Journal of Geotechnical and Geoenvironmental Engineering

Blasting has long been used as a ground improvement technique to densify loose granular soils. Explosives detonated at depth induce liquefaction in the target sand followed by pore pressure dissipation and attendant consolidation. Improvement typically is verified via measured surface settlements and increased penetration resistance with in situ tests such as the cone penetration test (CPT). However, an increase in penetration resistance is often not observed for months or even years after blasting. The results of instrumentation at a production blast zone at the Oakridge Landfill are reported herein. Instrumentation included surface settlements, pore pressure measurements inside and outside the blast zone, preblast and postblast piezocone (${\mathrm{CPT}}_u$) soundings, and gas concentrations of pore fluid. No increase in penetration resistance with the ${\mathrm{CPT}}_u$ was detected 53 days after the final blast, despite more than 0.5 m of settlement and an increase in relative density to 80% or more in some areas. Results of gas chromatography tests on the collected pore fluid samples showed that gas produced by the explosives was trapped in the target layer and saturated the groundwater with ${\mathrm{N}}_2$. Consequently, no more ${\mathrm{N}}_2$ could dissolve into solution, and ${\mathrm{N}}_2$ gas existed as occluded bubbles. The presence of occluded bubbles created an unsaturated condition, which in turn affected the mechanical behavior of the densified soil and reduced the CPT tip resistance during postblast testing.

This paper describes a new approach for interpreting cone penetrometer data in soft-stiff-soft clay deposits. The identification of layer boundaries and interpretation of shear strength profile were of particular interest. The proposed approach was based on an extensive parametric study using large deformation finite-element (LDFE) analyses, with a standard cone penetrometer penetrated continuously from the soil surface. The LDFE model has been validated against existing theoretical solutions and numerical results, with good agreement obtained. Regardless of strength ratio between two successive layers, the interface of soft-stiff layers can be identified at $0.8D$ ($D$ is the cone diameter) below the kink in penetration resistance curve in the top soft layer. The interface of stiff-soft layers can be demarked at $1.3D$ above the kink in the penetration resistance profile in the bottom soft layer. The undrained shear strength of a soft clay layer can be interpreted using a single-layer approach and the resistance profile without the influence of the adjacent stiff layer. The interpretation for the interbedded stiff layer necessitates implementing a correction factor, which is shown to be a function of the thickness and rigidity index of the stiff layer and the strength ratio between that layer and the bottom layer. The proposed design framework is illustrated through a flowchart to be used in practice.

The increased demand for higher speeds and increased freight capacity in railroad transportation exacerbates the rate of ballast degradation that leads to unacceptable track deformation and frequent maintenance. In response, a series of large-scale cyclic triaxial tests was performed to investigate the combined effect of train speed (frequency), axle load, and confinement on the deformation and degradation of ballast. In these tests, the load frequency $f$ was varied from 5 to 60 Hz to simulate train speeds of $40–400\mathrm{km}/\mathrm{h}$. Two sets of deviator stress magnitude $q_{\max ,cyc}$ (230 and 370 kPa) were applied to resemble axle loads of 25 and 40 t, respectively, and the effect of three levels of confining pressure ${\sigma }_3^{\prime }$ (10, 30, and 60 kPa) was examined. The results indicate that three distinct categories of permanent deformation mechanisms exist at various levels of $f$, $q_{\max ,\mathrm{cyc}}$, and ${\sigma }_3^{\prime }$. Ballast degradation was more pronounced at higher $f$, and it had a more profound effect on cyclic densification. The resilient modulus also increased with an increase of $f$, $q_{\max ,\mathrm{cyc}}$, ${\sigma }_3^{\prime }$, and number of load cycles ($N$). An empirical relationship for the dynamic amplification factor (DAF) was proposed to properly reflect the role of train speed on axle load.

Microbial-induced calcite precipitation (MICP) is a novel ground improvement method to increase strength and stiffness of sand using natural biogeochemical processes. Cementation level and confining pressure are two important factors that control the behavior of MICP sand. The monotonic mechanical response of MICP cemented sand is systematically investigated using four cementation levels (untreated, lightly treated, moderately treated, and heavily treated) and three levels of effective confining pressure (100, 200, and 400 kPa). The results indicate that the stiffness, peak shear strength, and dilation increases with an increase in calcite content at a given effective confining pressure and the dilation is suppressed with an increase in effective confining pressure. This behavior is consistent with soil-like behavior; therefore, all the MICP soils presented herein are evaluated using critical-state soil mechanics and not an analogous fracture-mechanics framework. The experimental results also indicate that the improvement in peak and residual friction angles and initial elastic modulus, $E_i$, are dependent on the levels of cementation and effective confining pressure. The uniformity of MICP cementation in the laboratory specimens is also discussed.

