Numerical Modeling the Behavior of Ground Improvement in Soft Clay

Transcription

1 Numerical Modeling the Behavior of Ground Improvement in Soft Clay Jason Y. Wu Department of Civil Engineering and Engineering Informatics Chung Hua University No.707, Sec. 2, Wufu Rd., Hsinchu City 300, Taiwan (R.O.C.) Tel.: Fax.: Abstract Compaction grouting has become a popular method for site improvement around the world. However, in saturated soft clays, the treatment often induces higher excess pore water pressure and limits the efficiency of the improvement. To take advantage of the prefabricated vertical drain (PVD) and compaction grouting, a radial ground improvement technology (RGIT) was proposed for treatment of a site that was located on a thick saturated soft estuarine deposit. The treatment turns the compaction grouting into a radial accelerating consolidation process that increases improvement efficiency significantly. This paper describes a phase I study of RGIT by using finite element analysis to examine the field performance and develop design criteria for the construction. Based on the research findings, RGIT appears to be practical for soft clay improvement and demonstrated advantages over conventional compaction grouting. Confidence in what was achieved allowed RGIT to be used for the proposed site. Keywords: numerical analysis, soft clay, ground improvement. 1. Introduction Highly compressible soft deposits are widely distributed in lowland estuarine formations. Ground modification is often mandatory for these imperfect geological areas to ensure the safety of structures. There are many ground improvement technologies developed to date that fulfill the safety requirement. These safety measures vary from simple mechanical compaction to a sophisticated ground freezing treatment. However, due to the heterogeneous natures of subsurface strata and the different engineering considerations, one particular treatment seldom satisfies all the requirements of cost, quality, schedule, and improvement efficiency. Compaction grouting has been used around the world for over 50 years. It has been documented in numerous case histories as an effective ground improvement technology [1]. The key feature of compaction grouting is the injection of low slump, cementitious grouts under high pressure to increase the density of problem soil strata [2]. Such treatment has been used for structural protections and rectifications for various purposes. In recent years, it has become a popular method for site improvement such as the mitigation of liquefaction potential and the densification of soil in order to increase bearing

2 capacity and reduce settlement. In saturated clays, however, compaction grouting often induces higher excess pore water pressure that accumulates in the soil and limit the effectiveness of the improvement. Case studies also indicated that significant post-treatment settlements are very likely for thick saturated clayey deposits and reduces the grouting efficiency [3-5]. Expediting the dissipation of excess pore water pressure is mandatory if compaction grouting is proposed for use in these problematic soils. In recent years, the prefabricated vertical drain (PVD) is probably the most widely used technique to shorten the consolidation time and to strengthen the weak clayey soil in situ. The use of PVDs reduces the length of the drainage paths and takes advantage of the usually higher permeability of the soils in the horizontal direction, thus resulting in a shorter consolidation time. PVDs have been used successfully to accelerate settlement and improve the engineering properties in soft clayey soils for many projects [6-7]. To take advantage of PVDs and compaction grouting, a radial ground improvement technology (RGIT) was proposed for treatment of a site that was located on a thick saturated soft estuarine deposit. RGIT turned the compaction grouting into a radial accelerating consolidation process, leading to the reduction of post-treatment settlement and an increase in the overall subsoil bearing capacity. The principle of RGIT appears to be practical; however, its theoretical mechanism still needs to be examined to develop suitable design criteria for field performance. This paper presents the results of the first phase of finite element analysis using computers to simulate the performance of RGIT in soft clay. Development of the model is first described, and followed by performance checks involving rows of compaction grout holes surrounded by different numbers of PVDs in a fine-grained compressible soil. 2. Model Description 2.1 Overview Model studies for compaction grouting have been presented by many researchers [2,8-9]. In general, the intruded grouting mass can be considered as a spherical or columnar shape in uniform soil. The expanding mass of grout results in a complex system of radial and tangential stresses within the soil. The soil adjacent to the grout bulb experiences enormous shear strains. A zone of plastic failure associated with major disruption, shearing, and plastic deformation is developed around the grouted mass [2]. As the distance from the soil-grout interface increases, the deformation essentially becomes elastic. Because soil behavior is inherently density dependent, an elasto-plastic model for the soil that represents the effect of density is necessary. The analysis was simplified by adopting a cavity expansion approach assuming either a cylindrical or spherical movement, and causing a conical shear failure above the grout bulb [29]. For radial consolidation associated with PVD, Barron s theory provides basic solutions for the rate of settlement and excess pore water pressure dissipation. Subsequent developments in radial consolidation theory and numerical approaches have offered more specific considerations of stress and strain conditions. However, difficulties remain inhibiting the determination of an appropriate constitutive model, and the model s parameters have limited the use of these methods in practice. Simplified models tailored to the specific problem are generally preferred [7,10-11]. 2.2 Geometry and Soil Parameters In this study, the PLAXIS FEM program was

