Journal of Geotechnical and Geoenvironmental Engineering

Permanent ground deformations in unsaturated, compacted hillside fills under seismic loading conditions are discussed, with emphasis given to fill performance during the 1994 Northridge earthquake. These movements represent a significant yet often unrecognized hazard to developed hillside areas, as relatively modest deformations induced widespread damage totaling hundreds of millions of dollars during the Northridge event. The development of grading standards in the Los Angeles area is reviewed to place the seismic fill deformation problem in context with other issues that have shaped design and construction practices for hillside fills. Field observational data on fill performance during the Northridge earthquake is presented, and typical ground distress patterns are found to include cracking near cut/fill contacts, lateral extension and settlement of fill pads, and bulging of fill slope faces. For most sites, the prevalent mechanism of permanent ground deformation responsible for the fill movements is contractive volumetric strain accumulation within the unsaturated fill soils during strong earthquake shaking (that is, seismic compression).

Current design software (FloridaPier, Com624P) requires p-y curves to estimate the foundation lateral load resistance. Input parameters used to develop these curves can be obtained from in situ [standard penetration test (SPT) and cone penetration test (CPT)] correlations. This paper presents an evaluation of predictions using input soil parameters from SPT and CPT correlations versus field measured values. A lateral load test database consisting of 24 SPT and 6 CPT data sets was developed. The comparisons showed that four different SPT correlations for ϕ coupled with three different k-values all produce similar R-values. (R-value = measured/predicted × 100%). Therefore, little difference exists between the SPT correlation combinations, albeit the estimated k value has a greater effect on predicted deformation. Similar combinations of CPT correlations also show little effect among the commonly used correlations. SPT predictions are quite conservative at low load levels (R-values ≈ 53%) and remain conservative (R-values ≈ 87%) at high load levels. Also, the scatter (standard deviation) is high (≈40%). CPT evaluations gave unconservative predictions (R-values ≈ 105 to 154%). In addition, the scatter (standard deviation) is high (≈34 to 74%).

The in situ stiffness and apparent maximum preconsolidation stress of many soils and sediments appear to be higher than in the laboratory tests. We seek to verify experimentally whether this also holds for deep marine sediments. We also discuss an alternative explanation for this effect to the classical one, implying the sample damage during coring. We test numerically the explanation, suggesting possible unaccounted changes in stiffness, occurring in sediments in situ when subjected to aging, or secondary compression for geological scale time periods. Results of “aging tests” on sandy and clayey sediments are presented, involving secondary compression at the constant in situ stress level, during which strain develops together with other changes in properties. Only two weeks of aging produced a notable increase in the apparent maximum preconsolidation stress and in the stiffness below it, and above the in situ stress. A framework for a mathematical model is proposed, based on the supposition that during aging the sediment develops a secondary microstructure through reactions of local dissolution/precipitation of less stable minerals.

The paper presents a synthesis of the field measurements obtained in a series of large-scale pile-driving tests. The field test was performed at the Chiayi-Taipo area of the high speed rail system currently under construction in Taiwan. The test project included a complete instrumentation of three precast concrete piles with a network of piezometers, inclinometers, level posts, and velocity sensors. The instruments were used to monitor the variations of the dynamic soil pore water pressure, lateral ground deformations, vertical surface displacements, and ground vibrations induced by the pile on the surrounding soil medium during the driving process. Based on the test results, a correlation between the pile-driving resistance and the geological condition of the test site is identified.

The interactions between plastic soils and polyvinyl chloride (PVC) geomembranes were studied using a direct shear device under as-compacted conditions. The PVC geomembranes had smooth or textured surfaces, and the soils were of plasticity index (PI) ranging from 35 to 100%. The peak and residual failure envelopes were expressed using Coulomb failure criteria. The adhesion and angle of friction increased for PIs up to 70% and subsequently recorded a decrease. The adhesion is larger for the peak strength compared to the residual strength, but it was the reverse for the angle of friction. The efficiency in terms of adhesion appeared more relevant than that of the angle of friction in expressing the interactions between geomembrane and cohesive soils. The smooth and textured geomembranes showed little difference in results at the residual state.

