Effects of the tip depth of a pre-existing fracture on surface fault ruptures in cemented clay

Ground deformation due to faulting can cause serious damage to buildings and structures. Much attention has been devoted to understanding fault rupture propagation in uncemented soil. However, the effects of a pre-existing fracture in cemented soil on surface fault ruptures are not fully understood. This paper describes a numerical parametric study to investigate the mechanism of normal fault rupture propagation through cemented clay. Special attention was paid to the effect of the location (or tip depth) of a pre-existing fracture on the mechanism. The numerical model and model parameters adopted were verified through two centrifuge model tests. The results show that a zone of influence consisting of a tensile failure zone and a differential settlement zone was induced by normal faulting in cemented clay both with and without a pre-existing fracture. The width of this zone of influence increased with the tip depth of the pre-existing fracture. The effects of the tip depth of a pre-existing fracture on the width of the zone of influence were more significant when the tip was located at a shallow depth.     

Numerical simulation of cone penetration testing using a new critical state constitutive model for sand

A new perspective on the numerical simulation of cone penetration in sand is presented, based on an enhanced critical state model implemented in an explicit-integration finite element code. Its main advantage, compared to similar studies employing simpler soil models, is that sand compressibility can be described with a single set of model parameters, irrespective of the stress level and the sand relative density. Calibration is based on back-analysis of published centrifuge experiments, while results of the methodology are also compared against independent tests. Additional analyses are performed to investigate sand state effects on cone penetration resistance, in comparison with empirical expressions from the literature.     

水下爆破对大坝影响的离心模拟试验研究

由于离心机能够在原型应力条件下模拟爆破对大坝的影响。本文使用中国水利水电科学研究院研制的离心模拟爆破系统,在不同重力加速度之下研究了在不同水深,距坝面不同距离的情况下雷管爆破对于大坝的影响。     

Centrifuge modelling of plutons intruding shear zones: application to the Fürstenstein Intrusive Complex (Bavarian Forest,Germany)

Models consisting of a thick overburden resting on a buoyant layer were sheared and centrifruged in order to study the relationship between strike-slip shear zones and intrusions of buoyant material. Three experiments were carried out: In model 1, where the overburden consisted of a viscous material, no diapirs formed even after shearing for 40 mm (? = -1.07) and 27 min centrifuging. In models 2 and 3, where the overburden was semi-brittle, prescribed cuts at two different orientations (model 2: parallel to s1; model 3: perpendicular to s1) were initiated in the overburden in order to see whether such cuts acted as pathways for intrusion. In model 2 the prescribed cuts were used by the buoyant material as pathways when the cuts opened during shearing. Continued shearing widened the cuts and allowed the buoyant material to extrude on the surface of the model forming a coalesced elliptical sheet. In model 3. the cuts were closed during shearing and prevented the intrusion of the buoyant material. During further shearing, the cuts rotated and activated as strike-slip faults bounding pull-apart basins. Such pull-apart basins were not deep enough to tap the buoyant material. Nevertheless, the results of the experiments suggest that magma ascends in shear zones not as diapirs, but rises along preexisting pathways as dykes. Model results were used to evaluate emplacement of the Fürstenstein Intrusive Complex (FIC) in the Bavarian Forest, whose magnetic and structural inventory have been investigated in detail. The pluton consists of 5 magma batches, each with distinct magnetic fabrics. which are interpreted as the result of magma intrusion along opening and rotating tension gashes within the BPSZ stress field. Shear failure of the crust in the FIC area due to thermomechanical weakening provided the space for the emplacement of the last and biggest granite magma batch. Overall, the emplacement history of the FIC fits perfectly with the observations made during experiment 2 and indicates that magma ascent in shear zones is bound to tension gashes.     

Performance of a transparent Flexible Shear Beam container for geotechnical centrifuge modeling of dynamic problems

A transparent Flexible Shear Beam (FSB) container was designed and constructed to simulate the dynamic response of a stratum of soil under horizontal, one-dimensional (1-D) earthquake shaking in a geotechnical centrifuge. A stack of four rectangular, acrylic frames separated by layers of flexible, high-strength rubber was used to form the transparent container. The fundamental natural frequency of the container was estimated to be similar to a layer of sand in its softened or liquefied state. The suitability of the container in simulating 1-D site response with minimal boundary effects was evaluated by monitoring the uniformity of the induced accelerations and settlements across the soil specimen. Further, the measured lateral displacements were compared with equivalent-linear site response analyses. The new FSB container was found to provide satisfactory boundary conditions for studying complex Soil–Structure-Interaction problems, while simultaneously enabling researchers to visualize deformations of the soil and buried structures during shaking.     

