Wave-Induced Seabed Instability: Difference Between Liquefaction and Shear Failure

This study investigated the macroscopic physical and mechanical properties of Guiyang red clay during surcharge loading, lateral excavation and lateral unloading with axial loading, and clarified the failure mechanism of microstructure before and after shear under different stress paths of CTC, RTC and TC. Consolidated undrained triaxial shear permeability, SEM scanning, XRF fluorescence spectrum analysis and XRD diffraction tests were conducted to simulate the actual engineering conditions. The stress–strain curve, shear strength, pore water pressure variation rule and macroscopic failure mode of soil samples under different stress paths were analysed. In addition, Image Pro Plus 6.0 and PCAS were used to study the relationship between the macro mechanical properties and micro microstructure failure under different stress paths. The stress–strain curves from CTC, RTC and TC in CU tests were different, with the peak values of shear stress under the three stress paths being P-increasing, equal P-path and P-decreasing path. Moreover, the internal friction angle and cohesion of the increasing P path were higher than those of equal P path and decreasing P path, hence, the influence of stress paths on the cohesion is greater than that of internal friction angle. The pore water pressure is strongly dependent on the stress path, and the variation characteristics of pore water pressure are consistent with the change in the law of the stress–strain curve. Under the same confining pressure in the P-increasing path, the shear failure zone runs through the whole soil sample, and the shear failure zone is significant, whereas under the condition of the P-reducing path, the shear failure angle of soil sample is about 65°, 55° and 45°, and in the equal P path, the soil sample is dominated by the confining pressure, with no obvious microcrack on the surface of the soil sample. The difference is that the distribution of pores in the path of increasing P and equal P is directional, and the anisotropy rate is small, while the distribution of pores in soil samples with shear failure and before shear is random and the anisotropy rate is high.

Sand strata located 5 to 10 miles seaward of the major active passes of the Mississippi River have been buried by more than 100 ft of rapidly deposited clay. As a result of rapid deposition the clay is underconsolidated and contains pore water at pressures in excess of hydrostatic values. Results of soil borings drilled as part of foundation investigations in the region have indicated that in some instances the sand strata also contain excess pore water pressures. The existence of pressurized shallow sands of this type has been verified by direct measurement at a location in 200 ft of water near Southwest Pass. Using a piezometer installed at a penetration of over 200 ft in the bottom of a borehole, pore pressure 2.6 ksf greater than the hydrostatic was recorded. Excess pore pressures of this magnitude may cause problems during pile installation if jetting is used to clean out the soil plug. Sand below the pile tip may become quick and flow into the pile as a result of the excess head in the sand relative to the pile. The excess pore pressure would reduce pile end bearing and skin friction capacity in the sand. The stability of soil deposits beneath a sloping seafloor may also be adversely affected by excess pore pressures. The geologic conditions leading to the development of pressurized sands are examined using an example from the Mississippi River delta. A method of estimating the in situ excess pore pressure in clay soils using laboratory test data is presented. The method is applied to the clay soils which confine a pressurized sand stratum and the results are compared to a piezometer measurement of excess pore pressure in the sand. INTRODUCTION Soil borings performed in the Gulf of Mexico as a part of foundation investigations usually penetrate 300 to 400 ft below the seafloor as required for the design of structure-support piling. Sand strata which are encountered in such investigations offshore Texas and western Louisiana generally appear to have interstitial pore pressures at normal hydrostatic pressures. In such conditions, the pore pressure at any point in the strata is equal to the product of the unit weight of seawater and the distance below sealevel. The relatively slow sedimentation processes on the continental shelf have allowed time for these sands and clays to adjust to the increasing weight of overburden through drainage of water from the pores of the soil to the seafloor, and for transfer of the loads to intergranular stresses. Exceptions exist near the mouths of rivers where rapid sedimentation results in the formation of under-consolidated clays, in soils charged by gas and in artesian aquifers charged by onshore sources at higher elevations. Excess pore pressures may also form in deep-lying confined beds where the drainage paths for the escape of water are so great in length that sedimentation and changes in tectonic stresses induce pore pressures which cannot be effectively dissipated. High pore pressure sands are not unusual, for example, at depths below 8000 ft in some Louisiana Gulf Coast reservoirs.

