Journal of Geotechnical and Geoenvironmental Engineering

A foundation system comprising a group of four drilled shafts, each 750 mm in diameter by 5.5 m long, constructed in silt/sand/gravel with a rigid pile cap, was subjected to constant force shaking by a servo-hydraulic actuator reacting against a heavy anchor block. Forces of up to 150 kN peak to peak were achieved with resulting displacements of up to 2 mm and accelerations of up to $0.5g\text{.}$ The displacement response of the pile cap was measured over a range of frequencies from 2 to 12 Hz and the resonant frequency and damping ratio were determined for three different levels of excitation. The foundation behaved much like a simple harmonic oscillator with a natural frequency of approximately 10 Hz. This resonant frequency was found to decrease slightly with increasing excitation while the damping ratio decreased slightly. Values for generalized stiffness, mass, and damping for an equivalent simple harmonic oscillator were back-calculated from the test data. The results of this study suggest that it may be possible to predict values for generalized mass and damping a priori by using simple empirical methods.

Piles in a pile raft are sometimes considered as settlement reducers, not load-carrying members. In design, one often tries to minimize the number of piles. This often results in a high axial stress in the piles that may deter their use due to the limits on pile stress in practice. An alternative is to consider the pile as reinforcement in the base soil, and not as a structural member. Serving as a soil stiffener, the pile can tolerate a lower safety margin against structural failure without violating building codes. Previous numerical studies on the use of disconnected piles as settlement reducers have shown the effectiveness of such piles. This study aims to verify experimentally the effectiveness of such piles through load tests of model rafts resting on pile-reinforced sand. By varying factors such as raft stiffness, pile length, pile arrangement, and pile number, results of the investigation indicate that structurally disconnected piles are effective in reducing the settlement and bending moments in the model rafts.

This paper proposes a method for estimating the degree and depth of improvement resulting from dynamic compaction on sand. The method is based on extensive two-dimensional finite element analyses, which are benchmarked using centrifuge model results. By appropriately normalizing the results from a wide range of soil properties, initial conditions and operating parameters such as momentum per blow, energy per blow, and number of blows, the results for numerous cases can be summarized into a relatively small number of plots, which can be used for predictive purposes. A preliminary assessment of the reliability of these curves was conducted using results from a centrifuge and a field study. The results of this comparative exercise indicate that the predictions using the method are reasonably realistic.

A series of resonant column tests was conducted on chemically stabilized specimens of sulfate-rich expansive clay from southeast Arlington, Tex. Specimens were tested for different stabilizer types, stabilizer dosages, compaction moisture contents, and confining pressures. Three chemical stabilization methods were used: sulfate resistant type V cement, low calcium class F fly ash, and lime mixed with polypropylene fibers. Results in the small-shear strain amplitude range $(<0.0001\%)$ were analyzed to assess the influence of compaction moisture content and confining pressure on the linear shear modulus $G_{\max }$ and material damping $D_{\min }$ of stabilized soil. Tests were also conducted at small- to mid-shear strain amplitude levels (0.0001–0.01%) to assess the threshold strain limit ${\gamma }_{\mathrm{th}}$ for each treatment method, and to study the effects of torsional shearing on the rate of degradation of normalized modulus $G/G_{\max }$ of treated soil. A 10%-by-weight dosage of sulfate resistant type V cement was found to give the highest modulus and lowest damping when compacted at 95% of maximum dry unit weight ${\gamma }_{d-\max }$ on the wet side of Proctor optimum.

A parametric study was conducted using Monte Carlo simulation to assess how uncertainty in design parameters affects the probability of internal failure of mechanically stabilized earth (MSE) walls. Bishop’s simplified method was used to conduct the internal stability analyses. The results of the analyses indicate that the mean and coefficient of variation of the backfill friction angle, mean and coefficient of variation of the tensile strength of reinforcement, mean unit weight of the backfill, mean surcharge, mean reinforcement vertical spacing, and mean reinforcement length have a significant effect on the probability of internal failure of MSE walls. Based on the results of the parametric study, a series of additional simulations were conducted where the significant parameters were varied over a broad range. The results of these simulations were used to develop a set of reliability-based design (RBD) charts for internal stability of MSE walls. A method to adapt these charts to address model bias and model uncertainty is also presented. A MSE wall was designed using the RBD method and two other deterministic design methods. The required tensile strength of the reinforcement obtained from the RBD method fell between the strengths determined from the deterministic methods.

A methodology to identify the collapse potential of unsaturated soils is proposed in this paper on the basis of pressuremeter test results associated with independent measurements of the in situ matric suction. A solution combining the expansion of a cylindrical cavity to a modified Cam clay critical state model has been introduced and accommodated to the framework of unsaturated soil behavior. This accounts for changes in soil properties induced by suction changes. Interpretation of pressuremeter tests performed under unsaturated and soaked conditions links the amount of collapse to strength and stiffness changes and provides assessment to the constitutive soil parameters that are necessary to define the yield envelopes of the soil. A comprehensive site investigation program comprising field and laboratory tests carried out in two residual soil sites is discussed in order to validate the proposed methodology. Values of shear strength, in situ stress, and yield pressure derived from both field and laboratory data are used as input parameters of a constitutive model adopted for describing the yield envelopes of these unsaturated residual soil sites.

