Centrifuge modelling of sand compaction piles in soft clay under embankment load

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1 Centrifuge modelling of sand compaction piles in soft clay under embankment load T.M. Weber, J. Laue & S.M. Springman Institute for Geotechnical Engineering, ETH Zurich, Switzerland ABSTRACT: Sand compaction piles are used in practice for ground improvement of soft subsoil in order to accelerate consolidation, reduce compressibility, and increase strength. The current design procedure of sand compaction piles is based on simple empirical calculations, which does not take full account of the sand pile behaviour. This research project investigates the behaviour of a base reinforced embankment constructed on a soft clay layer, which is improved with sand compaction piles. In order to gain a better understanding of interactions within the structure, physical investigations are being conducted by means of centrifuge modelling. Tests were performed under plane strain conditions in a strong box, using a newly developed sand compaction pile installation tool for in-flight pile construction. Initial analysis of test data shows the change in stress state due to sand compaction pile installation and the influence on clay behaviour. 1 INTRODUCTION Sand compaction piles are widely used in practice in order to improve soft ground, such as soft clay or peat. In particular, when less settlement-sensitive structures such as embankments are designed, the more cost effective method of sand piles is often preferred, in comparison to creating a piled embankment with reinforced concrete piles. There are several methods available for sand compaction pile installation. They can be described as displacement and replacement methods with a wet or dry installation process. Furthermore the compaction of the sand pile is achieved by ramming or vibrating (Van Impe et al. 1997, Van Impe 1989, Mitchell 1981). One of the most frequently used installation methods for sand compaction piles is the bottom feed vibrodisplacement method. This method uses compressed air to flush a vibrating probe into the ground and the pile is inserted by displacing the soil. Sand or gravel is filled through an inner tube into the probe while a lock closes the probe to the top against the inner air pressure. The probe has an opening at the bottom and the gravel remains in the soil when the probe is withdrawn. The column is compacted by vibrating and reinsertion of the probe. The design procedure for sand compaction piles is based on empirical and analytical approaches (Priebe 1995, Aboshi et al. 1991, Balaam & Booker 1985). Some key assumptions such as even settlement of the soil body and the pile grid do not apply to embankment loads. Also, the change in stress state due to the construction process of the sand piles is not fully taken into account. The method after Priebe (1995) is widely used in German speaking countries and gives an improvement factor referring to the unimproved ground. The calculation is fairly robust and conservative. In order to overcome the deficiencies of the existing methods and to gain a better understanding of the interaction behaviour of sand compaction piles, physical investigations are being carried out by using the Zurich geotechnical drum centrifuge (Springman et al. 21). An installation tool for sand pile construction has been developed and several tests have been conducted modelling one-dimensional consolidation of pile grids and embankments on improved clay. The results of the latest test are presented in this paper. 2 SAND COMPACTION PILE INSTALLATION In order to model the sand compaction pile installation process, an installation tool has been developed for the geotechnical drum centrifuge. This tool reproduces the construction process of a displacement pile using a dry method similar to the bottom feed vibrodisplacement method. Earlier methods of sand pile installation in the centrifuge were conducted under 1 g, for instance the sand piles were frozen and pressed into holes previously augered into the clay, or simply sand was

poured into predrilled holes (Kimura et al. 1985). These methods are not able to model the stress changes that occur during the installation process.

(1998) developed a sand compaction pile installer for a beam centrifuge using a different system of pile installation from the one described in this paper. 1 6 2 Fig. 3).

