Journal of Geotechnical and Geoenvironmental Engineering

An instrumented geogrid-reinforced wall constructed on a highly compressible foundation was deconstructed 16 months after its completion, providing a unique opportunity to exhume and examine the instrumented geogrids that were used to construct the wall. The objectives of this post mortem study were: (1) to inspect the condition of the strain gauges that were attached to the geogrid layers before construction and to verify the reliability of their output; (2) to develop a procedure in which the residual (plastic) strains along exhumed geogrid panels could be determined; and (3) to assess the in situ strain and force distribution along geogrid panels based on the measured residual strains from the exhumed geogrids. After exhumation, it was observed that many of the attached strain gauges failed due to full or partial debonding from the geogrid, thus rendering outputs which potentially underestimated the actual strain. Combining aperture measurements of virgin and exhumed geogrids, all from the same manufacturing lots, enabled the assessment of residual strains following stress relaxation. Laboratory simulation of loading and unloading, including creep and relaxation, yielded a relationship between the measured residual strains and the in situ strain and force distribution; i.e., the residual strain fingerprint provided insight into the behavior of the geogrids within the wall prior to its deconstruction. The mobilized maximum tensile strains in the geogrid panels along the height of the wall were roughly uniform, in the range $4\pm 1\mathrm{\%}$ . These findings imply that if the same type of reinforcement had been used throughout the height of the wall, the mobilized force along the height would have been relatively uniform. The back-calculated maximum force in the geogrids indicated that the factor of safety on the long-term strengths of the geogrids ranged from about 1.4 on the stronger/stiffer geogrid to about 1.8 on the weaker/softer geogrid.

An experimental and analytical program was designed and conducted to evaluate the magnitude and distribution of seismically induced lateral earth pressures on cantilever retaining structures with dry medium dense sand backfill. Results from two sets of dynamic centrifuge experiments and two-dimensional nonlinear finite-element analyses show that maximum dynamic earth pressures monotonically increase with depth and can be reasonably approximated by a triangular distribution. Moreover, dynamic earth pressures and inertia forces do not act simultaneously on the cantilever retaining walls. As a result, designing cantilever retaining walls for maximum dynamic earth pressure increment and maximum wall inertia, as is the current practice, is overly conservative and does not reflect the true seismic response of the wall-backfill system. The relationship between the seismic earth pressure increment coefficient $\left(\Delta K_{\text{AE}}\right)$ at the time of maximum overall wall moment and peak ground acceleration obtained from our experiments suggests that seismic earth pressures on cantilever retaining walls can be neglected at accelerations below 0.4 g. This finding is consistent with the observed good seismic performance of conventionally designed cantilever retaining structures.

Two series of centrifuge model tests were conducted using Nevada sand. Four saturated models placed in a mildly inclined laminar box and simulating a 6-m-thick deposit were shaken inducing liquefaction effects and lateral spreading. The sand was deposited at a relative density, $D_r=45$ or 75%; two of the 45% models were subjected to overconsolidation or preshaking. The second series involved in-flight measurements of static cone tip penetration resistance, $q_c$ , simulating the standard cone penetration test (CPT) 36-mm cone. Values of $q_c$ increased with $D_r$ , overconsolidation, and preshaking. A normalized resistance, $q_{c1N}$ , was assigned to each of the four liquefaction/lateral spreading models. Increases in $D_r$ , overconsolidation, and preshaking decreased liquefaction and ground deformation, but relative density alone captured these effects rather poorly. Conversely, $q_{c1N}$ predicted extremely well the liquefaction and lateral spreading response of the four models, confirming Seed’s hypothesis to explain the success of penetration-based seismic liquefaction charts. The depth to liquefaction measured in the four centrifuge models is consistent with the field CPT liquefaction chart.