This paper presents an original analysis of the flotation potential for a pipeline buried in soil which may not be fully liquefied, but nevertheless exhibits a significant excess pore pressure induced by wave-induced soil shearing. For pipelines that are light enough to be subjected to flotation, the accumulation of pore pressure enhances the flotation potential in two ways:The pore pressure accumulation in the seabed increases the upwards buoyancy force exerted on the pipeline.The increase of excess pore pressure in the soil reduces the soil resisting force which restrains the pipeline to prevent flotation. Ultimately, the excess pore pressure may become equal to the initial effective stress level, which implies full liquefaction. It is commonly accepted that this implies a high risk of pipeline flotation if the submerged specific gravity of the pipe is less than the total unit weight of the soil. However, the combination of an increased buoyancy force and a reduced soil resistance as excess pore pressures accumulate mean that the buried pipeline may reach an unstable state before full liquefaction is reached. Hence pipelines may also float in soils that are not fully liquefied. This paper will present the theoretical background for assessing both the uplifting force exerted on the pipeline and the corresponding soil resisting force, as a function of both the excess pore pressure level in the seabed and the cover depth over the crown of the pipeline. The model used for assessing pore pressure accumulation in the seabed is also described and then an example application is presented, comprising a typical pipeline in a backfilled trench subject to a design storm. Introduction Offshore pipelines in shallow waters are often buried in order to protect them against damage by fishing equipment or drag anchors and to improve their on-bottom stability under storm conditions. A vital aspect of buried pipeline design is the assessment of pipeline flotation potential under design storms or seismic events since these may govern the pipe specific gravity, the burial depth and the selection of engineered backfilling materials. Two fundamental components need to be addressed in a pipeline flotation assessment: what is the storm generated excess pore pressure distribution in the surrounding soil and what resistance can be mobilised in the soil in order to resist pipeline flotation. While the mechanisms of excess pore pressure generation under seismic or storm conditions have been studied extensively (e.g. Seed & Idriss, 1971; Ishihara and Yamazaki, 1983), the existing methods for assessing the potential for soil liquefaction have mainly been developed for silica sands with relative density or cone resistance as the fundamental input soil parameter. In addition, the standard procedures are only applicable where fully undrained conditions pertain in the soil, whereas partially drained conditions would generally be expected under storm conditions for most liquefiable soil types. For the carbonate soils encountered in tropical waters (e.g. on the North West Shelf of Australia), these standard approaches are not applicable since the liquefaction properties of carbonate soils are quite different to those for a silica soil of equivalent cone resistance. In addition, it is often found that carbonate materials with a high fines content are much more liquefiable than equivalent non-carbonate clayey soils. Hence, the potential for pipeline flotation during a storm is generally much higher in uncemented carbonate soils compared to most silica sands.

In the Gulf of Alaska region, sediment has rapidly accumulated (>1 km/my) in the trench sourced from intensified glaciation in the past ∼1.2 million years. This rapid sediment accumulation increases overburden and should accelerate dehydration of hydrous minerals by insulating the underlying sediment column. These processes have the potential to generate fluid overpressures in the low permeability sediments entering the subduction zone. A 1‐D model was developed to simulate dehydration reaction progress and investigate excess pore pressures as sediments approach the trench and are subducted. At the deformation front, simulated temperatures increase by ∼30°C due to the insulating effect of trench sediments. As a result, opal‐A begins to react to form quartz while smectite remains mostly unreacted. Loading due to the trench sediments elevates excess pore pressures to ∼30% of lithostatic pressure at the deformation front; however, deformation front excess pore pressures are sensitive to assumptions about the permeability of outer wedge sediments. If the outer wedge sediments are coarse‐grained and high‐permeability rather than mud‐dominated, excess pore pressures are lower but still have an insulating effect. During early subduction, simulated pore pressures continue to rise and reach ∼70% of lithostatic by 60 km landward. The 1‐D modeling results suggest that the elevated pore pressures are primarily due to loading and that dehydration reactions are not a significant component of excess pore pressure generation at this margin.

The piezoresistive silicon pressure transducers based on the piezoresistive effect have demonstrated their potential in the accurate monitoring of pressure. However, their usage in the pore and earth pressure monitoring at the pile surface under hydraulic jacking has not yet been explored. In this study, two self-made model piles (one is a closed-ended pile and the other is an open-ended pile) were instrumented with piezoresistive silicon earth transducers and pore pressure transducers and then driven into the soil using a hydraulic jack. A comprehensive investigation was first carried out for the structure of the model piles, the installation procedure of the transducers, and the composition of the test system. The pore and earth pressure measurements of the transducers were used for the evaluation of the distribution of the pore pressure, excess pore pressure, radial earth pressure, and radial effective earth pressure. The model test results indicate that the piezoresistive silicon pressure transducers are suitable for monitoring the pore and earth pressure at the pile surface during jacking. In addition, the pore pressure, excess pore pressure, radial earth pressure, and effective radial earth pressure along the test piles were affected by the penetration depth and the pile end form.

