Journal of Geotechnical and Geoenvironmental Engineering

This paper examines a commonly used procedure, proposed by Baecher, for separating the random measurement error associated with a particular test from the inherent spatial variability of the geological material. By comparing the two techniques of random field theory and geostatistics it is shown that the method, while well founded, requires a number of factors to be investigated before conclusions can be made regarding the random measurement error of a particular test. These factors include the nugget effect, the sample spacing, and the stationarity of the data. Two case studies are presented that are based on closely spaced data derived from a series of vertical and horizontal cone penetration tests performed in a relatively homogeneous, overconsolidated clay. The sensitivity of the results obtained from the procedure of Baecher is tested with regard to the degree of trend removed from the data and the spacing of the samples in the data set.

Geosynthetic clay liners (GCLs), which consist of a thin layer of bentonite attached to one or more geosynthetic materials, are receiving increased use as low-permeability barrier layers in waste-containment systems. Tests were performed in tanks to measure the hydraulic conductivity of GCLs that were subjected to differential settlement. In most cases the GCLs maintained a hydraulic conductivity of 1 × 10⁻⁷ cm/s or less when subjected to tensile strains of 1–>10%, depending on the material and test conditions. Overlapped GCL panels maintained their hydraulic integrity despite in-plane slippage of up to 25–100 mm. In general, the ability of GCLs to withstand differential settlement appears to be greater than that of compacted clay liners, but less than that of geomembranes. GCLs are a promising barrier material for situations in which differential settlement is expected, for example, in landfill final covers.

Secondary compression is important in peat deposits because they exist at high void ratios and exhibit high values of compression index C_c, display the highest values of C_α/C_c among geotechnical materials, and primary consolidation is completed in weeks or months in typical field situations. Secondary compression of Middleton peat was investigated by oedometer tests on undisturbed specimens. The observed secondary compression behavior of this fibrous peat, with or without surcharging, is in accordance with the C_α/C_c concept of compressibility. Near the preconsolidation pressure, the secondary compression index, C_α, increases significantly with time. In the compression range C_α decreases only slightly with time, and for most practical purposes a constant C_α with time can be used to compute secondary settlement. Postsurcharge secondary compression index, ${\mathit{C}}_{\alpha }^{\prime }$, always increases with time. This is predicted by the C_α/C_c concept of compressibility. A secant postsurcharge secondary compression index, ${\mathit{C}}_{\alpha }^{″}$, is therefore introduced for a simple computation of secondary settlement.

Vertical vibration of pile generates the cyclic shear in soil and can induce excess porewater pressure if the soil is submerged and cohesionless. The induced porewater pressure softens the soil and reduces the maximum skin friction allowed at the soil-pile interface. The behavior of a pile foundation under such an environment is numerically studied herein. A soil-pile interaction model and its formulations are first developed in the frequency domain adopting Winkler's hypothesis. They are computationally efficient yet capable of taking into account the effects of porewater pressure rationally. Parametric studies show that the most significant effects associated with porewater pressure are the reduction of the maximum skin friction allowed at the interface and the drastic alteration of the pile response due to slippage. It is found that the skin friction can be fully mobilized far more easily with porewater pressure than without it. This suggests a great advantage in utilizing vibration for driving a pile in submerged cohesionless soil.

The paper presents and analyzes results from nine constant rate of strain (CRS) consolidation tests on resedimented Boston blue clay (BBC) using a specially instrumented, computer-automated hydraulic consolidation device. Specimens were loaded using one of three vertical strain rates (0.1, 1, and 3%/h), and pore pressures were measured at three points through the specimen height and at the base. The CRS results were compared to the end of primary states in four incrementally loaded (IL) oedometer tests performed using standard procedures. At the two faster CRS rates, excess pore pressures (Δu) developed and two methods were used to compute vertical effective stress $\left({\sigma }_{\mathit{v}}^{\prime }\right):$ nonlinear CRS theory of Wissa et al. that depends in part on Δu measured only at the base; and direct integration of the measured Δu distribution through the specimen (the “isochrone method”). The results show that at the two faster rates, the CRS tests based on nonlinear CRS theory are consistent with those obtained using the isochrone method; results from CRS tests at all three rates compare well with the IL test results, despite base Δu values exceeding 70% of applied vertical stress in the 3%/h CRS tests. The CRS behavior of resedimented BBC is virtually independent of strain rate across the range tested; the value of preconsolidation pressure $\left({\sigma }_{\mathit{p}}^{\prime }\right)$ is rate independent as is the virgin compressibility. At ${\sigma }_{\mathit{v}}^{\prime }$ levels at and above ${\sigma }_{\mathit{p}}^{\prime }\text{,}$ CRS tests loaded at 1 and 3%/h yielded coefficients of consolidation and hydraulic conductivity values that agreed well with IL test values. The paper supports the validity of nonlinear CRS theory and suggests that less structured clays such as those tested may have less rate dependence during consolidation than soils with strong interparticle bonding and structure.