2 poured into predrilled holes (Kimura et al. 1985). These methods are not able to model the stress changes that occur during the installation process. Only piles installed in flight in the centrifuge are able to represent the stress situation of the prototype pile (Lee et al. 24, Kusakabe 22, Lee et al. 21). Ng et al. (1998) developed a sand compaction pile installer for a beam centrifuge using a different system of pile installation from the one described in this paper Fig. 3). During the test, the positions of the drawing pins are approached by the tool and one pile after another can be produced without stopping the centrifuge. axes θ z ω flexible sand hose r r-z-working table pore pressure transducers laser scanner filling tube lost tip soil model tool table drum channel Figure 2. Construction of sand piles in the drum centrifuge. Figure 1. Photo of the sand compaction pile installation tool mounted on the working table. The newly developed tool designed to construct sand compaction piles is shown in Figure 1, mounted on the working table in the middle of the drum centrifuge. The installation tool itself consists of a mounting (1) at the working table (2), a collection tube (3), a transition piece (4) and a filling tube (5). Also a laser scanning device (6) is fixed on top of the tool in order to measure heave or settlement of the model surface. A flexible hose (7) delivers the sand through a coupling from outside of the centrifuge. The construction process of a sand compaction pile is shown schematically in Figure 2. In order to prevent plugging of the filling tube with clay during installation of the pile, a lost tip system is used. A drawing pin is stuck onto the surface of the clay model before pile installation. The cap of the drawing pin closes the opening of the filling tube when the tool is driven into the clay. When the desired depth is reached, sand is filled into the inlet sand hose from outside the centrifuge. Slowly the installation tool is withdrawn and the sand flows out of the filling tube. The drawing pin remains in the soil model. Compaction of the sand pile is achieved during withdrawal through re-insertion of the installation tool into the soil (see Fig. 5b). Depending on the relationship between the degree of withdrawal and reinsertion, different sizes and densities of the sand pile can be obtained (Weber 24). In order to produce a grid of sand compaction piles, the dimensions of the grid must be marked with drawing pins on the surface of the clay model in the z-θ axes before the beginning of the test (see 3 CENTRIFUGE TEST This centrifuge test was performed at 5-times gravity. Referring to scaling laws (Taylor 1995), the model represented a prototype structure of 7 m clay layer depth with a sand compaction pile grid spacing of 1.7 x 1.7 m and an average pile length of 5 m with.6 m pile diameter. This gives an area improvement ratio of 1 %. The model embankment produced an overburden pressure of about 9 kpa, representing a prototype embankment constructed of sand of about 5 m in height. A model geosynthetic was incorporated on the clay surface as base reinforcement to the embankment. Measuring in model scale, the thickness of the clay layer after consolidation was 14 mm. The centre to centre grid spacing of the sand compaction piles in the model was 34 x 34 mm, giving an area improvement ratio of 1 %, as in prototype scale, while an average sand pile had a diameter of about 12 mm. The average pile length was about 1 mm representing floating piles. The groundwater table was set approximately to the clay surface. The grid layout of the sand compaction piles and the clay model are shown in Figure 3. The embankment was constructed with lead balls of 2 mm diameter to 3 mm thickness with a mean density of 6.7 g/cm 3. Due to the restricted height of the model in the drum centrifuge, a denser material than sand was chosen in order to model stresses of a higher embankment. The slope angle was 18 because of the in-flight construction of the embankment. The test was conducted in a modelling container which was divided into 2 sections. In one section, the soft clay was improved with sand compaction

piles, while in the other section, the clay was left unimproved for comparison. Natural clay material from Birmensdorf (Switzerland) was used for the clay model.

plastic limit w P [%] 16.7 liquid limit w L [%] 42.4 clay content < 2 μm [%] 42.3 angle of friction ϕ' [ ] 27.6 1-D compression modulus M' [kn/m 2 ] 929 permeability k [m/s] 1.

3 piles, while in the other section, the clay was left unimproved for comparison. Natural clay material from Birmensdorf (Switzerland) was used for the clay model. Selected soil parameters are stated in Table 1 (Messerklinger et al. 23). Table 1. Material properties of Birmensdorf clay taken from Messerklinger et al. (23) and consolidation step 5-1 kpa. plastic limit w P [%] 16.7 liquid limit w L [%] 42.4 clay content < 2 μm [%] 42.3 angle of friction ϕ' [ ] D compression modulus M' [kn/m 2 ] 929 permeability k [m/s] coefficient of consolidation c v [m/s 2 ] saturated density γ sat [g/cm 3 ] 1.84 water content w [%] 43 The clay model was prepared outside the centrifuge in the model container, mixed to slurry of 1 % water content and pre-consolidated under a press. The container has the dimensions of 4x4 mm in plan and 2 mm in height. The load steps of the preconsolidation procedure were 6, 12, 25, 5 and 1 kpa vertical stress. The soil parameters derived from the last consolidation step are given in Table 1. Sieved quartz sand of the fraction.5-1. mm diameter was used for the sand piles. The friction angle ϕ' of the sand is about 37. Figure 3. Preparation of the sand pile grid, separation of model container into two parts, left side unimproved and right side improved. The procedure of the centrifuge test started with an initial consolidation phase at in-situ stresses in the centrifuge. Following consolidation of the clay model, the sand compaction piles were installed. After that, the embankment was constructed and the time settlement behaviour of the embankment was monitored. In order to obtain the undrained shear strength of the clay model, a T-Bar test was conducted during the centrifuge test at the end of the initial consolidation phase (Fig. 4). The profile of the undrained shear strength s u derived after Stewart & Randolph (1994) from this test gave a fairly narrow range from 18 kpa at the surface to 23 kpa at the base of the clay layer. model depth [mm] s u [kpa] Figure 4. Result of a T-bar penetrometer test in the clay model. 4 RESULTS Results from the centrifuge test can be shown in order to understand the system behaviour. All values are given in model scale except where stated otherwise. a) excess pore water pressure [kpa] penetration depth [mm] P P P 1 12 (1) (2) b) (3) Figure 5. Construction of a single sand compaction pile: a) excess pore water pressure development in the clay model during pile construction, pore pressure transducers P 1, P 2 and P 3 are located at model depths of 12, 7 and 25 mm respectively, distance to pile axis is 2 mm or twice the sand pile diameter, P 1 P 2 P 3

