Journal of Geotechnical and Geoenvironmental Engineering

A field test for investigating the soil arching effect behind the diaphragm wall was carried out in an average 17-meter-deep, braced excavation in the normally-consolidated clayey ground. Horizontal and vertical ground movements, lateral earth pressures, diaphragm wall deformations, and strut loads were measured and analyzed. The soil arching zone and the loosened zone behind the wall are determined by the force and displacement boundary conditions of soil mass. The arching stress transferred along the arching zone drags the soil against the bracing and increases the lateral earth pressure around the upper parts of the wall. This results in a passive state of stress and horizontal compression and vertical extension of the soil around the crest of the wall. The top of the arching zone extends vertically and is on the same vertical line as the maximum ground surface settlement. The three-dimensional arching is essentially formed by the superposition of multiple transverse two-dimensional archings along the longitudinal direction of excavation, of which two arching feet were located near the corner.

Deep dry soil mixing is a ground improvement technique commonly used for treating soft soils. Portland cement is the most commonly used binder, but its long-term use is unsustainable due to the high ${\mathrm{CO}}_2$ emissions associated with its manufacture. Alkali-activated cements are a low-carbon alternative that involve the use of pozzolanic industrial by-products and wastes. This study provides insights into the one-dimensional compressibility, internal cemented structure and leaching characteristics of an alluvial soil stabilized with a new 100% waste-based cementitious binder, comprising biochar as the alkali activator and blast furnace slag as the pozzolan. The binder recently was demonstrated by the authors to satisfy European soil stabilization 28-day compressive strength requirements when using dosages of 7.5% and 10% by dry weight. The biochar successfully activated the pozzolanic properties of the slag, whereby the stabilized soil mixtures developed a cemented microstructure which resulted in improvements in compressibility and stiffness. Oedometer data sets for untreated and biochar–slag- and CEM-II-stabilized alluvium were processed successfully through a framework developed by the authors to quantify their artificially cemented internal structure, for use as an input parameter in advanced constitutive soil models. Leaching results indicated that the heavy and trace metal content of 1- and 28-day cured biochar–slag-stabilized samples complied with UK and European waste acceptance criteria, and with mean baseline heavy metal concentrations for groundwater resources in England and Wales. This study advocates the new biochar–slag binder as a suitable replacement for portland cements in soil stabilization, contributing to the path toward net zero carbon emissions for the ground engineering sector and improving the circular economy.

Microbially induced carbonate precipitation (MICP) has been widely studied as an innovative method for ground improvement applications by taking advantage of the cementation effect of the porous geomaterial and thus enhancing its stiffness and strength. Yet, a quantitative and precise description of the interparticle stresses induced by ${\mathrm{CaCO}}_3$ cementation, and the effective stress of MICP-treated sands remain elusive and obscure. Note that bonding among particles induced by the calcium carbonate bridging is ubiquitous in a more general term of cementing granular materials commonly seen by capillary water, ice or gas hydrates, polymers, etc. In this work, the concept of cementation stress for MICP-treated sands was proposed, which was found to be equal to the isotropic tensile strength of MICP-treated sands, to unify the effective stress in uncemented and MICP-treated sands. The cementation stress characteristic curve can be defined as the relationship between the cementation stress and ${\mathrm{CaCO}}_3$ content to describe how ${\mathrm{CaCO}}_3$ cementation develops for sands as ${\mathrm{CaCO}}_3$ precipitates. Cementation stress characteristic curves were used to predict the shear strength of MICP-treated sands at different ${\mathrm{CaCO}}_3$ contents and the contour of shear strength distribution of a MICP-treated sand box, which compared favorably with the measured shear strengths from triaxial, direct shear, and unconfined compression tests. The validity of the cementation stress characteristic curve was confirmed for several sands and test conditions, considering factors of ${\mathrm{CaCO}}_3$ contents (up to 25%), poorly graded sands with particle size ranging from 0.1 to 2 mm, confining pressure (up to 400 kPa), and concentrations of urea and calcium chloride between 50 mM and 1 M. The cementation stress characteristic curve provides a potentially simple and practical way to describe the state of stress in MICP-treated sands and contributes a unified method to quantify the ${\mathrm{CaCO}}_3$ cementation effects and predict the shear strength of MICP-treated sands.

Microbially induced carbonate precipitation (MICP) is a novel soil-strengthening technique that involves a biogeochemical process. Temperature plays a crucial role in influencing the biological and chemical processes involved in the formation of carbonate precipitates, which in turn affect the mechanical properties of the treated soil. The aim of this study was to investigate the impact of temperature on the cementing structure of MICP-treated soils and its subsequent effects on their strength parameters. The results revealed that temperature considerably affected the content, size, and distribution of ${\mathrm{CaCO}}_3$ crystals produced, resulting in variations in the friction angle, cohesion, stiffness, peak strength, residual strength, and dilation of the MICP-treated soil samples. Lower strength enhancement was observed when fewer and smaller carbonate crystals were produced at 4°C and 50°C. In contrast, higher numbers of larger crystal clusters were produced at 20°C and 35°C, which effectively bonded the soil particles. Increasing the number of bacterial injections at 50°C promoted the formation of larger crystals and enhanced strength effectively. This study highlights the temperature effects on calcium carbonate growth in biocemented soils, which is a critical step in determining the field-scale application of this innovative soil stabilization technique.

