Utilizing Direct CPT Methods Considering Consolidation and Aging Setup

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

Prestressed high-strength concrete (PHC) pipe pile with the static press-in method has been widely used in recent years. The generation and dissipation of excess pore water pressure at the pile–soil interface during pile jacking have an important influence on the pile’s mechanical characteristics and bearing capacity. In addition, this can cause uncontrolled concrete damage. Monitoring the change in excess pore water pressure at the pile–soil interface during pile jacking is a plan that many researchers hope to implement. In this paper, field tests of two full-footjacked piles were carried out in a viscous soil foundation, the laws of generation and dissipation of excess pore water pressure at the pile–soil interface during pile jacking were monitored in real time, and the laws of variation in excess pore water pressure at the pile–soil interface with the burial depth and time were analyzed. As can be seen from the test results, the excess pore water pressure at the pile–soil interface increased to the peak and then began to decline, but the excess pore water pressure after the decline was still relatively large. Test pile S1 decreased from 201.4 to 86.3 kPa, while test pile S2 decreased from 374.1 to 114.3 kPa after pile jacking. The excess pore water pressure at the pile–soil interface rose first at the initial stage of consolidation and dissipated only after the hydraulic gradient between the pile–soil interface and the soil surrounding the pile disappeared. The dissipation degree of excess pore water pressure reached about 75–85%. The excess pore water pressure at the pile–soil interface increased with the increase in buried depth and finally tended to stabilize.

Based on the wave flume experiments, the response of excess pore water pressure was studied under the regular breaking wave action on a 1:30 slope sandy seabed. The topographic change was observed in the wave broken zone and its causes were analyzed. The results show that the variation gradient of excess pore water pressure are larger in the surface layer and the changing amplitude of variation gradient of excess pore water pressure is bigger in the wave broken zone. The water depth is the most influential factor of excess pore water pressure and the wave height and period has little influence on excess pore water pressure in the same wave steepness range in the wave broken zone. The topographic change is greatly in the wave breaking zone. Excess pore water pressures changing amplitude at the top are significantly higher than it at the bottom. When the seabed scours, the amplitude of the excess pore water pressure increases; and when the sand seabed accumulates, the amplitude of the excess pore water pressure decreases.

Effect of water content and soil texture on consolidation in unsaturated soils

When the slurry shield tunneling in water rich sandy gravel stratum, it is easy to have problems such as difficult to form the filter cake and poor stability of the excavation face, and the filter cake is the key factor to maintain the stability of the slurry shield excavation face. Using finite element software, a three‐dimensional fluid solid coupling model of slurry shield tunneling is established. The influence of slurry parameters (slurry pressure and permeability coefficient of filter cake) on pore water pressure and excavation face stability during slurry shield tunneling is studied. The variation laws and internal relations of slurry parameters, pore water pressure and displacement are obtained. The results indicate that the slurry pressure and the permeability coefficient of the filter cake play an important role in maintaining the stability of the excavation face. Too large or too small slurry pressure will lead to forward or backward displacement of the excavation face, which is not conducive to the stability of the excavation face and causes large surface displacement. The excess pore water pressure will occur during tunneling, and the excess pore water pressure will decrease with the distance away from the center of the excavation face. When the slurry pressure is small, the excess pore water pressure is larger. With the shield away, the excess pore water pressure gradually recovers. When the permeability coefficient of the filter cake is large, it will accelerate the seepage of pore water in the excavation face, which will increase the excess pore water pressure value and lead to the increase of surface settlement.