The postcyclic reconsolidation response of low-plastic Fraser River silt was examined using laboratory direct simple shear testing. Specimens of undisturbed and reconstituted natural low-plastic Fraser River silt and reconstituted quartz powder, initially subjected to constant-volume cyclic loading under different cyclic stress ratios (CSRs) and then reconsolidated to their initial effective stresses $\left({\sigma }_{\text{vo}}^{\prime }\right)$ , were specifically investigated. The volumetric strains during postcyclic reconsolidation $\left({\epsilon }_{\text{v-ps}}\right)$ were noted to generally increase with the maximum cyclic excess pore-water pressure $\left(\Delta u_{\max }\right)$ and maximum cyclic shear strain experienced by the specimens during cyclic loading. The values of ${\epsilon }_{\text{v-ps}}$ and maximum cyclic excess pore-water pressure ratio $\left(r_{u\text{-}\max }\right)$ were observed to form a coherent relationship regardless of overconsolidation effects, particle fabric, and initial (precyclic) void ratio of the soil. The specimens with high $r_{u\text{-}\max }$ suffered significantly higher postcyclic reconsolidation strains; ${\epsilon }_{\text{v-ps}}$ ranging between 1.5 and 5% were noted when $r_{u\text{-}\max }>0.8$ . The observed ${\epsilon }_{\text{v-ps}}$ versus $r_{u\text{-}\max }$ relationship, when used in combination with the observed dependence of cyclic excess pore-water pressure on CSR and number of load cycles, seems to provide a reasonable approach to estimate postcyclic reconsolidation strains of low-plastic silt.

An ultrasonic $p$ -wave reflection imaging system is used to noninvasively image submerged soil models with embedded anomalies and complex geometric layer contacts. The ultrasonic transducers emit compressive waves into water that subsequently transmit into the underlying soil, and measurements of the reflections are used to construct the images. The properties of the transducers and data acquisition hardware and software are explained. Fast signal stacking is used to improve signal-to-noise ratio and provide clearer images. Transducer directivity is explained as a wave passage effect, and transfer functions are derived for square and circular transducers to quantify directivity. The transfer functions agree reasonably with measured amplitude data. The cause of errors in the imaged position of dipping reflectors is explained, and a Kirchhoff migration algorithm is implemented to correct these errors. A soil model consisting of embedded high- and low-impedance anomalies, dipping soil layer contacts, and an undulating concrete base layer was imaged using 500- and 100-kHz transducers. The geometric features of the model are clearly visible in the images recorded with the 500-kHz transducers and less clear with the 100-kHz transducers. The lateral spatial resolution of the migrated images is shown to be much larger than one wavelength.

The containment function of clay barriers used for waste containment applications (e.g., landfills) can be enhanced if such clays exhibit membrane behavior or the ability to restrict the migration of solutes (e.g., contaminants). In this regard, compacted specimens of a locally available natural clay known as Nelson Farm Clay (NFC), as well as NFC amended with 5% (dry weight) sodium bentonite, were evaluated for hydraulic conductivity, $k$ , and the potential for membrane behavior. The membrane efficiencies of specimens of both soils compacted such that $k$ was less than ${10}^{-7}\text{cm}/\text{s}$ were measured by establishing steady salt (KCl) concentration differences, $-\Delta C_o$ , ranging from 3.9 to 47 mM across the specimens in a flexible-wall cell under closed-system boundary conditions. The measured membrane efficiency for the unamended NFC was negligible (i.e., $\le 1.4\mathrm{\%}$ ), even though the $k$ was suitably low (i.e., $k<{10}^{-7}\text{cm}/\text{s}$ ). In contrast, compacted specimens of the bentonite amended NFC exhibited not only lower $k$ but also significant membrane behavior, with membrane efficiencies as high as 97.3% for $-\Delta C_o$ of 3.9-mM KCl. The results suggest that natural clays typically suitable for use as compacted clay liners (CCLs) are not likely to behave as semipermeable membranes unless the clay is amended with bentonite or the clay is inherently rich in high swelling clay minerals (e.g., sodium smectite). The potential benefit resulting from membrane behavior in a CCL constructed with the bentonite amended NFC is illustrated analytically in terms of liquid flux.