The validity of using the existing numerical $p\text{-}y$ methods [American Petroleum Institute (API) and Reese methods] for the design of offshore wind turbines’ large-diameter monopiles in sands is questionable, as many researchers have raised concerns related to the diameter effects in $p\text{-}y$ models. This study presents the development of experimental $p\text{-}y$ relationships for large-diameter monopiles in dense sands based on results from centrifuge tests exhibiting a softer monopile behavior than those proposed by the API and Reese methods. The effect of socketing the tip of a pile in rock bearing layers was also investigated. The initial gradients of the $p\text{-}y$ relationships in dense sand layers were shown to become significantly stiffer as the depth reaches the much stiffer and stronger rock-bearing layer. The lateral load-displacement curves obtained based on the developed experimental $p\text{-}y$ relationships were found to be well matched with the measured lateral load-displacement curves; therefore, it was concluded that the developed experimental $p\text{-}y$ relationships reasonably predict the lateral responses of large-diameter monopiles.

The installation of foundations and ground improvement systems alters soil stresses affecting their soil-structure interaction and behavior under vertical loading. The installation effects have been investigated for several foundation types, especially for driven piles; however, experimental full-scale investigations of drilled displacement piles are very limited. This paper focuses on investigating the short-term installation effects of controlled modulus columns (CMCs) using an instrumented full-scale field test unit. The soil was instrumented using push-in pressure sensors (PS) and shape acceleration arrays (SAAs) to monitor the evolution of soil horizontal stresses, pore water pressures, and lateral displacements during installation and vertical load test. These sensors were installed at approximately 1D, 2D, 3D, and 4D from the outside surface of the CMC shaft, where D is the diameter of the CMC. The measurements presented in this paper clearly show that the soil experienced an increase of horizontal stresses and lateral movement throughout the CMC installation. During mandrel advancement, the horizontal stresses gradually increase until the mandrel reaches the location (depth) of the pressure sensors, which was followed by a horizontal stress decrease. The measurements presented in this paper indicate that the zone affected by the CMC installation extend to 2 to 3D from the outer surface of the CMC shaft.

Cyclic hollow cylinder torsional tests were performed in undrained conditions on 12 compacted clay specimens collected from two types of dams (an earth-core rock-filled dam and an earth-zone dam) in Thailand. The cyclic degradation, cyclic pore water pressure generation, and volumetric cyclic threshold shear strain were studied via multistage strain-controlled tests. The testing program encompassed five overconsolidation ratios (OCRs), 1, 1.5, 2, 3.5, and 4, where six to eight cyclic strain-controlled stages were applied in each test with frequency $f=2\mathrm{Hz}$. The results reveal that the volumetric threshold shear strain amplitude of these compacted clays ranged between approximately 0.009 and 0.025% and generally increased substantially with the plasticity index (PI). The cyclic pore water pressures normalized by ${\sigma }_c^{\prime }$ are consistently lower than previous ones using other types of clays tested with other types of devices at comparable cyclic shear strain amplitudes. The cyclic degradation and cyclic pore water pressure in compacted clay are significantly affected by the OCR.

The effective mitigation of the liquefaction hazard requires an improved understanding of the consequences of liquefaction in terms of ground shaking, permanent displacement, and building performance. In this paper, results from centrifuge experiments of a shallow-founded structure on liquefiable sand are used to evaluate the predictive capabilities of a state-of-the-art numerical tool. Solid-fluid, fully-coupled 3D nonlinear numerical simulations were performed using the PDMY02 soil model implemented in a software modeling domain. The numerical model captured excess pore pressures and accelerations well in the free-field, but largely underestimated volumetric settlements due to loss of water during shaking. This was associated with the drastic increase in soil hydraulic conductivity when approaching liquefaction, which was not taken into account numerically, as well as the underestimation of soil volumetric compressibility. The contribution of volumetric strains to total building settlement was, however, relatively minor. Hence, by capturing the excess pore pressures and accelerations under the foundation, the model could capture deviatoric displacements and hence building’s settlement and tilt response. The results of the experiments and numerical simulations are combined in this paper to provide guidance on the evaluation of building response on liquefiable ground.

