Journal of Geotechnical and Geoenvironmental Engineering

An extensive program of centrifuge testing was conducted to quantify the improvements to seismic slope performance that can be achieved by installing a row of discretely spaced vertical precast RC piles. A key novelty of the work presented is the use of recently developed microreinforced concrete to produce realistically damageable model piles. Pile-reinforced slopes are a good example of a problem in which relative soil-pile strength is important in determining whether the soil or pile yields first, and in which the performance of a slope with structurally damaged piles may be of interest. The new model RC allows these factors to be properly accounted for in a reduced-scale physical model for the first time. Two different reinforcement layouts were considered, representing (1) a section specifically detailed to carry the bending moments induced by the slipping soil mass, and (2) a nominally reinforced section with low moment capacity. These were supported by further tests on conventional elastic piles that were instrumented to measure seismic soil-pile interaction. It was demonstrated that dynamic ground motions at the crest can be significantly reduced in amplitude by up to 20% with elastic piles spaced at $s/B=3.5$, both in terms of the peak acceleration and across the full response spectrum. Permanent deformations at the slope crest (e.g., settlement) were also reduced by up to 35% at $s/B=3.5$. These findings are consistent with previous suggestions made for the optimal $s/B$ ratio for encouraging soil arching between piles at maximum spacing both under monotonic conditions and from numerical investigations of the seismic problem. The improvements to slope performance were reduced slightly using the designed microreinforced concrete piles owing to deterioration in the bending properties arising from fatigue under the cyclic kinematic loading. This suggests that idealization of RC piles as elastic elements will likely be only an approximation of their true behavior. The importance of reinforcement detailing was also highlighted, with the nominally reinforced section yielding early in the earthquake, and the damaged piles subsequently only offering a small (although measureable) reduction in seismic slope performance compared with the unreinforced case. In addition to these findings, the data presented will be useful in validating future improved numerical models for predicting the performance of piled slopes and for the aseismic design of pile-reinforcing schemes.

Reduced-scale shaking table testing is a useful tool for understanding the seismic behavior of geosynthetic-reinforced soil walls. This paper presents the results from a series of reduced-scale shaking table tests on eight different configurations. The effects of change in peak ground acceleration, reinforcement length and spacing, model scale, treatment of the top two facing block layers on the accelerations on a wall face, maximum displacements of the wall face during shaking, permanent displacements, and strains in reinforcement are investigated. Maximum accelerations measured on the wall face during shaking increased from bottom to top. Geotextile length and spacing did not affect the maximum accelerations and face displacements when the geotextile length met the minimum requirements of established design procedures. No significant permanent displacements were observed. Decreasing the geotextile length and increasing the geotextile spacing increased the geotextile strains when the geotextile was long enough to provide anchorage beyond the potential failure surface.

This paper calculates the thrust of lateral earth pressures exerted by unstable slopes comprised of $c\text{-}\varphi$ soil and subjected to seismic (pseudostatic) loading conditions. Although the proposed method can be used for seismic stability analysis of geosynthetic-reinforced earth structures (GRESs), the formulation and results are also applicable to many other relevant earth retention systems. To study the impact of cohesion, the authors develop formulations to determine the seismic active earth pressure coefficient for $c\text{-}\varphi$ soils resulting in a closed-form solution. The pseudostatic formulation considers the effect of tensile cracks, backslope inclination, and batter while assuming a log spiral failure surface. Two formulas are presented. One is for the conventional inclination of thrust. The second formulation considers a more feasible inclination of the thrust when it is likely to act against facing units with large setbacks constructed for large batter walls. The authors perform parametric studies and investigate the effects of relevant parameters. The results are compared to those obtained from an alternative limit equilibrium method and a rigorous upper-bound solution in limit analysis of plasticity. The proposed method is used to illustrate the impact of cohesion on the seismic design of GRESs. The formulas are incorporated into a spreadsheet, thus providing a tool to assess the thrust for $c\text{-}\varphi$ soils. However, given the high impact of cohesion combined with the significant uncertainties related to cohesion in partially saturated soils during the lifespan of the structure, cohesion’s design value warrants careful consideration. Consequently, cohesion should be used in design only when its value during the lifespan of the structure can be assessed with confidence or when provisions taken to prevent potential surface water infiltration will sustain a verified value of apparent cohesion.

A laboratory modeling program has been conducted to assess the mechanics of initiating the piping erosion process in sandy soils. The models were performed on several soils, differing in gradation, grain size, grain shape, and specific gravity. Observations and monitoring of pore pressures within the samples during the modeling identified four stages in the development of piping initiation: initial movement, progressive heave, boil formation, and total heave. By linking the observed behavior with the measured pore-pressure regime in the sample, a model for the mechanics of piping development has been developed. Finite-element seepage analyses were performed to model the progression of piping development in the laboratory models and corroborate the developed model of mechanics. The findings of the study identified a new model for the initiation of piping development that can be applied to the assessment of piping in the field.

