single consolidating stratum, with drainage occurring at the top and bottom. 2. One of the preloading embankments adopted (cf. Fig. 2) was used as a trial embankment for compa-ring the efficiency of the four types of drains installed. The characteristics of the latter are given in Table II. TABLE II Drain Characteristics TYPE SPACING (1) DIAMETER MANDREL DIAMETER {em) (em) (em) JETTED SAND DRAINS soo 30.0 (2) 1~ (2) GEODRAINS 380 6.3 SOILDRAINS 380 5.5 10 SANDWICKS 420 7.0 9 (1) EQUILATERAL TRIANGULAR ARRAY (2) EQUIVALENT DIAMETER (DRAINS ARE BAND-SHAPED) After the embankment was placed, its behaviour was monitored by means of 45 piezometers of va-rious types, surface settlement plates, deep set tlement sensors of the below-hose type and verti cal and horizontal inclinometer tubes. This in--strumentation allowed the measurement of surface settlements, vertical strains, pore pressures and horizontal displacements. Some details of the mea surements may be found in Hansbo et al. (1981). -Unfortunately, the long-term performance of the major part of the installed piezometers was unsa-tisfactory in this environment because of the pre sence of isolated pockets of organic gas. It must be assumed that the drains installed contributed to the diffusion of the gas in the soil mass by connecting the isolated pockets. A typical example of the unsatisfactoryperformance of the piezometers is shown in Fig. 3 which com-pares measured pore pressures with measured sur-face settlements and maximum lateral displace-ments. It appears that in spite of the progress in consolidation indicated by the surface settle ments (which are combined with small lateral di= First International Conference on Case Histories in Geotechnical Engineering Missouri University of Science and Technology http://ICCHGE1984-2013.mst.eduLATE"AL DISPLACEMENT, em CROSS-SECTION CONSTRUCTION SCHEOU~E --MEASURED SETTLEMENTS ----FITTED SETTLEMENTS S[CmJ INITIAL SETTLEMENT OF' CLAV + SETTLEMENT OF SAND LAYERS AT THE END OF LOADING (POINT VJ •II em 4UU/m2:J 0 - PIEZOMETER EP2 C 18.80 BELOW G.L. J -CENTER OF AR1EA WITH JETnDI SAND DRAINS :_~/---·- r ·-O M APR MAV JUN JUL p T NOV F B 1877 19TB Fig. 2. Geometry of Trial Embankment Fig. 3. Settlement and Pore Pressure Records splacements of the soft layer) , the piezometers do not indicate any appreciable dissipation of the excess pore pressures caused by the applied loads. 3. In the present paper an attempt is made of a nalysing the piezometer data measured during the construction of the trial embankment with the aim: a. To compare the undrained excess pore pressure which may be predicted on the basis of diffe-rent theories, ~u~, with the maximum excess pore pressure measured in the piezometers at the end of the loading ramp, ~umax• b. To evaluate from the piezometer readings the horizontal coefficient of consolidation, ch, assuming that the influence of gas on the pie zometer performance is negligible during em--bankment construction when the soil is sUbjec ted to a simultaneous increase of both total-stresses and pore pressures. 4. The prediction of the undrained pore pressure response of saturated or nearly saturated soft cohesive soils under an imposed surface load is possible by one of the following four approaches (Foot and Ladd, 1973): ~- Empirical approach which assumes that ~u~=~crv (=increment of total vertical stress at the level of the considered piezometer) • b. Approach in which it is assumed that the soil behaves as an elastic perfectly plastic mate-rial governed by the Tresca failure criterion. In this case, as long as the soil around the piezometer responds "elastically", i.e. f=~01--~03/2cu < 1, ~uP=~cr0 • When a condition of con tained plastic ~low is reached (f=1), then-~u~=Llcrv. increment of mean total stress around the piezometer undrained shear strength of soil 276 Llcr 1} increments of major and minor