Journal of Geotechnical and Geoenvironmental Engineering

This paper reports the results from a series of model tests undertaken to provide insight into the behavior of torpedo anchors during dynamic installation and pullout in lightly overconsolidated kaolin clay and calcareous silt. The tests were carried out in a drum centrifuge at 200g, varying the drop height (hence the impact velocity) and the time delay for consolidation before pullout. The pullout angle at the mudline was also varied to encompass various mooring systems, including catenary (0°), taut leg (45°), and tension leg ($\sim 80°$). Two geometries of torpedo anchors were explored, varying the fin and tip geometry. The results demonstrated that the anchor embedment depth increased as the drop height (and hence the impact velocity) increased and the soil undrained shear strength decreased. In stronger silt, the cavity above the installing anchor remained open, whereas in soft clay, it was fully backfilled and replenished. The corresponding anchor embedment depth was also about 0.63 times compared with that in clay. The anchor holding capacity was found to increase with increasing postinstallation consolidation time, depth of embedment, and soil undrained shear strength and with reducing pullout angle at the mudline. The anchor rotation during pullout, and hence the pullout distance for attaining the maximum capacity, reduced as the load inclination at the mudline increased. The fin geometry (rectangular or elliptical) and tip geometry (conical or ellipsoid) were shown to have remarkable influence on the holding capacity, with rectangular fins and conical tip proving to be more effective. The negligible adherence of silt along the anchor sides and the presence of semiliquid material around the extracting anchor resulted in a significantly (34–47%) lower holding capacity in the calcareous silt, with significantly higher intact undrained shear strength.

This paper presents the results from a series of centrifuge tests and three-dimensional finite-element (FE) analyses, which examined the benefits of combining a footing with a monopile as a solution for foundations that are subjected to large moment loading, such as those used for towers and wind turbines. The experiments were carried out in silica sand and involved monotonic application of lateral loads at an equivalent prototype height of 26 m above the foundations. Tests were conducted on piled footings, monopiles, and unpiled footings. These experimental results together with the findings from the FE analyses show that the footing interacts positively with the piled foundation and that both the rotational stiffness and capacity of the combined piled footing system are greater than the sum of the individual contributions. Increased capacity arises as the footing causes a significant reduction in moment loading on the pile (hence facilitating the application of larger loads), primarily owing to an increased footing effective area arising from the tension capacity of the pile.

A methodology has been presented for determining the stability of unsupported vertical cylindrical excavations by using an axisymmetric upper bound limit analysis approach in conjunction with finite elements and linear optimization. For the purpose of excavation design, stability numbers ($S_n$) have been generated for both (1) cohesive-frictional soils and (2) pure cohesive soils, with an additional provision accounting for linearly increasing cohesion with increasing depth by means of a nondimensional factor $m$. The variation of $S_n$ with $H/b$ has been established for different values of $m$ and $\varphi$, where $H$ and $b$ refer to the height and radius of the cylindrical excavation. A number of useful observations have been gathered about the variation of the stability number and nodal velocity patterns as $H/b$, $\varphi$, and $m$ change. The results of the analysis compare quite well with the different solutions reported in the literature.

Previous numerical analyses of the bearing capacity of embedded footing under combined vertical, horizontal, and moment loading have been based on the assumption that the foundation is wished-in-place, surrounded by undisturbed soil. Under these conditions, the large displacement and remolding of the soil during the footing installation are not accounted for. This assumption results in an overestimation of the capacity. This paper presents results of the size and shape of the bearing capacity surface of spudcan footing in soft clay that accounts for the effects of installation in the modeling. Results for soil sensitivities between one and five and for embedment depths up to three diameters are provided. The findings have practical application in the site-specific assessment of mobile jack-up platforms in soft clay.