3 used to simulate the performance of RGIT in a soft clay layer. Imposing a specific cross sectional area and soil properties to the interface element, the effect of grouting improvement with radial pore water pressure dissipation can be simulated. To model the performance of the proposed matching scheme, the overall average consolidation and heave caused by grouting, were evaluated in terms of the net deformation during the grouting process. The Elastic-plastic Mohr-Coulomb model was employed for the numerical analysis. It basically involves five input parameters, Young s modulus (E) and Poisson s ratio (ν) for soil elasticity; angle of shearing resistance (φ) and cohesion (c) for soil plasticity, and ψ as an angle of soil dilatancy. In the PLAXIS, it is also possible to specify undrained behavior in an effective stress analysis using effective model parameters. Staged constructions also can be specified by activating or deactivating loads to simulate the staged grouting processes. In this study, a stage-up mode was used with procedures recommended by [12]. Figure 1 shows the geometry used for RGIT study and Table 1 presents the associated parameters used for the analysis. An equivalent horizontal permeability (k h ) as recommended by Lin et al. (2000) was used for clay to account for the effect of PVD on radial consolidation. It also has included the smear effect caused by the installation disturbance. The vertical permeability of PVD was taken based on a calculated discharge capacity of 10m 3 /year [13]. 2.3 Model Performance The proposed RGIT model was used to evaluate the grouting criteria for a site in the southern part of Taiwan where an office complex was to be developed for a college campus. The subsurface of the site in general is underlain by four layers of soils: (1) medium dense silty sand or sandy silt (fill) varying in thickness from 4 to 6m; (2) very soft silty clay varying in thickness from 7 to 11 m; (3) dense sand varying in thickness from 4 to 8 m; and (4) very dense to dense gravel. The average depth to the bearing stratum is about 20m across the site. To eliminate large potential settlements caused by the underlying soft clay, piles or ground modification must be used to support structures. However, further analysis revealed that large negative skin friction forces will be present and thus excluded the use of piles.rgit was finally selected based on the result of value engineering analysis for various ground modification technologies. Model simulations were conducted as pilot study to evaluate the feasibility of this innovation technology. Commercial general criteria for compaction grouting were used as a baseline for the evaluation. The general criteria were (1) inject grout at each stage until 4,500 kpa is reached; or (2) 0.14 m 3 per 0.3 m grout stage is injected; or (3) 6 mm of structural movement or 13 mm of ground surface heave is observed; and (4) if excessive grout takes are recorded with little or no increase of injection pressure, then additional compaction grout holes will be required in a given area. Figure 2 shows representative distorted meshes with and without the installation of PVDs after expanding the compaction grout element under a grouting pressure of 80 kpa. It was observed that the inclusions of PVDs have reduced the heave significantly. Based on the model analysis, the variations of grouting pressure with the radius of grout bulb for a given overburden 5m are presented in Figure 3. The volume of grout bulb increases with the increase of pressure with an ultimate pressure (P u ) at about 200 kpa corresponding to a grout bulb radius of about 0.2m. This is equivalent to a staged grouting volume of 0.15 m 3 approximately meeting the criteria described above. The ultimate pressure observed in

