Search

A manually operated penetrometer (DMS) fitted with cylindrical (T-bar) and ball penetrometer tips was developed for measuring the profiles of undisturbed and remolded undrained shear strength within box-core samples. This paper summarizes the findings of a series of miniature penetrometer tests and vane shear tests that were carried out on reconstituted clay from a local site in Western Australia. The aim of the tests was to evaluate the potential of the DMS in characterizing the shear strength of seabed surficial sediments. It was found that the DMS gave essentially identical T-bar and ball penetration resistances but these were up to 17% lower than the net cone resistance. From the comparison between the T-bar and ball penetration resistance and the shear strengths measured from vane shear tests, average N factors of 11 and 14 were obtained for intact and fully remolded conditions, respectively. The test results suggest that the DMS is a reliable and efficient means of obtaining intact and remolded shear strength profiles. DOI: 10.1061/(ASCE)GT.1943-5606.0000379 CE Database subject headings: Clays; Penetration tests; Shear strength; Cyclic tests; Soft soils; Measurement. Author keywords: Clays; Penetrometers; Penetration resistance; Shear strength; Cyclic tests.

Pipelines are frequently subjected to active loading from slide events both on land and in the offshore environment. Whether the pipeline is initially buried or lying close to the surface, and whether it crosses the unstable region or lies in the path of debris originating from further away, the main principles are unchanged. The pipeline will be subjected to active loading over some defined length, related to the width of the slide, and as it deforms will be restrained by transverse and longitudinal resistance in adjacent passive zones. Ultimately the pipeline may come to a stable deformed shape where the continued active loading from the slide is equilibrated by the membrane tension in the pipeline in addition to the passive resistance. This problem has been explored by various writers and these principles are well established. However, to date no attempt has been made to develop a standard set of parametric solutions, which is the purpose of the current paper. Both analytical and numerical solutions of the problem have been developed, initially for slides acting normal to the pipeline but later extended to general conditions with the slide impacting the pipeline at some angle. It is shown that analytical solutions based on certain idealizations maintain their accuracy over a wide parameter range, and the net effect of the slide in terms of stresses induced in the pipe wall and maximum displacement of the pipeline may be captured in appropriate dimensionless groups. Design charts are presented for slide widths of up to 10,000 times the pipeline diameter for a practical range of other parameters such as the ratios of passive normal and frictional resistance to the active loading. Although the solutions are limited by some of the idealizations, they should provide a useful starting point in design, providing a framework for a more detailed numerical analysis for the particular governing conditions. DOI: 10.1061/(ASCE)GT.1943-5606.0000314 CE Database subject headings: Design; Underwater pipelines; Soil-pipe interactions; Landslides; Lateral loads. Author keywords: Pipeline design; Submarine pipelines; Soil-pipe interactions; Landslides; Lateral loads.

Three-dimensional large deformation finite-element (FE) analyses were performed to investigate plate anchor capacity during vertical pullout. The remeshing and interpolation technique with small strain approach was expanded from two-dimensional to threedimensional conditions and coupled with the FE software, ABAQUS. A modified recovery of equilibrium in patches technique was developed to map stresses after each remeshing. Continuous pullout of rectangular plate anchors was simulated and the large deformation results for strip, circular, and rectangular anchors were compared with model test data, small strain FE results, and plastic limit solutions. Interface conditions of no breakaway (bonded) and immediate breakaway (no tension) were considered at the anchor base. The effects of anchor roughness, aspect ratio, soil properties, and soil overburden pressure were investigated. It was found that the anchor roughness had minimal effect on anchor performance. For square and circular deep anchors under immediate breakaway conditions, the maximum uplift capacity increased with soil elastic modulus, which suggests that lower bound limit analysis and small strain FE analysis may overestimate the capacity. The soil beneath the anchor base separates from the anchor at a certain embedment depth near the mudline, once tensile stresses were generated. The ratio of separation depth to anchor width was found to increase linearly with the ratio of soil undrained shear strength to the product of soil effective unit weight and anchor width and was independent of the initial anchor embedment depth. DOI: 10.1061/(ASCE)GT. 1943-5606.0000210 CE Database subject headings: Plate anchor; Large deformation analysis; Finite element method; Capacities; Clays. Author keywords: Anchors; Soil deformation; Finite-element method; Clays; Three-dimensional analysis.