This paper develops Bayesian approaches for facilitating the determination of characteristic (or nominal) values of geomaterial properties in geotechnical analysis and design when extensive testing cannot be performed, which is the case for a majority of geotechnical projects, particularly those of a small or medium size. These Bayesian approaches aim to characterize probabilistically the undrained shear strength, $S_u$, of clay using a limited amount of liquidity index (LI) test data, and to provide a logical route to determine the characteristic values for analysis and design, particularly those using probability-based design codes. The proposed approaches include (1) a Bayesian model comparison approach that selects the most appropriate likelihood model, a key element in the Bayesian framework, using a limited number of LI data obtained from a specific project site, and (2) a Bayesian equivalent sample approach that uses the selected likelihood model, integrates the sound engineering judgment/local experience with the project-specific LI data, and transforms the integrated knowledge into a large number of equivalent samples using Markov chain Monte Carlo simulation. Conventional statistical analysis of the equivalent samples is subsequently performed to determine characteristic values of the $S_u$ profile. The proposed approaches use engineering judgment/local experience in a quantifiable and transparent manner and effectively tackle the difficulty of generating meaningful statistics and probability distributions of soil properties from a usually limited number of test data obtained during geotechnical site investigation.

Geosynthetic layers above a pipe can potentially reduce the deflection and strain in the pipe attributable to static loads. This paper discusses the laboratory results of shallowly buried steel-reinforced high-density polyethylene (HDPE) pipes subjected to static loads with or without geogrid. In the testing, static loads were applied to a steel plate seated on the ground with a 0.61-m-diameter steel-reinforced HDPE pipe buried in a compacted-sand trench. Four static loading tests were run with two different base courses and geogrids inside and above the trench. The test section was instrumented to record pipe deflections, earth pressures, and strains in the pipe wall and geogrid. Installation deflections were monitored and compared with a theoretical model. The measured earth pressures were compared with those estimated by the current AASHTO live-load distribution method. Reduced deflections and strains of the pipe were recorded as a result of the geogrid reinforcement. The type of base course also affected the performance of the shallowly buried pipe. The important finding of this study is that the geogrid reduced the longitudinal strains in the plastic shell of the buried pipe subjected to surface static loading.

The constant rate-of-strain (CRS) laboratory consolidation test is used to measure consolidation properties of fine-grained soils. Although the CRS test offers many advantages over the conventional incremental-loading consolidation test, uncertainties associated with the method of data analysis have presented an obstacle to more widespread use of the CRS test in practice. This paper presents results from a numerical investigation of the accuracy of linear and nonlinear data analysis methods for the CRS consolidation test. Numerical simulations were conducted using a validated large strain consolidation model for CRS loading conditions, published material properties for two reconstituted clay soils, and three applied strain rates. Based on the numerical results, recommendations are provided for analysis of CRS consolidation data, including changes to the ASTM D4186 equations for nonlinear data analysis. The most appropriate analysis method for soils with linear compressibility is the current ASTM D4186 linear theory method. The most appropriate analysis method for soils with nonlinear compressibility is the proposed modified nonlinear (MNL) theory method. These recommended congruent analysis methods provided good to excellent results for normally consolidated, steady-state conditions, including constant or variable coefficient of consolidation, but yielded large errors for overconsolidated conditions near the preconsolidation stress. As a precaution, CRS tests for overconsolidated soils should generally be conducted using lower strain rates.

There is an increasing interest in the reuse of coal combustion byproducts, such as high carbon fly ash, which is generated in large quantities as a result of equipping power plants with low nitrogen oxide burners. In this study, a laboratory investigation was conducted to evaluate the use of high carbon fly ash as a reactive medium for passive groundwater remediation applications via permeable sorptive barriers. Specifically, batch and column sorption-desorption experiments were performed using three fly ashes with a range of carbon contents, and naphthalene and o-xylene as model contaminants. Results from the batch and column tests were consistent in showing adsorption/desorption hysteresis, and a strong positive correlation between the hydrocarbon adsorption capacity and the carbon content of the fly ashes. However, the hydrocarbon adsorption capacity in the column sorption-desorption tests was overpredicted when parameters from batch tests were used in model simulations, possibly because of the limited accessibility of sorption sites in the columns compared with the batch tests.

