Journal of Geotechnical and Geoenvironmental Engineering

This paper presents the shear strength characteristics of a low lime class F fly ash modified with lime alone or in combination with gypsum. Unconfined compression tests were conducted for both unsoaked and soaked specimens cured up to 90 days. Addition of a small percentage of gypsum (0.5 and 1.0%) along with lime (4–10%) enhanced the shear strength of modified fly ash within short curing periods (7 and 28 days). The gain in unsoaked unconfined compressive strength $\left(q_u\right)$ of the fly ash was 2,853 and 3,567% at 28 and 90 days curing, respectively, for addition of 10% lime along with 1% gypsum to the fly ash. The effect of $24\mathrm{h}$ soaking showed reduction of $q_u$ varying from 30 to 2% depending on mix proportions and curing period. Unconsolidated undrained triaxial tests with pore-pressure measurements were conducted for 7 and 28 days cured specimens. The cohesion of the Class F fly ash increased up to 3,150% with addition of 10% lime along with 1% gypsum to the fly ash and cured for 28 days. The modified fly ash shows the values of Skempton’s pore-pressure parameter, $A_f$ similar to that of over consolidated soils. The effects of lime content, gypsum content, and curing period on the shear strength parameters of the fly ash are highlighted herein. Empirical relationships are proposed to estimate the design parameters like deviatoric stress at failure, and cohesion of the modified fly ash. Thus, this modified fly ash with considerable shear strength may find potential use in civil engineering construction fields.

The ductility, toughness, and resistance to tensile cracking of clays can be improved with the inclusion of short fibers. Tire buffings are derived from the tire retread process and because of their elongated shape, may be used as fiber inclusions. The objective of this study was to evaluate the drained and undrained shear strength of mixtures of clay and tire buffings. Mixtures of silty low plasticity kaolinitic clay and 10% by dry weight of tire buffings were compacted at both Standard and Modified compaction energy. Consolidated-drained and consolidated-undrained triaxial tests were run at confining stresses ranging from $50\text{to}300\mathrm{kPa}$ . Preshear and postshear permeability tests were conducted. Results showed that the peak strength of the composite is comparable to or greater than that of clay alone when tested at confining stresses below $200–300\mathrm{kPa}$ . Above this threshold, the presence of inclusions tends to degrade the strength of the clay. Changes in permeability were not significant.

Solutions for the problem of tunneling effects on existing pipelines are given. The solution utilizes a boundary integral formulation for describing the elastic continuum, in conjunction with a limiting force to consider relative pullout failure. The solution requires estimation of soil and pipe elastic properties, relative pipe-soil uplift capacity, and the green field soil settlement profile given in the current paper as a modified Gaussian curve. Normalized graphs for the solution are given as a function of these input parameters. The solution method is compared and evaluated against a limited number of finite-element analysis.

In this paper, the results of a series of experiments using one-dimensional oedometer testing, mercury intrusion porosimetry (MIP), and scanning electron microscopy are reported on kaolinite samples with known and controlled fabric associations to reexamine the dual-porosity hypothesis for the underlying mechanisms of secondary consolidation. The oedometer testing results indicate that the pH 7.8 sample (face-to-face aggregated structure) has the smallest values of compression index, $C_c$ , and secondary compression index, $C_{\alpha }$ . The pH 4 sample (flocculated but dispersed structure) has the largest values, while the pH 7.8 with salt sample (flocculated and aggregated structure) has medium values. The $C_{\alpha }∕C_c$ ratios for these three samples are similar regardless of the structure and consolidation pressure. The MIP results on pore-size evolution in the pH 7.8 with salt sample show that both primary and secondary consolidation processes preferentially occur in the larger and weaker interaggregated pores instead of in the smaller and stronger intraaggregate pores. These oedometer testing and MIP results do not support the dual-porosity hypothesis, whereas they suggest that the primary and secondary consolidation processes involve the same physical factors.

