Mohamed A. Meguid

Investigation of Tunnel-Soil-Pile Interaction in Cohesive Soils

Underground tunnels are considered to be a vital infrastructure component in most cities around the world. Careful planning is always necessary to ensure minimum impact on nearby surface and subsurface structures. This study describes the experimental investigation carried out to examine the effect of existing piles installed in cohesive soil and extended to bedrock on the circumferential stresses developing in a newly constructed tunnel supported by a flexible lining system. A small scale testing facility was designed and built to simulate the process of tunnel excavation and lining installation in the close vicinity of preinstalled model piles. Lining stresses were measured for different separation distances between the lining and the existing piles Consistent decrease in the lining load was observed when the piles are located within a distance of one tunnel diameter from the tunnel. The results presented in this study indicated that measuring the lining response near existing pile foundations may be used to evaluate the extent of the interaction between the lining and the surrounding piles.

Discrete Element and Experimental Investigations of the Earth Pressure Distribution on Cylindrical Shafts

AbstractExperimental and numerical studies have been conducted to investigate the earth pressure distribution on cylindrical shafts in soft ground. A small-scale laboratory setup that involves a mechanically adjustable lining installed in granular material under an axisymmetric condition is first described. The earth pressure acting on the shaft and the surface displacements are measured for different induced wall movements. Numerical modeling is then performed using the discrete element method to allow for the simulation of a large soil displacement and particle rearrangement near the shaft wall. The experimental and numerical results are summarized and compared against previously published theoretical solutions. The shaft-soil interaction is discussed, and conclusions regarding soil failure and the earth pressure distributions in both the radial and circumferential directions are presented.

Algorithm to Generate a Discrete Element Specimen with Predefined Properties

The discrete element method is a powerful numerical tool in simulating the behavior of granular materials. It bridges the gap between continuum mechanics and physical modeling investigations. In spite of the significant achievements to date, some major problems are yet to be solved including the development of realistic large-scale models with initial conditions similar to those encountered in real problems. This paper introduces a computational method to generate a large-scale packing with predefined porosity and grain-size distribution in three-dimensional space based on a small initial sample packing. The developed method is implemented into an open- source computer code and used to generate specimens with known properties. The results showed that, under static condition, specimens generated using the proposed algorithm exhibited realistic behavior suitable for geotechnical applications. In addition, the controlled structure of the initial sample packing is successfully transferred to the final packing. DOI: 10.1061/ASCEGM.1943-5622.0000028 CE Database subject headings: Discrete elements; Geotechnical engineering; Grain size; Porosity; Algorithms; Granular materials. Author keywords: Discrete element method; Geotechnical engineering; Packing; Grain-size distribution; Porosity; Dynamic packing.

Three-Dimensional Analysis of Geogrid-Reinforced Soil Using a Finite-Discrete Element Framework

AbstractThree-dimensional analysis of soil-structure interaction problems considering the response at the particle scale level is a challenging numerical modeling problem. An efficient framework that takes advantage of both the finite- and discrete-element approaches to investigate soil-geogrid interactions is described in this paper. The method uses finite elements to model the structural components and discrete particles to model the surrounding soil to reflect the discontinuous nature of the granular material. The coupled framework is used in this study to investigate two geotechnical engineering problems, namely, strip footing over geogrid-reinforced sand and geogrid-reinforced fill over a strong formation containing void. The numerical model is first validated using experimental data and then used to provide new insights into the nature of the three-dimensional interaction between the soil and the geogrid layer.

On the role of sphericity of falling rock clusters—insights from experimental and numerical investigations

In this study, the dynamic behavior of rock clusters falling on rough slopes and impacting a vertical barrier is investigated experimentally and numerically using discrete element analysis. A specially designed laboratory setup that involves a flume of adjustable slope lined with a bumpy surface and equipped with an instrumented wall at the toe is used in the experimental investigation. The velocity profiles and impact forces were measured for three inclination angles using two different rock clusters. Three-dimensional discrete element analysis is then conducted to investigate the mechanical behavior of the rockfall and examine the role of sphericity of the rock cluster on the overall behavior of the system. This was achieved by explicitly simulating the complex shapes of the used rocks and the rough surface of the slope. The material coefficient of friction was measured using heap tests, and the results are compared with those obtained numerically using four different particle sphericities. Conclusions are made regarding the effect of slope inclination angle and the volume of the cluster on the impact forces exerted on rigid barriers. This study suggests that rock sphericity plays important roles on the dynamic behavior of the system and should be taken into consideration in simulating rockfall problems.

