Effect of Field Drainage on Seismic Pore Pressure Buildup and Kσ under High Overburden Pressure

Four centrifuge experiments were conducted at Rensselaer Polytechnic Institute to explore the impact of stratum thickness on the behavior of silty sand soil under seismic conditions, specifically focusing on pore water pressure dynamics during and after shaking. This research also aimed to calculate the overburden pressure correction factor (Kσ) and to assess how variations in stratum thickness affect this parameter under field-simulated single drainage conditions. The experimental design includes tests on 2.5 and 5 m soil strata subjected to low (1 atm) and high (6 atm) overburden pressures. All tests maintained a consistent relative density of 45% and were conducted under a centrifugal acceleration of 45 g, with each test subjected to a 10-cycle sinusoidal wave base shaking. Instrumentation captured data that enabled detailed analysis of acceleration time histories, excess pore pressure ratio time histories, pore pressure profiles, pore pressure dissipation, settlement, shear stress ratios, and shear strains. Notably, despite the presence of a free boundary at the top of the silty sand layer, the maximum excess pore pressure ratio (ru) consistently occurred in the upper half of the stratum across all tests. The dissipation of excess pore pressure was observed to be faster in the 2.5 m tests as compared to the 5 m counterparts. A key finding from this study is the inverse relationship between stratum thickness and the field Kσ correction factor, with higher Kσ values observed for the 2.5 m stratum compared to the 5 m stratum. This outcome suggests that partial drainage may influence this factor, potentially elevating it to values above 1, contrary to the values typically predicted by current state of practice (SoP) that rely heavily on undrained triaxial tests. The SoP neglects the critical effects of drainage and stratum thickness. This study underscores the need for a reevaluation of current methodologies to incorporate these influential factors more accurately in the assessment of soil liquefaction potential under seismic loading.

This article presents numerical simulations investigating pore pressure buildup of a sand layer with a free drainage boundary at the top under both low and high overburden pressures and subjected to earthquake base excitation. The numerical runs simulate two centrifuge experiments previously conducted and reported. In these tests, a 5-m layer of clean Ottawa sand with relative density Dr=45% was tested under overburden pressures of ∼100 and ∼600 kPa (1 and 6 atm). The simulations were performed using Dmod2000, a nonlinear effective stress numerical one-dimensional (1D) site response analysis code. The tests revealed that the response was partially drained rather than undrained, with much more partial drainage at ∼600 kPa (6 atm) compared to ∼100 kPa (1 atm). The simulations correctly modeled this behavior, with very good agreement between simulated and measured centrifuge excess pore pressures. A key aspect of this good accord in the simulations was the correct selection in the simulations of the 1D drained volumetric stiffness of the sand, M′=1/mv, because the coefficient of consolidation, cv, is proportional to M′. Both cv and M′ were 2.5–3 times greater at ∼600 kPa (6 atm) than at ∼100 kPa (1 atm) in both centrifuge tests and simulations. Any future simulation of pore pressure response of sand under field drainage conditions needs to consider this large increase in volumetric stiffness at high overburden pressure. Good agreement was found between values of M′ back-calculated from the centrifuge tests and from a consolidometer test on a different sand reported in the literature. The value of M′ seems to increase approximately with the root square of the overburden pressure, and future simulations for high overburden and realistic field drainage conditions should account for this increase. The proper high-pressure correction factor, Kσ, to be used in conjunction with liquefaction charts may be higher than 1 for some realistic field drainage conditions due to this substantial decrease of sand compressibility under high overburden pressure.

This article is the first of two companion papers studying the effect of overburden pressure on the liquefaction behavior of saturated Ottawa sand. A series of four centrifuge tests were conducted simulating a 5-m layer of this sand having two different relative densities, and subjected to overburden effective pressures of ~100 and 600 kPa (1 and 6 atm). The objective was to study the pore pressure response of the soil to base acceleration under low and high pressure, including evaluation of the overburden pressure factor Kσ for idealized field drainage conditions. The sand layer had a bottom impervious and a top pervious boundary, approximating a common field situation. A novel experimental technique was developed using a dry lead shot layer to provide the necessary high level of pressure. The performances of the sand layer under low and high confining pressure were compared in terms of times histories and profiles of excess pore pressures, cyclic stress ratios (CSR), and cyclic shear strains γc, with some of the parameters determined using system identification. It was found that pore pressure dissipation started earlier at shallower depths, and that partial drainage was more significant in the 6-atm than in the 1-atm tests. Field overburden pressure correction factors at 6 atm, Kσ, obtained from the centrifuge tests for (ru)max=0.8 in 10 cycles of shaking and including the partial drainage effect, were found to be higher than 1.0 for both Dr=45% and 80% This is different from the usual laboratory undrained Kσ<1 based on cyclic triaxial and simple shear laboratory tests and reflected in the current state of practice. The discrepancy is related to the more significant effect of partial drainage and deviation from the undrained assumption at the higher confining pressure for the field drainage and other conditions of these centrifuge tests.