This study investigated the behavior of a 38-m deep twin circular peanut-shaped cofferdam interconnected with a rectangular section for cut-and-cover tunnel construction using distributed fiber-optic sensors (DFOSs) based on optical frequency domain reflectometry (OFDR). The distributed sensors revealed that temperature changes on the two sides of the diaphragm wall were different upon its exposure by excavation, while the measured strains were used to evaluate the wall deflection and bending moments. The high spatial resolution achieved by DFOS measurements revealed unique aspects of the wall response, which are difficult to obtain by conventional types of instrumentation. In particular, the strains along vertical and lateral directions of the wall panels were measured, the latter of which indicated eccentric compression in the concrete panels that arises from the distinctive peanut-shaped geometry. Developments of hoop forces and circumferential bending moments in the panels at various construction stages are discussed, with particular focus on the release of such during partial demolition of a temporary cross wall to facilitate the assembly and launching of tunnel boring machines. The mechanisms of stress developments and release were simulated using three-dimensional finite-element models that, together with the field measurements, enhance the understanding of the behavior of multicell cofferdams.

The multiaxial behavior of an artificially lightly cemented sand was investigated using the cubical cell apparatus (CCA). Two different cement–sand blends were prepared by compaction at different densities and cement contents but featuring a similar porosity/volumetric cement content index. The two blends were subjected to a semirosette of probing stress paths in the octahedral plane with orientation from the vertical stress axis of 0°, 30°, 60°, 90°, 120°, 150°, and 180° in order to explore the multiaxial response and a full failure envelope for artificially lightly cemented sands. The stress–strain behavior and the strength failure envelope revealed the presence of some material cross-anisotropy. The experimental data were well fitted by a modification of well-established theoretical multiaxial failure envelopes to account for the material anisotropy. The shape of the strength envelope in the octahedral plane seemed to be independent of the individual blend compositions (porosity and cement content) because the porosity/cement content index was the same. Further tests exploring the material mixture space are required to validate this initial observation.

Destructive underwater mass movements can impose a threat to off-shore infrastructure and near-shore communities. Yet predicting their formation and failure mechanisms remains a major challenge, in part due of the large variety of factors affecting their stability over time. Long-term processes such as sedimentation as well as short-term events such as earthquakes can impact the stability of the slope highlighting the need for an integrated analysis procedure to quantify their impact. In this article, such a framework is presented to simulate the evolution of subaqueous landslides, ranging from sediment deposition to seismic triggering to the postfailure evolution of the collapsing soil mass. Each stage is simulated in an individual step, based on different finite element-based methodologies, to best model the governing processes. The steps are linked in a consistent manner to facilitate the simulation of the landslide evolution as a continuous process. The presented framework is applied to analyze three historical landslide events in Swiss lakes. The model predictions compare well with the in situ landslide deposits. The simulation results provide insight into slope failure mechanisms and effects of seismic ground motion characteristics on the stability of the analyzed slope failures.

Abstract

Coastal environments are characterized by a variety of sediment deposits with highly diverse geotechnical properties. Particularly in energetic coastal environments, sediment type and properties may vary on small spatiotemporal scales, limited previous information may be available, and sediment coring may be difficult. This study proposes a single data analysis framework to estimate geotechnical strength parameters in the uppermost layers of the seabed surface for a wide variety of sediment types from portable free fall penetrometer (PFFP) measurements without need for previous or complementary data. The framework builds on existing approaches and modifies and integrates them toward one universal analysis procedure. Target results are to distinguish sediment type, estimate relative density and critical friction angle for coarse-grained sediments, and undrained shear strength for fine-grained sediments. Data sets and sediment samples collected from seven different sites across the US served the validation and assessment of the framework. It was found that for the PFFP used a penetration depth less than 20 cm indicated coarse-grained sediments with a certainty of 96%, whereas a penetration depth greater than 25 cm was associated with fine-grained sediments with a certainty of 100%. Friction angles yielded a favorable match with laboratory triaxial and direct shear testing for the sand within $\pm 1°$ and undrained shear strength values resulted in mismatches $<10\%$ of minivane shear results for most fine-grained sediments using cone factor of 10 for two groups of sediment strength-depth profiles. Higher mismatches observed for the third type of distinct profiles (up to 48%) suggested lower cone factors, likely related to a change in sediment properties.

