Initial Horizontal Stress Research Articles

Numerical study of hydraulic fracturing in the sectorial well-factory considering well interference and stress shadowing

In the Changqing Oilfield in northwest China, when traditional petroleum exploitation encounters forestry reserves or water source protection areas, sectorial well-factory design is proposed. The most distinct feature of a sectorial well-factory is the deviation of the well from the minimum horizontal principal stress, resulting in hydraulic fracture deflection after the initiation, along with possible well interference (i.e., fracture hit) and fracture coalescence in the oblique wells. Four indexes describing well deflection are then proposed according to fracture morphology. Several fracturing designs, including stage arrangement, fracturing sequences, and fracturing techniques are applied to study the feasibility of the sectorial well-factory design. The results show that the “gradual” or “sparse” stage arrangement, large injection rate, and simultaneous multifracture treatment can help to optimize the fracture morphology and stimulation design. However, the subsequent stress shadowing effect usually adversely affects the fracturing of adjacent wells. With a small initial horizontal stress difference, large injection rate and staggered stage arrangement can achieve ideal stimulation performance. Our results can provide a guidance for optimizing stimulation design in unconventional well-factory while taking into account environmental protection.

Seismic deformation analysis of earth dams subject to liquefaction using UBCSAND2 model

The prediction of expected seismic deformation subjected to liquefaction plays a crucial role in the design stage of earth dams located in regions of moderate to high seismicity. This technical note is a short communication following the effective stress model called UBCSAND2, which is used to assess the liquefaction-induced damages occurring at a well-documented case study of earth dams: Lower San Fernando Dam (LSFD) near the epicenter of the 1971 San Fernando earthquake. The model itself can predict the effect of the initial horizontal stress ratio K0 (= σ′x/σ′y) of soil elements beneath the slopes under direct simple shear conditions. This is usually not included in predicting liquefaction when using alternative practical constitutive models. Before the numerical modeling of the case study, the model was calibrated with typically drained and undrained behaviors of single elements under monotonic and cyclic Direct Simple Shear (DSS) loading using generic input parameters for different relative densities. It was then calibrated by empirical data from liquefaction-triggering weighting curves. The numerical results indicate that the shear band development of the failure on the upstream side was consistent with the observed post-liquefaction slide analysis.

Reflection and transmission of plane waves in stressed media with an imperfectly bonded interface

SUMMARY The reflection and transmission of elastic waves are of great significance for predicting reservoir physical properties, interpreting seismic data and detecting crustal structures. Most studies only consider the initial vertical stress state when studying the reflection and transmission of elastic waves at imperfectly bonded interfaces, but few studies consider the influence of initial stress on boundary conditions. Moreover, the effect of initial stress on the energy distribution of elastic waves at imperfectly bonded interfaces has rarely been investigated. We propose a unified method to calculate the energy reflection and transmission coefficients for different incident waves at welded or imperfectly bonded interfaces in stressed media. The effects of initial stress on the equation of motion, the elastic properties of the medium and the boundary conditions at the interface are considered. The elastic properties of rocks under initial stress are described by acoustoelasticity theory. In addition, we define a new stress tensor to modify the linear-slip model for describing boundary conditions at the imperfectly bonded interface in the presence of initial stress. Numerical results show that the energy reflection and transmission coefficients at the non-welded interface in stressed media depend on the elastic properties of the incident and transmitted media, the initial stress, the type and magnitude of the interfacial compliance and the frequency and propagation direction of the incident wave. The initial vertical and horizontal stresses dominate the reflection and transmission coefficients at small and large angles, respectively. The discontinuity in displacement across the imperfectly bonded boundary results in the frequency dependence of the reflection and transmission coefficients. Imperfect bonding enhances P-wave and SV-wave energy reflection and weakens P-wave energy transmission. However, imperfect bonding can enhance the energy transmission of the SV wave for the imperfectly bonded interface with high contrast between tangential and normal compliance and a resonance peak appears at a specific frequency. We also notice that imperfectly bonded interfaces with interfacial compliance less than $1.0 \times {10}^{ - 11}$ m Pa−1 can be regarded as welded interfaces in the seismic frequency band (lower than 100 Hz). In addition, the initial stress greatly influences the reflection coefficients at high frequencies and the transmission coefficients at low frequencies. The initial vertical stress can reduce the energy transmission of SV waves at imperfectly bonded interfaces. In contrast, the initial horizontal stress can significantly increase the energy transmission of low-frequency SV waves and may lead to the disappearance of the resonant peak in the transmission coefficients.