Earthquake ground motions in the near-fault region frequently have intense, double-sided pulses in the velocity-time series that can be very damaging to structures. Many of these velocity pulses are attributed to the effects of forward directivity, which occurs when a fault ruptures toward a site. However, pulses are not always observed in the forward directivity region, and some pulses cannot be explained by forward directivity. The relative contribution of pulse-type motions to the overall seismic hazard should be considered when selecting records in a suite of design ground motions for a site in the near-fault region. This study uses a new scheme to classify records from an enhanced database of records from shallow crustal earthquakes that have the closest site-to-source distances less than 30 km with moment magnitudes greater than 6.0 as either pulse or nonpulse motions. The resulting database of 673 records from 52 earthquakes contains 141 pulses, including 74 explained well by forward directivity. Logistic regression is used to develop a simple model to estimate the proportion of pulse motions as a function of closest site-to-source distance and epsilon of the seismic hazard. The resulting relationship can be used to estimate the number of pulse-type motions that should be included within a suite of ground motions to represent the proper contribution of pulse motions to the seismic hazard. Guidance is also provided for selecting pulse records.

An experimental investigation was performed on five widespread soil types common in the United States to characterize the effects of freezing temperatures on the unconfined compressive strength ($q_u$), the modulus of elasticity ($E$), and strain at the unconfined compressive strength (${\epsilon }_{q_u}$). Soil specimens were subjected to monotonic and cyclic loading with varying strain rates at temperatures ranging from 20 to $-23°\text{C}$ (68 to $-9.4°\text{F}$). When compared with test results at 20°C (68°F), testing at $-20°\text{C}$ ($-4°\text{F}$) showed an increase in $q_u$ by a factor of 100, an average increase in $E$ by a factor of 300, and an average decrease in ${\epsilon }_{q_u}$ by 5% strain. Increase in the soil compaction, moisture content, and applied strain rate amplified the cold temperature effects on $q_u$. Additional testing at $-20°\text{C}$ ($-4°\text{F}$) resulted in an increase in ${\epsilon }_{q_u}$ with no change in $E$ when the applied strain rate was increased. Cyclic experimentation produced data trends comparable to the monotonic experimentation for the mechanical properties but allowed residual deformation as a function of cold temperature to be identified. To assist with current seismic design practice, experimental trends were incorporated into a $p\text{-}y$ curve development and the impact of observed soil response as a function of temperature is demonstrated using a series of pushover analyses on a column continued into the subsurface as a drilled shaft foundation.

While the liquefaction potential index (LPI) has been used to characterize liquefaction hazards worldwide, calibration of LPI to observed liquefaction severity is limited, and the efficacy of the LPI framework and accuracy of derivative liquefaction hazard maps are thus uncertain. Herein, utilizing cone penetration test soundings from nearly 1,200 sites, in conjunction with field observations following the Darfield and Christchurch, New Zealand, earthquakes, this study evaluates the performance of LPI in predicting the occurrence and severity of surficial liquefaction manifestations. It was found that LPI is generally effective in predicting moderate-to-severe liquefaction manifestations, but its utility diminishes for predicting less severe manifestations. Additionally, it was found that LPI should be used with caution in locations susceptible to lateral spreading, because LPI may inconsistently predict its occurrence. A relationship between overpredictions of liquefaction severity and profiles having soils with high inferred fines-content was also investigated. It was determined that the LPI procedure might be improved if it accounted for the characteristics of soils in the crust and interbedded nonliquefiable layers, as well as the characteristics of the soils predicted to liquefy. Further research is needed in this regard. Finally, it was found that for the in situ conditions in Christchurch, the computed LPI values were relatively sensitive to estimates of groundwater depth because of the proximity of liquefiable strata to the ground surface.

A new concept with respect to offshore wind turbines, the bucket foundation, is known in the oil and gas industry, although the load conditions for wind turbines are significantly different. An extensive number of loading tests with small-scale bucket foundations subjected to combined loading were carried out in the geotechnical laboratory at Aalborg University in Aalborg, Denmark. Tests were performed on buckets of various sizes, embedment ratios, and load paths on saturated dense Aalborg University Sand No. 1. The experimental results were used to evaluate the behavior of the bucket foundations in accordance with the macromodel approach. An expression to describe the combined capacity of bucket foundations was developed by calibrating the failure criteria for bucket foundations.

