Rate-dependent mechanical behavior of jointed rock with an impersistent joint under different infill conditions

Transition in the rate-dependent mechanical response of rock was investigated due to the presence of impersistent joint with different infill conditions.Four types of samples,i.e.intact,jointed with no grouting,jointed and grouted with cement,and jointed and grouted with epoxy,were fabricated using model material.A series of dynamic split Hopkinson pressure bar(SHPB) tests was conducted on prepared samples with strain rates varying between 53-130 s^-1 along with static uniaxial compres...     

临近基坑开挖复合地基侧向力学性状离心试验研究

临近基坑开挖引起复合地基CFG桩变形和内力改变,现有理论缺乏对该问题的研究。通过开展离心模型试验,对临近基坑开挖条件下,复合地基变形、CFG桩内力和变形、土压力等分布和变化规律进行深入分析。结果表明：开挖引起CFG桩弯矩增大,近基坑桩增幅明显|开挖引起桩土不同步沉降,导致CFG桩上刺入褥垫层,桩受到褥垫层的“嵌固拉结”作用,同时远基坑桩、褥垫层、加载气囊一起提供了“摩擦拉结”作用,从而在近基坑桩上出现负弯矩|而远基坑处不均匀沉降小,“嵌固拉结”作用小,且“摩擦拉结”作用是利于正弯矩产生,桩上未出现负弯矩|开挖引起支护背后土压力分为增长区和减小区两部分,在上部土体中,土体卸荷,土压力减小,而下部土体受支护挤压,土压力有所增大|开挖引起地表沉降呈指数形式,临近基坑地表沉降最大,在显著变形区域内,支护水平位移基本呈直线形式,各阶段最大水平位移均出现在支护顶端|开挖引起的CFG桩水平变形大小和范围随距基坑边距离的增大而减小。     

A new estimation method and an anisotropy index for the deformation modulus of jointed rock masses

The deformation modulus of a rock mass is an important parameter to describe its mechanical behavior.In this study,an analytical method is developed to determine the deformation modulus of jointed rock masses,which considers the mechanical properties of intact rocks and joints based on the superposition principle.Due to incorporating the variations in the orientations and sizes of joint sets,the proposed method is applicable to the rock mass with persistent and parallel joints as well as that with nonpersistent and nonparallel joints.In addition,an anisotropy index AIdmfor the deformation modulus is defined to quantitatively describe the anisotropy of rock masses.The range of AIdmis from 0 to 1,and the more anisotropic the rock mass is,the larger the value of AIdmwill be.To evaluate the proposed method,20 groups of numerical experiments are conducted with the universal distinct element code(UDEC).For each experimental group,the deformation modulus in 24 directions are obtained by UDEC(numerical value)and the proposed method(predicted value),and then the mean error rates are calculated.Note that the mean error rate is the mean value of the error rates of the deformation modulus in 24 directions,where for each direction,the error rate is equal to the ratio of numerical value minus predicted value to the numerical value.The results show that(i)for different experimental groups,the mean error rates vary between 5.06%and 22.03%;(ii)the error rates for the discrete fracture networks(DFNs)with two sets of joints are at the same level as those with one set of joints;and(iii)therefore,the proposed method for estimating the deformation modulus of jointed rock masses is valid.     

Experimental and numerical analyses of an opening in a jointed rock mass under biaxial compression

Tunnel construction in a rock mass produces damage around the tunnel by concentration of in situ stress and by construction activity such as blasting. The generated damage changes the mechanical and hydraulic properties of the rock mass. In this study, the rock fracture and joint sliding behaviors of jointed rock masses with an opening under biaxial compression are investigated through experimental and numerical analyses. The tested rock models have a persistent joint set with dip angles of 30°, 45°, and 60° to the horizontal. Under the applied biaxial compression, tensile crack initiation and propagation are the dominant fracture behaviors around the hole in a low joint dip angle rock model (i.e., 30° to the horizontal). The propagation direction of the tensile cracks is roughly normal to the joint surface, and with propagation of tensile cracks, removable rock blocks are generated. The experimental results are simulated using a discrete element code. The numerical analysis simulates several aspects of rock mass cracking and the joint sliding processes around an opening: progressive fracture behaviors in a low joint angle rock model, abrupt initiation and propagation of tensile cracks and joint sliding in a high joint angle rock model (i.e., 60° to the horizontal), propagation of tensile cracks normal to the joint surface, generation of removable blocks in rock segments, an increase of lower hoop stress threshold inducing tensile fractures with a decrease in the joint angle, and an increase of the damage zone around the hole with a decrease in the joint angle.     

