An investigation on the strain accumulation of the lightly EICP-cemented sands under cyclic traffic loads

Industrial production of chemical cement leads to extreme emissions of greenhouse gases.Biological or bioinspired sustainable materials for soil treatment projects can be employed instead of chemical cement to heal the carbon cycle in the ecosystem.The enzyme-induced calcite precipitation(EICP)method is one of the novel bio-inspired technologies that can be employed in soil treatment projects to increase desired properties of soils.While the monotonic and cyclic behavior of the enzymatically treated sands has been investigated comprehensively,the strain accumulation pattern in these improved soils under cyclic traffic loads has not been evaluated yet.In this paper,confined and unconfined cyclic compression tests are applied to the enzymatically lightly cemented sands,and the effects of the different parameters on their strain accumulation pattern are investigated for the first time in the literature.This study uses two types of specimens with unconfined compression strengths(UCS)equal to 42 kPa and 266 kPa.It is shown that the treated specimens have a rate-dependent behavior where cyclic loads with low frequencies lead to more resilient and plastic strains in the specimens.The results show that by approaching the maximum applied stresses to the UCS of the specimens(by breaking more calcite bonds between sand particles),the rate dependency behavior of specimens will reduce.Investigation of the effects of the cementation level demonstrated that by increasing the amount of the precipitated calcite from 0.38%to 0.83%,accumulated plastic strains are reduced almost 95%under the same loading condition.Effects of the initial static loads,confining pressures,the number of cycles,and amplitudes of the cyclic loads are also evaluated.

Numerical simulation study on particle breakage behavior of granular materials in confined compression tests

In order to study the fragmentation law,the confined compression experiment of granular assemblies has been conducted to explore the particle breakage characteristic by DEM approach in this work.It is shown that contact and contact force during the loading process gradually transform from anisotropy to isotropy.Meanwhile,two particle failure modes caused by different contact force states are analyzed,which are single-through-crack failure and multi-short-crack failure.Considering the vertical distribution of the number of cracks and the four characteristic stress distributions(the stress related to the maximum contact force,the major principal stress,the deviatoric stress and the mean stress),it is pointed out that the stress based on the maximum contact force and the major principal stress can reflect the distribution of cracks accurately.In addition,the size effect of particle crushing indicates that small size particles are prone to break.The lateral pressure coefficient of four size particles during the loading process is analyzed to explain the reason for the size effect of particle breakage.

Experimental Simulation and Numerical Modeling of Deformation and Damage Evolution of Pre-Holed Sandstones After Heat Treatment

The deformation and damage evolution of sandstone after heat treatment greatly influence the efficient and safe development of deep geothermal energy extraction.To investigate this issue,laboratory confined compression tests and numerical simulations were conducted on pre-holed sandstone specimens after heat treatment.The laboratory test results show that the failure modes are closely related to the heat treatment temperature,with increasing treatment temperature,the failure modes change from mixed and shear modes to a splitting mode.The cracks always initiate from the sidewalls of the hole and then propagate.The failure process inside the hole proceeds as follows:calm period,particle ejection period,rock fragment exfoliation period and rock failure period.The specimens have different final failure features for the entire rock after heat treatment,but have the same failure features inside the hole.These phenomena can be explained by numerical simulations.The numerical simulations reveal that the failure modes in the numerical results agree very well with those observed in the experimental results.The damage zone always occurs at sidewalls of the hole and then propagates to the entire rock affected by shear or tensile damage.From 20℃to 200℃,thermal effect may promote shear damage and restrain tensile damage,while from 200℃to 800℃,thermal effect promotes tensile damage and restrains shear damage.Notably,the damage zone near the sidewalls of the hole has the same distribution range and pattern.Finally,the differences in the mechanisms due to increasing heat temperature are evaluated using scanning electron microscope(SEM)observations.

Confined compressive strength model of rock for drilling optimization

The confined compressive strength(CCS)plays a vital role in drilling optimization.On the basis of Jizba's experimental results,a new CCS model considering the effects of the porosity and nonlinear characteristics with increasing confining pressure has been developed.Because the confining pressure plays a fundamental role in determining the CCS of bottom-hole rock and because the theory of Terzaghi's effective stress principle is founded upon soil mechanics,which is not suitable for calculating the confining pressure in rock mechanics,the double effective stress theory,which treats the porosity as a weighting factor of the formation pore pressure,is adopted in this study.The new CCS model combined with the mechanical specific energy equation is employed to optimize the drilling parameters in two practical wells located in Sichuan basin,China,and the calculated results show that they can be used to identify the inefficient drilling situations of underbalanced drilling(UBD)and overbalanced drilling(OBD).

A study of influence of gravity on bulk behaviour of particulate solid

This paper examines the influence of gravity on the bulk responses of a granular solid. The loading scenarios in this study include confined compression, rod penetration into a granular medium and discharging through an orifice. Similar loading and flow conditions are likely to be encountered in the stress and deformation regimes that regoliths are subjected to in extraterrestrial exploration activities including in situ resource utilisation processes. Both spherical and non-spherical particles were studied using the discrete element method （DEM）. Whilst DEM is increasingly used to model granular solids, careful validations of the simulation outcomes are rather rare. Thus in addition to exploring the effect of gravity, this paper also compares DEM simulations with experiments under terrestrial condition to verify whether DEM can produce satisfactory predictions. The terrestrial experiments were conducted with great care and simulated closely using DEM. The key mechanical and geometrical properties for the particles were measured in laboratory tests for use in the DEM simulations. A series of DEM computations were then performed under reduced gravity to simulate these experiments under extraterrestrial environment. It was found that gravity has no noticeable effect on the force transmission in the confined compression case; the loading gradient in the rod penetration is linearly proportional to the gravity; the mass flow rate in silo discharge is proportional to square root of the gravity and the angle of repose increases with reducing gravity. These findings are in agreement with expectation and existing scientific evidence.