The hydraulic conductivity of calcium and sodium bentonites was investigated for sand-bentonite mixtures, a thin bentonite layer simulating a geosynthetic clay liner (GCL), and bentonite-cement mixtures simulating backfill for a vertical cutoff wall. The permeant liquids were tap water and distilled water containing 0.25 M calcium chloride. In general, the hydraulic performance of calcium bentonite was not significantly better than the performance of sodium bentonite for either the clay-amended sand or the GCL application, and was substantially worse than the performance of sodium bentonite in the bentonite-cement mixture. A drained angle of internal friction of 21° was measured for calcium bentonite, compared to 10° for sodium bentonite. Except for a larger drained shear strength, no advantage of calcium bentonite over sodium bentonite could be identified from the results of this study.

This study was carried out to evaluate the effect of foundry sand and fly ash on permeability of flowable slurry mixtures. In this work, two reference flowable fly ash slurry mixtures were proportioned for strength levels in the range of 0.34–0.69 MPa (50–100 psi) at 28 d using two different sources of ASTM Class F fly ash. Other mixtures contained clean and used foundry sands as a replacement for fly ash in the range of 30–85%. The permeability of the flowable mixtures was affected by an increase in either the water to cementitious materials ratio or the foundry sand content. The permeability values were either comparable to or lower than those reported for granular compacted fills up to 85% fly ash replacement with foundry sand. The type of foundry sand (clean or used) did not materially affect permeability of the mixtures tested. The permeability values for the mixtures tested varied from 3 × 10⁻⁶ to 74 × 10⁻⁶ cm/s.

A study of liquefaction at five sites in the Moss Landing area during the 1989 Loma Prieta earthquake is presented. Results of this case history study include: (1) a large set of cone penetration test (CPT) data for sites where evidence of liquefaction was and was not observed; (2) information regarding the influence of thin soil strata on overall site behavior; (3) inclinometer data on deformations at depth due to lateral spreading of a shoreline; (4) a description of the apparent liquefaction of a marshland clayey silt deposit; (5) seismic shear-wave velocity data at sites where evidence of liquefaction was and was not observed; and (6) an evaluation of how interpretation methods affect the development and use of CPT-based liquefaction correlations.

Desiccation and freeze-thaw of compacted clay barriers may result in cracks that serve as preferential flow paths. A series of infiltration tests on compacted kaolin samples was conducted to explore the importance of preferential flow paths during infiltration, and their effect on the infiltration rate. Clod size at the time of compaction was found to have a strong influence on both the rate and depth of infiltration. We suggest that flow and infiltration through fractured clay may be described in terms of two stages: an initial dynamic stage in which the infiltration rate is initially high but decreases rapidly due to the clay swelling and closing fractures, and a steady-state stage usually characterized by k_sat, during which the infiltration rate is relatively constant. Our study has shown that cracks do not fully heal upon hydration and readily reopen during subsequent dehydration. Infiltration rates during the dynamic stage of infiltration, while cracks are closing, are orders of magnitude higher than the steady-state rate used to estimate k_sat, for barrier evaluation.

The present study provides a methodology to include the smear effect of vertical drains in a two-dimensional (2D) plane-strain finite-element model, employing the modified Cam-clay theory. The analysis is conducted by converting the radius of the smear zone and its permeability (axisymmetric) into equivalent plane strain parameters. The introduction of smear effects improves the accuracy of the numerical model that is tested for a Malaysian soft clay, in this study.

Triaxial compression tests following stress paths of constant σ₁ were conducted to determine the shear strength of five processed scrap tire products having different gradations and particle shapes. The interparticle frictional component was separated from the total shear strength according to the energy correction concept proposed by researchers. The experimental results show that all five tire chip products have ultimate internal friction angles of 45° to over 60°. The interparticle frictional component of the strength was fully mobilized and nearly reached a constant value after approximately 5% axial strain. The experimental results confirmed that the strength parameter obtained with constant σ₁ was more reasonable. This finding is supported by field observation in which the tire chips have an angle of repose ranging from 37° to 43° (loosely stock piled) and up to 85° (compacted). The engineering applications of the experimental results are discussed.