4 this study was much less in comparison with other studies. The lower value can be attributed to the fact of the lower shear strength in the surrounding clay and the relatively shallower grouting depth. Essler et al. (2000) also expressed the injection pressure as six times the in-situ shear stress plus the horizontal overburden stress (6C u + σ h ) close to the findings in this study. Parameter Table 1. Parameters used for material modeling Material Type Fill Sand Clay PVD Dry Density, γ d (kn/m 3 ) Saturated Density, γ t (kn/m 3 ) Young s Modulus, E ( kpa) ,000 30,000 15,000 Poisson s Ratio, ν Angle of Shearing Resistance, φ ( ) Dilatancy, ψ ( ) Undrained Cohesion, C u (kpa) Horizontal Permeability, k h (m/s) Vertical Permeability, k v (m/s) Compaction Grout Hole 2.5m 2.5m 5 m Fill 9 m Clay 6 m Dense Sand PVDs 45 m Figure 1. Typical geometry for RGIT

5 PVDs Compaction Grout Holes (a) Compaction grouting Compaction Grout Holes (b) RGIT Figure 2. Deformed finite element mesh In Figure 4, it is seen that the amount of ground heave increases with the increase of the grouting pressure. A rapid increase of heave is observed when pressure is over 80 kpa; indicating the soil is undergoing plastic movement. This is approximately the weight of the overburden above the grout bulb. Based on the theory of conical failure, EI-Kelesh et al. (2001) explained that at the onset of ground surface heave, the upward pressure of the grout bulb equals the weight of the cone of the soil above the grout bulb plus the downward shearing resistance along the cone surface [2]. Since the undrained shear strength on the site is relatively small, the overburden pressure above the grout bulb was considered as the control value for minimum heave. Figure 4 also indicates that the amount of heave increases with the number of concurrent grouting applications. However, heave tendency with pressure remains unchanged. The grout injection holes were designed with a 10 m center to center spacing. Based on the results of the analysis, the limiting pressure should be no more than 96 kpa in order to satisfy the design criteria of heave. The more the grout holes, the less the limiting pressure. The effect of PVDs on the compaction grouting is presented in Figure 5. Observe that the

6 ground heave with PVDs was almost identical with those without PVDs for pressures below 80 kpa. However, at the previous described limiting pressure (96 kpa), the installation of PVDs drops the ground heave by about 46% in comparison with that without PVDs. To meet the design criteria of ground heave, the limiting pressure can be increased to 126 kpa if PVDs are installed. Increasing the number of PVDs further drops the ground heave, but its reduction tends to be diminished with the increased number. The accumulations of excess pore water pressure and its associated settlement are the primary negative issues for the long-term efficiency of compaction grouting. However, with the installation of PVDs, the excess pore water pressure induced by the injection can be dissipated in a timely manner. The presented ground heaves in Figure 5 are net vertical movements resulting from the iteration of upward and downward deformation caused by ground heave and settlement. Au et al (2003) stated that the effectiveness of grouting can be evaluated by the amount of soil heave observed for a given injected grout volume [5]. The reduction of ground heave indicate that the efficiency of compaction grouting can be significantly improved with PVDs. 100 Bulb Radius (cm) Grout Pressure (kpa) Figure 3. Effect of grout pressure on bulb radius at depth 5m. Ground Heave (mm) Hole 2 Holes 3 Holes Grout Pressure (kpa) Figure 4. The variations of ground heave with grout pressure Ground Heave (mm) W/O PVD 2 PVDs 4 PVDs Grout Pressure (kpa) Figure 5. The effect of PVDs on ground heave 3. Conclusions In this study, finite element modeling with PLAXIS was used to examine the performance of RGIT. The efficiency of compaction grouting and the effect of PVDs on the reduction of excess pore water pressure were evaluated to verify design criteria for field control. Based on the model study, the following conclusions are drawn: 1. The volume of the grout bulb increases with the increase of the injection pressure with an ultimate pressure equal to 200 kpa which is similar to those findings in the literature for soft clay. They can be approximately expressed as six times the in-situ undrained shear strength plus the effective horizontal stress (6C u + σ h ). Such pressure corresponding to a staged grouting volume of 0.15 m 3 also approximates the proposed design criteria. 2. The amount of ground heave increases with the