This paper reports advances in the prediction of spudcan resistance profiles during installation and preloading, based on the soil flow mechanisms observed in model tests and numerical analysis, and taking into account strain softening and rate dependency on the soil strength. Deep penetration of spudcan foundations was simulated for 14 case histories reported from different locations in the Gulf of Mexico, where the soils were single layer, predominantly normally to lightly overconsolidated clay. Axisymmetric and three-dimensional analyses were carried out using two large deformation finite-element (LDFE) approaches, modifying the simple elastic-perfectly plastic Tresca soil model to allow strain softening and strain-rate dependency of the shear strength. The field data were compared with the results of the finite-element analyses and with predictions using various design approaches. Excellent agreement was found, particularly when a mechanism-based design approach was used that differentiated between conditions before and after backflow of soil above the penetrating spudcan.

Permeable granular columns are used to increase the time rate of consolidation, reduce liquefaction potential, improve bearing capacity, and reduce settlement. However, their behavior depends on the confinement provided by surrounding soil, which limits their use in very soft clays and silts, and organic and peat soils. This research effort aims to develop a new ground-improvement method using pervious concrete piles. Pervious concrete piles provide higher stiffness and strength that are independent of surrounding soil confinement while offering permeability comparable to granular columns. This proposed ground-improvement method can improve the performance of different structures supported on poor soils. To achieve the goal of the research project, four vertical load tests were performed on one granular column and three pervious concrete piles. In this paper, the material properties of pervious concrete, the developed installation method, and the vertical load response of pervious concrete and aggregate piles are presented, and the variation of soil stresses and displacement during pile installation are briefly discussed. The experimental test results show that the ultimate load capacity of the pervious concrete pile was 4.4 times greater than that of an identical granular column. In addition, the ultimate load capacity of a pervious concrete pile installed using the developed technique was 2.6 times greater than a precast pervious concrete pile. The used installation method created nonuniform lateral soil displacement and increased vertical and horizontal soil stresses.

The influence of fines on the undrained behavior of sand was investigated experimentally over a range of fines content and effective mean stress. The role of fines on compressibility and formation of meta-stable soil fabric was analyzed. For a fines content in the range of 5–30%, most of the specimens manifested reverse behavior opposed to the normal behavior of clean sand. The addition of fines led to shifting of both the isotropic consolidation line and the steady-state line, increased compressibility, and occurrence of reverse behavior. All these behavior attributes are linked when the test results are interpreted in the context of the critical-state soil mechanics framework. In particular, reverse behavior is associated with an isotropic consolidation line that locates above the steady-state line. Several quantitative measures for characterizing the undrained deviator stress-strain responses were proposed, and these measures manifested correlation with the state parameter at the start of undrained shearing. These correlations were also tested with two published datasets.

A comprehensive experimental program that consists of consolidation and hydraulic conductivity tests has been conducted on mature fine tailings (MFTs) dewatered with super absorbent polymer (SAP) as well as MFTs dewatered with SAP and subjected to freeze-thaw cycles. The consolidation behavior and hydraulic conductivity of MFTs dewatered by using recycled SAP are also investigated. The obtained results indicate that dewatering by using SAP or a combination of dewatering by using SAP and a freeze/thaw process can significantly increase the solid content and decrease the initial void ratio and compressibility of the MFTs. Moreover, the freeze–thaw process can significantly increase the hydraulic conductivity of the MFTs, which, in combination with reduced compressibility, will improve the MFT consolidation.

Fines content can have a great influence on the mechanical behavior of sand. The effect of fines content has not been taken into account in most of the strength-dilatancy relationships for sand. An attempt was made in this paper to account for the influence of fines content on the strength-dilatancy relationship of silty sands. The test data on Ottawa sands with nonplastic fines were reanalyzed to examine the strength and dilatancy relationship for silty sand with different fines contents. The ratio of excess angle (which is the difference between the peak friction angle and the critical state friction angle) to the relative dilatancy index was found to be a function of fines content. Furthermore, a strength-dilatancy relationship was also modified to account for the influence of confining pressure on the strength-dilatancy relationship of silt sands.