Partial drainage during a penetration test increases the penetration resistance, and also affects the excess pore pressures generated and the form of any subsequent dissipation test. This complicates interpretation of such tests, particularly in soils of intermediate consolidation characteristics, because it is difficult to obtain an accurate estimate of the coefficient of consolidation from which to assess the degree of consolidation during penetration. The effect of partial consolidation during penetration on subsequent dissipation tests is investigated here through a number of model piezocone and piezoball tests conducted in a single soil sample under centrifuge conditions. Backbone curves of penetration resistance and excess pore pressure are devised, and comparisons are made of the form of the dissipation response for the two types of penetrometers and for two different pore pressure monitoring positions on the piezoball. Different methods are proposed for assessing the degree of consolidation during the penetration phase, thus allowing the consolidation coefficient to be estimated with reasonable accuracy. Advantages of monitoring pore pressure at two different locations on the piezoball are argued.

The effects of soil nonlinearity on the dynamic behavior of fixed-head floating inclined pile groups are investigated experimentally in the form of horizontal impedance functions (IFs) and internal forces in piles. A $2\times 2$ model of inclined pile groups embedded in dry cohesionless soil and subjected to a broad range of lateral harmonic loadings at the pile-cap level are examined. Piles with a spacing-to-the-diameter ratio ($\text{s}/\text{d}$) of 5 with practical pile inclinations of 0, 5, and 10° along the direction of loading are reported. Results show a profound impact of local nonlinearity and resonance of soil on the horizontal impedance functions. The inclined pile groups show higher values of horizontal IFs than the vertical pile group, in accordance with the general understanding. Influence of pile inclinations on the horizontal IFs is further examined through the variation of soil reactions along the depth of the piles. Semiempirical expressions for estimating the soil reactions, as a function of pile inclinations, are proposed. As for the internal forces in piles, bending strains for the inclined piles are smaller than the vertical pile; the axial strains show an opposite behavior.

Scour is a process of soil erosion, which can occur around the foundations of bridges or offshore structures through the action of flowing water; it can, therefore, reduce the capacity of the foundations and sometimes lead to the failure of structures. During an analysis of scour effects on laterally loaded piles, scour hole geometry is often ignored. Instead, scour effects are considered by simply removing the whole soil layer to the scour depth. However, a scour hole has not only depth, but also width and a slope. At present, the widely used $p\text{-}y$ method for analyzing laterally loaded piles cannot consider three-dimensional scour hole dimensions. For this reason, a simplified method was developed herein for the analysis of laterally loaded piles in sand under a scour condition. The $p\text{-}y$ curves in this method were based on a wedge type of failure. The effects of the scour hole dimensions on the response of laterally loaded piles in sand were evaluated using this simplified method. The computed results from the simplified method compared well with those from the three-dimensional finite-difference method. Scour depth was identified as the most influential factor on the pile response compared with scour width and scour hole angle. Based on the parameters investigated, a model that removes the entire soil layer to the scour depth resulted in 49–68% higher groundline lateral displacements of the pile than a model that incorporates the three-dimensional scour hole dimensions.

The constant $m_i$ is a fundamental parameter required for the Hoek-Brown (HB) criterion to estimate the strength of rock materials. To calculate $m_i$ values, triaxial tests need to be carried out; however, triaxial tests are time-consuming and expensive. In this research, a simplified method is proposed that can estimate $m_i$ values using information on rock types and the uniaxial compressive strength (UCS) of intact rocks. To evaluate the reliability of the proposed method, $m_i$ values estimated from the proposed method are used in the HB criterion to predict intact rock strength. The predicted intact rock strength is then tested against experimental intact rock strength using existing triaxial tests and tests in the authors’ laboratory. Results from the comparison show that $m_i$ values calculated from the proposed method can be used reliably in the HB criterion for estimating intact rock strength when triaxial test data are not available.

A large database has recently been published that details the development of new empirical expressions for the stiffness reduction with strain of clays and silts. In this note, the same database is used to examine two major considerations for engineers using these expressions in numerical analyses: the transformation from secant to tangent stiffness and the effect of stress history.

Scour can remove soils around bridge foundations and result in reduced bridge capacity. Because most bridge failures have been associated with scour events, it is important to evaluate scour effects on bridge safety. In practice, an evaluation of the performance of bridge piles under scoured conditions is usually achieved by simply removing soils from around the piles while possible changes in stress history of the remaining soils are ignored. In reality, the remaining soils undergo an unloading process when scour removes soils from above the remaining soils. This process increases the overconsolidation ratios of the remaining soils. In this study, the stress history of soft clay was considered in the analysis of laterally loaded piles using the $p\text{-}y$ method. To account for the stress history effect, the conventional $p\text{-}y$ curves for the soft clay were modified to evaluate the undrained responses of the laterally loaded piles at different scour depths. A case study was used to compare the computed results by considering and ignoring the stress history effect. The results showed that consideration of the stress history effect resulted in a 21.8–32.7% reduction in ultimate soil resistance. Ignoring the stress history of the soft clay led to an unconservative design of the laterally loaded piles under scour conditions.