Experimental study on the effective stress law and permeability of damaged sandstone under true triaxial stress

The effective stress law and the permeability play a notable role in dealing with the gas-solid coupling problem of porous media . The key parameter to quantify the influence of pore pressure to effective stress is the effective stress coefficient α ij . However, current relevant researches are mainly focused on the pore elastic flow characteristics in the elastic stage or hydrostatic pressure under conventional triaxial stress state, which are difficult to reflect the actual formation stress condition. To this end, the present study was conducted to investigate the influence of induced damage on the effective stress coefficient and the permeability of sandstone under true triaxial stress. The results showed that the effective stress coefficient and the permeability were closely related to the damage evolution and fracture propagation . α ij exhibited obvious anisotropy, α 1 was larger than α 2 and α 3 , and α 3 > α 2 appeared in the horizontal direction. α ij increased with increasing the fracture density and opening, even greater than unity. Increasing the initial horizontal stress could reduce α ij under the damage state. The elastic modulus E ij also exerted anisotropy during loading, E 1 basically exhibited an increasing trend followed by a decrease as the axial strain increased, E 2 and E 3 were continue to degrade with increasing the axial strain. Loading the deviatoric stress could cause damage, which induced the initiation and propagation of microcracks , and in turn led to the nonlinear stress-strain relationship and volume expansion of the rock, so the permeability increased, and it had increased sharply before reaching the peak strength. Under the influence of σ 2 , α ij first increased and then decreased with increasing the intermediate principal stress coefficient b under the damage stage. The porosity effect also affected the relationship between α ij and the permeability related to the induced damage, and α ij revealed an increasing trend as the permeability increased. • Effective stress coefficient a ij increases with fracture growth, even beyond unity. • a ij and E ij exhibit anisotropy with loading. a 1 is the largest, and a 3 > a 2 . • Increasing the horizontal stress can reduce a ij related to the induced damage. • a ij first decreases and then increases with increasing b under the damage state. • a ij increases with the increase in permeability attributed to the induced damage.

Geomechanical Response Induced by Multiphase (Gas/Water) Flow in the Mallik Hydrate Reservoir of Canada

Summary Natural gas hydrate (NGH) is regarded as an important alternative future energy resource. In recent years, a few short-term production tests have been successfully conducted with both permafrost and marine sediments. However, long-term hydrate production performance and the potential geomechanical problems are not very clear. According to the available geological data at the Mallik site, a more realistic hydrate reservoir model that considers the heterogeneity of porosity, permeability, and hydrate saturation was developed and validated by reproducing the field depressurization test. The coupled multiphase and heat flow and geomechanical response induced by depressurization were fully investigated for long-term gas production from the validated hydrate reservoir model. The results indicate that long-term gas production through depressurization from a vertically heterogeneous hydrate reservoir is technically feasible, but the production efficiency is generally modest, with the low average gas production rate of 4.93 × 103 ST m3/d (ST represents the standard conditions) over a 1-year period. The hydrate dissociation region is significantly affected by the reservoir heterogeneity and reveals a heterogeneous dissociation front in the reservoir. The depressurization production results in significant increase of shear stress and vertical compaction in the hydrate reservoir. The response of shear stress indicates that the potential region of sand migration is mainly in the sand-dominant layer during gas production from the hydraulically heterogeneous hydrate reservoir (e.g., sand layers interbedded with clay layers). The maximum subsidence is approximately 78 mm and occurred at the 72nd day, whereas the final subsidence is slowly dropped to 63 mm after 1-year of depressurization production. The vertical subsidence is greatly dependent on the elastic properties and the permeability anisotropy. In particular, the maximum subsidence increased by approximately 81% when the ratio of permeability anisotropy was set at 5:1. Furthermore, the potential shear failure in the hydrate reservoir is strongly correlated to the in-situ stress state. For the normal fault stress regime, the greater the initial horizontal stress is, the less likely the hydrate reservoir is to undergo shear failure during depressurization production.