4 b) penetration path of the installation tool into the clay model during pile construction, including vertical location of pore presser transducer. The development of excess pore water pressure due to the construction of a single sand compaction pile is shown in Figure 5a, with pore water pressure readings at different depths in the clay model. The arrows mark the 3 stages of the construction process which can also be seen in Figure 5b. Arrow (1) indicates the penetration of the filling tube into the clay model. Arrow (2) marks when the tool reached the final depth and the sand was filled into the centrifuge. After that, the compaction process starts. Arrow (3) highlights the time when the filling tube reached the clay surface and the tool was extracted from the soil surface. The maximum excess pore water pressures during the penetration phase of the tool develop when the tip of the tool passes the depth of the pore pressure transducer. Significant excess pore water pressures also develop during the compaction phase as the tip of the tool reaches the transducer depth again. After that, a regularly serrated curve forms, showing each stroke of the tool during the compaction process. High excess pore water pressures dissipate fairly quickly after the construction of the pile, but a small excess pressure remains in the clay for a longer period. After embankment construction, surface settlements were monitored with a laser scanning device (see Fig. 1, no. 6). For illustration, the time settlement curves of two key points from the embankment section are displayed in Figure 6, point (1) at the toe and point (2) at the crest of the embankment. The readings from the two model sections of improved and unimproved clay are shown. settlement [mm] Figure 6. Linear time settlement curves of the crest (2) and the toe (1) of the embankment after construction; comparison between improved (----) and unimproved sub soil ( ). The curves for the toe of the embankment in Figure 6 point (1) show fairly similar behaviour in the two model sections. A small amount of heave occurs just after embankment construction on the unimproved side, which later disappears due to a broad (1) (2) (1) (2) settlement trough. This gives an indication of the undrained (constant volume) plastic behaviour of the saturated clay. Calculating the stiffness of the clay by taking the settlement curve for the embankment crest (Fig. 6), a one-dimensional compression modulus M' of 45 kn/m 2 for the improved clay and 22 kn/m 2 for the unimproved clay can be determined due to the embankment load. This calculation is a straightforward analysis simply assuming 1-D conditions under the embankment centre, neglecting spatial effects, horizontal movements, the stress history of the clay, and the compressibility of the embankment material. The increase in stiffness and the reduction of settlements result in a ground improvement factor n of about 2 at the end of consolidation. Analysing the development of pore water pressures after embankment construction in Figure 7, the excess pore pressure within the sand pile grid dissipates much faster than in the unimproved clay. pore water pressure [kpa] Figure 7. Pore water pressure dissipation after embankment construction; readings in improved ground (----) within the sand pile grid in comparison with unimproved ground ( ) at certain model depths d i [mm]: d 1 =12, d 2 =7, d 3 =25. For the unimproved clay, a consolidation time t 9 of 4 min can be calculated when 9 % of the consolidation has occurred. This also corresponds to the analysis of the time-settlement curve in Figure 6. Calculating the consolidation time t 9 for the improved clay within the sand pile grid from the pore water pressure measurements, one can obtain t 9 of about 76 min. Comparing this with the timesettlement behaviour from Figure 6, a difference in t 9 can be observed, giving a value for t 9 of 38 min from these readings. The reason for this discrepancy is probably the floating pile construction and ongoing consolidation in the lowest layer of the clay model, which is not improved. Transferring the consolidation into prototype scale, consolidation time t 9 can be accelerated from 23 to 4.4 months using ground improvement with sand drains at the given grid spacing. This gives a time improvement factor of 5. d 1 d 2 d 3

