Shallow Spread Footings Research Articles

The Effect of Soil Liquefaction and Lateral Spreading to the Seismic Vulnerability of RC Frame Buildings Considering SSI

ABSTRACT Evidence of past earthquakes highlighted the seismic vulnerability to buildings due to soil liquefaction and lateral spreading phenomena. Damage to buildings on a liquefiable site, especially on a gently sloping site, can be severe. The liquefied soil can undergo permanent lateral ground displacements as large as several meters in some cases, which can be severely disastrous to buildings. Recent physical tests and effective stress analyses of nonlinear dynamic soil-structure interaction (SSI) proved that simplified empirical processes are often improper for evaluating liquefaction-induced building settlements. A better understanding of the SSI effects is also essential in cases where the underlying soil is susceptible to liquefaction. In this respect, the interaction of the earthquake excitation and ground failure (i.e. soil liquefaction and lateral spreading) for buildings considering SSI is investigated herein. Typical low-code reinforced concrete (RC) frame buildings of different heights with shallow spread footings on liquefiable level-ground or sloping soils subjected to ground shaking are considered. The direct SSI approach is implemented. Predictive relationships are established between the maximum normalized differential displacements (horizontal or settlement) and the permanent ground displacement (PGD) vector at the level of foundation for low-code RC frame buildings with shallow spread footings on soils with high liquefaction potential subjected to ground shaking. Finally, vulnerability is expressed in terms of fragility and vulnerability curves as well as in terms of fragility surfaces as a function of two intensity measures.

Design of Short Aggregate Piers to Support Highway Embankments

When new embankments are constructed over soft soils and existing roadways and embankments are widened rapidly, stability and differential settlement problems often result. Past approaches to mitigate these problems include overexcavation and replacement, preloading, lightweight fill, piles and concrete caps, geosynthetic-reinforced soil and pile-supported embankment systems, stone columns, lime-cement columns, and deep soil mixing. The advantages and limitations of these approaches are well documented in the literature. An alternative approach to support highway embankments that is less well documented, but that has seen increased use in recent years, is short aggregate piers. Although the technique has been used to support shallow spread footings, design approaches and parameter values have not been widely available for embankment support applications and are not well known in the field. The engineering properties and an approach used to design short aggregate piers for the support of highway embankments are summarized, and the mechanisms of load-settlement behavior are described. Techniques for evaluating overall stability, control and time rate of settlement, and bearing capacity are summarized. Performance measurements are referenced for embankment support projects recently constructed in Iowa.

Spread Footing Foundation for Highway Bridge Applications

Shallow spread footings on soil are becoming an attractive choice for supporting highway bridge structures. However, to encourage their utilization, well-documented, comprehensive case histories must be established and made accessible to practicing bridge and geotechnical engineers. In addition, performance prediction methods for spread footing foundations must be verified further using carefully compiled field data. This paper is the writers' attempt toward achieving these goals, presenting details of a recently completed research project on 30 bridge spread footings in Ohio. The project data show that spread footings can be used successfully to support highway bridge structures on both cohesionless and cohesive soils, provided that the subsoil conditions are suitable and that the existing performance prediction methods are accurate.

Numerical Modeling of Spread Footings at Bridge–Embankment Interfaces

The use of spread footings on slopes is a common practice in the design of bridge foundations. Footings generally are constructed in the slope or at its crest to minimize the bridge length, thereby minimizing costs. However, the analytical methods that frequently are used to predict the ultimate bearing capacity of shallow spread footings on slopes yield a wide range of solutions. There has been an increased effort in recent years to evaluate and improve techniques for predicting the deformation and ultimate bearing capacity of shallow spread footings located on or near slopes. Research in the development of numerical techniques used in conjunction with sophisticated elastoplastic constitutive models continues to be an area in which additional effort is needed. Physical model tests were used to obtain maximum bearing pressures of shallow spread footings located near sand slopes. These results were compared with classical bearing capacity solutions and numerical predictions obtained with finite element discretizations. Deformed meshes and incremental nodal displacements are presented.

Large Model Spread Footing Load Tests on Geosynthetic Reinforced Soil Foundations

The potential benefits of geosynthetic reinforced soil foundations are investigated using large-scale model footing load tests. A total of 34 load tests were performed to evaluate the effects of single and multiple layers of geosynthetic reinforcement placed below shallow spread footings. Two different geosynthetics are evaluated: a stiff biaxial geogrid and a geocell. Parameters of the testing program include the number of reinforcement layers, spacing between reinforcement layers, the depth to the first reinforcement layer, plan area of the reinforcement, the type of reinforcement, and soil density. Test results indicate that the use of geosynthetic reinforced soil foundations may increase the ultimate bearing capacity of shallow spread footings by a factor of 2.5.

Verrazano-Narrow Bridge: Design of Tower Foundations and Anchorages

The world's largest suspended span rests on large concrete caissons, 229 ft by 129 ft in plan. These are founded at a depth of 105 ft below sea level on a sand-gravel stratum and at a depth of 270 ft on a compact granular stratum, respectively. The outer walls are 5 ft thick, the interior walls are 3 ft and 4 ft thick. The bottom of the walls is heavily armored with structural steel to form cutting edges. The caisson is topped by a 4-ft thick distribution slab. Two granite-faced pedestals support the steel tower. The anchorages are large blocks of concrete resting on shallow spread footings. A judiciously selected construction sequence balanced anchorage loads with cable pull to control soil pressures. Extensive soils explorations provided basic information on stratification and character of underlying soils. Laboratory soils tests permitted evaluation of expected settlements of tower foundations and anchorages and of a sliding friction factor for design of the anchorages.

Verrazano-Narrow Bridge: Construction of Tower Foundations and Anchorages

The tower foundations for the world's longest suspension span are open-dredged concrete caissons, constructed by the sand island method. The caissons are 229 ft by 129 ft in plan. They are founded at a depth of 105 ft below sea level on a sand-gravel stratum and at a depth of 270 ft on a compact granular stratum, respectively. Concrete was supplied by a floating batch plant and placed in 10-ft lifts. Excavation was performed through 17-ft-diameter dredging wells by crane and bucket. The base was sealed by tremie concrete. The caissons are topped by a 4-ft-thick distribution slab that supports the two granite-faced tower pedestals. The two anchorages are composed of several massive concrete blocks that resist the pull of the bridge cables by their weight and the friction at the bottom of their foundations. Both anchorages rest on shallow spread footings. Excavation was done primarily by scrapers. Concrete batching and mixing plants at the site of each anchorage were equipped to control concrete placing temperatures by addition of flaked ice. Triangulation across the Narrows was checked by Tellurometer.