This paper discusses the state-dependent strength of sands from the perspective of unified modeling in triaxial stress space. The modeling accounts for the dependence of dilatancy on the material internal state during the deformation history and thus has the capability of describing the behavior of a sand with different densities and stress levels in a unified way. Analyses are made for the Toyoura sand whose behavior has been well documented by laboratory tests and meanwhile comparisons with experimental observations on other sands are presented. It is shown that the influence of density and stress level on the strength of sands can be combined through the state-dependent dilatancy such that both the peak friction angle and maximum dilation angle are well correlated with a so-called state parameter. A unique, linear relationship is suggested between the peak friction angle and the maximum dilation angle for a wide range of densities and stress levels. The relationship, which is found to be in good agreement with recent experimental findings on a different sand, implies that the excess angle of shearing due to dilatancy in triaxial conditions is less than 40% of that in plane strain conditions. A careful identification of the deficiency of the classical Rowe’s and Cam-clay’s stress–dilatancy relations reveals that the unique relationship between the stress ratio and dilatancy assumed in both relations does not exist and thereby obstructs unified modeling of the sand behavior over a full range of densities and stress levels.

Tests have been performed in a biaxial pipe test cell to develop baseline information on profiled pipe behavior under biaxial loading. These include a lined corrugated high-density polyethylene pipe and a helically wound ribbed polyvinyl chloride pipe. Results of the tests are utilized to examine the effectiveness of the two-dimensional methods of buried pipe analysis. Calculations of pipe responses by the two-dimensional finite element method and a set of simplified design equations are compared with the measurements of pipe strains and deflections. The study reveals that the two-dimensional finite element analysis can effectively be used to calculate pipe deflections and circumferential strains. The simplified equations appear suitable as design tools for standard buried thermoplastic pipe installations. Janbu’s nonlinear soil model with Mohr–Coulomb plasticity provided an effective simulation of the nonlinear soil behavior A study of pipes with low stiffness soil support under the haunches shows that this leads to strain concentrations in the pipe walls near that zone. Higher values of empirical strain factor, $D_f\text{,}$ are estimated to include this strain concentration during design.

The soil-water characteristic curve (SWCC) has traditionally been represented using equations whose fitting parameters do not individually correspond to clearly defined soil properties or to features of the curve. As a result, unique sets of parameters are often nonexistent, and sensitivity analyses and statistical assessments of SWCC parameters become difficult. In order to overcome these difficulties, a new class of equations to represent unimodal and bimodal SWCCs is proposed. The chosen fitting parameters are the air-entry value, the residual suction, the residual degree of saturation, and a parameter that controls the sharpness of the curvatures. The physical meaning for the soil parameters is discussed for different soil types. A unique relation between each of the equation parameters and the individual features of SWCCs is assured. The proposed equations are fitted to data corresponding to a variety of soil types and a good fit is observed.

Estimation of thermal resistivity of soils is normally done in the laboratory with the help of a laboratory thermal probe. However, as naturally occurring soils consist of various size fractions, ranging from clay to gravel, this probe cannot be used efficiently for measuring their thermal resistivity. This necessitates fabrication of a field probe that can be used to measure thermal resistivity of a soil either in its remolded state or under in situ conditions. With this in view, efforts were made to develop a field probe that works on the principle of the transient method and is a magnified version of the laboratory probe developed by Rao and Singh in 1999. The results obtained have been validated using the findings of Van Pelt in 1976 and Johansen in 1975 for gravels and crushed rocks.

Ring shear tests were conducted on five samples of different nature with a modified Imperial College type ring shear machine. The three different testing methods used, (1) individual sample testing for each normal stress, (2) increasing load multistage ring shear test, and (3) reducing load multistage ring shear test, all showed similar effective residual internal friction angle for the samples, irrespective of testing method. However, effective residual shear intercept was different according to the testing methodology. The internal friction angle did not vary, particularly after the first minimum point in the stress displacement curve, although the residual shear intercept decreased with increase in the displacement. The thickness of the shearing zone increased along with the displacement. The remolded peak shear strength for saturated conditions at field dry density varied with the consolidation history. Measurement of remolded peak shear strength was possible in a single sample using the increasing load multistage ring shear test at normal consolidation. The equilibrium water content of the sample after the ring shear test was nearly equal to the plastic limit.

The compression index, $C_c$ $(C_c^{\prime }$ in the case of remolded clay), which is obtained from the $e-\log p$ (e: void ratio, p: consolidation pressure) curve of the consolidation test, is used to calculate the consolidation settlement. The initial porosity, $n_0\text{,}$ was introduced as a factor which is satisfied with the condition and the type of soil, and the relationship between $C_c^{\prime }/n_0$ and $C_c^{\prime }$ was expressed with a good correlation for several remolded clays having a wide range of plasticity. Moreover, it was found that the relationship between $C_c/n_0$ and $C_c$ for undisturbed Ariake clay was also expressed with a good correlation independent of sensitivity ratio, $S_t\text{.}$