Vertically installed plate anchors have been investigated in this paper by numerical analysis and centrifuge modeling. In the numerical analysis, the large deformation finite-element method (remeshing and interpolation technique with small strain) was used to simulate strip plate anchor rotation. In the centrifuge model tests, transparent soils were used to observe square anchor rotation. The loss in anchor embedment during anchor keying was assessed for anchors in uniform and normally consolidated soils with anchor pullout angle varying from 30[degrees] to 90[degrees] to the horizontal. It was found that the loss in anchor embedment during anchor keying may be expressed in terms of a nondimensional anchor geometry factor, which is a function of the eccentricity of the padeye, angle of loading, and the net moment applied to the anchor at the stage where the applied load balances the anchor weight. However, once the anchor geometry factor reaches a certain value, the loss in anchor embedment stabilizes at 0.25-0.5 times the anchor width. The loss in anchor embedment decreases linearly with decreasing pullout angle. Simple formulae and design procedures have been proposed to estimate the loss in anchor embedment during keying. DOI: 10.1061/(ASCE)GT.1943-5606.0000098 CE Database subject headings: Anchors; Clays; Centrifuge models; Deformation; Finite element method; Imaging techniques; Rotation.

Spudcan foundations for offshore mobile drilling rigs are large saucer-shaped foundations that can penetrate several tens of meters into soft sediments. The penetration depth is typically predicted by considering a wished-in-place foundation at different depths and following traditional bearing capacity approaches to assess the depth at which the estimated capacity matches the applied loading. However, the geometry of the spudcan and its progressive mode of penetration lead to soil failure mechanisms that differ markedly from those relevant to onshore practice. This paper presents a new rational design approach for assessing spudcan penetration in single layer clays based on a study combining centrifuge model testing and large deformation finite-element (FE) analysis. The design approach takes account of the evolving failure mechanisms in the soil, which start with cavity formation and surface heave at shallow penetration, gradually transforming to backflow of soil over the spudcan. A detailed FE parametric study has explored the relevant range of normalized strength, strength nonhomogeneity, and spudcan base roughness, with results validated against centrifuge model test data. The penetration response curves are presented in terms of profiles of bearing capacity factors, forming nondimensional design charts along with simplified expressions for convenient use in practice. Comparisons with approaches suggested in the SNAME design code suggest an urgent need to update current practice. DOI: 10.1061/(ASCE)GT.1943-5606.0000054 CE Database subject headings: Load bearing capacity; Centrifuge models; Clays; Finite element method; Heterogeneity; Foundation design.

The undrained remolded shear strength of soft clays is of importance in geosystem design, particularly for offshore structures. Common methods to estimate remolded shear strength, such as correlations with cone penetration data, direct measurement with an in situ field vane shear device, and laboratory measurements, produce varied results and can be particularly costly and time consuming. Full-flow penetrometers (T-bar and Ball) provide an alternative rapid method to estimate remolded shear strength and soil sensitivity through remolding soil by repeated cycling of the penetrometer up and down over a given depth interval. The cyclic penetration resistance degradation curve inherently contains information regarding remolded strength and sensitivity. The objective of this paper is to assess the ability of full-flow penetrometers to predict remolded strength and soil sensitivity, and to develop a suite of predictive correlations in which these properties can be estimated in the absence of complementary laboratory or in situ test data. To accomplish this, full-flow penetration profiles and cyclic tests were performed at five well characterized soft clay sites, which together represent the broad range of soils in which the penetrometers will be often used. A previously developed model for the reduction in penetration resistance with cycling is modified to predict the entire degradation curve, including the remolded penetration resistance using only measurements obtained during initial penetrometer penetration and extraction. Using field vane shear strength as the reference measurement, correlations are developed to predict soil sensitivity and remolded shear strength based solely on full-flow penetrometer data, which is particularly useful in site investigation programs where site specific data are not yet available or are sparse. Finally, the usefulness of these relationships is demonstrated by implementing them for two additional soft clay sites. DOI: 10.1061/(ASCE)GT.1943-5606.0000037 CE Database subject headings: In situ tests; Drainage soft soils; Clays; Sediments.