Geomechanics in Depleted Faulted Reservoirs

This paper examines the impact of the effective stresses that develop during depletion of a faulted reservoir. The study is based on finite element modeling using 2D plane strain deformation analysis with pore pressure and elastoplastic deformation of the reservoir and sealing shale layers governed by the Drucker–Prager plasticity model. The mechanical properties and response of the rock formations were derived from triaxial test data for the sandstone reservoirs and correlation functions for the shale layers. A normal fault model and a reverse fault model were built using seismic data and interpretation of field data. The estimated tectonic in-situ stress field was transformed to the plane of the modeled geometry. Sensitivity studies were performed for uncertainties on the values of the initial horizontal stress and for the friction of the fault surfaces. It was found that the stress path during depletion is mainly controlled by the initial lateral stress ratio (LSR). The developed effective stresses with depletion are influenced by the fault geometry of the compartmentalized blocks. Plastic deformation develops for low LSR whereas for high values the system tends to remain in the elastic region. When plastic deformation takes place, it affects mainly the region near the fault. The reservoir deformation is dominated by vertical displacement which is higher near the fault region and nearly uniform in the remote area. The volumetric strain is dominated by compaction. More volatile conditions in relation to change of the friction coefficient and LSR were found for the normal fault geometry.

Deformation and mechanical characteristics of tunneling in squeezing ground: A case study of the west section of the Tawarazaka Tunnel in Japan

Ground squeezing induced by tunnel excavation is a common phenomenon in weak and jointed rock masses. The prediction of convergence during tunnel construction in squeezing ground is important for the selection of temporary support and excavation methods. In the present study, the main factors that lead to ground squeezing are discussed based on a case study of the west section (WS) of the Tawarazaka Tunnel. A systematic analysis of convergences measured on site is presented, and numerical modeling using the finite difference method (FDM) is performed. The results show that ground squeezing is prone to occur in Paleogene mudstone formations, especially when slickensides are strongly developed. Ground squeezing is affected by the initial horizontal stress ratio, and a larger initial horizontal stress ratio lead to a larger convergence ratio, which is defined as the ratio of horizontal convergence to vertical convergence. Tunnel convergence values linearly increase with the decrement of bedding joint stiffness, and tunnel convergence increases with the increment of internal friction angle of bedding joints. Ground squeezing in the WS is mainly affected by the modulus values of nearby rocks and the existence of bedding joints. For the WS of the Tawarazaka Tunnel, the convergence ratio tends to stabilize when a critical overburden depth of 100 m is exceeded.

Influence of sidewall size on spalling in deep D-shaped tunnels: An experimental simulation

A series of true triaxial compression tests was performed herein on cubic red sandstone samples containing a D-shaped hole with three sidewall sizes to study the influence of sidewall size on spalling in D-shaped tunnels. The initial horizontal stress of the tests refers to the in-situ stress at 500 m depth. The hole axis was arranged along the direction of the maximum horizontal principal stress. The vertical stress was increased to simulate the increasing surrounding rock stress after tunnel excavation. During the test, a self-made monitoring device was used to monitor and record the failure process of the tunnel models in real time. Furthermore, the spalling process of the D-shaped tunnels with different sidewall sizes was reproduced indoors. The failure process and the stress and fracture characteristics of the tunnel models with three different sidewall sizes were then analyzed and summarized. The effect of the sidewall size on spalling in D-shaped tunnels will be discussed in this paper. The results show that the failure of the tunnel models with different sidewall sizes first occurred on the sidewall near the corner then gradually developed upward and axial along the sidewall with the increase of the vertical stress. The width and the depth of the failure zone gradually increased, and a V-shaped notch was finally formed on the sidewall. No failure occurred on the roof and the floor of the tunnel models. The dominant failure modes were tensile failure. The failure forms were mainly spalling occasionally accompanied by slab ejection. The slabs were shaped in thin plates. The vertical stress that caused the initial failure of the tunnel models gradually decreased with the increasing sidewall size. Furthermore, the ability of the surrounding rock to store elastic strain energy decreased. The tensile characteristics became more prominent, and the size of the plate-shaped slabs increased. In the same stress environment, the greater the sidewall size, the larger the failure width and depth on the sidewall, and the more visible the V-shaped failure zone. The stability of deep D-shaped tunnels can be improved, and spalling can be mitigated by reducing the sidewall size. However, the risk of a rockburst will increase. This finding has an important engineering significance for the optimization design of a tunneling cross-section and the prevention of spalling and rockburst in D-shaped tunnels.

