International Journal of Geomechanics

This paper investigates the influence of anisotropy and destructuration on the behavior of a test embankment on soft clay. The test embankment at Murro, Finland, was commissioned in 1993 by the Finnish Road Administration and has been monitored for over $10\text{years}$ . The construction and consolidation of Murro test embankment is analyzed with finite element method using three different constitutive models to represent the soft soil. The results are compared with field observations. The constitutive models used include two recently proposed constitutive models, namely S-CLAY1 that accounts for initial and plastic strain induced anisotropy and its extension, called S-CLAY1S. The S-CLAY1S model accounts, additionally, for interparticle bonding and degradation of bonds. For comparison, the test embankment is also analyzed using the isotropic Modified Cam Clay model. The simulations demonstrate that for this type of problem, it is important to account for the anisotropy, whereas destructuration appears to have less influence on predicted deformations. However, only a model incorporating destructuration can explain the decrease in undrained shear strength during consolidation that was measured in field.

Uplift induced slope failure occurs due to high water pressures in the sand layer below the soft top layers in a polder. The top layers are lifted due to water pressure invoking slope instability. The mechanism is significant in the Dutch lowlands. In this paper a new stability design model has been presented and corresponding geocentrifuge testing, a full-scale field test, and numerical validation are discussed, as well as the influence of the human factor. Comparisons of the new model with four existing models are shown for ten Dutch representative polder dikes. Furthermore, safety aspects are discussed and conclusions are drawn. The new design model has been incorporated in the Dutch national guideline for dike design.

A parametric finite element study has been carried out to demonstrate the importance of clay strength and depth of clay layer on the earth pressures, strut loads, and bending moments for a strutted sheet pile wall in soft, essentially normally consolidated clay. The clay is modeled as nonlinear and anisotropic. The modeled excavation is $10\mathrm{m}$ deep. For a shear strength profile giving a close to failure condition the maximum bending moment is found to be 6 times larger than for a clay profile with 40% higher strength, and the maximum strut loads are up to twice as large. The maximum strut loads are significantly higher than those given by existing empirical design rules. Comparative analyses with an isotropic linear elastic–plastic soil model show relatively small differences in moments and strut loads. Comparisons against analyses with a beam-on-spring type finite element model show significant differences to the continuum FEM analyses. The main reason is that beam-on-spring models cannot capture the significant effect of arching on earth pressures, strut loads and bending moments.

This paper describes the analytical formulation of a modified consolidation theory incorporating vacuum pressure, and numerical modeling of soft clay stabilized by prefabricated vertical drains, with a linearly distributed (trapezoidal) vacuum pressure for both axisymmetric and plane strain conditions. The effects of the magnitude and distribution of vacuum pressure on soft clay consolidation are examined through average time-dependent excess pore pressure and consolidation settlement analyses. The plane strain analysis was executed by transforming the actual vertical drains into a system of equivalent parallel drain walls by adjusting the coefficient of permeability of the soil and the applied vacuum pressure. The converted parameters are incorporated in the finite element code ABAQUS, employing the modified Cam-clay theory. Numerical analysis is conducted to study the performance of a full-scale test embankment constructed on soft Bangkok clay. The performance of this selected embankment is predicted on the basis of four different vacuum pressure distributions. The predictions are compared with the available field data. The assumption of distributing the vacuum pressure as a constant over the soil surface and varying it linearly along the drains seems justified in relation to the field data.

Land reclamation is a major civil engineering activity in Singapore. Due to depletion of suitable local fills and the cost of imported sand, dredged and excavated clay fills, in spite of their poor engineering properties, are being evaluated as a fill material. To reduce double handling, it is desirable for the clay to be used directly in a lump form, instead of the more conventional slurry fill. While the performance of a slurry fill is relatively well understood, the behavior of lumpy fill is not. This paper reports the results of a laboratory study carried out on lumpy fill made of cubical clay lumps of size ranging from $12.5\text{to}50\mathrm{mm}$ . The study showed that the interlump voids are substantially closed at a consolidation pressure much lower than the preconsolidation pressure of the lumps. The study also shows that at a consolidation pressure of about $100\mathrm{kPa}$ , the permeability of a lumpy fill is reduced to an order similar to that for homogeneous clay. However, the shear strength profile obtained using the cone penetration test indicates that the fill is still highly heterogeneous under a pressure of $100\mathrm{kPa}$ . When the preconsolidation pressure of the lumps is exceeded, the strength profile becomes uniform. The degree of swelling of the lumps plays a significant role. For fully swollen lumps, the consolidation pressure required to close the interlump voids is considerably less than that if the lumps were not allowed to swell. The coefficient of secondary compression of the lumpy fill is comparable to the homogeneous clay indicating that secondary compression is not a serious issue.