total pri~ 6 cipal stresses around the piezometer, 03 respectively c. Semi-empirical approach which assumes that as long as f < 1, the value of ~u~ is governed by the following formula: flu~ = ~cr0 + a Ll-r 0 (1 l where: LlT 0 increment of octahedral shear stress around the piezometer a Henkel's pore pressure coefficient. d. Approach in which it is assumed that the soil behaviour is in agreement with the modified Cam Clay model (MCCM), see Burland (1967) and Wroth and Parry (1977). The evaluation of flu~ by means of approaches a. to c. is quite obvious, but the use of the MCCM merits some further comments. Since the Porto To le soft silt~_clay is virtually NC, it may be as sumed that its ~rrent initial effective stress: at or very close to the yield surface ( cf. Fig. 4 )am! i i behaviour may be approximated by that of a compl tely plastic "wet" clay under plane strain cond tions (Burland, 1967). . ·.:g·t.:..•:.c--·~ AIIIC• TOTAl. STI'II8S PATH .. Fig. 4. Pore Pressure Prediction according to Mo dified Cam Clay Model First International Conference on Case Histories in Geotechnical Engineering Missouri University of Science and Technology http://ICCHGE1984-2013.mst.eduIn such circumstances the undrained effective stress path equation coincides with that of the yield surface (Burland, 1967) (see Fig. 4): 2 s (.!:) 2 (!:!__) (~ - 1) (2) s 3 s where: s = 0 s M [ (~)i] si 1 + ~2 (3) avo (1 + K 0 ); 2 (4) 1/2 (o1 + cr3); t = 1/2 (a1 - a 3) slope of the critical-state line (CSL) as determined from a triaxial compression test ovo= effective overburden stress On the basis of experimental evidence (Burland, 1967), it was assumed that the Coulomb-Terzaghi failure criterion holds for the ratio t/s at fai lure: t 3M . -<5> f = 6 + M = s~n cp (5) where $ is intended as the effective angle of friction for plane strain conditions. It should be remembered that (t/s) f defined by eqn. (5) is lower than the (t/slcs ratio at the critical st~ te, the latter being equal to M/(3 (see Fig.4). Moreover, the total(plastic) stress path (AB on Fig.4) has been computed assuming a ratio ~a3/ /~a1 corresponding to elastic conditions. 5. As an example, Table III gives a comparison between ~u~ predicted by means of the four above mentioned approaches and ~Umax measured by the electropneumatic piezometer EP-2 located at a depth of 1 9. 60 m below ground level under the cen terline of the embankment, in a sector where -jetted sand drains where installed. TABLE III. Predicted vs Measured Excess Pore Pressure of Piezometer EP-2 (~~ax = 7.85 t/m2) APPROACH a b c d ~u /~up 0.91 1.11 0.89 0.78 max o ~up I ~a 1.00 0.82 1.03 1.17 0 v On the basis of the data shown in Table III, the following comments can be made: a. The elasto-plastic approach, b. ,leads to an un reliable prediction of ~u0 for it is obvious that ~Umax should be less than ~ug, particu-larly in the presence of vertical drains. b. As far as approaches a. and c. are concerned, they satisfy the above condition; however, they lead to a degree of consolidation Uh= 1 - Umax/ub of 0.1, approximately, at the end of the loading ramp which is too low if compared with the consolidation settlements of the soft cohesive stratum measured in the same period of time (cf. Fig. 3). c. The evaluation of ~ug by means of the MCCM a£ proach leads to ~Umaxl~ub ratios which are qu~ litatively in good agreement with the values of the consolidation settlements observed at the end of the loading ramp. d. The latter approach indicates ~u~/~av ratios which are consistently higher than 1. This may be attributed at least partly to the occurrence of contained plastic flow and even a slight strain softening of the in situ clay of Porto Tol le. Additional experimental evidence supporting the above observations may be found in Table IV in which ~u~,obtained on the basis of the MCCM a£ proach,is compared with the values of AUmax and Aav of other three piezometers. The same table also shows the time from the be-ginning of the loading ramp after which the soil around the piezometers reached a state of contai ned failure (f=1). -TABLE IV. Examples of Excess Pore Pressures Predicted by the MCCM Approach SECTOR PIEZO- DEPTH ~a ~up ~u ~0 Ll.Umax tf METER BELOW v 0 max Ll.av ~up 0 G.L. Nr. m t/m2 t/m2 t/m2 days GEODRAINS BAT-2 15.27 9.05 10.27 7.32 1.13 0.71 32 GEODRAINS BAT-3 18.40 8.70 10.18 7.73 1.17 0.76 36 JETTING EP-2 19.60 8.60 10.10 7.85 1.17 0. 