Effect of Installation Method on External Shaft Friction of Caissons in Soft Clay

The influence of the installation method on the soil flow pattern, resulting external radial total stress changes, and final external shaft friction after consolidation has been investigated for caissons in soft clay by means of centrifuge model tests, large deformation finite-element (FE) analysis, and a simple cavity expansion approach. Both the centrifuge measurements and the FE results show that more soil is forced into the caisson under suction than under jacking. However, the difference in the resulting external radial total stress changes or penetration-induced excess pore-water pressure is much less significant, since the expansion-induced excess pore pressure is smaller for thin-walled caissons than for driven piles. After subsequent consolidation, the influence of the installation method reduces further, and the final shaft friction ratios are close for the two installation methods. Based on the magnitude of heave ratios derived from the centrifuge measurements and the FE analysis, a simple form of cavity expansion approach can reasonably estimate external radial stress changes during installation and after consolidation, and final shaft friction ratios for the caissons. An approach for estimating the external shaft friction ratios for vertical pullout of sealed caissons is proposed.     

Assessment of the Axial Load Response of an H Pile Driven in Multilayered Soil

Most of the current design methods for driven piles were developed for closed-ended pipe piles driven in either pure clay or clean sand. These methods are sometimes used for H piles as well, even though the axial load response of H piles is different from that of pipe piles. Furthermore, in reality, soil profiles often consist of multiple layers of soils that may contain sand, clay, silt or a mixture of these three particle sizes. Therefore, accurate prediction of the ultimate bearing capacity of H piles driven in a mixed soil is very challenging. In addition, although results of well documented load tests on pipe piles are available, the literature contains limited information on the design of H piles. Most of the current design methods for driven piles do not provide specific recommendations for H piles. In order to evaluate the static load response of an H pile, fully instrumented axial load tests were performed on an H pile (HP?310×110) driven into a multilayered soil profile consisting of soils composed of various amounts of clay, silt and sand. The base of the H pile was embedded in a very dense nonplastic silt layer overlying a clay layer. This paper presents the results of the laboratory tests performed to characterize the soil profile and of the pile load tests. It also compares the measured pile resistances with those predicted with soil property- and in situ test-based methods.     

Uplift Capacity of Suction Caissons under Sustained and Cyclic Loading in Soft Clay

A series of centrifuge tests were conducted on instrumented model suction caissons in normally consolidated, lightly overconsolidated, and sensitive clays, to investigate the uplift capacity and external radial stress changes for sealed caissons subjected to sustained loading and cyclic loading. The external shaft friction ratio during vertical pullout for these two types of loading was analyzed from the radial stress measured at failure, then the corresponding reverse end-bearing capacity factor was derived from the pullout capacity. Tests results were compared with those under monotonic undrained pullout. For caissons under sustained loading, the holding capacity was found to be 72–85% of that under monotonic undrained loading. Radial stress reduction around the caisson shaft reduced the external shaft friction ratio to 0.67–0.75, while dissipation of the “passive” suction at the caisson tip reduced the reverse end-bearing capacity factor to 7.5–9.4. Under cyclic loading, the uplift capacity of the caisson was found to be 72–86% of the monotonic capacity. Repeated loading reduced the external shaft friction ratio to 0.65–0.80, while the reverse end-bearing capacity factor reduced to 6.4–9.0.     

Effects of Nonplastic Fines on the Liquefaction Resistance of Sands

A laboratory parametric study utilizing cyclic triaxial tests was performed to clarify the effects of nonplastic fines on the liquefaction susceptibility of sands. Studies previously published in the literature have reported what appear to be conflicting results as to the effects of silt content on the liquefaction susceptibility of sandy soils. The current study has shown that if the soil structure is composed of silt particles contained within a sand matrix, the resistance to liquefaction of the soil is controlled by the relative density of the soil and is independent of the silt content of the soil. For soils whose structure is composed of sand particles suspended within a silt matrix, the resistance to liquefaction is again controlled by the relative density of the soil, but is lower than for soils with sand-dominated matrices at similar relative densities. In this case, the resistance to liquefaction is essentially independent of the amount and type of sand. These findings suggest the need for further evaluation of the effects of nonplastic fines content upon penetration resistance, and the manner in which this relationship affects the simplified methods currently used in engineering practice to evaluate the liquefaction resistance of silty soils.     

