Sherif Elfass

A Simple Bearing Capacity Equation

The bearing capacity of a shallow foundation is usually assessed by applying various correction factors to the classical three-term bearing capacity equation. Correction factors are typically included to account for foundation shape and embedment, load inclination and eccentricity, and the inclination of the foundation base. However, the bearing capacity depends on the evaluation of the bearing capacity factors N γ , N q and N c , which differ depending on the source consulted (e.g., Terzaghi, Meyerhof, Hansen or Vesic). Unfortunately, there have been very few full scale tests that have documented the accuracy of such equations causing considerable uncertainty in the mind of the practicing engineer. In addition, complicating factors lead to confusion regarding the meaning and application of the assessed bearing capacity. Such factors include the choice of total or effective stress strength parameters, the use of direct shear, triaxial or plane strain friction angle for cohesionless material, the effect of confining pressure variation in different zones of the failure mass on the friction angle, and the effect of apparent cohesion, particularly in the interpretation of plate load test results. The following paper provides discussion and illustration of these various matters, including the introduction of a simple bearing capacity equation that can also a yield load-settlement response.

Estimation of in-situ shear strength parameters for subgrade layer using non-destructive testing

The Falling Weight Deflectometer (FWD) test has been widely used for evaluating the structural condition and load-carrying capacity of asphalt pavement systems as a non-destructive testing device. Conventionally, the in situ stiffness properties of the various pavement layers are estimated from the analysis of the surface deflection measurements using a backcalculation procedure. In this pilot study, an innovative and novel approach was investigated for estimating the shear strength parameters (C and ϕ) of the subgrade layer by means of FWD testing. Such parameters become important and necessary when assessing the risk of instantaneous shear failure in asphalt pavement layers under non-standard heavy vehicles. In order to assess the applicability of proposed approach, numerically simulated FWD test as well as measured FWD field data conducted on the APT asphalt pavement sections at the National Airport Pavement Test Facility (NAPTF) were analyzed. The surface deflection measurements from the FWD testing at multiple load levels were used in conjunction with backcalculation process to capture the stress dependent behavior of unbound layers and subsequently used to determine the shear strength parameters of the subgrade layer. It was found that the proposed approach was capable of estimating the in situ shear strength parameters of the subgrade material and the results were consistent with those obtained from conventional laboratory testing. Based on the findings from this study, work is currently undergoing to extend and validate the proposed approach for different type of asphalt pavement structures and subgrade properties.

Large Scale Laboratory Testing of Low Mobility Compaction Grouts for Drilled Shaft Tips

While post grouting of drilled shaft tips is an acknowledged means of mobilizing shaft base resistance at low values of displace- ment, much uncertainty exists among engineering professionals as to the conditions under which this option may be entertained. The impact of diverse factors such as soil gradation and soil relative density, overburden pressures, and grouting methods utilized needs to be properly understood. This paper details the laboratory test set-up utilized by researchers toward studying the impact of such factors on post grouting and resultant shaft base capacity increase. The focus of this paper is on the different components of the test set-up considered during the design and implementation stage. Several unique aspects dealt with as a part of this study and elaborated on herein include the testing chamber dimensions, method of overburden pressure application, soil deposition technique, grouting methodology employed for the low mobility compaction grout utilized, load testing protocol, and instrumentation. Sample test results included indicate satisfactory load resistance increase through grouting with a low mobility compaction grout. The performance of this set-up during the testing phase indicated applicability for use in large scale laboratory testing of low mobility grouts and drilled shaft foundations.

Large-Scale Laboratory Study on the Innovative Use of Compaction Grout for Drilled Shaft Tip Post Grouting

This paper presents the results of a comparative analysis carried out on the behavior of two 32.4 cm (12.75 inch) diameter model shafts load tested at the Large Structures Laboratory of the University of Nevada, Reno. While one of these shafts was grouted with a low mobility compaction grout at its tip prior to vertical loading, the other was not. Both model shafts were tested within a soil tank containing the same non-cohesive soil prepared to a similar target relative density. Evaluation of the load — displacement response of both shafts undergoing similar load-unload-reload cycles showed the tip capacity at a top displacement of 5% of the shaft diameter for the grouted shaft to be eight times that of the ungrouted shaft. This increased end bearing was found to be due to two reasons: 1) The effect of preloading of the soil caused by the grout induced tip pressures that effectively eliminate a great deal of plastic deformation of the supporting soil mass (as in strain hardening of steel); and 2) The increased tip bearing area, caused by the compaction post grout that, because of its very viscous but unconfined nature, forms an intact mass or bulb that is larger than the shaft area.