Use of the equivalent continuum approach to model the behavior of a rock mass containing an interlayer shear weakness zone in an underground cavern excavation

An interlayer shear weakness zone (ISWZ) is a weak zonal geotechnical system of variable thickness that occurs between different rock strata (e.g., tuff and basalt). At the site of the future Baihetan hydropower station, Sichuan Province, China, because of the relatively poor ISWZ mechanical properties, the overall stability of the underground powerhouse is potentially at risk. In this study, to evaluate the effects of ISWZs on the stability of the future underground powerhouse by means of three-dimensional continuum modeling (3-D continuum modeling), the concept of a virtual rock mass composed of ISWZ and host rock is proposed. An equivalent continuum approach, including a rock–soil composite material (RSCM) model, is elaborated, with corresponding expressions for the input parameters. Comparisons were made between the predictions from the RSCM model, the results obtained by an analytic method, and existing data from physical model tests. The comparison showed that all three types of information showed good consistency in terms of failure mode and strength. This indicates the suitability of the RSCM model for describing the behavior of a rock mass containing discontinuities. Furthermore, comparison between the predictions of the proposed equivalent continuum approach, the joint element approach, and the solid element approach for a deformation of a test tunnel section containing an ISWZ show that the results produced by the first two approaches are similar, but much smaller than that using the third approach. Further comparison of the actual state of the ISWZ-containing rock mass in the test tunnel section confirmed the applicability of the proposed equivalent continuum approach to prediction of deformation of the rock masses containing ISWZs at the future Baihetan underground powerhouse site.     

预应力锚杆柔性支护法力学行为影响的数值模拟研究

在岩质深基坑及超深基坑中,预应力锚杆柔性支护法具有造价低,工期短,施工简便,安全性高等诸多优点,经济效益和社会效益十分显著。结合大连胜利广场深基坑工程,采用 FLAC 软件进行数值模拟分析,重点研究了不同锚杆预应力对基坑力学行为的影响。结果表明：锚杆的预应力较小时,预应力越大,基坑侧壁的水平变形和地表的竖向沉降越小;预应力较大时,预应力的继续增加已不能进一步降低水平变形和减小竖向沉降;锚杆的预应力能有效减小岩土体内的最大剪切应变增量,在一定程度上延缓基坑剪切滑移破坏的发生;锚杆预应力存在一个合理的界限值。本文关于应力锚杆柔性支护法的设计方案及数值模拟研究结果对该技术的推广和应用提供了很好的借鉴作用。     

Mechanical behavior of a floating model pile in unsaturated expansive soil associated with water infiltration: Laboratory investigations and numerical simulations

Pile foundations are widely designed extending principles of saturated soil mechanics assuming drained conditions (i.e. effective stress) even in expansive soil deposits despite the fact the soil around the pile is in a state of unsaturated condition. The mechanical behavior of in-situ pile foundations are significantly influenced by suction changes associated with water infiltration. In this paper, a single model pile mechanical behavior variations associated with suction changes in an unsaturated expansive soil under a service load were investigated. Experimental results suggest pile settlement increases due to water infiltration. Such a phenomenon can be attributed to pile shaft friction reduction in the active zone associated with suction reduction due to water infiltration and volume expansion of the soil. Experimental results were also simulated and interpreted using a computation program, PEI (i.e. Pile behavior in Expansive soil upon Infiltration), which was proposed based on principles of unsaturated soil mechanics. A theoretical model was proposed for the estimation of threshold pile head load which distinguishes whether the pile would experience upward or downward movement upon water infiltration. The theoretical model and the numerical modeling approach presented in this are valuable for practicing engineers in the rational design of piles taking account of the mechanical behavior of expansive soils.     

Efficient 1D model for blind assessment of existing bridges: simulation of a full-scale loading test and comparison with higher order continuum models

Assessing the structural performance of existing concrete bridges is nowadays a major task. Nonlinear finite element (FE) analysis can quantify their capacity, evaluate strengthening interventions and prevent premature dismantle. However, this technique, mainly performed with 2D/3D FE, is seldom used at true scale due to the great complexity and computational costs involved. In this paper, the loading test of a strengthened concrete bridge in Sweden (Örnsköldsvik) is simulated using a 1D model. The bridge failed in combination of shear–bending–torsion triggered by fibre-reinforced polymer bond failure. Consecutive levels of refinement of the 1D model are presented and available results from higher order models are compared. The study of the structural response involved comparing displacements, strains, cracking patterns and failure mechanisms. The demonstrated robustness and efficiency of the proposed model makes it adequate for blind assessments of existing bridges.     

Numerical analyses of pillar behavior with variation in yield criterion,dilatancy, rock heterogeneity and length to width ratio

With recent advances in numerical modeling, design of underground structures increasingly relies on numerical modeling-based analysis approaches. While modeling tools like the discrete element method (DEM) and the combined finite-discrete element method (FDEM) are useful for investigating small-scale damage processes, continuum models remain the primary practical tool for most field-scale problems. The results obtained from such models are significantly dependent on the selection of an appropriate yield criterion and dilation angle. Towards improving its capabilities in handling mining-related problems, the authors have previously developed a new yield criterion (called progressive S-shaped criterion). The focus of the current study is to demonstrate its use in modeling rock pillars through a comparative analysis against four other yield criteria. In addition to the progressive S-shaped criterion, only one out of the four other criteria predicted a trend in strength consistent with an empirical pillar strength database compiled from the literature. Given the closely-knit relationship between yield criteria and dilation angle in controlling the overall damage process, a separate comparison was conducted using a mobilized dilation model, a zero degree dilation angle and a constant non-zero dilation angle. This study also investigates the impact of meso-scale heterogeneity in mechanical properties on the overall model response by assigning probability distributions to the input parameters. The comparisons revealed that an isotropic model using a combination of progressive S-shaped criterion and mobilized dilation angle model is sufficient in capturing the behaviors of rock pillars. Subsequently, the pillar model was used to assess the effect of L/W (length/width) ratio on the peak strength.