Under general conditions of loading, a plate anchor is subjected to six degrees of freedom of loading, three force components and three moment components. Prediction of the anchor performance under general conditions of loading requires realistic estimates of the anchor pullout capacity for each individual load component as well as the interaction effects when these loads are applied in combination. This paper presents an analysis of plate anchor capacity under these general conditions of loading. The study considers a range of plate width-to-length ratios ranging from 1:1 to 2:1. The anchor capacity estimates and interaction relationships were developed based on finite-element studies and upper bound plastic limit analyses. Interaction relationships developed from the numerical and analytical studies were fitted to a simple six degrees-of-freedom yield locus equation.

Piled raft foundations are often used when the supporting soil has adequate bearing capacity but the raft settlements exceed allowable values. In traditional practice, long piles with high load capacity are usually used that may lead to two structural problems: the structural collapse of the pile and large strains mobilized in the raft leading to an uneconomic design. This paper presents an experimental study of the effectiveness of using short piles either connected or unconnected to the raft (instead of long piles) on the behavior of an eccentrically loaded raft. The load configuration was designed to simulate rafts under vertical loads and overturning moment. Several arrangements of piles with different lengths and numbers along with the effect of the relative density of the soil and the load eccentricity were studied. Test results indicate that the inclusion of short piles adjacent to the raft edges not only significantly improves the raft bearing pressures but also leads to a reduction in raft settlements and tilts leading to an economical design of the raft. However, the efficiency of the short piles-raft system is dependent on the load eccentricity ratio and pile arrangement. Also, connecting short piles to the raft gives greater improvement in the raft behavior than unconnected piles. Based on test results, the effects of different parameters are presented and discussed.

In the United States, an estimated $1 billion is spent annually on repair and replacement of deep foundations. In a recent study, the possibility of using ultrahigh-performance concrete (UHPC) for deep foundation applications was explored with the objectives of increasing the service life of deep foundations supporting bridges to 75 years and reducing maintenance costs. This paper focuses on field evaluation of two UHPC piles and references a steel H-pile. An UHPC pile with an H shape was designed to simplify the process of casting the pile and reduce the volume (i.e., cost) of the material needed to cast the pile. Two instrumented UHPC piles were driven in loess on top of a glacial till clay soil and load tested under vertical and lateral loads. This paper provides a complete set of results for the field investigation conducted on UHPC H-shaped piles. The results presented in this paper prove that the designed UHPC piles can be driven using the same equipment used to drive steel H-piles through hard soil layers without a pile cushion. The vertical load capacity of the UHPC pile was over 80% higher than that of the steel H-piles.

This paper presents an empirical methodology that allows calculation of ground displacements behind corners of retained cut excavations. The increased stiffness of a retaining wall at the corner of an excavation normally leads to a corresponding reduction in ground movements behind the retaining wall. These “corner effects” derived from retained cut excavations can only be assessed at present by using three-dimensional numerical analysis or empirical methods. Significant cost and time can be taken carrying out three dimensional analysis, which, additionally, is not normally carried out at an early stage of the design of a project. Furthermore, numerical analysis must be undertaken by a competent person with appropriate training. An inappropriate analysis can yield to misleading and counterproductive results. This constitutes an expensive requirement on practitioners that often resort to more conservative designs which ignore corner effects. The methodology of adjusting calculated ground movements around corners of excavations described in this paper is simple to use and easy to program into software or spreadsheets. It can be used in conjunction with two-dimensional numerical analysis and also for calculating displacements in early stages of projects when numerical analysis has not yet been undertaken. This allows for more informed early discussion with third parties where approvals are sought on a given project.

To date the microporosity structures of coarse soils with various coarse/fines contents are still not fully understood. In this study, the pore-size distributions (PSDs) of five types of soil varying from gravel to clay were characterized using mercury intrusion porosimetry. The soil with a coarse content below 70% (i.e., fines content above 30%) is found to have a fines-controlled microstructure, which is sensitive to water content changes. Such soil forms a dual-porosity structure due to compaction, in which both intraaggregate pores and interaggregate pores are dominant. After saturation, the dual-porosity structure evolves into a unimodal porosity structure dominated by the intraaggregate pores. During drying, such soil exhibits a significant reduction of total volume. The soil with a coarse content above 70% instead has a coarse-controlled microstructure, which is stable upon water content changes. Such soil maintains dual-porosity structures no matter if the soil is compacted, saturated, or dried. As an example of application, the measured PSDs are used to predict the soil water characteristic curves (SWCCs) for the test soils and the predictions are consistent with the SWCCs measured in the laboratory.