Suction Caisson Capacity in Anisotropic, Purely Cohesive Soil

This paper presents a plastic limit analysis of the lateral load capacity of suction caissons in an anisotropic, purely cohesive soil assuming conditions of rotational symmetry about the vertical or gravity axis. The formulation utilizes a form of the Hill yield criterion that is modified to allow for different soil strengths in triaxial compression and extension. Using this yield criterion, energy dissipation relationships are formulated for continuous and discontinuous deformation fields. These dissipation relationships are then applied to a postulated caisson failure mechanism comprising a wedge near the free soil surface (mudline), a two-dimensional flow-around failure at depth, and a hemispherical slip surface at the base of the rotating caisson. The plastic limit analysis predictions compared favorably to predictions obtained from finite-element simulations employing a Hill yield criterion. For the range of anisotropic undrained strength properties commonly reported for normally K0-consolidated clays, parametric studies indicate that suction caisson horizontal load capacities predicted using a conventional approach (a von Mises yield surface fitted to the soil simple shear strength) will differ from anisotropic predictions by less than 10%.     

Liquefaction Resistance of Clean and Nonplastic Silty Sands Based on Cone Penetration Resistance

Liquefaction of granular soil deposits is one of the major causes of loss resulting from earthquakes. The accuracy in the assessment of the likelihood of liquefaction at a site affects the safety and economy of the design. In this paper, curves of cyclic resistance ratio (CRR) versus cone penetration test (CPT) stress-normalized cone resistance qc1 are developed from a combination of analysis and laboratory testing. The approach consists of two steps: (1) determination of the CRR as a function of relative density from cyclic triaxial tests performed on samples isotropically consolidated to 100 kPa; and (2) estimation of the stress-normalized cone resistance qc1 for the relative densities at which the soil liquefaction tests were performed. A well-tested penetration resistance analysis based on cavity expansion analysis was used to calculate qc1 for the various soil densities. A set of 64 cyclic triaxial tests were performed on specimens of Ottawa sand with nonplastic silt content in the range of 0–15% by weight, and relative densities from loose to dense for each gradation, to establish the relationship of the CRR to the soil state and fines content. The resulting (CRR)7.5-qc1 relationship for clean sand is consistent with widely accepted empirical relationships. The (CRR)7.5-qc1 relationships for the silty sands depend on the relative effect of silt content on the CRR and qc1. It is shown that the cone resistance increases at a higher rate with increasing silt content than does liquefaction resistance, shifting the (CRR)7.5-qc1 curves to the right. The (CRR)7.5-qc1 curves proposed for both clean and silty sands are consistent with field observations.     

Effects of Electrode Configuration on Electrokinetic Stabilization for Caisson Anchors in Calcareous Sand

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.     

Capillary Barrier Effect from Underlying Coarser Soil Layer

Infiltration tests were conducted on soil columns of silty sand over pea gravel, concrete sand over pea gravel, and silty sand over concrete sand to investigate the capillary barrier effect of an underlying coarser soil layer. Water movement across the interface occurred when the suction head at the interface reached the breakthrough head of the coarser lower soil layer, defined as the suction head at which the coarser layer first became conductive, regardless of infiltration rate or the properties of the overlying finer soil layer. Thus, the coarser lower soil layer controlled breakthrough in this study. After infiltration was terminated, the suction head near the interface increased above the breakthrough head and the barrier was restored. The breakthrough head did not change substantially after eight test cycles of breakthrough and restoration for a capillary barrier with a pea gravel as the coarser lower soil layer. The barrier formed with the concrete sand as the coarser layer permitted breakthrough at a greater suction head than did the barrier with the pea gravel, indicating that the more uniform and coarse the lower soil layer is, the more effective the capillary barrier.     