Practical Applications

This paper proposes a single framework that builds on and evaluates existing approaches to estimate geotechnical sediment properties in coastal environments using a portable free fall penetrometer. The framework identifies different soil types (coarse-grained versus fine-grained) and proceeds with data analysis toward strength parameters. Relative density and friction angle are estimated for coarse-grained sediments, and undrained shear strength is estimated for fine-grained sediments. Results of the framework were validated and assessed with laboratory testing based on a data set collected from over seven different sites across the US. The good agreement between field and lab results supports the feasibility for one data analysis framework for the use of portable free fall penetrometers for a rapid and preliminary seabed surface characterization, especially in areas with difficult access or under extreme conditions. This makes this method accessible to a broad group of users in the geotechnical and coastal engineering community.

This paper presents a numerical model, called consolidation settlement - elastic-viscoplastic (CS-EVP), for the consolidation of layered soft soils, including soil self-weight and time-dependent compressibility effects. CS-EVP was developed using the piecewise-linear method for large-strain consolidation and elastic-viscoplastic model for time-dependent soil compressibility. The model accounts for vertical strain, soil self-weight, nonlinear hydraulic conductivity and compressibility, nonlinear creep with limited creep strain, and time-dependent surcharge and/or vacuum loading for layered heterogeneous soils. The accuracy of CS-EVP is verified by comparing calculated values with the results from finite-element simulations and a large-scale laboratory vacuum consolidation test of soft soil slurry. Lastly, simulated settlements and excess pore pressure profiles are compared with field measurements for embankment loading in Väsby, Sweden. The results indicate that CS-EVP provides good estimates of long-term large-strain consolidation under both laboratory and field conditions.

Design guidelines for geosynthetic-reinforced soil (GRS) walls in a tiered configuration are often limited to two tiers with zero batter. To facilitate rational expansion of current design, this paper extends the current top-down procedure, based on limit equilibrium analysis, from single GRS walls to multitiered walls. For a given factor of safety, the required reinforcement tension distribution and connection load are determined for each reinforcement layer. The approach considers the impact of potential compound failures. Parametric studies are conducted to investigate the influences of backfill soil, wall geometry (i.e., wall batter, number of tiers, and offset distance), and reinforcement layout. The results demonstrate quantitively that increasing the internal friction angle of backfill soil, wall batter, number of tiers, and offset distance reduce the required maximum tension. The tiered configuration in GRS walls leads to localized increase in connection load at the toe elevation of each tier. Using close reinforcement spacing could significantly reduce the connection load. The critical offset distance is realized when the reinforcement in each tier acts internally independent of others. Its value decreases with increasing the internal friction angle of backfill soil, wall batter, and number of tiers. The observations in this study are significant in the context of optimal design of multitiered GRS walls.

The effect of vertical effective consolidation stress, ${\sigma }_{vc}^{\prime }$, on the cyclic resistance of nonplastic to plastic normally consolidated (NC) and overconsolidated (OC) intact and NC reconstituted silt was investigated using a series of constant-volume, stress-controlled cyclic direct simple shear (CDSS) tests. The results were interpreted considering the changes of specimen properties [e.g., void ratio, $e$, and overconsolidation ratio (OCR)] associated with the increased ${\sigma }_{vc}^{\prime }$. Despite increasing density, all specimens exhibited a reduction in cyclic resistance as ${\sigma }_{vc}^{\prime }$ increased. The reduction in cyclic resistance for intact specimens occurred due to the detrimental effect of yielding of the natural soil fabric, reduction in OCR, and the potential suppression of dilative tendencies, which outweighed the beneficial effect of reduced $e$. Tests of uniformly prepared reconstituted NC specimens reduced the number of factors contributing to the reduction of cyclic resistance (i.e., destruction of natural soil fabric, and reduction of OCR) with increased ${\sigma }_{vc}^{\prime }$, and were used to confirm the sensitivity of cyclic resistance to ${\sigma }_{vc}^{\prime }$, in which the detrimental effect of suppressed dilative tendencies on cyclic resistance dominated the beneficial effect of reduced $e$. The overburden correction factor, $K_{\sigma }$, was observed to decrease with increased plasticity index, PI, highlighting the role of compressibility on cyclic resistance.

Identifying the precise elevation of the seabed is important for many applications, including navigation and the investigation of seabed surface processes such as erosion and scour. Certain sediment conditions, such as fluid mud or very soft soils with high water content, can challenge the accuracy and consistency of seabed or mudline identification. Portable free-fall penetrometers (PFFPs) can assist with determining the mudline; however, this is typically done during processing by an expert user, leading to possible uncertainties and bias. The determined mudline location also affects PFFP processing such as zero-offsetting to account for water column effects as well as the determined penetration depth. In this work, an automated methodology is proposed to reliably predict the impact point. The method’s performance is compared with human processors. The proposed method shows good agreement with human processors for soil with fines content less than 50% and undrained strengths greater than 1.5 kPa. At higher fines content and lower undrained strength, the automation identified the mudline generally later and at a greater depth, representing a more conservative but less ambiguous choice.