This article is the second of two companion papers studying the effect of a high overburden pressure on the liquefaction behavior of saturated Ottawa sand. A series of four centrifuge experiments were conducted simulating a 5-m prototype layer of this sand in the field, having two relative densities and subjected to overburden effective pressures, σv0′, of ~100 and 600 kPa (1 and 6 atm). The layer was on a rigid impervious base and could drain freely at the top. This was supplemented by undrained stress-controlled and strain-controlled cyclic triaxial tests on the same sand consolidated at 1 and 6 atm. The laboratory undrained overburden pressure factor at 6 atm obtained from the triaxial tests on loose sand, Kσ=0.85, is consistent with the state of practice (SoP), which assumes that Kσ<1.0 and Kσ decreases with σv0′. However, in the centrifuge experiments and σv0′=6 atm, the field Kσ=1.28 for loose sand and Kσ>1.15 for dense sand. The discrepancy is due to more significant partial drainage during shaking in the 6-atm centrifuge models. Although the excess pore pressures at the bottom of the sand layer seem to have been close to undrained in the four experiments, they were much smaller at shallower elevations in the 6-atm tests compared with the 1-atm tests. Further analysis is conducted by evaluation in the four centrifuge experiments of the coefficient of consolidation, cv, during the dissipation phase. This is done using the recorded pore pressure and settlement data. It is concluded that cv was two to four times greater during dissipation in the 6-atm centrifuge tests. The reason is the increase—also by a factor of two to three—of the drained constrained volumetric stiffness of the sand, M′=1/mv, when going from 1 to 6 atm. This finding plus other data from the literature suggest that for a range of sands, layer thicknesses, field conditions, earthquake shaking, and values of σv0′, both M′ and cv may increase proportionally to √σv0′, with the field Kσ>1.0, and with Kσ increasing instead of decreasing with σv0′.

The seismic stability of an embankment resting on saturated sand layers was studied by carrying out shaking table tests using large models. This paper is concerned with the failure mode of embankments during earthquakes ; the slope stability analysis method used here is based on the assumption of failure along a circular arc (Circular Arc Analysis). For each test a large model of an embankment on a saturated sand layer was placed in a shaking table box (8 m in length, 4 m in width and 2 m in height). The failure of the embankment was investigated by measuring the rapid rapid increase in residual settlement and lateral displacement in accordance with the increase in table acceleration. The pore pressure ratio was found to be greater than 0.5 when significant settlement and lateral displacement of the embankment took place, and the failure plane was determined by locating the breaks in the horizontal white sand layers (5 mm in thickness) in a cross-section of the embankment. The test results were in good agreement with the results calculated by a method of circular arc analysis which takes into account the effects of the seismic forces and the excess pore pressures. It was concluded that the stability of embankments on saturated sand layers during earthquakes can be evaluated by using this type of circular arc analysis method.

Study on seismic response of liquefiable soil-pile group considering pile cap and soil contact

Various surgical treatments have been proposed for the treatment of chronic subdural haematoma (CSDH). Herewith, we set out to compare the efficacy of an enlarged single burr hole versus double burr hole drainage for the treatment of CSDH. We studied patients with symptomatic CSDH proven by CT scan that were treated in our institute between January 2002 and January 2009. All patients were treated by an enlarged single or double burr hole drainage. A subdural drain was placed in all cases. A total of 245 patients were included in the study. Double hole drainage was performed in 156 (63.7%) patients (group A) and an enlarged single burr hole drainage in 89 (36.3%) patients (group B). There were nine recurrences in group A and five in group B; however, the difference was not statistically significant. There was no significant relationship between recurrence rate and age, gender, bilateral haematoma and antiplatelet or anticoagulant therapy. There was a trend towards higher risk of recurrence for patients with residual clots on postoperative CT scan. The mean hospitalization time was 6.2 days, and there was no significant difference between the two groups. No significant difference was found between patients' outcome, as assessed by Glasgow outcome scale score, and treatment method. Enlarged single burr hole and double burr hole drainage had the same efficacy in the treatment of CSDH.

