Mien Jao

STABILITY OF STRIP FOOTINGS ABOVE CONCRETE-LINED SOFT GROUND TUNNELS

Abstract There are cases that footings are underlain by shallow tunnels. The presence of underground tunnels may adversely affect the stability of the overlying footings. Presently, there is no widely accepted analysis or design methodology available to ensure the stability of such footing-soft ground tunnel systems. As a step toward the development of such a methodology, this study was undertaken to investigate the effect of underground tunnel on the bearing capacity of strip footings. In this study, a two-dimensional plane-strain elasto-plastic finite element computer program was used for analysis. Using this computer program, the stability of strip footings was analyzed for a wide range of soil type and tunnel conditions, including tunnel size, tunnel location, and lining thickness. Based on the results of analysis, the mechanistic behavior of the footing-tunnel system was evaluated. Furthermore, bearing capacity equations for strip footings above concrete-lined soft ground tunnels were formulated. The developed equations are shown to be capable of predicting the bearing capacity of footings located above concrete-lined tunnels reasonably well

BEHAVIOR OF SOFT GROUND TUNNEL UNDER STRIP FOOTING

There are cases that soft-ground tunnels are overlain by footings. The tunnel and footing may interact so that they may manually influence the individuals stability adversely. Presently, there is no widely accepted analysis or design methodology available to ensure the stability of such footing/soft ground tunnel systems. As a step toward the development of such a methodology, this study was undertaken to investigate the interaction between the soft-ground tunnel and the overlying strip footing. In this study, a two-dimensional plane-strain elasto-plastic finite element computer program was utilized to analyze the behavior of the tunnel for a wide range of soil type and tunnel conditions. Based on the results of the analysis, the mechanistic behavior of the footing/tunnel system was evaluated. It was concluded that the presence of the underground tunnels causes stress transfer from the tunnel crown to the spring line. The degree of stress redistribution depends on soil properties, tunnel location, tunnel size, and lining thickness. It was also concluded that for a given footing and tunnel condition, different lining thick nesses may be required for different soil types. Along a concrete lining, critical locations, which required special attention in the lining design, are in the intrados between crown and shoulder as well as between haunch and invert. This paper presents the results of the study and their practical applications.

Stability of Footings Adjacent to Pile Walls

AbstractThe stability of a footing may be destabilized near a retaining wall because of increased lateral displacement and decreased soil confining pressure. The stability of footing within this footing–soil–wall system is complex and not predicted using the classical solutions utilized for a semi-infinite footing–soil system. In this study, the bearing capacity of a concrete footing placed near a wall of spaced driven concrete piles using three-dimensional finite-element (FE) analysis with ANSYS 13.0 is investigated. All system materials are idealized as nonlinear, elastic–perfectly plastic, and rate independent using the upper bound function of the Drucker–Prager yield criterion. Three different soil deposits are studied: kaolin, silty clay, and kaolin–sand. A total of 144 unique FE analysis runs are executed. The footing bearing capacity is computed from the output footing pressure–settlement curve using several well-known postprocessing linearization criteria from which the lowest value is chosen. Soi...

Characterizing the soft clays in the texas gulf coast using the Texas Cone Penetrometer (TCP)

In this study, properties of soft clay soils (undrained shear strength (Su) < 25 kPa) from three locations along the Texas Gulf Coast were investigated using laboratory and field tests. The data and samples for this study was obtained from 46 boreholes in three counties along the Gulf coast. The soils were characterized based on their geotechnical properties including undrained shear strength using statistical methods. The field tests were performed using the Texas Cone Penetrometer (TCP), which is a dynamic method using a cone of 3-in (75 mm) in diameter. Both CL and CH soft soils were present in these locations and there was large variation in the index properties of the soils. Physical, index and strength properties of CL and CH soft soils were quantified using mean, variation, coefficient of variation and probability distribution function. The natural moisture content of 95% of the soft clays was lower than the liquid limit and the coefficient of variation for the Su of CH soil was about 25%. Based on available data the undrained shear strength of soft clay soil (Su) was related to the TCP blow count.

Design of Confined Placement Areas in Sabine-Neches Waterway, Texas

The objective of this study is to use sediment settling data to analyze and predict performance of 15 Confined Placement Areas (CPAs) as part of the 50 year dredged material management plan. Two modules of the Automated Dredging and Disposal Alternatives Management System (ADDAMS), DYECON and SETTLE, were used to design CPAs. Design variables for CPAs include dike height, weir length, surface area, flow path length and effluent total suspended solids (TSS) concentration requirements. After sensitivity analyses of design parameters were performed, design curves were developed and presented as formatted graphs showing the relationship among maximum dredged volume, dike height and effluent TSS concentration for all 15 CPAs. It was found that the number of hours of dredging per day and the re-suspension factor could result in sharp changes in projected effluent TSS concentrations. The influence of these design variables was not well understood by operation engineers and SETTLE users before.