Theoretical Equations for the Ratio of Undrained Shear Strength to Vertical Effective Stress for Normally Consolidated Saturated Cohesive Soils

Two theoretical equations are developed to calculate the ratio of undrained shear strength to the vertical effective stress (the ratio of (su/sv’)) for normally consolidated saturated cohesive soils. The effective stress approach is used as the basis in the development of the theoretical equations. The theoretical equations are developed by relating the total and the effective stress paths. The development of the excess pore-water pressure is quantified using Skempton A and B pore-water pressure parameters. The theoretical equations are developed for two initial stress conditions: (i) an initially hydrostatic condition and (ii) an initially Ko (non-hydrostatic) condition. The performance of the theoretical equations of this study is compared with field and laboratory measurement data obtained from the literature. The close results between the theoretical equations and the measurements show that the theoretical equations of this study can compute the ratio of (su/sv’) well. Using the theoretical equations, the values of the ratio of (su/sv’) commonly used in engineering practice can be explained from the soil mechanics framework. Keywords: Saturated cohesive soils, c/p ratio, normally consolidated soil, undrained shear strength, effective shear strength, theoretical equation. Abstrak Dua persamaan teoritis dikembangkan untuk menghitung rasio kuat geser tak teralirkan dengan tegangan efektif vertikal (rasio (su/sv’)) untuk tanah kohesif jenuh terkonsolidasi normal. Pendekatan tegangan efektif dijadikan dasar dalam pengembangan kedua persamaan teoretis ini. Persamaan teoretis tersebut dikembangkan menghubungkan lintasan tegangan total dan lintasan tegangan efektif. Kenaikan tekanan air pori ekses dikuantifikasi menggunakan parameter tekanan air pori A dan B dari Skempton. Persamaan teoretis dikembangkan untuk dua kondisi tegangan awal: (i) tegangan awal hidrostatik dan (ii) teganan awal Ko (non hidrostatik). Kinerja kedua persamaan teoretis tersebut dibandingkan terhadap data pengukuran lapangan dan pengujian laboratorium yang diperoleh dari literatur. Persamaan teoretis dari studi ini memiliki kinerja yang baik dalam memperhitungan rasio (su/sv’) yang ditunjukkan dengan dekatnya hasil perhitungan menggunakan persamaan teoretis dan hasil pengukuran lapangan maupun pengujan laboratorium. Dengan persamaan teoretis tersebut, nilai rasio (su/sv’) yang biasa digunakan dalam rekayasa praktis bisa dijelaskan secara mekanika tanah. Kata-kata Kunci: Tanah kohesif jenuh, rasio c/p, tanah terkonsolidasi normal, kuat geser tak teralirkan, kuat geser efektif, persamaan teoretis.

Empirical Correlation between Shear Wave Velocity (Vs) and Uncorrected Standard Penetration Resistance (SPT-N) for Dinajpur District, Bangladesh

In seismic analysis, shear wave velocity ("V" _"s" ) is a valuable parameter for measuring the site effect of earthquake microzonation. Various methods have been explored to measure this key factor directly. Since measuring shear wave velocity directly is time consuming and expensive, researchers in various regions have been attempting to update empirical relationships between shear wave velocity ("V" _"s" ) and other soil geotechnical properties such as SPT blow count, depth, vertical effective stress and so on. Geophysical tests associated with direct methods are not feasible in developing countries like Bangladesh, so the indirect method is more important. Due to the location of a fault line nearby, the Dinajpur district has previously undergone anomalous earthquakes. As a result, site characterization is important for enhancing seismic design considerations. Focused on an indirect approach, this paper mainly aims to propose a general correlation between shear wave velocity and standard penetration number in 13 upazilas of Dinajpur district for four soil categories (“all soils”, sand, clay, and silt). Finally, an approach was developed to find the overall correlations for the Dinajpur district by combining all of the data. The regression coefficient ("R" ^"2" ) values were observed to vary in between 0.04416-0.6134 for “all soils”, 0.0593-0.668 for sand, 0.5911-0.7149 for clay and 0.5547-0.6794 for silt. The correlations obtained in this study can be used for seismic hazard analysis in both the study region and other areas with identical soil strata.