A numerical approach for simulation of stress-controlled cyclic shear loading in a soil column considering partial drainage

The Palu 28 September 2021 M 7.5 Earthquake has brought several new challenges to the understanding of liquefaction and its following geotechnical phenomena. In addition, that main shock was followed by a series of aftershocks within a short time frame. The common geotechnical conditions of Palu area include layered soils conditions, and the associated variability of geotechnical conditions exists. This paper reports the dynamic effective stress analysis (ESA) study of four different liquefiable layered sand columns, and the above three conditions (layered soils, variability, aftershocks) are explicitly modeled. The dynamic ESA employs the PM4Sand constitutive model for liquefiable sands, implemented in the OpenSees platform. Three ground motion sets (“main shock only”, “main shock plus aftershock”, “aftershock” only) of variable amplitude, single frequency harmonic motions are used. The models are validated by comparing qualitatively their results against laboratory test results and field measurements. The saturated sand layers in all cases subjected to “main shock only” are liquefied with different detailed excess pore pressure (EPP) responses, highlighting the importance of the system response of liquefying sand columns. The cases subjected to “main shock plus aftershock” show a much a longer higher EPP state, while cases subjected to both “main shock plus aftershock” and “aftershock only” indicate a longer liquefaction state during the aftershock. The implication of the longer duration in the higher EPP state and the longer liquefaction state is that a longer duration of lower shear strength conditions would exist. The different EPP responses resulted from different geotechnical conditions represented by the four sand columns suggest that the variability of geotechnical conditions would have an important influence on the system response.

Drainage air-water capillary-pressure curves were obtained for Pittsburgh and Pocahontas coals at various overburden pressures. Capillary-pressure data were used to investigate pore-size characteristics. Results were indicative of the complex pore structure of coal, consisting primarily of a network of macro- and microfractures. In most cases, however, displacement pressure and residual water saturation increased at higher overburden pressure. Reasonable agreement between measured relative permeabilities and those calculated from capillary-pressure data with Purcell's model was obtained for only a few samples. Fracture permeabilities computed from pore-size distribution were lower than permeabilities pore-size distribution were lower than permeabilities actually measured at the same overburden pressure. Helium porosity was considerably higher than porosity determined by water saturation, indicating porosity determined by water saturation, indicating inaccessible pore volume to water. Pore compressibility was determined under triaxial stress-loading conditions. Changes in porosity with overburden pressure were more significant at pressures below 1,500 psig. Above this pressure, pore compressibility appeared to approach a pressure, pore compressibility appeared to approach a constant value averaging about 0.5 × 10(−4) psi(−1) for the coal samples studied. Introduction Increased interest in underground coal gasification and coal-seam degasification for the purpose of producing clean energy stimulated fundamental producing clean energy stimulated fundamental research into the phenomena of multiphase fluid flow through coal. Two previous papers presented results of investigation of the air- and water-permeability and relative-permeability characteristics at various overburden pressures for two different types of coal. However, to understand the mechanisms of two-phase flow (usually gas and water) through a complex porous system such as coal, one needs a clear insight into the internal pore structure of coal and the interaction between pore structure of coal and the interaction between this structure and the associated fluids. Such knowledge of the make-up of the pore structure helps in modeling fluid flow through the system and in interpreting permeability and relative-permeability data. Interaction between the pore structure and fluids results in the capillary-pressure phenomena. Capillary-pressure data have been used extensively to determine the pore characteristics of many petroleum reservoir rocks and to relate these petroleum reservoir rocks and to relate these characteristics to the single- and two-phase flow behavior in the rock. It is also known that natural fracture systems are the principal source of flow capacity of many petroleum reservoir rocks and contribute materially petroleum reservoir rocks and contribute materially to the storage capacity of some. Changes in fracture capacity resulting from changes in net overburden pressure have an important influence on the flow pressure have an important influence on the flow properties of the rock, as reported by Jones. In our properties of the rock, as reported by Jones. In our previous work with coal, which is a naturally previous work with coal, which is a naturally fractured system, absolute and effective permeabilities were found to be highly sensitive to overburden pressure (pov). Thus, it would be expected that the pressure (pov). Thus, it would be expected that the effect of Pov on the fracture flow capacity, capillary pressure, and pore compressibility is more dramatic pressure, and pore compressibility is more dramatic for coal. The internal structure of coal has been studied by microscopic methods, gas sorption measurements, and by mercury porosimetry. Data on helium porosity of different types of coal also can be porosity of different types of coal also can be found in Ref. 5. However, we are not aware of any determinations of capillary pressure in coal at different overburden pressures. In this paper gas-liquid capillary-pressure relationships for coal at different overburden pressures are presented. pressures are presented. SPEJ P. 261

