Dynamic analysis of beams vibrating on nonlinear poroelastic multi‐layered continuum

ABSTRACTAlthough the uplift behavior of offshore plate anchors under undrained conditions has been investigated well in the past, studies on the behavior of anchors under long-term sustained loading are in relatively few numbers. The time required for consolidation under sustained load is important because the shear strength of soil changes after dissipation of excess pore pressure. In this paper, small strain finite-element analyses have been performed to investigate the consolidation time history above and beneath strip anchors. The modified cam clay plasticity constitutive model is used for modeling coupled pore fluid stress analysis. The effects of magnitude of preloading with embedment level have been studied. As expected, the FE results have shown that excess pore pressure dissipation time for soil above the anchor increased with the increase in embedment depth and the magnitude of preload. Rapid dissipation of negative excess pore pressure beneath the anchor was observed with increasing embedment depth, if the preload magnitude is equal to or more than 60% of the undrained capacity. Observed consolidation responses are presented as nondimensional design charts and simplified equations for ease of practice.

The current design of MSE walls, which is based on limit state analysis, does not apply to undrained conditions. Laboratory and numerical pullout tests are performed to determine the relation between drained and undrained pullout capacities for different soil types (clean sand, 5, 10, 15 and 35 % silty sand), overburden pressures (30, 100 and 200 kPa), and scale and permeability effects in the dissipation of excess pore pressures. The results of the pullout tests show that both drained and undrained pullout capacities change as silt content changes since the pullout capacity increases as the internal friction angle of the soil increases. It is also observed that the pullout capacity increases as the overburden pressure increases. Undrained conditions significantly reduce the pullout capacity as much as 50%. This is caused by the generation of excess pore pressures in the soil under rapid loading which decrease the effective stress at the soil-reinforcement interface. The magnitude of the pullout reduction is related to the permeability of the soil since for large permeabilities the dissipation of excess pore pressures is very rapid and no reduction in pullout is produced; in contrast for low permeabilities the dissipation of excess pore pressures is slower than the rate of pullout and thus a reduction occurs. This is confirmed by the experiments that show no reduction in pullout capacity for clean sand, and a large reduction for silty sands. The ratio of undrained to drained pullout capacity changes with silt content and overburden pressure; for 100 and 200 kPa overburden pressure, the ratio is 1.0 for clean sand, 0.67~0.69 for 5 % silty sand, 0.77~0.78 for 10 %, 0.72~0.73 for 15 %, and 0.57~0.59 for 35 % silty sand. For 30 kPa overburden pressure, the ratio is 1.0 for clean sand, 0.5 for 5 % silty sand, 0.67 for 10 %, 0.78 for 15 %, and 0.72 for 35 % silty sand. It is observed in the numerical analyses that the dissipation of pore pressures is very rapid for permeabilities larger than 10-2 cm/sec, and significantly slow for permeabilities smaller than 10-3 cm/sec. Scale effects are extremely important since as the length of the reinforcement increases the time for pore pressures to dissipates increases.

Post-cyclic loading settlement of saturated clean sand

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 finite element analysis is performed on the excess pore pressure dissipation for pressed pile installation using the ABAQUS finite element software. The modified Cam-Clay model is used to model the soil behavior. The finite slide contact model is used to model the pressed pile installation process. Based on the geology stratum of soils and drainage conditions, the excess pore pressure dissipation process is analyzed using the proposed method. The initial excess pore pressure distribution along the pile depth and the pile radius direction is obtained. The excess pore pressure dissipation after 98 days is analyzed.

The development of negative skin friction (NSF) on single piles is investigated based on an uncoupled method of analysis with the Mikasa (1963) generalized nonlinear consolidation theory in terms of finite strain and the nonlinear load-transfer method. Predicted results are compared with results based on the conventional linear consolidation theory with infinitesimal strains. It is found that predicted development of dragload using the conventional consolidation theory is slightly greater and conservative compared to that using the nonlinear consolidation theory based on effective stress (β method). Effective stress predictions using the conventional theory are larger due to the faster dissipation of excess pore pressures, with the assumption of constant coefficient of consolidation and permeability. However, since the relative displacements required to mobilize the ultimate skin friction are small, and piles are usually installed near the final stages of soil consolidation, the differences in the predictions for the development of dragload on piles between the two consolidation theories are overshadowed. Using the uncoupled model for pile NSF, it is therefore found that the most significant factor for the estimation of dragload and downdrag is the proper selection of the β value rather than the consolidation theory used.

A case history of repeated dynamic and static loading tests in Alberta on two pipe piles during dissipation of driving-induced pore pressures is presented together with three reanalyzed published case histories involving similar records. The four case histories demonstrate that, for each case, the same effective-stress proportionality coefficients, beta-coefficients, fit the capacities at different degrees of dissipation of excess pore pressures. For two of the test sites, the beta-coefficients back-calculated from the tests differed considerable from the values determined from the soil plasticity relation, while for two, the agreement is good. For one case, the back-calculated shaft resistance agreed well with the values of vane shear strength, while a less good agreement was found for the other tests. Neither case showed good agreement was found for methods combining undrained shear strength and effective overburden stress. Capacity calculations for two cases employing methods based on CPT (cone penetrating testing) soundings gave excellent agreement with one test and a poor agreement with the other. The increase of capacity due to aging after dissipation of excess pore pressures did not agree with cited recommendation for calculations of aging effect.

