Pipeline Floatation Assessment in Liquefiable Soil

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.

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.

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.

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.

Previous studies investigating dynamic responses of gravelly soil were limited to high-strain conditions, in which a high level of pore-water pressure is developed, leading to a significant reduction in shear strength and subsequent liquefaction. This paper presents a series of dynamic centrifuge modeling tests performed on loose gravel–sand mixtures to evaluate progressive response under various shear strains. The centrifuge models simulated a uniform soil profile of gravel–sand mixtures with gravel contents of 20%, 40%, 65%, 80%, and 100% that were subjected to incrementally increasing shaking amplitudes from 0.01 to 0.40 g. Due to the influence of composition on the void ratio of the specimens, the results were analyzed in terms of their dominant behaviors (i.e., sandlike, gravellike, or transition soil). Although the soils had comparable initial relative densities, the sandlike soils had the lowest void ratio, and the void ratio increased when the gravel content was greater than 65%. Resonant column testing results indicated that the soils had comparable dynamic properties because of their loose condition. The results showed that dynamic shaking generates comparable shear strains ranging from 0.03% to 3.8% in all models, but the accumulation of pore pressure leads to upward flow in sandlike soils, whereas transient pore-pressure behavior leads to oscillatory flow in gravellike soils. Differences in the stress–strain response and the effects of the number of shaking cycles were observed in different soil mixtures depending upon the level of excess pore pressure. At low shaking amplitude and low excess pore pressure, stiffness degradation was observed while the stress–strain loop was symmetric. At high shaking amplitude and high excess pore pressure, significant stiffness degradation was observed followed by shear-induced dilation resulting in an asymmetrical stress–strain loop. This study clarifies the differences in the dynamic responses and behaviors of sandlike, gravellike, and transition soil over a wide range of strains.

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.

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.

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.

Post-cyclic loading settlement of saturated clean sand

Shear stress history controls the pore pressure response in liquefiable soils. The excess pore pressure does not increase under cyclic loading when shear stress amplitude is lower than the peak prior amplitude—the shielding effect. Many sophisticated constitutive models fail to capture the shielding effect observed in the cyclic liquefaction experiments. We develop a data-driven machine learning model based on the LSTM neural network to capture the liquefaction response of soils under cyclic loading. The LSTM model is trained on 12 laboratory cyclic simple shear tests on Nevada sand in loose and dense conditions subjected to different cyclic simple shear loading conditions. The LSTM model features include the relative density of soil and the previous stress history to predict the pore water pressure response. The LSTM model successfully replicates the pore pressure response for three cyclic simple test results considering the shielding and density effects.

Insights from the numerical analysis of axially loaded piles in liquefiable soils

The estimation of undrained shear strength ( ) from Cone Penetration Test with pore pressure measurement (CPTu) for two sites was investigated in this research. The CPTu cone parameters: net cone resistance ( ), excess pore pressure ( ) and effective cone resistance ( ) were used to estimate the for the two sites and the values obtained were compared with the undrained shear strength in triaxial compression ( calculated from Anisotropically Consolidated Undrained Compression (CAUC) triaxial test results. For the clayey silt site, the undrained shear strength from the CAUC tests are higher compared to the values obtained from the CPTu, and the highest correlation was obtained from the effective cone resistance parameter . For the quick clay test site, the value and were relatively the same and showed stronger correlation as compared to the clayey silt site. The measured from the cone parameter had the highest correlation. It was concluded that and yields good correlation with the CAUC results for the clayey silt and quick clay test sites respectively as compared to the other con parameters.

Soil liquefaction has conventionally been studied in cyclic laboratory tests as a pure undrained condition, assuming that excess pore water pressure is unable to dissipate during rapid shearing. As such, dissipation of excess pore water pressure generated during shearing is often not modelled in classic cyclic simple shear and triaxial tests. In contrast, dissipation of excess pore pressure does occur in the field following major earthquakes in the form of severe ground settlement and sand boils where fine granular soils were found. In this theme lecture, two aspects of soil liquefaction are discussed. First, a study on the interaction of excess pore water pressure generation and dissipation on liquefiable clean sand in the cyclic triaxial test setup is conducted. Results show that pore water dissipation at merely a fraction of the permeability of the soil can have a significant effect to the soil’s susceptibility to liquefaction. This calls for future cyclic laboratory tests to adopt similar near-perfect undrained condition to better reflect a more realistic representation of soil liquefaction in the field. Second, a critical cyclic shear stress ratio is obtained, where excess pore pressure dissipation dominates excess pore pressure generation, beyond which the soil is no longer susceptible to liquefaction. Complementing this ratio is the identification of a critical void ratio in which the excess pore pressures do not build up despite considerable shearing amplitude, implying the soil is no longer susceptible to liquefaction at that degree of densification. Such information would serve as a guidance to mitigate soil liquefaction in soil densification operations.KeywordsSoil liquefactionPore pressureGenerationDissipationCyclic shear stress ratioVoid ratio

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.