5 In addition, the isochrones of excess pore water pressure over model depth can be constructed from pore water pressure readings shown in Figures 8, 9. For the unimproved clay (Fig. 8), a classical parabolic shape for the excess pore water pressure distribution with depth can be drawn, except that the first curve at t 1 shows some distortion at the top and bottom of the layer. Excess pore water pressure dissipation occurs mainly in a vertical direction. Drawing isochrones for the improved clay within the sand pile grid (Fig. 9), the curves have a different shape from those in the unimproved clay. Excess pore water pressure dissipates mostly horizontally towards the sand piles, so that the isochrones are straighter and near-parallel. At the base of the column (1 mm), there is a footing effect and the excess pore pressure dissipation develops slower than at the top of the clay layer. This can be explained by to the limited depth of the sand compaction piles and resulting longer net drainage paths in that zone. Figure 1 shows the excavation of the soil model after the centrifuge test. The embankment is schematically drawn on top of the clay layer because the loose embankment material collapses when the drum centrifuge stops. The diameter of the sand compaction pile becomes narrower towards the base. The reason for this is the constant compaction regime of the sand piles when the tool is withdrawn from and reinserted into the clay model. The ratio that was used in this test was 15 mm to 1 mm withdrawal to reinsertion. The confining pressure of the clay is higher at greater depth in the model and therefore the column is smaller at the base and broader at the top. The pile lengths are different depending on the pile position within the grid. Due to technical constraints of the sand compaction pile installation tool, the pile lengths vary between 8 and 12 mm, giving an average length of about 1 mm mm 12 mm 4 model depth [mm] t 7 t 6 t 5 t 4 t 3 t 2 t 1 1 mm excess pore water pressure [kpa] Figure 8. Isochrones of excess pore water pressure in unimproved ground at certain times t i [min] after embankment construction: t 1 =, t 2 =6, t 3 =13, t 4 =22, t 5 =44, t 6 =88, t 7 =12. model depth [mm] t 7 t 6 t 5 t 4 t 3 t 2 t excess pore water pressure [kpa] Figure 9. Isochrones of excess pore water pressure in improved ground within the sand pile grid at certain times t i [min] after embankment construction: t 1 =, t 2 =4, t 3 =6, t 4 =1, t 5 =2, t 6 =4, t 7 =12. Figure 1. Cross section of soil model after the centrifuge test. The density of a sand compaction pile was measured as 1.75 g/cm 3. An average diameter in the middle of the pile was measured as 12 mm. In un-compacted sand piles from previous tests (Weber et al. 25, Weber 24), a density of 1.5 g/cm 3 and a diameter of 9 mm were measured. The difference in the densities and diameters of compacted and un-compacted piles shows notably the compaction effect of the pile installation method. 5 CONCLUSIONS The sand compaction pile installation tool was successfully applied in a centrifuge test to construct sand piles in-flight. The measurement of excess pore water pressure gives an indication of the change in the stress state around a sand pile during construction. Further analysis is needed in order to estimate the effect of stress change in the soil on the sand compaction pile behaviour. Due to the ground improvement in this centrifuge test, a settlement reduction factor of 2 was measured for an area improvement ratio of 1%. In comparison to empirical and analytical methods, similar val-