This paper presents a numerical investigation of the effect of foundation size on the response of shallow circular foundations on siliceous and calcareous sands. The study is based on the predictive capabilities of the MIT-S1 soil model for simulating both the compression and shear behaviors of natural sands over a wide range of densities, [K.sub.0] values and confining pressures. The paper highlights the variations in the deformation mechanisms for the siliceous and calcareous sands cases. The assessment of the bearing capacity factor, [N.sub.[gamma]], is examined, showing a dramatic decrease in the values with increasing foundation size for the case of footings on calcareous sands, eventually converging to a terminal [N.sub.[gamma]] value. At this stage the sand resistance is insensitive to variations in initial density and foundation size because the sand tends to loose its initial characteristics due to grain crushing, leading the material rapidly toward ultimate conditions. In the silicious sand case, it is found that, eventually, for extremely large footing diameters, the deformation mechanism progresses toward a punching shear mechanism, rather than the classical rapture pattern accompanied by surface heave as employed in current bearing capacity equations. A dimensional transition between the failure mechanisms can clearly be defined, referred to as a 'critical size' in the [N.sub.[gamma]]-D relationship. CE Database subject headings: Sand; Calcareous soils; Silica; Shallow foundations; Size effect; Finite element method; Numerical analysis.

A CPT-based method is presented for estimating the self-weight penetration and suction required during the installation of suction caissons in medium dense to dense sand. Existing methods, although fundamental in nature, suffer from the difficulty in estimating key input parameters such as an appropriate bearing factor for the tip resistance and frictional coefficients for skirt friction. Instead, the proposed method is based directly on the cone resistance profile, starting with the Det norske Veritas approach for calculating the installation resistance of skirted foundations. The approach is modified to include a simple linear reduction in end beating on the skirt tips and internal friction with the level of suction, with those components of resistance assumed to reduce to zero once the suction reaches the critical level corresponding to hydraulic failure of the soil plug. The method is coupled with a seepage model so that, for a given pumping rate, the penetration and corresponding resistance can both be expressed as a function of time. Results of calculations with this complete model are shown to compare well with data from centrifuge model tests and with published data from suction caisson installations. CE Database subject headings: Suction; Caissons; Sand; Cone penetration; Installation; Pumps; Centrifuge models.

This paper discusses the development of a framework for classifying soil using normalized piezocone test (CPTU) data from the corrected tip resistance ([q.sub.t]) and penetration pore-water pressure at the shoulder ([u.sub.2]). Parametric studies for normalized cone tip resistance (Q = [q.sub.cnet]/[[sigma]'.sub.v0]) and normalized excess pressures ([DELTA][u.sub.2]/[[sigma]'.sub.v0]) as a function of overconsolidation ratio (OCR=[[sigma]'.sub.vy]/[[sigma]'.sub.v0]) during undrained penetration are combined with piezocone data from clay sites, as well as results from relatively uniform thick deposits of sands, silts, and varietal clays from around the globe. The study focuses on separating the influence of yield stress ratio from that of partial consolidation on normalized CPTU parameters, which both tend to increase Q and decrease the pore pressure parameter ([B.sub.q] = [DELTA][u.sub.2]/[q.sub.cnet]). The resulting recommended classification chart is significantly different from existing charts, and implies that assessment of data in Q-[DELTA][u.sub.2]/[[sigma]'.sub.v0] space is superior to Q-[B.sub.q] space when evaluating piezocone data for a range of soil types. Still, there are zones of overlap for silty soils and heavily overconsolidated clays, thus requiring that supplementary information to Q and [DELTA][u.sub.2]/[[sigma]'.sub.v0] be obtained in unfamiliar geologies, including variable rate penetration tests, dissipation tests, CPT friction ratio, or soil sampling. CE Database subject headings: Soil classification; Cone penetration tests; Silts; Sand; Clays; Overconsolidated soils.