Undrained Point Load-Point Displacement Response of Piles/Drilled Shafts in Clay and Sand

The undrained load-settlement response of a pile or drilled shaft tip in saturated clay or sand can be assessed based on a simple Mohr circle model of the stress state at and below the pile/shaft tip coupled with established stress-strain formulation employed with a corresponding mobilized Schmertmanns strain triangle. Such analysis is presented and used to compare with tests from the literature for clay and from Statnamic results from Washington DOT tests on two drilled shafts after rate or velocity effects have been taken out but undrained load-deflection behavior ABSTRACT: The undrained load-settlement response of a pile or drilled shaft tip in saturated clay or sand can be assessed based on a simple Mohr circle model of the stress state at and below the pile/shaft tip coupled with established stress-strain formulation employed with a corresponding mobilized Schmertmanns strain triangle. Such analysis is presented and used to compare with tests from the literature for clay and from Statnamic results from Washington DOT tests on two drilled shafts after rate or velocity effects have been taken out but undrained load-deflection behavior

Specification and Design Aspects of the Academic Researcher's Assistant (ARA) Software for Mobile Devices

Mobile devices are being widely and increasingly used in many areas of human activity. Designing applications for mobile devices has introduced several new challenges that are currently being addressed by interested researchers and developers. This paper explores different human-computer interaction challenges in designing an academic researchers assistant (ARA) software application for mobile devices. ARA is a tool for mobile devices designed to provide academic researchers with a practical portable assistant that helps them organize their daily research-related activities. The paper provides details of ARAs organizing principles, software specification, design, and prototype implementation. Several directions of future work are also presented.

Undrained Tip Response of Drilled Shaft in Cohesionless Soils

A method for assessment of the undrained drilled shaft tip load — tip displacement response in sands under earthquake or impact loading has not been available heretofore. Statnamic test results on two WSDOT drilled shaft with the shaft tips in sand below the water table have yielded field evidence with which to compare/calibrate such assessment. A method for tip load — tip displacement evaluation based on the assessed undrained stress-strain-strength of a sand as established from drained tests results and formulation is presented here. It is used to provide assessed response to compare with field recorded behavior.

The Effects of Compaction Post Grouting of Model Shaft Tips in Fine Sand at Differing Relative Densities - Experimental Results

This paper describes a laboratory study conducted on the effects of compaction grouting of model shaft tips embedded in a dry cohesionless soil prepared to different target relative densities. These tests were conducted within a circular steel test chamber 2.4 m in diameter and 2.7 m high. The soil used in this series of tests was a dry sub-angular fine sand (SP) having a D50 of 0.24 mm and a Cu of 1.64. The soil was prepared to relative densities of 50%, 70%, and an initial test on a variable but high relative density specimen. The chamber was filled with the sand in one foot lifts, and densified by vibration in the case of tests on soils at 50% and 70% relative densities, and by hand tamping for soil in the initial variable high density test. The model shaft was placed within the chamber as filling of the tank progressed. Densities were measured using a combination of nuclear density gage measurements and dynamic cone penetrometer (DCP) readings. Nuclear density gage readings were made at 300 mm height intervals as filling progressed while DCP recordings, when possible, were made subsequent to the completion of tank filling operations and application of overburden pressure. At both 50% and 70% relative densities two shafts were load tested, with one being tip post-grouted while the other was not. Additionally a lone ungrouted shaft was load tested within soil of the Initial (variable high density) Test. Grouting of the shaft tip involved pumping a low mobility compaction grout, into the said tip using a system of hoses and pipes as currently used in the industry. Tip pressures induced during the grouting stage together with volume of grout pumped and strains induced within the shaft and test chamber were

Physical and Numerical Modeling of Rock Joints and Slope Stability

In nature, rock behavior and its general modes of failure can be observed and identified. Once identified, the failure process can be explored via a physical model. For this purpose the geotechnical group at the University of Nevada, Reno designed and built a gravity table to examine the stability of rock slopes. Experimentation with the physical models provided the opportunity to observe rock joints and slope stability behavior. The physical modeling was conducted in conjunction with computer simulation using Interactive Physics. Both models were introduced during the engineering camp for high school students where they were very well received.

Computer Simulation of Soil Liquefaction

Soil liquefaction is a phenomenon that develops in loose saturated sand deposits during earthquakes (or other disturbances) that can cause great damage. In an effort to raise awareness of such a hazard, a computer simulation model of soil liquefaction was developed using STELLA (a computer program for model building and simulation). The model demonstrates the effect of the different parameters on the factor of safety against liquefaction. Students have the opportunity to draw conclusions and establish correlations by changing input parameters. The model was presented during the University of Nevada, Reno College of Engineering’s summer camp on geotechnical and earthquake engineering.