One of the challenges in upstream tailings dam projects is to ensure the allowable rate of deposition of tailings in the pond (i.e., pond filling rate) while maintaining the stability of the dam. This is due to the fact that an upstream tailings dam is constructed by placing dikes on top of previously deposited soft tailings, which could lead to a decrease in dam stability because of the build-up of excess pore water pressure. The main purpose of this work is to investigate the effects of pond filling rates on excess pore water pressure and the stability of an upstream tailings dam by a numerical study. A finite element software was used to simulate the time-dependent pond filling process and staged dam construction under various pond filling rates. As a result, excess pore water pressure increased in each raising phase and decreased in the subsequent consolidation phase. However, some of the excess pore water pressure remained after every consolidation phase (i.e., the build-up of excess pore water pressure), which could lead to a potentially critical situation in the stability of the dam. In addition, the remaining excess pore water pressure varied depending on the pond filling rates, being larger for high filling rates and smaller for low filling rates. It is believed that the approach used in this study could be a guide for dam owners to keep a sufficiently high pond filling rate but still ensure the desirable stability of an upstream tailings dam.

Stresses and pore water pressure induced by machine foundation on saturated sand

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.

This study investigates the results of experimental and numerical studies on effect of Variable Confining Pressure (VCP) on Excess Pore Water Pressure (EPWP) increase in granule-clay mixtures. EPWP plays an important role in the seismic stability of the dams constructed with a core of granule-clay mixture. These dams would experience VCP during an earthquake. This is unlike the previous studies on the material behaviour which only included Constant Confining Pressure (CCP). VCP experiments showed that EPWP increases in the granule-clay mixture compared with the pure clay which agrees with the CCP-based literature in this regard. By comparison of experiments and numerical models, it was shown that the pore pressure build-up between the grains in a granule-clay mixture can be numerically modelled by just modelling two adjacent grains. Numerical models showed that EPWP decreases with an increase in granule size, and increases with an increase in granule content. However, the amount of EPWP increase in a mixture compared to the pure clay is higher in CCP loading, compared with the VCP loading. It means the rate of EPWP build-up in the mixed and pure clay get closer in a VCP loading, compared to the CCP loading.

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.

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 changing rule of excess pore water pressure during dynamic compaction is important for construction. The following conclusions could be drawn by observing and analyzing the excess pore water pressure in test area. Under ramming strike energy of 2000kN•m, the best hit number during the 1~2 times is 8~9, the best hit number during the third time is 6~8. After compaction, the shallow excess pore water pressure is basically larger than the deep. The shallow excess pore water pressure needs more time to dissipate. The horizontal distance of the shallow excess pore water pressure is larger than that of the deep. During the dissipating process of the pore water pressure, we should pay attention to avoid vibration interference of load in the surface and prevent liquefaction of the powder soil. Under ramming strike energy of 2000kN•m, the biggest influence depth of dynamic compaction is 8～9m, the effective reinforcement depth is 6～8m. The conclusions can provide the theoretical basis for dynamic compaction construction.

Ground improvement using jet grout columns is a well-known technique to mitigate liquefaction hazard in sand stratum under existing building. However, the performance of conventional jet grout reinforcement technique has not achieved the sufficient level yet in terms of reducing shear strains and excess pore water pressure generated within the liquefiable soil layer. Therefore, a new countermeasure method, using small diameter jet grout column with additional horizontal slab, is introduced to control the shear deformation and excess pore pressure more effectively. To determine the efficiency of the new countermeasure method, numerical studies on unimproved and improved ground were separately performed in this study. The effectiveness of jet grout column with horizontal slab was evaluated by comparing the changes in excess pore water pressure, acceleration as well as distribution of shear stress and shear strain in the liquefiable soil before and after improvement. The results showed that the new liquefaction mitigation method offers positive effect on control of excess pore water pressure and shear deformation.

The small hydrate concept (SHC) was proposed to predict the generated excess pore water pressure due to dissociation of gas hydrate within sandy soils. This concept was then used to calculate excess pore water pressure in the Storegga slide of which gas hydrate dissociation may have contributed. Since the excess pore water pressure calculated by the SHC resulted in the considerable loss of stiffness and strength in the gas hydrate zone, the submarine slope failure that occurred at the Storegga slide was successfully simulated. Key words: Gas hydrate, dissociation, submarine slope, excess pore water pressure.