Dredging operations in European harbors for maintenance of navigable water depth produce vast amounts of harbor mud. Between 2005 and 2007, the second largest harbor construction project in Germany was designed as a pilot study to use dredged harbor mud as backfill material to avoid expensive disposal or ex situ treatment. During this project, a partial collapse of the backfill highlighted the need for an improved assessment of undrained shear strength of naturally occurring liquid harbor mud. Using vane shear testing, this study evaluates the effect of shear rate on the undrained shear strength of harbor mud. It is shown that measured values for both peak and residual shear strength are significantly influenced by shear rate effects. Furthermore, the influence of shear rate on the peak shear strength is found to be independent of water content while the influence of the shear rate on the residual shear strength strongly depends on water content. New shear rate dependent correction factors $\mu$ are proposed using the test results and the observed time to failure in the harbor basin. The proposed correction leads to significant lower design undrained shear strengths than the classical Bjerrum correction and would have predicted the failure during the construction.

The combined effect of soil moisture in unsaturated soil layers and pore-water pressure in saturated soil layers is critical to predict landslides. An improved infinite slope stability model, that directly includes unsaturated zone soil moisture and groundwater, is derived and used to analyze the factor of safety’s sensitivity to unsaturated zone soil moisture. This sensitivity, the change in the factor of safety with respect to variable unsaturated zone soil moisture, was studied at local and regional scales using an active landslide region as a case study. Factors of safety have the greatest sensitivity to unsaturated zone soil moisture dynamics for shallow soil layers $\left(<2\text{m}\right)$ and comparatively deep groundwater tables (1 m). For an identical groundwater table, the factor of safety for a 1 m thick soil mantle was four times more sensitive to soil moisture changes than a 3-m thick soil. At a regional scale, the number of unstable areas increases nonlinearly with increasing unsaturated zone soil moisture and with moderately wet slopes exhibiting the greatest sensitivity.

Lime slurry pressure injection (LSPI) is a stabilization operation used in problematic soils by transportation industries with the aim of improving the geotechnical properties and bringing excessive maintenance costs to an acceptable standard. This paper presents detailed field and laboratory studies of a lime/fly ash stabilized site at Breeza, NSW, Australia. The mixing of slurry into the soil with depths was investigated by excavating a trench while the improvement of geotechnical properties was determined in detailed field and laboratory tests. Visual observations of the surfaces of an excavated trench showed slurry to be distributed within the shrinkage cracks in the desiccated upper soil horizon whereas slurry was conveyed through planes of hydraulic fracture in the soils at greater depths. Laboratory swell tests on the stabilized soils demonstrated a statistically significant reduction of the intrinsic swell properties in the upper horizon of highly plastic clayey soils by LSPI. A gain in soil strength was observed in cone penetrometer test soundings conducted in stabilized soils. Scanning electron microscope and x-ray diffraction studies proved the underlying physicochemical and cementitious reaction processes in stabilized soils. Aggregation of the soils was observed with the outward diffusion of calcium cations within proximity of slurry seams and resulted in a subdued shrink/swell propensity.

Gravimetric and volumetric sampling techniques are reliable for the measurement of porosity in fully saturated granular filters. However, both methods require a significant effort to gather and prepare samples, are time intensive to process, and do not capture real-time changes. Portable impedance probes serve as a valuable alternative to these destructive and laborious sampling methods. These probes measure the dielectric properties of the soil-water mixtures from which the porosity of filters may be inferred. This study demonstrates that generalized calibrations can result in large errors for porosity estimation when using diverse and small-scale filter types. By comparing with gravimetric and volumetric based porosity measurements for saturated granular filter porosity, impedance probes with filter-specific calibration offer the reliability and confidence owing to its reduced error in a quick, nondestructive fashion. This paper also presents the results of a laboratory investigation using an impedance probe to monitor real-time changes in the porosity of saturated granular filters subjected to cyclic train loading.