Verification of Fracture Reorientation and Analysis of Influence Factors in Multiple Fracturing Treatment

A fracture will be initiated and propagated along the direction of maximum horizontal stress in fracturing treatment; however, in refracturing stimulation, the new fracture may be initiated and propagated along a different direction from the initial one. This is defined as a fracture reorientation. It is difficult to predict fracture reorientation due to the variation of formation properties after long-term production. To verify the existence of fracture reorientation and analyze its influencing factors in multiple fracturing treatment, experimental and numerical simulations are presented in this paper. Firstly, multiple fracturing stimulation is carried out with a self-assembled large true triaxial apparatus, and the fracture reorientation is successfully induced by changing the injection pressure and initial stresses in multiple fracturing processes. Then, numerical coupled hydromechanical modeling of the actual field production and injection well pattern is performed. In particular, the stress reversal region, which indicates the distance of fracture reorientation, and the factors that influence the reorientation are analyzed. The laboratory experiment and numerical simulation results show that the fracture reoriented angle obtained can be perpendicular to the initial fracture. Stress field and formation pressure are the two main factors that influence the fracture reorientation. With higher pressure differences and lower initial horizontal stress differences, the area in which it is possible to initiate reoriented fracture will be larger. The fractures of wells in the early production stage are hard to reorient due to the high formation and borehole pressure difference, and the fracture reorientation area will be expanded until the pressure difference is low to a certain value. This research result can guide oilfield stimulation treatments.

Influence of fracture nucleation and propagation rates on fracture geometry: insights from geomechanical modelling

We combine a power-law microfracture size distribution function with an expression for fracture propagation rate derived from subcritical fracture propagation theory and linear elastic fracture mechanics, to derive a geomechanically based deterministic model for the growth of a network of layer-bound fractures. This model also simulates fracture termination due to intersection with perpendicular fractures or stress-shadow interaction. We use this model to examine key controls on the emergent geometry of the fracture network. First, we examine the effect of fracture propagation rates. We show that at subcritical fracture propagation rates, the fracture nucleation rate increases with time; this generates a very dense network of very small fractures, similar to the deformation bands generated by compaction in unconsolidated sediments. By contrast, at critical propagation rates, the fracture nucleation rate decreases with time; this generates fewer but much larger fractures, similar to the brittle open fractures generated by tectonic deformation in lithified sediments. We then examine the controls on the rate of growth of the fracture network. A fracture set will start to grow when the stress acting on it reaches a threshold value, and it will continue to grow until all the fractures have stopped propagating and no new fractures can nucleate. The relative timing and rate of growth of the different fracture sets will control the anisotropy of the resulting fracture network: if the sets start to grow at the same time and rate, the result is a fully isotropic fracture network; if the primary fracture set stops growing before the secondary set starts growing, the result is a fully anisotropic fracture network; and if there is some overlap but the secondary set grows more slowly than the primary set, the result is a partially anisotropic fracture network. Although the applied horizontal strain rates are the key control on the relative growth rates of the two fracture sets, we show that the vertical effective stress, the initial horizontal stress, the elastic properties of the rock and the inelastic deformation processes, such as creep, grain sliding and pressure solution, all exert a control on the fracture growth rates, and that more isotropic fracture networks will tend to develop if the vertical effective stress is low or if the fractures are critically stressed prior to the onset of deformation. Thematic collection: This article is part of the Naturally Fractured Reservoirs collection available at: https://www.lyellcollection.org/cc/naturally-fractured-reservoirs

Three-dimensional stress-transfer mechanism and soil arching evolution induced by shield tunneling in sandy ground

Tunnel excavation inevitably causes variations in stress and displacement fields in soil, which could affect the serviceability and safety of the adjacent underground structures. Change of the earth pressure acting on the existing structures is closely related to soil arching effect caused by a new tunnel excavation, but it is still lack of full understanding. In this study, a finite element method (FEM) model was first verified by field measurements based on a case history of Metro Line 2 construction in Changsha, China. Then, a series of simplified three-dimensional (3D) FEM models were established to investigate the stress redistribution and the soil arching evolution induced by earth pressure balance shield (EPBS) tunneling. The changes of the earth pressure, the coefficient of lateral earth pressure, and the settlement of soil mass above the tunnel during tunneling were analyzed and the soil arching zone was determined. 3D stress-transfer mechanism in soil arching zone during tunneling was revealed. In addition, the influences of shield-driving parameters (i.e. support pressure, grouting pressure) on soil arching evolution were also investigated. The results show that the loosened zone extends to nearly 0.73D (D = tunnel diameter) above the tunnel crown in vertical direction. The height of the arch zone above the loosened zone is about 1.27D. The horizontal soil arching in front of EPBS occurs near the ground surface. With EPBS advancing, the horizontal soil arching gradually transforms into the vertical soil arching. After the installation of lining, the earth pressure is finally mainly transferred by vertical soil arching in the transverse section. The proper combination of shield-driving parameters for controlling the expansion of the loosened zone is support pressure of 0.6P1 − 2.2P1 and grouting pressure of 1.0P2 − 1.8P2, where P1 is initial horizontal stress at the tunnel axis and P2 = 1.2P1.