It is well known that the bearing capacity and stiffness of displacement piles depends on short term effects during and shortly after pile installation as well as long term effects, a phenomenon usually referred to as set-up. During pile installation the surrounding soil is influenced by the installation process. The soil state e.g. the stress state, pore pressure or void ratio are changing due to complex mechanical processes in the soil. The change of the soil state has a great influence on the pile capacity and the behavior of the pile under vertical and horizontal loading. During the installation process phenomena like a temporary and locally limited liquefaction of the soil can occur. After the pile installation the bearing capacity may increase significantly with time due to set-up effects. In cohesive soils the set-up is commonly explained by consolidation processes in which the dissipation of the excess pore pressures around the pile leads to an increase in effective radial stresses. Case histories show that set-up may also occur in sand over a period which exceeds by far the consolidation process. Therefore, other effects apart from the dissipation of excess pore pressures must contribute to this long-term set-up effect. An understanding and a correct estimation of these effects is of great importance for a economical design of pile-founded structures e.g. offshore wind turbines. For the investigation of these phenomena different methods like numerical simulations, model scale or full scale tests are applicable. In this paper two methods are used: The installation process is investigated with numerical simulations and the set-up effects are investigated by long term in-situ measurements.

Influence of biodegradable natural fibre drains on the radial consolidation of soft soil

Previous studies have revealed that prefabricated vertical drains (PVDs) are effective in stabilizing the soft subgrade soils under traffic-induced cyclic loads. A consolidation model of soft clays under cyclic loading incorporating both radial and vertical drainages, and nonlinear variation of compressibility and permeability is presented in this paper. The prediction of the model matches well with the result from a large-scale cyclic triaxial test conducted on a soil sample installed with a single PVD. Parametric analysis has also been carried out to explore the impacts of cyclic degradation parameter, cyclic stress ratio, nonlinear compressibility and permeability, cyclic loading frequency, drainage condition, and number of loading cycles on the behaviors of the PVD-reinforced soil. Results indicate that radial drainage helps to decelerate the accumulation of excess pore pressures under cyclic loading due to two factors: increased rate of excess pore pressure dissipation and decreased rate of internal excess pore pressure generation. In addition, radial drainage also facilitates the rapid dissipation of excess pore pressures during rest period, leading to a denser soil which is stronger for resisting the subsequent set of cyclic loads. The findings of this study demonstrate the usefulness of PVDs in reinforcing soft subgrades under cyclic loading conditions, e.g., road pavement, railway embankment, etc.

Centrifuge model test and numerical interpretation of seismic responses of a partially submerged deposit slope

The authors thank Professor Mesri for his discussion of their paper on the combined effects of strain rate and temperature on the behaviour of St-Roch-de-l’Achigan clay. Two aspects were discussed: the method of analysis and the value of m ′. These two aspects will be considered in the following paragraphs. The discusser presents eq. [D1] showing the variation of σ p′ with strain rate. It is equivalent to the linear relationship between the logarithm of preconsolidation pressure and the logarithm of strain rate, as described by eq. [2] in our paper, with m ′ = Cc /Cα. The discusser indicates that eq. [D1] would be valid only after primary consolidation, which corresponds to the discusser’s own view that viscosity applies only after dissipation of excess pore pressures. Previous studies (Leroueil 1988, 1996, among others) have shown that this is not the case and, in this sense, eq. [2] is more general than eq. [D1]. The discusser questions the analysis of the test results, alleging that the evaluation of the preconsolidation pressure could be erroneous if u b ′ /σv is larger than 15%. In fact, in all the CRS tests performed, the ratio u b ′ /σv remained smaller that 10% at the passage of the preconsolidation pressure. The evaluation of σ p′ values and of m ′ have thus not been influenced by the method of analysis. As indicated in the paper, the obtained value of m ′ = 16 for St-Roch-de-l’Achigan clay is lower than expected for inorganic clays and indicates a strain-rate effect larger than that usually observed. The authors agree on this point with the discusser. It is however not clear why this m ′ value is low — possibly soil variability and the small number of CRS tests carried out; or possibly the limited range of strain rate that was considered. However, as previously explained, it is not due to excessive pore pressures. In any case, as the m ′ value does not change, all of the conclusions in the paper concerning the combined effects of strain rate and temperature on the mechanical behaviour of natural clays remain valid.

Influence of excess pore pressures on axial offshore pile response : Lee, C Y; Poulos, H G Univ Sydney School of civil and mining engng research reportR577, July 1988, 27P

Laboratory tests were carried out to simulate pressuremeter testing so that the generation and dissipation of pore water pressure during pressuremeter tests could be studied. A modified triaxial cell was used to accomodate 150 mm hollow cylinder specimens prepared with known stress history. Pore pressures on the cavity wall as well as in the soil mass were monitored during the expansion. The results show that a large magnitude of excess pore pressure gradient is set up around the model pressuremeter where the maximum value is at the cavity wall. This is then followed by radial dissipation of pore pressure, the rate of which depends on the pore pressure gradient. (Author/TRRL)

Numerical analysis of consolidation behavior of soil-bentonite backfill in a full-scale slurry trench cutoff wall test