International Journal of Geomechanics

This paper presents the effect of surcharge support pressure on pile load–settlement response during static load testing. A three-dimensional nonlinear finite-element model is developed to investigate the loading sequences of an axial compressive static load test, using surcharge loads as the reaction system. Results of a static load test at a construction site for which the geotechnical data are available were used for verification of the numerical model. The results show that the surcharge pressure on supports has little impact on the ultimate capacity of the pile, as the shaft skin friction is slightly influenced by the surcharge and the effect on the tip resistance is negligibly small. The initial stiffness of the pile, however, might be considerably affected by the surcharge pressure distributed around the pile shaft. The influence on stiffness increases with increase in the pile diameter. The effect of surcharge pressure is more pronounced for piles embedded in frictional soils compared with cohesive soils. Comparing the results with those available in the literature for pile testing using the reaction pile system shows that the pile stiffness is 15 to 25% less affected in the surcharge reaction system.

Gene expression programming (GEP) models, a robust variant of genetic programming, are developed in this study to correlate resilient modulus with routine properties of subgrade soils and state of stress for pavement design applications. A database used for building the model was developed that contained grain size distribution, Atterberg limits, standard Proctor, unconfined compression, and resilient modulus results for 97 soils from 16 different counties in Oklahoma. Of these, 63 soils (development data set) are used in training, and the remaining 34 soils (evaluation data set) from two different counties are used in evaluation of the models developed. Two different correlations were developed using different combinations of the influencing parameters. The proposed constitutive models relate the resilient modulus of routine subgrade soils to moisture content $w$, dry density ${\gamma }_d$, plasticity index (PI), percent passing a No. 200 sieve ($P_{200}$), unconfined compressive strength $U_c$, deviatoric stress ${\sigma }_d$, and bulk stress $\theta$. The results are compared with those from artificial neutral network (ANN) models. Overall, GEP models show good performance and are proven to be better than ANN models. The GEP-based design equations can be readily used for pavement design purposes or may be used as a fast check on solutions developed by more in-depth deterministic analyses.

This paper suggests that the isotropic hardening rule with a modified hardening parameter can be used to simulate anisotropic hardening behavior from both theoretical and experimental viewpoints. It is believed that the isotropic hardening rule cannot simulate anisotropic hardening behavior mainly because its yield function is expressed in terms of the principal stresses and its hardening parameter is expressed in terms of the principal plastic strains. Contrary to this belief, the first part of this paper theoretically suggests that the isotropic hardening rule with a modified hardening parameter, expressed in terms of the plastic strains in the coordinate system defined by the principal stress axes, can simulate anisotropic hardening behavior. In the second part of this paper, it is experimentally shown that the isotropic hardening rule with the modified hardening parameter can simulate the actual anisotropic hardening behavior of presheared clay. The second part also discusses the microscopic mechanism of the hardening for the presheared clay with reference to the isotropic hardening rule with the modified hardening parameter.

Focusing on three-dimensional finite-element analysis (FEA) for the assessment and optimization of reinforcement treatment using deep soil-mixing and umbrella vault methods to stabilize a cross-passage excavation in soft alluvial soils, this paper presents parametric studies, interpretation analyses, and details of the modeling technique for the reinforcement treatment and the excavation process. Displacements at the crown of the cross passage are estimated and used as the basis for assessment and optimization for the reinforcement. Optimization could account for savings of about 50% in the cost of soil mixing. The analysis indicates that simulated displacements can be reduced significantly when both the soil-mixing and the umbrella vault techniques are used. Deep soil mixing is more effective and adaptable, whereas the umbrella vault method has to be used jointly with other techniques to stiffen the upper soil layers. The simulation results further demonstrate that the progress of the excavation influences the crown stability, but such influence also depends on the soil conditions after reinforcement. Based on the findings in this paper, it has been concluded that construction sequence and risk-control measures can be arranged based on the simulation outcomes to achieve savings in construction time and cost.

Modeled rockfill materials were obtained from nine sites (two riverbed and seven quarried) from different projects in India and Nepal, and drained triaxial tests were conducted with specimens that were 381 mm in diameter and 813 mm high. Index properties of the rockfill materials, namely, unconfined compressive strength (UCS) and uncompacted void content (UVC), were determined. An elastoplastic hierarchical singe-surface (HISS) constitutive model was used to characterize the behavior of modeled rockfill materials. The predicted stress-strain-volume change behavior was compared with the observed behavior, and it was found that both observed and predicted behavior matched closely. Procedures have been developed to predict the shear strength and elastic parameters of rockfill materials using the index properties, namely, UCS, UVC, and relative density (RD), and predictions were made satisfactorily. Predicted shear strength and elastic parameters were compared with the experimental values, and it was observed that both values matched closely. Then these procedures were used to predict the elastic and shear strength parameters of large-size prototype rockfill materials. Relationships were also developed between index properties and strength parameters of modeled rockfill materials and were used to predict the strength parameters for the prototype rockfill materials. Using the predicted material parameters, the stress-strain-volume change behavior of prototype rockfill material was predicted using the elastoplastic HISS constitutive model.