The paper describes a performance-based approach to the evaluation of liquefaction potential, and shows how it can be used to account for the entire range of potential ground shaking. The result is a direct estimate of the return period of liquefaction, rather than a factor of safety or probability of liquefaction conditional upon ground shaking with some specified return period. As such, the performance-based approach can be considered to produce a more complete and consistent indication of the actual likelihood of liquefaction at a given location than conventional procedures. In this paper, the performance-based procedure is introduced and used to compare likelihoods of the initiation of liquefaction at identical sites located in areas of different seismicity. The results indicate that the likelihood of liquefaction depends on the position and slope of the peak acceleration hazard curve, and on the distribution of earthquake magnitudes contributing to the ground motion hazard. The results also show that the consistent use of conventional procedures for the evaluation of liquefaction potential produces inconsistent actual likelihoods of liquefaction.

Percolation from a landfill final cover containing a geosynthetic clay liner (GCL) as the hydraulic barrier is described. The GCL was covered with $760\mathrm{mm}$ of vegetated silty sand and underlain with two gravel-filled lysimeters to monitor percolation from the base of the cover. Higher than anticipated percolation rates were recorded in both lysimeters within $4–15\text{months}$ after installation of the GCL. The GCL was subsequently replaced with a GCL laminated with a polyethylene geofilm on one surface (a “composite” GCL). The composite GCL was installed in two ways, with the geofilm oriented upwards or downwards. Low percolation rates $(2.6–4.1\mathrm{mm}∕\text{year})$ have been transmitted from the composite GCL for more than $5\text{years}$ regardless of the orientation of the geofilm. Samples of the conventional GCL that were exhumed from the cover ultimately had hydraulic conductivities on the order of $5\times {10}^{-5}\mathrm{cm}∕\mathrm{s}$ . These high hydraulic conductivities apparently were caused by exchange of Ca and Mg for Na on the bentonite combined with dehydration. The overlying and underlying soils likely were the source of the Ca and Mg involved in the exchange. Column experiments and numerical modeling indicated that plant roots and hydraulic anomalies caused by the lysimeters were not responsible for the high hydraulic conductivity of the GCL. Despite reports by others, the findings of this study indicate that a surface layer $760\mathrm{mm}$ thick is unlikely to protect conventional GCLs from damage caused by cation exchange and dehydration. Accordingly, GCLs should be used in final covers with caution unless if cation exchange and dehydration can be prevented or another barrier layer is present (geomembrane or geofilm).

So far, earthquake-induced slope instability has been evaluated by force equilibrium of soil mass in engineering practice, which cannot evaluate failure deformation once large failure occurs. An energy approach is proposed here, in which the amount of earthquake energy is evaluated in conjunction with the gravitational potential energy dissipated in slope displacement including large flow deformations. Shake table tests of dry sand slopes are carried out in which the earthquake energy used for slope failure can be successfully quantified. Measured slope displacement can be reliably evaluated by the proposed energy approach based on a rigid block model if an appropriate friction coefficient of the slope is specified. The energy approach is then applied to hypothetical slopes, indicating that even if extremely large earthquake energy is considered, slope failures with long run-out distance will not occur unless friction coefficients reduce near to or smaller than slope inclinations.

This paper describes the elastic response of a block sample of compressible Chicago glacial clay under a variety of stresses and its relationship with the deformation characteristics at relatively large strains. The elastic shear stiffness was obtained from bender element tests during consolidation and shearing in drained triaxial stress probe tests. An empirical correlation was established based on the elastic shear stiffness in a preyield condition. By comparing the empirical correlation with the measured elastic shear stiffness in the stress region during probing, the changes of elastic shear stiffness were investigated. The departure of elastic shear stiffness from values computed by the empirical relation based on $K_0$ loading directly relates to the yielding characteristics of the clay. The large-scale change of soil structure at yielding alters the well-established relationship between the elastic shear stiffness and stresses in the preyield condition. The mechanical yielding response of clays can be detected based on the systematic analysis of the elastic shear wave velocities.