78 38 JETTING BP-2 24.10 7.90 9.72 7.32 1.23 0~ 75 46 BAT = ELECTRIC PIEZOMETER EP = ELECTROPNEUMA'l'IC PIEZOMETER 6. Fig. 5 shows the range of measured values of Aumax (normalized with respect to ~u~) evaluated by the MCCM approach, as recorded unaer all four sections of the trial embankment. From this figu re it may be argued that, although different ty= pes of drains are involved, the response of the normalized pore pressure is substantially similar for all piezometers at the end of the loading ramp, with AUmax/Ll.u; ranging from 0.75 to 0.65. These values, corresponding to a local degree of consolidation for radial flow of Uh ~ 0.25 to 0.35, compare well with the average degree of con solidation U estimated on the basis of the obser= ved consolidation settlements shown in Fig. 3. The U·valueswere est:i!mated from a range of CR com-prised between o. 17 5 (lower bound of labor a tory d~ termined values) and o. 24 (average CR corrected for sample disturbance according to Schmertmann (1955). The substantially uniform pore pressure response of all piezometers (Fig. 5) led to the conclusion that, at least during construction of the trial embankment, the performance of the prefabricated drains was the same as that of the jetted sand drains. This evidence is of practical interest in that it shows that: a. The well resistance of the drains does not af-fect the consolidation process. This statement is also supported by the fact that the ratio ~umax/Aug for the piezometers installed at dif ferent depths between the prefabricated drains is substantially independent of depth; see for example Fig. 6 for band-shaped Geodrains. b. Fig. 6 confirms indirectly that the high dis-charge capacities recently measured (Han.sbo, 1983; Jamiolkowski et al. 1983-bl for these drains in the laboratory may be considered to be reliable, at least for a short period after drain installation. First International Conference on Case Histories in Geotechnical Engineering Missouri University of Science and Technology http://ICCHGE1984-2013.mst.eduO MARCH APRIL MAY JUNE JULY Fig. 5. Normalized Pore Pressures Measured During Construc-tion Loading Fig. 6. ch Values from Back-Ana lysis for Area of Gee--drains Fig. 7. ch Values from Back-Analysis for Area of Jetted Sand Drains The above statements may not be entirely true for a longer period of time when the prefabricated drains suffer some deterioration. 7. Assuming that the MCCM approach predicts reli~ ble values of ~ug and considering radial pore wa-ter flow only, it is possible to evaluate ch con-trolling consolidation during the construction p~ riod. This has been done using the simple uncou-pled radial consolidation theory which allows to take into consideration a linear increase of the surface load with time (Schiffman, 1958). There sulting ch values for the jetted sand drains are shown in Fig. 7. If for these drains,instead of~ sing ~uP according to the MCCM,~u~ is assumed to be equa~ to ~crv, one obtains for the same piezo-meters Ch = (7 to 15) 1o-3 cm2/sec; ch which al-lows to fit the rate of the consolidation settle ment from point 1 to point Y in Fig. 3 is equal to = 20-30 . 