Experimental Study of the Earth Pressure Distribution on Cylindrical Shafts

This paper describes the results from an experimental program that has been conducted to investigate the distribution of earth pressure on a cylindrical wall embedded in granular material and subjected to radial displacement. The model shaft has been designed and built using mechanically adjustable segments to control the magnitude and uniformity of the wall movement during the tests. A series of experiments have been performed, and the progressive changes in earth pressure along the shaft have been continuously measured for different wall displacements. Results indicated a rapid decrease in lateral earth pressure when a small wall movement was introduced. When the wall movement reached about 2.5% of the shaft radius, the earth pressure distribution along the shaft became uniform and independent of any additional wall displacement. The experimental results are also compared with some of the available theoretical solutions, and the applicability of these solutions is then examined.

Application of a multilaminate model to simulate the undrained response of structured clay to shield tunnelling

A constitutive model based on the multilaminate framework has been implemented into a finite element pro- gram to investigate the effect of soil structure on the ground response to tunnelling. The model takes into account the elas- tic unloading-reloading, inherent and induced anisotropy, destructuration, and bonding effects. The model is successfully calibrated and used to investigate the undrained response of structured sensitive clay in the construction of the Gatineau tunnel in Gatineau, Quebec. Numerical results were compared to the field measurements taken during tunnel construction. To improve the performance of the numerical model, an implicit integration algorithm is implemented and proven to be very effective when coupled with the multilaminate framework as compared to the conventional explicit integration meth- ods. The effect of different soil parameters including bonding and anisotropy on the tunnelling induced displacements and lining stresses is also examined using a comprehensive parametric study. The results indicated that soil bonding and aniso- tropy have significant effects on the shape of the settlement trough as well as the magnitudes of surface displacements and lining stresses induced by tunnelling.

Evaluation of Soil–Pipe Interaction under Relative Axial Ground Movement

AbstractThe expansion of urban communities around the world resulted in the installation of utility pipes near existing natural or artificial slopes. These pipes can experience significant increase...

Modeling the Impact of a Falling Rock Cluster on Rigid Structures

Rockfall is a common geological hazard in mountainous areas and can pose great danger to people and properties. Understanding the impact forces induced by a single rock or a rock cluster on retaining structures is considered key in the analysis and design of protection barriers. This study presents the results of small-scale laboratory experiments conducted to measure the impact forces induced by a group of rocks moving down a rough slope on a barrier wall. The effect of slope inclination angle and wall location on the impact pressure acting on the wall was examined. A three-dimensional discrete element model was then proposed and used to study the behavior of the rock cluster under different geometric conditions. Rocks were modeled using polydisperse clumps in which each clump consisted of several overlapping spherical particles to account for the shape effect of the falling rocks. First, the model was validated by comparing the measured and calculated forces, and then, it was used to investigate the role of different material and geometric parameters on the impact behavior. Conclusions were made regarding the role of modeling the irregular rock shapes and the roughness of the slope surface on the behavior impacted by the travel mode for different slope angles. DOI: 10.1061/(ASCE)GM.1943-5622.0001045.

On the Effects of Subgrade Erosion on the Contact Pressure Distribution under Rigid Surface Structures

The performance of rigid surface structures such as concrete pavements and slabs-on-grade supported by a deteriorated subgrade and experiencing local contact loss is investigated experimentally and numerically in this study. A laboratory setup has been designed to facilitate the simulation of subsurface erosion and measure the changes in contact pressure at selected locations under a slab-on-grade supported on granular material. The presence of erosion voids under a slab-on-grade can lead to rapid increase in the contact pressure in the immediate vicinity of the void in addition to an increase in tensile stresses at the outermost fibers of the slab. This preliminary study suggests that efforts to detect and arrest the growth of erosion voids under slabs-on-grade should be made before the voids reach the size where significant loss of support develops and the tensile strength of the slab material is exceeded.