The propagation of shallow-water waves may cause liquefaction of the seabed, thereby reducing its support capacity for pipelines and potentially leading to pipeline settlement or deformation. To ensure the stability of buried pipelines, it is crucial to consider the excess pore pressure induced by irregular waves thoroughly. This paper presents the findings of an experimental study on excess pore pressure caused by irregular waves on a sandy seabed. A series of two-dimensional wave flume experiments investigated the excess pore pressure generated by irregular waves. Based on the experimental results, this study examined the influences of irregular wave characteristics and pipeline proximity on excess pore pressure. Using test data, the signal analysis method was employed to categorize different modes of excess pore-water pressure growth into two types and explore the mechanism underlying pore pressure development under the influence of irregular waves.

Despite certain restrictions on its validity, the use of a triaxial apparatus is still a commonly used laboratory test method to determine soil behaviour. Solutions were obtained many years ago for evaluating mean excess pore pressure values in soils subject to continuous rates of loading. They have been used to predict mean excess pore pressures in consolidation tests with constant rates of loading. Such solutions may be suitable for predicting excess pore pressures in soil specimens, subject to continuous loading, in drained triaxial stress path tests. This note proposes the use of these solutions to select the rate of loading for continuously loaded, drained stress path triaxial tests. The analytical solutions of the equations in the known mathematical model may be used to predict the average excess pore pressure in a triaxial sample, subjected to time-dependent loading, at any time during a continuous triaxial stress path. This requires an estimate of the imposed excess pore pressure. A calculation method is given for this pressure, together with a proposed method for predicting deviation from the required effective stress path. As an illustrative example, a prediction of the effective stress path, during a typical drained triaxial stress path test, is presented.

A solution is derived for the heat flow and consolidation which occur when a heat source is buried deep in a porous thermoelastic soil having anisotropic flow properties. This solution is used to examine the pore pressure generation and dissipation near both point and cylindrical heat sources. An increase in temperature will tend to generate an increase in excess pore pressure. However, the pore water will tend to flow from regions of high excess pore pressure to regions of low excess pore pressure, and so consolidation will occur, and temperature‐generated excess pore pressures will tend to dissipate. Many natural soils exhibit horizontal layering and so have a higher horizontal than vertical permeability. It is shown that in soils the excess pore pressure generated by a heat source is significantly less than that in an isotropic soil having an equal vertical permeability.

The self-boring pressuremeter can be used to determine in situ consolidation characteristics of clays (Clarke et al. 1979). The test procedure includes undrained loading from which total horizontal stress, shear modulus and strength can be determined. Excess pore pressures are generated during loading. These decay during the holding portion of the test when the membrane is held at about 10% cavity strain. The rate of decay is used together with a theoretical prediction of consolidation around a cavity to determine the horizontal coefficient of consolidation. The main disadvantage of this is the difficulty with measuring the pore water pressure. As the excess pore pressures dissipate the total applied pressure within the membrane is reduced. The total pressure can be monitored more accurately than the pore pressure and, hence, holding tests based on total pressure decay would be a preferred field technique. This paper discusses work carried out at the SERC Bothkennar soft clay site where undrained SBP and holding tests were carried out down to a depth of 19.6 m. The tests have been interpreted using total pressure observations. The tests have also been interpreted using a finite element package which includes the Cam Clay model and coupled consolidation. This interpretation allows a comparison to be made between the use of pore and total pressure decay in the analysis and recommendations for an alternative test procedure are made.

Excess pore pressure generation in sand under non-uniform cyclic strain triaxial testing

The influence of porosity reduction processes on the hydrologic characteristics of the earth's crust is examined. We present a simple mathematical model that shows the effect of porosity reduction on pore pressure development in the crust. Pore pressure in excess of hydrostatic can be generated if porosity reduction rates are sufficiently high. Elevated pore pressure could in turn affect properties such as strength and seismic reflectivity. Indirect evidence from laboratory, theoretical, and field studies indicates that porosity reduction processes are active, or very likely to be so, in large portions of the crust. Porosity, permeability, and pore pressure in the earth's crust may well be time‐dependent, even in regions not subjected to large deviatoric stresses.