10- cm2/sec. As far as the prefabricated drains are concerned, one obtains the same range of ch values if the soil disturbance caused by drain installation is neglected. If, however, for these drains the existence of a less permeable smear zone around the drains is assumed, one obtains the very lar-ge ch values shown in Fig.6. The above clearly shows how difficult and uncer-tain the evaluation of ch may be in the presence of vertical drains, even in the case of a well instrumented trial embankment as the one conside red here. The problem becomes particularly com--plex in the case of prefabricated drains where, in addition to the effects of possible well re-sistance and. drain deterioration, various other factors must be considered, such as the problem of accurately estimating ~u~, smear around the drains and its influence on soil permeability, and also errors in estimating the distance bet-ween the considered piezometer and the nearest drain. In the case considered here, the estimate of Ch was also difficult because of the presence of gas and its effect on the piezometers. In any case, the following conclusions are possible: a. The observed settlement rates and the measured pore pressure responses during embankment con-struction both indicate a substantially simi-lar performance of all four types of installed drains. 278 b. The pore pressures measured during construct: indicate that prefabricated drains are not s1 jected to the phenomenon of well resistance. c. The evaluation of ch on the basis of ~Umax leads to "reasonable" values of the consolid< tion coefficient for jetted drains. The same plies also to prefabricated drains if the ef· feet of smear is neglected. d. The estimate of ch on the basis of pore pres~ re response strongly depends on the assumed , p lue of lm0 • REFERENCES Battaglia, M., !1aniscalco, R. ( 1983), "Il piezacono- esec zione ed interpretazione", .Proc. Conferenze di Geatecnic di Torino, Dipartimento Ingegneria Strutturale Politenic di Torino, N.607. Bilotta, E., Viggiani, c. (1975), "Un'indagine sperimenta in vera grandezza sul camportamento di un banco di argil normalmente consolidate", Proc. XII Italian Conference o Soil Mechanics, Cosenza, 1, 223-240. Burland, J.B. (1967), "Deformation of Soft Clay", Ph.D. T sis Cambrdige University. Foot R. and Ladd, C.C. (1973) ,"The Behaviour of Altchafa ya Test Embankments During Construction. Soil Publicatio 322, MIT, Boston, Mass. Hansbo, s., Jamiolkowski, M., Kok, L., (1981), "Consolida tion by Vertical Drains", Gee technique, N. 1. Hansbo, S. (1983), Discussion Presented to Specialty Ses-sion N°4, Proc. VIII ECSMFE. Henkel, D.J. (1960), "The Shear strength of Saturated Re-moulded Clays" , Proc. ASCE, Research Conf. Shear Strengt of Cohesive Soils, Boulder Colorado, 533-554. Jamiolkowski, M., Lancellotta, R. and Tordella, M.L. (198, "Geotechnical Properties of Porto Tolle NC Silty Clay", Proc. VI Danube Europ. Conf. Soil Mech., varna, Bulgaria Jamiolkowski, M., Lancellotta, R. and Wolski, W. (1983-a) State of the Art Report and General Report on "Precompre sian and Speeding-Up Consolidation", Proc. VIII ECSMFE, Helsinki. Jamiolkowski, M., Lancellotta, R. and Wolski, W. (1983-b) "Precompression and Speeding-Up Consolidation - Closure : marks", Specialty Session N°6, Proc. VIII ECSMFE, Helsink Schiffman, R.L. (1958), "Consolidation of Soil Under Time· Dependent Loading and Varying Permeability", Proc. High-way Research Board, V.37, pp.584-617. Schmertmann, J .M. (1955), "The Undisturbed Consolidation • Clay", Trans. ASCE, Vol. 120, pp.1201-1233. Wroth,C.P. and Parry, R.H.G. ( 1977) , "Shear Properties of Sof· Clay",Proc. Int. Symp. onSoftClay, Bangkok, Thailand. First International Conference on Case Histories in Geotechnical Engineering Missouri University of Science and Technology http://ICCHGE1984-2013.mst.edu

Embankment on Vertical Drains - Pore Pressures During Construction

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Embankment on Vertical Drains - Pore Pressures During Construction

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