6 ues for the improvement factor were calculated. Analysis after Priebe (1995) gave a good match with a factor of 1.8, while that due to Balaam & Bocker (1985) slightly underestimated the improvement with a factor of 1.4. The acceleration of consolidation time t 9 was measured to be factor 5 for the described test. These findings are a part of a test series in which several parameters of the model are varied to give conclusions on the system behaviour. Based on these results from past and future tests, supplemented by numerical calculations, comprehensive analyses shall provide a basis for recommendations on the design procedure of sand compaction piles in soft clay under embankment loading. ACKNOWLEDGEMENT This research programme is supported by the Swiss National Research Foundation - No /1. The authors are very grateful to E. Bleiker, H. Buschor, R. Chikatamarla, A. Ehrbar, M. Iten, A. Privitello, C. Senn and A. Zweidler for their respective contributions. REFERENCES Aboshi, H., Mizuno, Y. & Kuwabara, M Present state of sand compaction pile in Japan. In Esrig, M.I. & Bachus, R.C. (Eds), Deep Foundation Improvements: Design, Construction and Testing, ASTM Special Technical Publication American Society for Testing and Materials: Baltimore. Balaam, N.P. & Booker, J.R Effect of stone column yield on settlement of rigid foundations in stabilized clay. International Journal for Numerical and Analytical Methods in Geomechanics 9(4): Kimura, T., Nakase, A., Kusakabe, O. & Saitoh, K Behaviour of soil improved by sand compaction piles. XI International Conference on Soil Mechanics and Foundation Engineering, San Francisco Balkema: Rotterdam. Kusakabe, O. 22. Modelling of soil improvement methods on soft clay. In Phillips, R., Guo, P.J. & Popescu, R. (Eds), International Conference on Physical Modelling in Geotechnics, St. John's, Newfoundland, Canada Balkema: Lisse. Lee, F.H., Ng, Y.W. & Young, K.Y. 21. Effects of Installation Method on Sand Compaction Piles in Clay in the Centrifuge. Geotechnical Testing Journal (ASTM) 24(3): Lee, F.H., Juneja, A. & Tan, T.S. 24. Stress and pore pressure changes due to sand compaction pile installation in soft clay. Géotechnique 54(1): Messerklinger, S., Kahr, G., Plötze, M., Trausch Giudici, J.L., Springman, S.M. & Lojander, M. 23. Mineralogical and mechanical behaviour of soft Finnish and Swiss clays. In Vermeer, P.A., Schweiger, H.F., Karstunen, M. & Cudny, M. (Eds), International Workshop on Geotechnics of Soft Soils-Theory and Practice, Noordwijkerhout, Netherlands Verlag Glückauf GmbH. Mitchell, J.K Soil Improvement - State-of-the-Art- Report. X International Conference on Soil Mechanics and Foundation Engineering, Stockholm, Sweden Balkema: Rotterdam. Ng, Y.W., Lee, F.H. & Young, K.Y Development of an in-flight Sand Compaction Piles (SCPs) installer. In Kimura, T., Kusakabe, O. & Takemura, J. (Eds), International Conference Centrifuge '98, Tokyo, Japan Balkema: Rotterdam. Priebe, H.J The design of vibro replacement. Ground Engineering 28(1): Springman, S.M., Laue, J., Boyle, R., White, J. & Zweidler, A. 21. The ETH Zurich Geotechnical Drum Centrifuge. International Journal of Physical Modelling in Geotechnics 1(1): Stewart, D.P. & Randolph, M.F T-Bar Penetration Testing in Soft Clay. Journal of Geotechnical Engineering (ASCE) 12(12): Taylor, R.N Geotechnical Centrifuge Technology. Blackie Academic & Professional: London. Van Impe, W.F Soil Improvement Techniques and their Evolution, Balkema: Rotterdam. Van Impe, W.F., Madhav, M.R. & Van der Cruyssen, J.P Considerations in Stone Column Design. In Davies, M.C.R. & Schlosser, F. (Eds), III International Conference on Ground Improvement Geosystems, London, UK Thomas Telford: London. Weber, T.M. 24. Development of a sand compaction pile installation tool for the geotechnical drum centrifuge. In Brandl, H. & Kopf, F. (Eds), XVI European Young Geotechnical Engineer's Conference, Vienna, Austria Weber, T.M., Laue, J. & Springman, S.M. 25. Modelling the inflight construction of sand compaction piles in the centrifuge. XVI International Conference on Soil Mechanics and Geotechnical Engineering, Osaka, Japan Millpress: Rotterdam.

CENTRIFUGE MODELING OF GROUND IMPROVEMENT UNDER EMBANKMENTS

POLLACK PERIODICA An International Journal for Engineering and Information Sciences DOI: 10.1556/Pollack.1.2006.2.1 Vol. 1, No. 2, pp. 3 15, (2006) www.akademiai.com CENTRIFUGE MODELING OF GROUND IMPROVEMENT