A centrifuge study of the influence of site response,relative stiffness,and kinematic constraints on the seismic performance of buried reservoir structures

The seismic performance of underground reservoir structures depends on the properties of the structure, soil, and ground motion as well as the kinematic constraints imposed on the structure. A series of four centrifuge experiments were performed to evaluate the influence of site response, structural stiffness, base fixity, and excitation frequency on the performance of relatively stiff reservoir structures buried in dry, medium-dense sand. The magnitude of seismic thrust increased and the distribution of seismic earth pressures changed from approximately triangular to parabolic with increasing structural stiffness. Heavier and stiffer structures also experienced increased rocking and reduced flexural deflection. Fixing the base of the structure amplified the magnitude of acceleration, seismic earth pressure, and bending strain compared to tests where the structure was free to translate laterally, settle, or rotate atop a soil layer. The frequency content of transient tilt, acceleration, dynamic thrust, and bending strain measured on the structure was strongly influenced by that of the base motion and site response, but was unaffected by the fundamental frequency of the buried structure (f_structure). None of the available simplified procedures could capture the distribution and magnitude of seismic earth pressures experienced by this class of underground structures. The insight from this experimental study is aimed to help validate analytical and numerical methods used in the seismic design of reservoir structures.     

Upper bound solution of a laterally loaded rigid monopile in normally consolidated clay

A centrifuge test is performed to investigate the ultimate lateral capacity of a free-head rigid monopile foundation in normally consolidated (NC) clay. Based on the test result, an upper-bound velocity field is constructed to illustrate the evolution of the soil resisting mechanisms with the progressive rotation and the effect on reaction forces during loading. Simultaneously finite element analyses are also employed to model the centrifuge test and verify the conclusion of the upper bound analysis. Finally, an empirical expression is presented for predicting the upper-bound collapse loads of monopiles with different eccentricity/length ratios and length/diameter ratios in NC clay.     

Micro- and macro-observations of liquefaction of saturated sand around buried structures in centrifuge shaking table tests

A novel experimental method was designed to study the micro-behavior of saturated sand around a buried structure in centrifuge shaking table tests under strong simulated earthquake loading, in addition to the traditional macro-measurements. One free field test was first carried out as a reference, followed by one test with a deep buried structure and one with a shallowly buried structure. During the tests with the buried structure, high quality pictures of moving sand around the structure were recorded by a newly developed image acquisition system. By analyzing the interesting pictures at reasonable intervals using an image analysis software, the evolutions of microstructural features were obtained such as the orientations of the long axes of particles, the orientations of contact normals between particles and the average contact number of the interesting group of particles. The results showed that the evolutions of the micro-features were consistent with those of the macro-measurements such as excess pore pressures and accelerations, which help illuminate the mechanism of sand liquefaction.     

Evaluation of the Effect of Penetration Ratios on Composite Grounds Improved with Sand Compaction Piles

In this paper, centrifuge model tests were conducted in order to understand the deformation characteristics and behavior of sand compaction piles (SCPs) reinforced grounds in relation to area replacement ratios and penetration ratios. To simulate ground stress conditions, preliminary compaction was conducted to form grounds that maintained a certain level of strength. SCPs were installed in the grounds using compaction methods, and the relationship between loads and settlement as well as stress under rigid loading conditions were compared and analyzed. In addition, finite element analyses were conducted in order to verify the results of the centrifuge model tests and assess the effects of penetration ratios and depths on variations in stress. According to the results of the analyses, stress concentration ratios gradually decreased as depths increased, and the decreasing rate increased as penetration ratios decreased. However, in regions close to the surface layer in depth in which SCPs were installed, stress concentration ratios showed almost the same range regardless of penetration ratios. Stress concentration ratios showed proportional relations with penetration ratios. However, they showed similar values in regions close in depth to the surface of the ground. In particular, they showed very close ranges at penetration ratios of 100% and 80%.     

Centrifuge modelling of capillary rise

This paper reports results from centrifuge tests designed to investigate capillary rise in soils subjected to different gravitational fields. The experimental programme is part of the EU-funded NECER project (Network of European Centrifuges for Environmental Geotechnic Research), whose objective is to investigate the appropriateness of geotechnical centrifuge modelling for the investigation of geoenvironmental problems, particularly with reference to partially saturated soils. The tests were performed at the geotechnical centrifuge laboratories of Cardiff, Bochum, Manchester, and LCPC in Nantes. The aim was to determine the scaling laws of capillary rise under both equilibrium and transient conditions.

In all laboratories, column wetting tests in fine poorly graded sands (Congleton Sand, Bochum Normsand, HPF5 Sand, and Fontaineblau Sand) were performed. Capillary rise above the phreatic surface of the sand model was distinguished in a continuous capillary zone (completely saturated) and a discontinuous capillary zone (partially saturated).

The Cardiff Geotechnical Centrifuge Laboratory used matrix potential probes to follow the capillary rise of the continuous zone and, therefore, determine the suction above the phreatic zone during centrifuge testing. At Bochum, two cameras were used for optical and volumetric measurements, in order to follow the rise of the visible wetting front (upper limit of discontinuous zone) in the sand within the sample column. At Manchester, the movement of the wetting front was observed by video cameras over periods up to 8 h, whereas in LCPC pore pressure transducers recorded the changes in pressure caused by capillarity.

A simple centrifuge similitude law for capillary rise in these sands has been established and the kinetic phenomena have been measured as a function of the gravitational field. The results from these experiments verify that both the continuous and discontinuous capillary zones are scaled at a factor 1/N whereas the time for rise seems to be scaled at a factor 1/N². This research suggests that capillary phenomena can be modelled using a geotechnical centrifuge. Therefore, centrifuge testing can be a useful tool for future modelling of boundary value problems involving complex transport phenomena.