The behavior of strip and circular plate anchors during vertical pullout in uniform and normally consolidated clays was studied in this paper by means of small strain and large deformation finite-element analyses. Both fully bonded (attached), and 'vented' (no suction on rear face), anchors were considered. The current numerical results were compared with existing laboratory test data, finite-element results, and analytical solutions. This study showed that, in small strain analysis, the scatter of existing data was mainly due to the effect of soil stiffness. In large deformation analysis, when soil and anchor base were attached with suction, the pullout capacity factor formed a unique curve independent of the soil strength ([s.sub.u]), soil effective unit weight ([gamma]') and anchor size (B=width of strip anchor and D=diameter of circular anchor). The transitional embedment depth ratio, [H.sub.SD]/B or [H.sub.SD]/D, (where [H.sub.SD]=transition depth between shallow and deep embedment) was 1.4 for a strip anchor and 0.75 for a circular anchor. The ultimate pullout capacity factors ([N.sub.c]) for deep embedment were 11.6 and 11.7 for smooth and rough strip anchors and 13.1 and 13.7 for smooth and rough circular anchors, respectively. However, when the anchor base was vented, the soil stayed attached to the anchor base for deep embedment, and the pullout capacity was therefore the same as for the attached anchor. The separation depth ratio, [H.sub.S]/B or [H.sub.S]/D, (where [H.sub.s]=embedment depth at which the soil and anchor base separated) was found to increase linearly with the normalized strength ratio, [s.sub.u]/[gamma]' B or [s.sub.u]/[gamma]'D. CE Database subject headings: Anchors; Pull out resistance; Soil strength; Deep water; Finite element method; Deformation.

The study is on the electrokinetic strengthening of caisson anchors embedded in offshore calcareous sand. The effects of electrode configuration on the effectiveness of electrokinetic treatment are investigated based on electric field analysis and are verified by results from a series of large scale laboratory tests on caisson models of 200 mm diam and 400 mm height, embedded in calcareous sand submerged under seawater. The electrokinetic treatment generates cementation of soil solids as well as bonding between soil and caisson shafts, which leads to increases in the side resistance and overall pullout resistance. The effectiveness of electrokinetic treatment is directly related to the electric field intensity. A linear relationship is observed between the increase in the side resistance and energy consumption. The study shows that the effectiveness of electrokinetic treatments can be maximized by the optimization of the electric field distribution through the electrode configuration. DOI: 10.1061/(ASCE)1090-0241(2008)134:3(352) CE Database subject headings: Soil stabilization; Caissons; Anchors; Cements; Soil-structure interaction; Pull-out resistance.

Suction caissons are increasingly being used for offshore anchors because of their ease of installation. However, for soil profiles that have sand overlain by layers of silt, there are reasons to believe that it may not be possible to install suction caissons. This may happen because the low permeability of the silt will create a hydraulic blockage, and thus diminish the upward hydraulic gradient required to reduce the penetration resistance in the sand to enable caisson penetration. The tendency of the silt to be sucked up, and its effect on the underlying sand, are not clearly understood. Furthermore, the blockage by the silt layer may be less than if a clay layer is present. This is because scouring of the silt may occur, allowing an upward seepage gradient to develop, and hence causing a reduction in penetration resistance. A series of suction caisson installation tests have been conducted in a geotechnical centrifuge to provide data on the penetration resistance and mechanisms for soil profiles where silt layers are present, either on top, or interbedded between layers of sand. In these tests, the thickness of the silt layer and its depth below the sand surface have also been varied. It has been found that if silt overlies the sand, a larger suction force is required for installation than when installation occurs in homogeneous sand, but that this force is still much smaller than the soil resistance when the caisson is pushed into the ground. When a silt layer is present, the suction pressures are observed to follow a similar trend that is independent of the position and thickness of the silt layer. Scouring of silt was evident in the tests, and significant upward movement of the soil plug inside the caisson was observed in all soil profiles when silt layers were present. The plug height was seen to reduce after the suction was turned off, suggesting that the resulting heave was unstable. DOI: 10.1061/(ASCE)1090-0241(2007)133:10(1183) CE Database subject headings: Suction; Caissons; Installation; Offshore structures; Layered soils; Sand; Silts.