Identification and Prediction of Allo-Source Overpressure Caused by Vertical Transfer: Example from an HTHP Gas Reservoir in the Ledong Slope in the Yinggehai Basin

The Yinggehai Basin is a typical high temperature and high pressure (HTHP) gas-bearing basin. The pressure coefficient exceeds 2.2 in deeply-buried Miocene reservoirs in the Ledong Slope, a nondiapir zone in the Yinggehai Basin. Determining the overpressure mechanisms and predicting the pore pressure are key issues for natural gas exploration and development in the Ledong Slope. In this paper, overpressure mechanisms were investigated according to the analysis of vertical effective stress-logging responses and geological evaluations, and the pore pressure was predicted using the Bowers method. The loading-unloading crossplots indicated that the overpressure that existed in reservoirs mainly consists of two types: neighbor-source and allo-source overpressure. The neighbor-source overpressure is mainly caused by the pressure transmission from the adjacent mudstone to the reservoir, with a pressure coefficient less than 1.5 ~ 1.6. The high-magnitude overpressure points with pressure coefficients greater than 1.6 show a typical unloading response, indicating elevated sandstone pressures rather than in situ mudstone pressures, which are most likely to be generated by overpressure vertical transfer. The high-magnitude overpressure fluid generated by the high mature ultradeep buried N1s source rock migrated to the shallower reservoirs via hidden faults/microfractures, which led to the vertical transfer of overpressure. Vertically transferred overpressure was generated at 1.5 ~0.2 Ma, which is beneficial for the preservation of overpressure in lenticular sandbodies. The estimated pore pressure by the Bowers method is in good agreement with the measured pressure and provides a meaningful reference for predrilling pressure prediction in nondiapir or diapir zones in the Yinggehai Basin.

Modified Fixed Wall Oedometer When Considering Stress Dependence of Elastic Wave Velocities

A modified oedometer cell for measuring the applied stresses and elastic waves at the top and bottom of the specimen is developed to evaluate the effect of the side friction on the stress dependence of the elastic wave velocities. In the modified cell, two load cells are installed at the top and bottom plates, respectively. To generate and detect the compressional and shear waves, a pair of piezo disk elements and a pair of bender elements are mounted at both the top and bottom plates. Experimental results show that the stresses measured at the bottom are smaller than those measured at the top during the loading and vice versa during unloading, regardless of the densities and heights of the specimens. Under nearly saturated conditions, the compressional wave velocities remain almost constant for the entire stress state. With plotting stresses measured at top, the shear wave velocities during unloading are greater than those during loading, whereas with plotting stresses measured at bottom, the shear wave velocities during unloading are smaller than those during loading owing to the side friction. The vertical effective stress may be simply determined from the average values of the stresses measured at the top and bottom of the specimens.

Chemical compaction of deep buried mudstone and its influence on pressure prediction

The chemical compaction of mudstones which is dominated by the transformation of clay minerals leads to significant changes in the mineral composition and microstructure of mudstone during process of deep burial. In particular, the transformation of smectite to illite in mudstones results in noticeable impact on the pore pressure formation and the overpressure logging responses. At present, the study about the pressurization mechanism of chemical compaction and the impact on overpressure logging responses is really weak, which made it hard to pore pressure identification and pressure prediction for deep buried formations. Taking the Paleogene Shahejie Formation in the Dongying depression of the Bohai Bay Basin in eastern China as typical case, this paper analyses the characteristics of clay mineral transformation of the Shahejie Formation in the Dongying depression, the logging responses of overpressures, and the influence of chemical compaction on the prediction of pore pressure. The results showed that the chemical compaction of mudstones changes the relationship between the petrophysical properties of mudstone and vertical effective stress and the logging responses of overpressure. The typical characteristic of chemical compaction manifested as density increase continuous with the depth. The normal compaction trends of the different compaction stages are the basis for overpressure mechanisms identification and pore pressure prediction. The depth of the rapid transformation of clay minerals has a good consistency with the top of overpressure zone (2000–2800 m) in Dongying depression, which indicates that the overpressure and its logging responses may be related to the chemical compaction of mudstones. The measured pressure in intervals deeper than 3000 m is closer to the predicted pressure based on the normal compaction trend of chemical compaction.