Strength Loss and Localization at Silt Interlayers in Slopes of Liquefied Sand

The role of void redistribution in the liquefaction behavior of saturated sand slopes with and without silt interlayers was investigated using a series of dynamic centrifuge model tests. Twelve centrifuge model tests are described that represent four different simple slope configurations, a range of initial relative densities (DR), and three different input motions with different sequences of application. These experimental results demonstrate that the potential for void redistribution induced shear localizations and slope instability depends on the sand’s initial DR, slope geometry (silt layer shape, sand layer thickness), and shaking characteristics (duration, intensity, and history). The archived experimental data set provides a good basis for assessing the ability of numerical modeling methods to distinguish between conditions leading to localization or not. Apparent residual shear strengths mobilized in the models were backcalculated using techniques common to practice. The experimental and analytical results demonstrate that the apparent residual shear strength is unlikely to correlate closely to pre-earthquake penetration resistance alone, but rather is a function of the initial shear stresses and numerous factors affecting the process of void redistribution and localization.     

Water Film in Liquefied Sand and Its Effect on Lateral Spread

A 1D saturated sand layer of 2 m in thickness, in which a silt seam is sandwiched, is liquefied by an instant shock. It is found that a water film is easily formed beneath the silt seam with a thickness as thin as a few millimeters just after liquefaction in loose sand and that the film lasts longer than the post-liquefaction settlement. The effect of the water film on pore-pressure distribution and sand settlement is intensively studied. 1g shake table tests are then carried out for 2D models with or without seams of silt within a saturated sand layer. In the former case, water films formed beneath silt seams just after liquefaction enable the soil mass above them to glide due to an unbalanced force along the water films, not only during but also after shaking. In the latter case, the soil deforms continuously, mostly during shaking, and stops afterward. Thus, a significant effect of water films formed beneath thin, low-permeability sublayers in a liquefied loose sand, on the failure mode and timing in lateral spread, is clearly demonstrated by these simple model tests.     

Effect of Penetration Rate on Cone Penetration Resistance in Saturated Clayey Soils

In this paper, the effects of penetration rate on cone resistance in saturated clayey soils are investigated. Shear strength rate effects in clayey soils are related to two physical processes: the increase of shear strength with increasing rate of loading and the increase of shear strength as the process transitions from undrained to drained. Special focus is placed on this second effect. Cone penetration tests were performed at various penetration rates both in the field and in a calibration chamber, and the resulting data were analyzed. The field cone penetration tests were performed at two test sites with fairly homogeneous clayey silt and silty clay layers located below the groundwater table. Additionally, tests with both cone and flat-tip penetrometers in sand-clay mixtures were performed in a calibration chamber to investigate the change in drainage conditions from undrained to partially drained and from partially drained to fully drained. A series of flexible-wall permeameter tests were conducted in the laboratory for various clayey sand mixtures prepared at various mixing ratios in order to obtain values of the coefficient of consolidation, which is required to estimate the penetration rates below which penetration is drained and above which penetration is undrained. A correlation between cone resistance and drainage conditions was established based on the results of the calibration chamber and field penetration tests.     