An electrokinetic geosynthetic (EKG), is a polymeric geosynthetic material, enhanced to conduct electricity, which can be used to transport water in fine-grained soils by electrokinetic means. This paper describes the design, construction details, and analysis of a reinforced soil wall using EKG and wet cohesive fill. In order to establish an initial design layout, a long-term stability analysis of the wall was carried out using the soil’s critical state shear strength parameters. The long-term design was then checked for short-term stability based upon a minimum required undrained shear strength for the clay utilizing four different short-term analytical methods: critical height, Coulomb, discrete theory, and composite theory. The electroosmosis design was then undertaken, based upon the water content—undrained shear strength curve for the fill material ascertained from laboratory testing. Using this curve the difference between the as-placed water content and the water content corresponding to an undrained shear strength of $20\mathrm{kPa}$ was calculated, giving the volume of water that needed to be removed from each lift of clay fill. Using this volume of water the electroosmosis calculations were undertaken. A simplistic analysis was undertaken using a linear voltage gradient and fixed soil parameters, followed by a more complex analysis using finite difference techniques to establish the voltage gradient. The variation in the value of electro-osmotic permeability $k_e$ were estimated using both an empirical model and a graphical interpretation of the actual variation of $k_e$ measured in the laboratory. The results of these analyses yielded estimated treatment times and undrained shear strength of the clay.

Soft sediments are frequently encountered in geotechnical practice, forming natural deposits or manmade mixtures. These high water content soil–water mixtures involve soils with high specific surface and experience relatively low effective stresses, hence the role of electrical interparticle forces is enhanced and nonstandard soil-like behavior may be observed. Testing such materials presents some unique challenges not faced in soils with a stronger granular skeleton. This experimental study of soft sediments involves kaolinite–electrolyte mixtures. Two test sets are conducted. The first one involves index tests and the second set of tests explores the combined utilization of elastic and electromagnetic waves to assess soft sediments and to monitor their evolution. The kaolinite mixtures tested in both sets of measurements were prepared using fluids of different ionic concentrations to create different soil fabrics. The mixtures change behavior when the ionic concentration of NaCl exceeds $\sim 0.1\mathrm{mol}∕\mathrm{L}$ , indicating internal changes in fabric formation. While the liquid limit is sensitive to fluid–particle interactions, it is less sensitive to fabric effects than viscosity or sedimentation tests. The maximum porosity a soil may attain is determined by the particle slenderness; in the case of kaolinite, a skeleton capable of transmitting a shear perturbation is observed at water content as high as six times the liquid limit. At this incipient skeletal condition, minor differences in shear wave velocity hint to the stiffness of different fabrics. High frequency permittivity is a good indicator of water content and can be effectively used to monitor the evolution of soft sediments. On the other hand, the evolution of the effective conductivity varies with pore fluid concentration: for high ionic concentration fluids, the effective conductivity increases as porosity increases, yet, the opposite is true for low ionic concentration fluids due to the contribution of surface conduction. In a wide range of water contents [0.6 liquid limit (LL) $<w<6\mathrm{LL}]$ , the shear wave velocity is primarily determined by the water content or porosity, and fabric appears to have a limited impact. Test procedures and results can be used to develop parallel field characterization tools and interpretation guidelines.

The degree of consolidation is usually used as one of the criteria for assessing the effectiveness of soil improvement work using the fill surcharge or vacuum preloading method. It is also often used as a design specification in a soil improvement contract. Degree of consolidation is normally calculated using settlement data. However, as the effect of vacuum preloading is controlled largely by pore water pressure changes, it is necessary to analyze the pore water pressure variations and to assess the degree of consolidation using pore water pressures. In this paper, the problems involved in the estimation of degree of consolidation using settlement data are discussed. A method to estimate the average degree of consolidation using pore water pressure data is suggested. Two case studies are presented to examine the characteristics of the pore water pressure variation of soil under vacuum loading. The degree of consolidation achieved in each of the two cases is assessed using pore water pressure data and compared with that estimated using settlement data. Factors affecting the degree of consolidation assessment are discussed.