Compaction grouting is an important soil improvement and underpinning technique that involves injection of a very stiff grout material into the soil. The objective is to displace and compact the surrounding soil without permeating or hydrofracturing it. Compaction grouting has been developed and used almost entirely on the basis of practical experience. The present study proposes a theoretical model that describes the mechanics of the compaction grouting process and rationally considers the different soil and grouting parameters that govern the overall design and performance of the method. The theoretical basis of the model draws from the theory of cavity expansion as well as the conical shear failure above the grout bulb. This model was validated using limited available data. The results indicate reasonable agreement and show a good potential of the proposed model for rationally optimizing the design of compaction grouting operations.

On a macroscale, the effect of installing prefabricated vertical drains (PVDs) in a subsoil is to increase the mass hydraulic conductivity of the subsoil in the vertical direction. Based on this concept, a simple method for modeling PVD improved subsoils is proposed, in which an equivalent vertical hydraulic conductivity k_ve for the PVD improved subsoil is explicitly derived. With the proposed simple method, analysis of PVD improved subsoil is the same as that of the unimproved case. The theoretical verification of the simple method was made under 1D condition. The calculated average degree of consolidation and excess pore pressure distribution in the vertical direction using the simple method are compared with existing theoretical solutions (combination of Terzaghi's consolidation theory and Hansbo's solution for PVD consolidation). It has been proved theoretically that, in terms of average degree of consolidation, in the case of one layer and ignoring the vertical drainage of natural subsoil, the maximum error of the proposed method is 10% compared with Hansbo's solution. For the case of one layer or multilayers and considering both vertical and radial drainages with the parameters adopted here, the maximum error of the proposed method is 5%. The multilayer case was analyzed by FEM method, and the proposed simple method is compared with that of using 1D drainage elements. Then, 2D finite-element analyses were conducted for three case histories of embankments on PVD improved subsoils. One case is discussed in detail. The analyses using both the simple method and 1D drainage elements, were conducted. It is shown that for all three cases, the simple method yielded results as good as those using 1D drainage elements.

A plasticity constitutive model for sands is proposed, which combines a bounding surface framework for large cyclic strains with a Ramberg-Osgood-type hysteretic formulation for relatively smaller strains. The distinction between small and large cyclic strains is based on the volumetric threshold cyclic shear strain γ_tv, a well-established geotechnical parameter. The state parameter ψ is used explicitly to interrelate the critical, peak, and dilatancy deviatoric stress ratios. The plastic modulus is expressed as a particular function of accumulated plastic volumetric strain, which simulates empirically the effect of fabric evolution during shearing. Extensive comparisons with experiments show accurate simulation of the basic aspects of cyclic behavior for a wide range of cyclic strain amplitudes, specifically, (1) the degradation of shear modulus and increase of hysteretic damping with cyclic shear strain amplitude; (2) the evolving rates of shear strain and excess pore pressure (or volumetric strain) accumulation with number of cycles; and (3) the resistance to liquefaction. The 14 model parameters are proven independent of initial and drainage conditions, as well as the cyclic shear strain amplitude. The simulation of monotonic shearing is equally accurate.

This paper examines the ultimate bearing capacity of a strip footing located at the surface of a homogeneous soil. The approach adopted involves a numerical solution of the equations governing elastic-plastic soils with a nonassociative flow rule and makes use of the finite-difference code FLAC. This code is utilized to obtain the three bearing capacity factors for a wide range of values of the friction angle for four different values of the angle of dilation. The values of the bearing capacity factors obtained from the numerical approach are then compared with results derived from classical solutions modified to incorporate the nonassociative plastic flow of soil.