This state-of-the-art paper presents an interpretation of the permeability, compressibility, and shear strength of fibrous peats using data from laboratory tests on undisturbed block samples of two fibrous peats, as well as extensive laboratory and field data from the literature on fibrous peat deposits. Engineering properties of fibrous peats are significantly different from those of most inorganic soils. However, the same fundamental mechanisms and factors determine behavior of both inorganic soils and fibrous peats. Fibrous peat deposits possess very high initial permeability, typically 1,000 times the initial permeability of soft clay and silt deposits. Upon compression, the permeability of fibrous peats decreases dramatically, with a ratio of permeability change index to in situ void ratio equal to 0.25, as compared to 0.50 for soft clay and silt deposits. Fibrous peats display extreme compressibility to the increase in effective vertical stress, with compression index values right after preconsolidation pressure 5 to 20 times the corresponding compressibility of typical soft clay and silt deposits. Among geotechnical materials, fibrous peats display the highest ratios of secondary compression index to compression index, in the range of 0.05 to 0.07. The values of coefficient of earth pressure at rest for normally consolidated young fibrous peat deposits are in the range of 0.30 to 0.35, as compared to 0.45 to 0.65 for inorganic soils. The values of friction angle from triaxial compression tests for fibrous peats are in the range of 40 to 60°, as compared to less than 35° for soft clay and silt compositions. For fibrous peats, the ratios of undrained shear strength in compression to preconsolidation pressure are usually in the range of 0.50 to 0.75, as compared to 0.32 for soft clay and silt deposits. For surficial fibrous peat deposits the ratio of vane shear strength to preconsolidation pressure is near 1.0, as compared to 0.12 to 0.35 for inorganic soft clay and silt deposits. For fibrous peats, the ratio of undrained Young’s modulus to undrained shear strength is in the range of 20 to 80.

This paper presents a methodology to investigate the collapse behavior of unsaturated soils using suction-monitored oedometer tests. By incorporating independent suction measurement, the oedometer apparatus is capable of following the same stress paths as in double oedometer tests, while continuously monitoring the suction. The proposed method has been used to investigate the collapse behavior of a compacted silty clay and to confirm the uniqueness of the loading-collapse surface as identified from loading and wetting paths. A new mathematical form of the yield surface within an elastoplastic framework is proposed on the basis of test results over a wide range of suctions (0 to $\mathrm{30,000}\mathrm{kPa}$ ) and net stresses (up to $\mathrm{7,000}\mathrm{kPa}$ ). The fundamental assumptions of the newer type of elastoplastic framework, which incorporate the degree of saturation within their stress variables, are evaluated, and the limitations of such models are identified. The collapse behavior of samples with different fabrics induced by differing compaction characteristics is also investigated within an elastoplastic framework. The difference in fabric, which is observed through a petrological microscope, can be presented in a quantitative way with different model parameters.

This paper presents a new method for applying reliability-based design approaches to slope stability analysis. In this method the soil properties are considered to be random variables. The factor of safety of the slope is found using Bishop’s simplified method for noncircular slip surfaces. By considering the variability of the soil properties, the probability of failure is determined from the reliability index $\left(\beta \right)$ . The minimization problem (determination of the lowest $\beta$ value for the range of variables and possible slip surfaces considered) is solved using a genetic algorithm approach, which simultaneously locates the critical slip surface and determines the reliability index. The performance of the new method is compared to some existing reliability approaches when applied to case histories of slope failures from the geotechnical literature. The new approach is seen to provide reasonable and consistent estimates of the reliability index and critical slip surface.

Currently available cone penetration test (CPT) stress normalization schemes exhibit no consensus on the estimation of the stress normalization component. Depending on which power law stress normalization exponent is used, very different interpretations may result in the analyses where normalized CPT data are used (e.g., CPT-based soil classification and seismic soil liquefaction initiation assessment). Within the confines of this paper, it is intended to clarify and resolve some of these differences, and to propose improved recommendations for CPT stress normalization. For this purpose, available stress normalization databases from theoretical, numerical, and field data analyses approaches were compiled. For the soil types, and stress conditions where compiled database is not conclusive, additional finite element simulations have been performed. The resulting relationship not only eliminates several sources of bias intrinsic to previous, similar correlations, and provides greatly reduced overall uncertainty and variance, it also helps to establish a consensus to the stress normalization issue that have long been difficult and controversial. Key elements in the development of these new correlations are: (1) accumulation of a significantly expanded database of analytical/numerical CPT simulation results, as well as field and chamber test data from homogeneous soil layers; (2) use of improved knowledge and understanding of factors affecting CPT and stress normalization; and (3) use of high-order probabilistic tools (Bayesian updating).