Effects of previously created fracture on the initiation and growth of subsequent fracture during TPMSF

Temporary plugging multi-cluster staged fracturing (TPMSF), using multi-cluster perforations and self-degradable diverting agents, is a novel technique to develop oil and gas reservoirs in deep wells. In this paper, the effects of previously created fractures on the initiation and growth of subsequent fracture were numerically investigated by utilizing XFEM-based CZM (i.e., cohesive zone model based on the extended finite element method). A 2D plane-plain model (coupling fluid flow and solid deformation) was established and truss model (truss elements distributed on the previous fracture surfaces) was used to maintain the different width of previously created fracture. Based on the model, multiple fracture simultaneous propagation was firstly simulated and then parametric studies were conducted for different fracture spacing, the propped width of the previous fracture, fracture created sequence, horizontal differential stress, Young’s modulus, and rock tensile strength. Numerical results show that multiple fractures cannot propagate uniformly because of fracture interaction even with equal flow rate distributions. During TPMSF, the propped width of the previous fracture, fracture spacing, and Young’s modulus primarily control the injection pressure of subsequent fractures; while rock tensile strength has a relatively weaker influence and horizontal stress contrast plays a negligible role. Moreover, the growth of subsequent fractures is dominated by fluid pressure, initial horizontal stresses and induced stresses.

In situ horizontal stress from CPT in sand: a new approach

Despite the various approaches in the literature adopted for analysing cone penetration test (CPT), determination of the initial horizontal stress and relative density of a sandy soil during this test has not been tackled yet. In order to propose a new method in this regard, a numerical study of CPT has been performed and the predicted results have been compared with several databases of comprehensive calibration chamber tests. The penetration mechanism has been then fully investigated by presenting different kinds of outputs for the surrounding soil. Finally, an innovative procedure has been suggested for estimating the initial horizontal stress and relative density during performing CPT in a sandy soil. The suggested method is based on recording the time of occurrence of a pre-specified displacement in the surrounding soil during the test. This recording can be carried out using high-precision digital inclinometer systems which are mounted in some boreholes in different directions and various distances around the location of CPT.

Cone penetration test in sand: A numerical-analytical approach

Separation of the effects of initial horizontal stress and relative density on cone tip resistance in sandy soils has been a complicated issue for many years. In order to overcome this problem, a numerical modeling of CPT which has been verified by calibration chamber tests, has been used in this paper to achieve a reliable analytical solution. The analytical solution has resulted in two relationships for sleeve friction and cone tip resistance in terms of the initial conditions of sandy soil. Based on the presented solution, the initial horizontal stress and relative density can be determined according to CPT measurements.

Finite-element analysis of a deep excavation case history

The design of deep excavations requires careful consideration of the influence of various soil/structure interaction mechanisms and detailed issues relating to the construction processes and the mechanics of the soil. Finite-element analysis provides a useful design tool for deep excavations, but care needs to be taken to ensure that an appropriate level of detail is included in the model. This paper describes a three-dimensional finite-element analysis of a deep excavation supported by a diaphragm wall, recently constructed in Shanghai. The principal purpose of the study is to investigate the level of detail that is required in the finite-element model to obtain results that provide a realistic representation of the wall and ground movements measured during the construction process. Studies are conducted on (a) the influence of the soil constitutive model on the quality of the results; (b) procedures to model the effect of post-cure shrinkage in the concrete floor slabs; (c) procedures to model the construction joints in the diaphragm wall; (d) the relative merits of using shell and solid elements to model the diaphragm wall; and (e) the sensitivity of the analysis to the assumed initial horizontal stresses in the soil.