7 increase of the grouting pressure. Without the installation of PVDs, the limiting pressure should be no more than 96 kpa in order to satisfy the proposed design criteria of heave. 3. The effect of PVDs on grouting is insignificant for pressures less than 80 kpa. However, at higher injection pressures, the installation of PVDs drops the ground heave approximately 46% allowing the limiting pressure to increase to 126 kpa. Increasing the number of PVDs further drops the ground heave, but its reduction tends to be diminished with the increased number. The reduction of ground heave indicates that the efficiency of compaction grouting is significantly improved with PVDs. The results of finite element analyses presented in this study proved that RGIT appears to be practical for soft clay improvement and demonstrated advantages over conventional compaction grouting. Confidence in what was achieved allowed RGIT to be used for the proposed site. Field verifications and further studies are underway to examine the accuracy of model analyses and improve design criteria for field performance. Acknowledgements The work presented in this study is sponsored by CUC Hi-Tech Corporation, R.O.C. The writer also wishes to express his appreciation to Yezhi Zhang of Chung Hua University for his assistance in the numerical analyses. References [1]. Nichols, S. C. and Goodings, D. J. (2000). Physical model testing of compaction grouting in cohesionless soil. J. Geotechnical and Geoenvironmental Eng., ASCE, 126(9), [2]. El-Kelesh, A., Mossaad, M. E., and Basha, I. M. (2001). Model of compaction grouting. J. Geotechnical and Geoenvironmental Eng., ASCE, 127(11), [3]. Essler, R. D., Drooff, E. R., and Falk, E. (2000). Compaction grouting, concept, theory and practice. Proc., Advances in Grouting and Ground Modification, ASCE, [4]. Komiya, K., Soga, K., Akagi, H., Jafari, M. R., and Bolton, M. D. (2001). Soil consolidation associated with grouting during shield tunneling in soft clayey ground. Geotechnique, 51(10), [5]. Au, S. K. A., Soga, K., Jafari, M. R., Bolton, M. D., and Komiya, K. (2003). Factors affecting long-term efficiency of compensation grouting in clays. J. Geotechnical and Geoenvironmental Eng., ASCE, 129(3), [6]. Zhou, W., Hong, H. P., and Shang, J. Q. (1999). Probabilistic design method of prefabricated vertical drains for soil improvement. J. Geotechnical and Geoenvironmental Eng., ASCE, 125(8), [7]. Fox, P. J., Nicola, M. D., and Quigley, D. W. (2003). Piecewise-linear model for large strain radial consolidation. J. Geotechnical and Geoenvironmental Eng., ASCE, 129(10), [8]. Nicholson, D. P., Gammage, C., and Chapman, T. (1992). The use of finite element methods to model compaction grouting. Proc., Grouting in The Ground, Institution of Civil Engineer, Thomas Telford, London, [9]. Shuttle, D. and Jefferies, M. (2000). Prediction and validation of compaction grout effectiveness. Proc., Advances in Grouting and Ground Modification, ASCE,

8 [10]. Chai, J. C. and Miura, N. (1999). Investigation of factors affecting vertical drain behavior. J. Geotechnical and Geoenvironmental Eng., ASCE, 125(3), [11]. Chai, J. C., Shen, S. L., Miura, N., and Bergado, D. T. (2001). Simple method of modeling PVD-improved subsoil. J. Geotechnical and Geoenvironmental Eng., ASCE, 127(11), [12]. Warner, J. and Brown, D. R. (1974). Planning and performing compaction grouting. J. Soil Mechanics and Foundation Div., ASCE,100(6), [13]. Lin, D. G., Kim, H. K., and Balasubramaniam, A. S. (2000). Numerical modeling of prefabricated vertical drain. J. Geotechnical Eng., Southeast Asian Geotechnical Society, 31(2), [14]. Chai, J. C. and Miura, N. (1999). Investigation of factors affecting vertical drain behavior. J. Geotechnical and Geoenvironmental Eng., ASCE, 125(3), [15]. Chai, J. C., Shen, S. L., Miura, N., and Bergado, D. T. (2001). Simple method of modeling PVD-improved subsoil. J. Geotechnical and Geoenvironmental Eng., ASCE, 127(11),