The process of tunneling is associated with ground movements which may lead to stressing of nearby existing buried infrastructure, and potentially poses a risk of damage. The need for an effective evaluation method of the potential risk increases with the ongoing expansion of underground space utilization. This paper presents a new approach for evaluating the interaction between an assumed input of greenfield tunneling displacements and an existing buried pipeline. The approach integrates new developments with previous research findings to establish a practical interaction analysis methodology that can be used in design. It involves the use of an elastic-continuum analysis to solve the soil-pipeline interaction together with an iterative calculation of the equivalent stiffness in order to consider soil nonlinearity. A set of simplified closed-form expressions, which can be used to evaluate maximum pipeline bending moments within the suggested framework, are presented in the paper. A comparison of the new method results against centrifuge test data and advanced discrete element-method simulations is presented in the paper. The obtained agreement provides validation of the new method over a wide range of tunneling-induced volume losses and pipeline parameters.

The inclusion of granular columns in soft clay deposits leads to improvements in bearing capacity and overall stiffness, along with a reduction in consolidation settlement. Many laboratory investigations have focused on aspects of bearing capacity, but published data on settlement performance are limited. This paper reports on some interesting findings obtained from a laboratory model study with respect to these issues. In this investigation, 300 mm diameter by 400 mm long samples of soft kaolin clay were reinforced with single or multiple granular columns of various lengths using the displacement and replacement installation methods. The experimental findings revealed that, for the same area replacement ratio, limited settlement reduction was achieved for single long floating columns and end-bearing column groups. Marginal improvements in settlement performance were also achieved for columns installed by the displacement method. No settlement reduction was achieved for short single floating columns, whereas short floating granular column groups produced increased settlements. These observations were verified using contact pressure measurements between the footing and column/surrounding clay.

The standard penetration rate used in cone penetration tests (CPTs) is $20\mathrm{mm}/\mathrm{s}$, regardless of soil type, which yields fully drained penetration in sand and fully undrained penetration in clay. However, for silty soils that represent an intermediate grain size composition and unique characteristics compared with sand and clay, the standard rate of penetration results in partially drained penetration, often leading to misinterpretation of this soil type. In this study, 15 CPTs, with penetration rates varying from 0.5 to $60\mathrm{mm}/\mathrm{s}$, were performed at a test site in northern Denmark, where the subsoil consisted primarily of sandy silt with clay bands. The results illustrated that when the penetration rate is reduced, the cone resistance increases, but the pore pressure decreases. The transition between undrained and fully drained penetration was determined by converting the results into a normalized penetration rate. Also evaluated and presented in this paper is how cone resistance obtained under partially drained conditions underestimates the interpreted relative density $D_r$ and friction angle $\varphi$. Triaxial test results on undisturbed silt samples were applied for this analysis.

Time domain reflectometry (TDR) has become an effective nondestructive testing method for soil-nailing inspection. Previous studies utilized a preinstalled, single-core, electric wire alongside the rebar within the soil nail. Measurements may be affected by possible grout defects and excessively overestimate the rebar length if the wire is coiled around the rebar. An improved TDR waveguide and a corresponding decoupled data reduction method (for both soil-nail length determination and grout condition inspection) are proposed herein. The feasibility and advantages of the new approach were experimentally verified with two types of TDR device. The proposed methodology makes it possible to use a portable, low-cost, and low-speed TDR device as a quick and economical tool for quality control of soil nailing.

Statistical analysis of 146 in situ shear wave velocity ($V_S$) profiles from 37 municipal solid waste (MSW) landfill sites worldwide yields a simple linear relationship between MSW $V_S$ and depth up to a depth of 30 m. This empirical linear relationship holds for both U.S. landfills and the worldwide data set. The linear model provides a significantly better fit to the data than either the exponential or power law models for both data sets. At depths greater than 30 m, the observed linear relationship no longer holds. However, $V_S$ data at depths greater than 30 m are sparse and dominated by only a few landfills. Additional investigation of $V_S$ at depths greater than 30 m is required to establish a consistent trend in this depth range. The statistically derived empirical $V_S$ profile developed in this paper is not a substitute for site-specific measurements, but may be useful for preliminary analysis and assessing the reasonableness of measured $V_S$ values.