Penetration Resistance of Offshore Skirted Foundations and Anchors in Dense Sand

Penetration of skirts is an essential design issue for offshore skirted foundations and anchors in sand. Skirts may not penetrate far enough into dense sand by the available submerged weight alone. It may therefore be necessary to apply underpressure inside the skirt compartment to produce an increased driving force and to reduce the penetration resistance. This paper recommends procedures to calculate penetration resistance and required underpressure for skirts penetrated in dense sand with and without interbedded clay layers. The recommendations are based on interpretation of skirt penetration data from prototypes, field model tests, and laboratory model tests in dense sand. The paper first presents a model to calculate the penetration resistance of skirts penetrated by weight, or other external vertical load that does not cause flow of water in the sand. Two models are considered; one based on bearing capacity equations with friction angles from laboratory tests, and the other one based on empirical correlations with CPT tip resistance. The bearing capacity model gives more consistent correlations with the empirical data than the CPT model. Thereafter, a model to account for the effect of underpressure applied inside the skirt compartment is proposed. This model is developed based on interpretation of available prototype and model test data from skirts penetrated by underpressure. The results show that underpressure facilitates skirt penetration in sand considerably by providing both an additional penetration force and a reduced penetration resistance. It is also shown that interbedded clay layers can prevent flow of water through the sand and eliminate the beneficial reduction in penetration resistance.     

Analysis of Load Tests on Piles Driven through Calcareous Desert Sands

The ultimate bearing capacity of short, precast concrete piles driven into calcareous sands was examined by pile-load tests carried out at two sites in Kuwait. The piles had a 0.3 m × 0.3 m square cross section and extended to a maximum depth of 12 m. They were driven through a loose-to-compact calcareous surface sand layer underlain by a competent dense-to-very-dense siliceous cemented sand deposit. The pile tips and part of the pile shafts were embedded in the lower layer. The base resistance and shaft friction were calculated using the Meyerhof method for a layered soil profile. The method employs the standard penetration test N values. The results indicate that a great portion of the pile capacity is due to base resistance. The skin friction mobilized is small and consists of two components corresponding to the two layers penetrated along the pile shafts. The calculated pile capacities were very close to the measured values. The unit skin friction is not constant along the pile shafts.     

Load Testing of a Closed-Ended Pipe Pile Driven in Multilayered Soil

Piles are often driven in multilayered soil profiles. The accurate prediction of the ultimate bearing capacity of piles driven in mixed soil is more challenging than that of piles driven in either clay or sand because the mechanical behavior of these soils is better known. In order to study the behavior of closed-ended pipe piles driven into multilayered soil profiles, fully instrumented static and dynamic axial load tests were performed on three piles. One of these piles was tested dynamically and statically. A second pile served as reaction pile in the static load test and was tested dynamically. A third pile was tested dynamically. The base of each pile was embedded slightly in a very dense nonplastic silt layer overlying a clay layer. In this paper, results of these pile load tests are presented, and the lessons learned from the interpretation of the test data are discussed. A comparison is made of the ultimate base and limit shaft resistances measured in the pile load tests with corresponding values predicted from in situ test-based and soil property-based design methods.     

Shear Strength and Shear-Induced Volume Change Behavior of Unsaturated Soils from a Triaxial Test Program

A series of unsaturated soil triaxial tests were performed on four soils including sand, silt, and a low plasticity clay. Attempts were made to correlate unsaturated soil properties from these tests and data from the literature with soil-water characteristics curve (SWCC), soil gradation, and saturated soil properties. The feasibility of estimating unsaturated soil property functions from saturated soil properties, SWCCs and gradation data, is demonstrated. A hyperbolic model for estimation of the unsaturated soil parameter, ?b, versus matric suction is presented. Shear induced volume change behavior was also studied, and results are included in this paper. Although not correlated with soil index properties, these shear-induced volume change data are important to complete stress-deformation analyses, and represent a significant addition to the existing data base of unsaturated soil properties.     

Experimental Investigations of Response of Suction Caissons to Transient Vertical Loading

Suction caissons are a relatively new design being considered for use as foundations in a wide variety of offshore applications. They are unusual in that, in contrast with the development of offshore piling, there is no onshore experience that can be used as the basis for the development of designs. It is therefore essential to identify key behavioral patterns and mechanisms that govern capacity under a wide variety of loading regimes, particularly those derived from the cyclic wave loading. The object here is to establish a broad framework of response. More detailed work specific to a site or project would be required for detailed design. This paper describes experimental investigations into the vertical loading response of a suction caisson foundation. The experiments are carried out on the laboratory floor using a sophisticated three-degree-of-freedom loading rig. The caissons are embedded in sand saturated with viscous silicon oil so that modeled drainage times are representative of typical offshore conditions. The experiments involve cyclic loading about different mean loads, including cycling into tension and monotonic loading at different rates. The relationship between the cyclic loading and monotonic loading is explored. One of the key conclusions from the results is that serviceability requirements will dictate design rather than capacity. Perhaps surprisingly, for the experiments undertaken, the rate of loading had little effect on the response.     