Deterministic and probabilistic analyses of the stability of gentle infinite slopes subject to seismically induced excess pore pressures and inertia forces are developed. In the deterministic analysis, classical equations for infinite slope stability are rewritten to explicitly include excess pore pressure and seismic acceleration. Equations for the factor of safety are developed that include these factors. In the probabilistic analysis, the seismic acceleration, excess pore pressure, and effective friction angle are considered random variables. Acceleration peaks are considered Rayleigh distributed. Excess pore pressure is predicted using a model that considers Rayleigh distributed shear stress peaks. The friction angle is modeled with a Beta distribution. Acceleration and pore pressure development within the gentle infinite slope are assumed the same as those in a horizontal deposit of the same average thicknesss. Finite element analyses are performed to investigate the limits of this assumption. Results from both analyses are compared to documented case histories of lateral spreading.

Soil liquefaction is a devastating earthquake hazard, commonly causing tilting, sinking and floating of infrastructure. The classical mechanism for liquefaction requires undrained and loosely packed soil, that upon shear experiences elevated, lithostatic, pore pressure and consequently zero effective stress. However, some field and experimental observations cannot be explained by this mechanism. These include liquefaction of pre-compacted soils, liquefaction under drained conditions, repeated liquefaction events, and liquefaction triggered by small seismic energy density. A recent study suggests a new mechanism for soil liquefaction that arises only from buoyancy effects of fluids plus grain accelerations, where the term liquefaction , used as its phenomenological field definition, refers to a macroscopic transition from rigid to fluid-like behavior. We extend that study and seek a unifying mechanism for field observed liquefaction that accounts both for the buoyancy effect and for elevated pore pressure, though not necessarily with lithostatic values. To achieve this goal, we use a coupled fluid flow and granular dynamics numerical model to study the effect of pore pressure on the sinking of a large object ( intruder ) into a drained densely packed granular system, undergoing cyclic shearing. Results show that despite the drained conditions pore pressure rises during shaking. Although pore pressure remains well below lithostatic values, the soil liquefies, as identified macroscopically by intruder sinking to its isostatic position. Even simulations with buoyancy effects alone show liquefaction and intruder sinking under certain conditions, yet inclusion of pore-pressure effects add to the buoyancy effect, and is seen to enhance liquefaction and promote intruder sinking.

The variation in percentage consolidation with time within a clay layer subjected to a non-uniform initial excess pore water pressure distribution can be difficult to evaluate, and as a result, often a uniform initial distribution is assumed in most analyses. However, by utilizing some of the key features of consolidation in terms of excess pore water pressure dissipation, it is possible to simply adjust the uniform case to account for any number of non-uniform initial excess pore pressure distributions. By observing the decay of excess pore water pressure with time resulting from various non-uniform initial distributions, it is clear that any initial asymmetry or skewness is quickly dispersed, and the distribution of excess pore pressure with depth becomes sinusoidal (or half-sinusoidal if singly drained) shortly after consolidation has commenced. In other words, once the pore pressure decay due to a non-uniform initial distribution has become sinusoidal, it will actually decay at the same rate as the uniform case – however, it will be “ahead” or “behind” the uniform case by some constant factor. Once this factor has been determined, it is possible to simply adjust the rate of consolidation resulting from a uniform initial pore pressure distribution (the values of which are widely available in literature) to account for any number of realistic non-uniform initial excess pore pressure distributions.

The performance, including a failure, of two embankments in New Liskeard, Ontario is analysed. The underlying medium-to-soft varved clay foundation is studied with respect to stability, undrained deformations, and excess pore pressure distribution.Total stress stability analyses were performed with undrained strengths based on measured and corrected field vane strengths, the average unconfined compression and unconsolidated undrained strength, and SHANSEP strength parameters with and without anisotropy. Several approaches yielded satisfactory factors of safety and reasonable critical arcs. Effective stress analyses using pore pressures predicted from finite element analyses showed that either the effective stress strength parameters were much lower than those measured in the laboratory or the predicted pore pressures at failure were much too low.Undrained deformations and excess pore pressures at end-of-construction were predicted with the finite element program FEECON. The values were generally consistent with field measurements for an embankment located 60 m from the failure area.