SOIL CONDITIONS

The designer of a steel structure can not proceed without knowing the physical properties of the steel. The soil and rock formation s under the structure are just as much a part of the structural system as the steel structure. However, whereas designers can control the character of the man-made materials, they have little control over the character the soil and rock. Therefore, either the design must be adapted to the site conditions or else the site conditions must be improved. In either case it is imperative that these conditions be evaluated accurately.

A complete investigation of underground conditions includes the following points:

1. Nature of the deposits (geology, recent history of filling, excavation, and flooding)
2. Depth, thickness, lateral extent, elevations, and composition of each soil and rock stratum.
3. Groundwater elevations and their differences across the site.
4. The engineering properties of the soil and rock strata that affect the performance of the structure.

An investigation consists of three steps:

1. Reconnaissance: To determine the geology of the formations and to estimate the soil rock, and water conditions through geologic study and site inspection,
2. Exploratory investigation: To determine the depth, thickness, and composition of the soils and rocks, the level of groundwater, and to estimate the engineering properties of the materials through soil boring and sampling..
3. Intensive investigation: To secure quantitative data on critical strata from which design computation can be made.

An examination of the site and the adjacent areas will real much valuable information. The topography, drainage pattern, erosion pattern, vegetation, and land use reflect the underground conditions, particularly the structure and texture of the soil and rock. Highway and railroad cuts and stream banks often disclose the cross section of the formations and indicate the depth of rock. Outcrops of rock or areas of gravel and boulders may indicate the presence of dikes and more resistant strata. Groundwater conditions are often reflected in the presence of seeps, springs, and the type of vegetation.

1. Composition of Soil: By definition, soils include all unconsolidated materials which are composed of many different ingredients in all three states - solids, liquid, and gases. The same applies to many rocks.
2. Engineering Soil Classification: Textural classifications group soils by their grain size characteristics. The gravel and large sizes are disregarded and the particles finer than 2 mm in diameter are divided into three groups, sand sizes, silt sizes, and clay sizes. The soils are then grouped by the percentage of each of these three components. Clayey soils are classified by plasticity characteristics which are based on the interaction of clay and water.
3. Bearing Capacity The bearing capacity of a soil is the ability of the soil to carry a load without failure within the soil. It is analogous to the ability of a beam to carry a load without breaking.

The foundation is the supporting part of a structure. The term is usually restricted to the structural member that transmits the superstructure load to the earth, but in a large sense it includes the soil and rock below. The design depends on the characteristics of both the structure and the soil and rock. There are two types of foundations:

1. Shallow foundation: Footing and mats are commons used. A footing is an enlargement of a column or wall in order to reduce the pressure on the soil to maximum allowable. A mat is a combined footing supporting multiple structural elements not located in the same line.
2. Deep foundation: Piles and Piers (Caisson) foundation are used. Piles are used in many ways. Bearing piles that support foundation loads by transferring the load of the structure through soft strata into stronger, incompressible soils or rock below. Friction piles transfer load to surrounding soil by friction or adhesion. Tension piles are used to resist upward forces. Laterally loaded piles support loads applied perpendicular to the axis of the pile in foundations subject to horizontal forces. The pier foundation is a relatively large, deep foundation. Its function is to transfer a foundation load through soft soil to hard soil or rock or to transfer a load through soils that may be scoured away by rivers or tidal currents.

Groundwater is a factor to be considered in several ways. First, excavation below the groundwater level is expensive and often hazardous because upward seepage loosens sands and tends to create a quick condition, and water standing over exposed clays softens them. Second, when the groundwater level is above the lowest level floor, seepage into the structure and hydrostatic uplift become series problems. Third, changes in the elevation of the water table have caused much trouble such as building settlement and decreasing of soil strength.

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