Full-Scale Field Tests on Flexible Pipes under Live Load Application

This paper describes the procedure and results of the field tests on high-density polyethylene (HDPE), PVC, and metal large diameter pipes subjected to a highway design truck loading. Numerical simulations using finite element method are performed to determine pipe-soil system response under live load application. Comparisons of field test data with the predicted responses are made for soil pressures around and above the pipes, deformed cross-sectional pipe profiles, and pipe deflections. The field test results indicated that the buried flexible pipes, embedded with highly compacted graded sand with silt, demonstrated good performance without exhibiting any visible joint opening or structural distress. Under shallow burial conditions, the AASHTO specified deflection limit of 5% is found to be adequate for installation of the flexible pipes during the construction phase, and a vertical deflection limit of 2% is suggested for HDPE pipes based on the truck load response and repeated loading effect.     

Case Study: Bed Stability around the East Caisson of the Tacoma Narrows Bridge

A study on the hydraulic and sediment conditions at the Tacoma Narrows Bridge, in Washington State, was carried out to examine the stability of the bed material around the bridge caissons. Specifically, this was conducted around the east caisson, where the maximum velocities around either of the two caissons are experienced. This was performed for the peak tidal exchange event of May 27th to 28th, 2002. During this max flow event, multibeam surveyed bathymetry and three-dimensional acoustic doppler current profiler velocity data were collected around the east caisson in the course of both the flood and ebb. The surface of the bed material surrounding the east caisson was videotaped during the slack conditions following the yearly maximum flow event, and used to determine the particle size distribution and spatial arrangement of those distributions around the caisson. This was done by lowering a submersible video camera and appropriate lighting to the bottom of the Narrows, a depth of approximately 45?m. Flow and sediment observations were coupled to determine the commencement of sediment motion for different size classes of sediment. Two methods were utilized to calculate friction velocity in order to assess the stability of different bed particle size fractions during these high flow conditions. Friction velocity was first calculated from measured velocity profiles at various locations around the east caisson. The second method was based on the concept of a free stream power-law expression for depth-averaged velocity. Stability was then examined using the critical shear stress concept and captured video data of the bed. General results showed the particles ? 30?mm in diameter were in motion during the flood and ebb. The work is here presented as a case study because of the unique large-scale flow conditions that are present around the east caisson of the Tacoma Narrows Bridge.     

Experimental Investigations of the Response of Suction Caissons to Transient Combined Loading

Combined loading of foundations is a fundamental problem in civil engineering, particularly in the offshore industry where harsh environmental conditions occur. Large moment and horizontal loads may be applied to the foundation as well as vertical loads. Also, as the waves pass a structure, there can be rapid changes in the loads, so that transient effects need to be considered. When designing shallow foundations, such as suction caissons, there is uncertainty in the current understanding of how the foundation responds to these loads. This paper presents experiments, performed on model suction caisson foundations, where typical cyclic loading conditions are applied. The footing is embedded in oil-saturated sand so that dimensionless drainage times are comparable with the typical offshore conditions. Most of the testing was carried out with the vertical load held constant, to mimic the structural dead weight, while realistic “pseudorandom” moment and horizontal cyclic loads were applied. Experiments were carried out at different vertical loads, showing that the response depends on the vertical load level. Nondimensional relationships were established which accounted for this dependency. Surprisingly, the rate of loading had little impact on the load–displacement behavior for the experiments undertaken.