A Study of Stress Change and Fault Slip in Producing Gas Reservoirs Overlain by Elastic and Viscoelastic Caprocks

Geomechanical simulations were conducted to study the effects of reservoir depletion on the stability of internal and boundary faults in gas reservoirs overlain by elastic and viscoelastic salt caprocks. The numerical models were of a disk-shaped gas reservoir with idealized geometry; they mimic the structure of a gas field in the northern Netherlands which has experienced induced seismicity during gas production. The geomechanical simulations identified the area of the internal fault most sensitive to fault reactivation as coinciding with the epicenters of the largest seismic events associated with gas production. Depletion-induced shear slip is initiated at the depth of the reservoir, in the fault areas where the vertical fault throw ranges between 0.5 and 1.5 times the reservoir thickness. The extent of reactivated area differs depending on whether the caprock is viscoelastic or elastic: when it is viscoelastic, there is more down-dip shear displacement. High initial horizontal stresses in the rock salt and lower stresses in the elastic side-seal and the reservoir promote unloading of the internal and reservoir-bounding faults even before the reservoir is depleted. Particularly prone to fault reactivation are the fault zones along the interface between the reservoir rock and the salt caprock, which may already be critically stressed before depletion and are likely to be reactivated early during gas production. Stress relaxation and associated geomechanical changes affecting fault stability and ground surface deformation may continue long after production has ceased, due to the viscous behavior of the salt.

A Numerical Investigation of the Progressive Failure of Jointed Rock Slope Subjected to Transient Seepage

Disastrous rock slope failures have been posing a hazard to people’s lives and causing enormous economic losses worldwide. Numerical simulation of rock slope failure can lead to improve the degree of understand of such phenomenon so as to predict and avoid the occurrence of these disastrous events. In order to simulate the global behaviors of rock slope failure under the high seepage pressure and the local behaviors of the occurrence of hydraulic fracture in the pre-existing rock joints effectively, a powerful finite element tools F-RFPA2D, is adopted. The simulation takes into account of the growth of existing fractures and the initiation of new fractures under various of hydraulic pressure in different heterogeneities medium. The behavior of fluid flow and damage evolution, and their coupling action are studied in small specimens that are subjected to both hydraulic and biaxial compressive loadings. The influence of the ratio (the initial horizontal stress to the initial vertical stress) and the distance between the two existing cracks on the fracture propagation behaviors are investigated. Moreover, based on the fundamental study of hydraulic fracture, the progressive failure of rock slope under the influence of the increase in hydraulic pressure was also studied in the paper.

Cone penetration tests under simulated field conditions

Cone penetration tests (CPT) performed in a conventional cavity-wall calibration chamber are inevitably affected by boundary conditions. The authors have developed an axisymmetric field simulator whereby CPT calibration tests can be conducted under substantially reduced boundary effects. The simulator consists of a cylindrical, physical sand specimen and a stack of expandable rings to simulate the soil from beyond the physical boundary. A series of CPT calibration tests has been performed in a clean, uniformly graded quartz sand using this simulator system. For a given relative density, the cone tip resistance (qc) was found to be most closely related to mean normal stress. The horizontal stress at the physical boundary increased during CPT under simulated field conditions. This increased horizontal stress had a much more significant and consistent effect on qc than the initial horizontal stress existing in the soil prior to CPT. The paper describes the results of CPT in the simulator, and discusses implications for the performance of CPT in a chamber and their interpretation.

Cone penetration tests under simulated field conditions

Cone penetration tests (CPT) performed in a conventional cavity-wall calibration chamber are inevitably affected by boundary conditions. The authors have developed an axisymmetric field simulator whereby CPT calibration tests can be conducted under substantially reduced boundary effects. The simulator consists of a cylindrical, physical sand specimen and a stack of expandable rings to simulate the soil from beyond the physical boundary. A series of CPT calibration tests has been performed in a clean, uniformly graded quartz sand using this simulator system. For a given relative density, the cone tip resistance (qc) was found to be most closely related to mean normal stress. The horizontal stress at the physical boundary increased during CPT under simulated field conditions. This increased horizontal stress had a much more significant and consistent effect on qc than the initial horizontal stress existing in the soil prior to CPT. The paper describes the results of CPT in the simulator, and discusses implications for the performance of CPT in a chamber and their interpretation.

Stress Diffusion Experiment in Sand

Plate bearing load tests in a large sand box with boundary lateral stress control showed, by measurements using buried stress cells, that the greater the initial horizontal stress the more rapidly the vertical loading from the plate diffuses or spreads horizontally with depth. The measurements also showed that with horizontal stress constant, an increasing plate load progressively focuses a greater percent of the load into the sand under and along the axis of the plate. These diffusion and focusing results have the same cause and can become linked. The paper compares the experiment results with two elastic half-space theories: simple Boussinesq, nonhomogeneous Boussinesq, and one particulate-probabilistic theory described in Harr. Only Harr predicted the first results qualitatively. None predict the second results. A preliminary study with the PLAXIS finite element program shows promise for predicting both results. The paper also discusses practical applications and notes two principles of load transmission in soils demonstrated in the experiment.