Cyclic thermomechanical response of fine-grained soil-concrete interface for energy piles applications

Understanding the behaviour of soil-structure interfaces is critical for addressing the analysis and design of energy geostructures. In this study, the interface failure mechanism of energy piles (where a shear band is detached from the surrounding soil that behaves under oedometric conditions) is experimentally analysed in laboratory for saturated conditions. The choice of material (clayey soil and concrete), temperature range, and stress level is based on conditions that are likely to be encountered in practice. Specifically, cyclic thermal tests under constant vertical effective stress in oedometric conditions as well as constant normal stiffness (CNS) interface direct shear tests (in which samples have been subjected to thermal cycles between 10 and 40 °C) are presented. From a practical perspective, the results show very low volumetric strain variations and negligible effects on shear strength. The volumetric aspects do not appear to have significant impact on the shear resistance of the interfaces against cyclic thermal loads. Fundamental insight on the effects of thermal cycles on the concrete-soil interface behaviour which are relevant to energy piles are presented. In addition, the proposed interpretation procedure provides a basis for the standardisation of thermomechanical testing in geotechnical engineering.

Effect of Cyclic Loading Frequency on Liquefaction Prediction of Sand

The frequency of ground motions during earthquakes is typically in the order of a few hertz. As the earthquake-induced liquefaction of soils is widely assessed by performing laboratory tests, it is necessary to consider various loading frequencies generated by real earthquakes. The effect of loading frequency has been studied by cyclic triaxial tests; however, it has rarely been investigated by cyclic direct simple shear tests, which are more similar to the cyclic loading conditions associated with earthquakes. In this study, a series of cyclic direct simple shear tests were performed on clean sand with a relative density (Dr) of 40% (loose sand) and 80% (dense sand), obtained from Nakdong River. The parameters considered are the initial vertical effective stresses (σv0′ = 50, 100, and 200 kPa) and the loading frequencies (f = 0.05, 0.1, 0.5, and 1 Hz) to evaluate the effect of the loading frequency on the liquefaction prediction of clean sand. The results showed that the liquefaction resistance of the sand increases with the increase in the loading frequency, regardless of the initial vertical effective stress and relative density. When the loading frequency increased from 0.1 to 0.5 or 1 Hz, the maximum increase in the cyclic resistances were 15%, and 19% for loose and dense sand, respectively. For a given loading frequency, the liquefaction resistance of the sand decreased when the initial vertical effective stress increased.

Three-Dimensional Wave-Induced Dynamic Response in Anisotropic Poroelastic Seabed

This paper presents a novel analytical solution, which is developed for investigating three-dimensional wave-induced seabed responses for anisotropic permeability. The analytical solution is based on the assumption of the poroelastic and the u − p dynamic form, which considers the inertia force of the soil skeleton. In this paper, the problem is regarded as an eigenvalue problem through a first-order ordinary differential equation in matrix form. The problematic eigenvector involved in the solution is dealt with using numerical computation, and a process is proposed to implement the present solution for the desired dynamic response. A verification, which is compared with two existing solutions, demonstrates an agreement with the present solution. The results show that the amplitude profile of seabed response for a shorter wave period varies significantly. A comparison between the anisotropic and transverse isotropic, as well as isotropic permeabilities reveals that the error of vertical effective stress on the seabed bottom can reach 74 . 8 % for the isotropic case. For anisotropic permeability, when the wave direction is parallel to the higher horizontal permeability direction, the amplitude profiles of pore pressure and vertical effective stress exhibit the greatest dissipation and increment, respectively. For transverse isotropic permeability, the vertical effective stress is independent of the wave direction, which results in the two horizontal effective stresses on the seabed bottom being identical to each other and independent of the wave direction. Our comprehensive analysis provides insight into the effect of anisotropic permeability on different wave periods and wave directions.

Non-isothermal volume change behavior of saturated sand subjected to minimal vertical effective stress

To date, the majority of the available studies on the volume change behavior of soils under non- isothermal conditions have focused on fine-grained soils. Only a limited number of investigations have been made available about coarse-grained soils despite their abundance in practice, with contradictory features. To enrich the current knowledge about the volume change behavior of coarse-grained soils under non-isothermal conditions, this investigation presents the results of oedometric tests with temperature control performed on saturated sand. The influence of relative density on the volume change behavior of the tested material under non-isothermal conditions is investigated under a minimal level of applied vertical effective stress. The results show an expansive volume change upon heating and contractive volume change upon cooling for all relative densities. The magnitude of the contraction is more significant than that of the expansion, leading to a residual contractive volume change after one cycle of heating and cooling. The results of this study enrich the current literature about the volume change behavior of coarse-grained soils under non-isothermal conditions. Such competence may be considered for applications at the interface of geomechanics and energy wherein temperature variations occur and characterize the response of coarse-grained soils.