Post-cyclic loading settlement of saturated clean sand

Effect of shaking strength on the seismic response of liquefiable level ground

Chronic subdural haematoma (CSDH) is defined as the haematoma in the subdural space which tend to occur in the elderly several weeks after head injury. The incidence of CSDH varied from 1.72 per 100,000 inhabitants per year in Finland to 13.1 per 100,000 inhabitants per year in Japan with a peak incidence in the sixth and seventh decade of life. CSDH is a common treatable cause of dementia. The principal techniques used in the treatment of CSDHs are presently burr hole, twist drill craniostomy, craniectomy and craniotomy. The aim of this study was to assess clinical outcome in unilateral chronic subdural haematoma psatients treated by single or double burr-hole drainage. This clinical trial was carried out at the department of neurosurgery, BSMMU from June 2010 to November 2011. A total of 40 consecutive patients with their age ranged from 50 to 70 years with GCS 9 to 13 &amp; haematoma volume greater than 30cc were included in this study and randomly divided into two groups. In group A, patients with chronic subdural haematoma (CSDH) were managed with double burr-hole drainage. In group B, patients were managed with single burr-hole drainage. Clinical outcome was measured on the 1st post operative day, 3rd post operative day and at the time of discharge (usually on the 7th post operative day) and at 1 month follow-up by measuring Glassgow coma scale (GCS), improvement of limb weakness and Markwalder grading scale. In this study double burr-hole drainage and single burr-hole drainage surgery shows equal success in the management of CSDHs. DOI: http://dx.doi.org/10.3329/bmj.v43i1.21370 Bangladesh Med J. 2014 January; 43 (1): 13-16

An analytical approach has been developed to predict permanent settlements of foundations subjected to earthquake shaking. The validity of this approach is evaluated through comparison with results from centrifuge tests simulating the response of a tank foundation resting upon saturated sand and subjected to earthquake‐like shaking. Analyses are carried out for different values of soil permeability to back‐calculate the in situ permeability of the sand and demonstrate the effect of partial drainage. Good correlation exists between predicted and measured response for in situ permeability within the range of laboratory measured values. The results also demonstrate that partial drainage may occur even during high‐frequency dynamic loadings, with significant effect on settlements and excess pore pressures.

Vibro-driving is a promising technique for installing monopiles for offshore wind turbines (OWTs). It offers a faster and quieter alternative to impact hammering, eliminating costly noise mitigation and reducing pile run risk in low resistance soils, enhancing safety and cutting costs and environmental impact. Despite its growing adoption, uncertainties remain regarding the influence of soil and vibro-driving parameters on pile penetration rates and the evolving soil state during installation in saturated sand. This study systematically investigates key factors, including soil permeability, dynamic force magnitude, vibration frequency, and varying hook load, on pile displacement amplitudes, penetration rates, and the underlying changes in soil state using large deformation numerical modeling. Results indicate that lower sand permeability generally leads to higher penetration rates but the response is more complex than drainage analysis based on well-known expressions can capture. Reducing dynamic forces (while keeping the frequency and static force unchanged) decreases pile penetration rates and excess pore pressures. In contrast, reducing the frequency while maintaining the dynamic force ratio results in significantly larger penetration rates due to greater excess pore pressures and consequently reduced vertical effective stresses. • Large deformation FE modelling used to study vibro-driving of open-ended piles in saturated sand. • In clean sand (∼10 −3 m/s), penetration rates are similar to fully drained response. • Lower permeabilities generally increase penetration rates, but trends are not monotonic due to complex pore pressure generation and drainage effects. • Reducing dynamic force while remaining the vibration frequency lowers penetration rates, increases drainage, and limits excess pore pressure buildup. • Lower frequency at a constant dynamic force increases penetration rates, driven by higher pore pressures and reduced effective stresses.