This paper discusses the effects of penetration rate on the penetration resistance in soft clay for various shaped penetrometers (cone, T-bar, ball, and plate) and for T-bars with different aspect ratios. Constant rate ('normal') and variable rate ('twitch') penetration tests, where the penetration rate was successively halved over eight steps with the penetrometer advanced by one or two diameters in each step, were undertaken in the beam centrifuge at the University of Western Australia. The tests were conducted on samples reconstituted from clay collected from the Burswood site in Western Australia. The twitch tests showed higher penetration resistance than the corresponding normal tests after the penetration rate had been reduced by a factor of 16 due to cumulative effects of partial consolidation. The penetration rate at which the resistance started to increase due to partial consolidation was used to estimate the consolidation coefficient, [c.sub.v], of the reconstituted clay. The interpreted [c.sub.v] values were similar to values estimated from other consolidation data, both in the centrifuge and from laboratory Rowe cell and constant rate of strain consolidation tests. In addition, results from in situ twitch tests at the Burswood site were examined to evaluate viscous effects on the penetration resistance. DOI: 10.1061/(ASCE)1090-0241(2006)132:9(1188) CE Database subject headings: Centrifuge model; Clays; Loading rate; Penetration resistance; Penetration tests; Viscosity.

There is little information in the geotechnical literature regarding the influence of the type of cement on the engineering behavior of cemented soils. This paper explores the mechanical behavior of a calcareous soil under triaxial loading after treatment with different types of cement, namely Portland cement, gypsum, and calcite. To identify the specific effects of each cement type a parametric study was undertaken, where factors such as density and unconfined compressive strength were maintained constant for each cementing agent. Samples of the cemented soil were examined under optical and electron microscopy to understand the bond mechanism created by each cement. Results from triaxial testing have shown that, despite having the same unconfined compressive strength and density, the effective stress paths and postyield response are significantly different, mainly because of the different volumetric response upon shearing. Samples prepared using Portland cement showed ductile yield and strong dilation afterwards; calcite and gypsum-cemented samples exhibited brittle yield, generally followed by contractive behavior. The paper discusses the results and explains the reasons behind the differences in the mechanical response. DOI: 10.1061/(ASCE) 1090-0241(2002) 128:6(520) CE Database keywords: Calcareous soils; Portland cements; Contraction; Triaxial tests; Gypsum.

The results of centrifuge modeling of pipe-soil interaction for shallowly embedded offshore pipelines are presented. A non-associated bounding surface model is constructed in vertical-horizontal (V-H) load space on the basis of test data and the theory of plasticity to simulate the response of a pipeline embedded in sandy soil under combined (vertical and horizontal) monotonic loading, taking into account possible pre-loading effects. The model needs nine parameters that can be back calculated from model or field tests and in some cases estimated theoretically. It provides a suitable basis for modeling the load-displacement response of shallowly embedded offshore pipelines. The model reproduces the key features of the load-displacement response of pipelines observed in centrifuge model tests. In particular, the adoption of bounding surface plasticity allows a gradual transition from elastic to plastic response to be simulated and the introduction of a non-associated flow rule allows the model to predict the strain-softening behavior of pipes under horizontal loading. The lateral breakout resistance predicted by the model agrees very well with experimental data. CE Database keywords: Offshore pipelines; Calcareous sand; Bounding surface; Models; Plasticity.

The piled raft is a geotechnical composite construction, consisting of the three elements piles, raft, and soil, which is applied for the foundation of tall buildings in an increasing number. In a parametric study, 259 different piled raft configurations have been analyzed by means of three-dimensional elastoplastic finite element analyses. In the study, the pile positions, the pile number, the pile length, and the raft-soil stiffness ratio as well as the load distribution on the raft has been varied. In the scope of this paper, the results of the parametric study are presented and design strategies for an optimized design of piled rafts subjected to nonuniform vertical loading are discussed. DOI: 10.1061/(ASCE)1090-0241(2004)130:1(1) CE Database subject headings: Foundation design; Pile foundations; Raft foundations; Vertical loads; Numerical analysis; Finite element method; Differential settlement, Bending moments.