The Severn-Trent sand model is an advanced elastoplastic constitutive model that accurately reproduces the nonlinear mechanical response of coarse-grained soils at small to relatively high strain levels under monotonic loading. The model, formulated within the framework of critical state soil mechanics, includes advanced plasticity modeling concepts such as kinematic hardening, multisurface plasticity, and bounding surface plasticity. This paper shows how the use of Severn-Trent can be extended in modeling cantilevered wall-supported excavations. Following a brief description of the approach taken to introduce the constitutive model into a finite-difference commercial code, guidelines for the model calibration are discussed. Accounting for the approximation, which affects the evaluations of some variables, several plane-strain numerical analyses are performed. The parametric study aims to evaluate to what extent the response of a cantilevered retaining structure in a dry, normally consolidated Toyoura sand layer is influenced by these uncertainties. The results encourage the use of the Severn-Trent model in solving boundary value problems, such as excavations, particularly where accurate displacement field predictions are required.

The one-dimensional wave equation is used to develop a simplified analysis of the standard penetration test (SPT), including the phenomenon of hammer bounce. The analysis identifies the main dimensionless variables involved. Results suggest that a large part of soil resistance is due to damping rather than soil strength. Results confirm recent findings from experiments that significant energy inputs can occur at the second and subsequent impacts; this can affect several factors including those for rod length correction and in a liquefaction-resistance assessment that uses SPT. Opportunities for further investigation of the SPT apparatus are discussed.

The design of foundations on soft ground reinforced by columns usually involves two important verifications, namely, checking for adequate bearing capacity and checking for acceptable settlement performance. This paper details a comprehensive methodology for determining the optimized portion of the ground area that should be improved by the installation of columns. The optimization is required to avoid an overly conservative design and, consequently, the use of uneconomical quantities of material to construct the columnar reinforcement. The basis of the suggested methodology consists of first estimating the minimum improvement area ratio (IAR) required to ensure attainment of the required design bearing capacity of the reinforced soil and then determining an upper-bound or maximum value of IAR by considering the issue of allowable settlement. Optimization is then performed on the IAR within the range defined by these bearing capacity and settlement limits. Analysis of three case studies provides an illustration of the implementation of this novel design methodology, which has been incorporated into software recently developed to assist in the design of soil foundations reinforced by columns and to provide cost-effective solutions for this type of foundation.

The design of pile foundations requires good estimation of the pile load-carrying capacity and settlement. Design for bearing capacity and design for settlement have been traditionally carried out separately. However, soil resistance and settlement are influenced by each other, and the design of pile foundations should thus consider the bearing capacity and settlement inseparably. This requires the full load–settlement response of piles to be well predicted. However, it is well known that the actual load–settlement response of pile foundations can be obtained only by load tests carried out in situ, which are expensive and time-consuming. In this paper, recurrent neural networks (RNNs) were used to develop a prediction model that can resemble the full load–settlement response of drilled shafts (bored piles) subjected to axial loading. The developed RNN model was calibrated and validated using several in situ full-scale pile load tests, as well as cone penetration test (CPT) data. The results indicate that the developed RNN model has the ability to reliably predict the load–settlement response of axially loaded drilled shafts and can thus be used by geotechnical engineers for routine design practice.

This paper reports the engineering behavior of proposed expanded polystyrene-based geomaterial (EPGM) with fly ash through a laboratory experimental study. The proposed geomaterial is prepared by blending fly ash with expanded polystyrene (EPS) beads and a binder such as cement. The effects of different compositions and different mix ratios between EPS beads and fly ash (0.5–2.5%), cement and fly ash (10–20%), and water and fly ash (50 and 60%) on density, compressive strength, and initial tangent modulus of the geomaterial formed are studied. The authors observe that the density of EPGM can be effectively controlled by the quantity of EPS beads added in making the material. With the inclusion of merely 0.5–2.5% of EPS beads to fly ash (by weight), the density of the geomaterial formed can be reduced from $\mathrm{1,320}\hspace{0.17em}\text{to}\hspace{0.17em}725\hspace{0.17em}\text{kg}/{\text{m}}^3$. The compressive strength of EPGM increases considerably if cement-to-fly ash ratios of 10, 15, and 20% are used. Compared with EPS block geofoam, EPS beads mixed geomaterial has higher density but higher compressive strength and higher stiffness. Thus the geomaterial developed in the current study can be used as a substitute for EPS geofoam block when strong fill materials with high strengths are required.