Engineering and Construction

Generally, soil groups listed toward the top of Figure 2.2 are more desirable for supporting building foundations than those listed further down. The higherlisted soils tend to have better  soil engineering properties, that is, they tend  to have greater loadbearing capacity,  to be more stable, and to react less to
changes in moisture content. Rock is  generally the best material on which  to found a building. When rock is too deep to be reached economically, the designer must  choose from the strata  of different soils that lie closer to the  surface and design a foundation to per- form satisfactorily in the selected soil.

Figure 2.5 gives some conservative values of  loadbearing capacity for  various types of soil. These values give only an approximate idea of the  relative strengths of different soils;  the strength of any particular soil is
also dependent on factors such as  the presence or absence of water, the  depth at which the soil lies beneath  the surface, and, to some extent, the manner in which the foundation acts  upon it. In practice, the designer may  also choose to reduce the pressure  of the foundations on the soil to well  below these values in order to reduce  the potential for building settlement.

Figure 2.5 Presumptive surface bearing values of various soil types, from the 2006 IBC. Classes 3, 4, and 5 refer to the soil group symbols in Figure 2.2.

The stability of a soil is its ability to retain its structural properties under  the varying conditions that may occur  during the lifetime of the building. In  general, rock, gravels, and sands tend  to be the most stable soils, clays the least  stable, and silts somewhere in between.

Many clays change size under chang-ing subsurface moisture conditions,  swelling considerably as they absorb  water and shrinking as they dry. In the  presence of highly expansive clay soils,  a foundation may need to be designed  with underlying void spaces into which  the clay can expand to prevent structural damage to the foundation itself.

When wet clay is put under pressure,  water can be slowly squeezed out of  it, with a corresponding gradual reduction in volume. In this circumstance, long-term settlement of a foundation bearing on such soil is a risk that must  be considered. Taken together, these properties make many clays the least predictable soils for supporting buildings. (In Figure 2.2, the fine-grained  soil groups indicated as having a liquid  limit greater than 50 are generally the  ones most affected by water content,  exhibiting higher plasticity (moldability) and greater expansion when wet  and lower strength when dry.

In regions of significant  earth- quake risk, stability of soils during seismic events is also a concern. Sands and  silts with high water content are particularly susceptible to liquefaction, that
is, a temporary change from solid to  liquid state during cyclic shaking. Soil  liquefaction can lead to loss of support  for a building foundation or excessive  pressure on foundation walls.

The drainage characteristics of a  soil are important in predicting how  water will fl ow on and under building  sites and around building substructures. Where a coarse-grained soil is composed of particles mostly of the
same size, it has the greatest possible volume of void space between particles, and water will pass through it most readily. Where coarse-grained soils are composed of particles with a diverse range of sizes, the volume of void space between particles is reduced, and such soils drain water less eficiently. Coarse-grained soils consisting of particles of all sizes are termed well graded or poorly sorted, those with a smaller range of particle sizes are termed  poorly gradedor well sorted, and those with particles mostly of one size are termed uniformly graded (Figure 2.6).

Figure 2.6 Two gravel samples, illustrating differences in range of particle sizes or grading. The left-hand sample, with a diverse range of particle sizes, comes from a well-graded sandy gravel. On the right is a sample of a uniformly graded gravel in which there is little variation in size among particles.

Because of their smaller particle size,fine-grained soils also tend to drain water less efficiently:  Water
passes slowly through very fine sands and silts and almost not at all through many clays. A building site with clayey or silty soils near the surface drains poorly and is likely to be muddy and covered with puddles during rainy periods, whereas a gravelly site is likely to remain dry. Underground, water passes quickly through strata of gravel and sand but tends to accumulate above layers of clay and fine silt. An excel-lent way to keep a basement dry is to surround it with a layer of uniformly graded gravel or crushed stone. Water passing through the soil toward the building cannot reach the basement without first falling to the bottom of
this porous layer, from where it can be drawn off in perforated pipes before it accumulates (Figures 2.60–2.62). It does little good to place perforated drainage pipes directly in clay or silt because water cannot fl  ow  through the impervious soil toward the pipes.

Figure 2.60 Two methods of relieving water pressure around a building substructure by drainage. The gravel drain (left) is hard to do well because of the dificulty of depositing the crushed stone and backfill soil in neatly separated, alternating layers. The drainage mat (right) is easier and often more economical to install.

Figure 2.62 A diagrammatic representation of the placement of sheet membrane waterproofing around a basement. A mud slab of low- strength concrete was poured to serve as a base for placement of the horizontal membrane. Notice that the vertical and horizontal membranes join to wrap the basement completely in a waterproof enclosure.

Rarely is the soil beneath a building site composed of a single type. Beneath most buildings, soils of various
types are arranged in superimposed layers (strata) that were formed by various past geologic processes. Frequently, soils in any one layer are themselves also mixtures of different soil groups, bearing descriptions such as well-graded gravel with silty clay and sand, poorly graded sand with clay, lean clay with gravel, and so on. Determining the suitability of any particular site’s soils for support of a building foundation, then, depends on the be-haviors of the various soils types and how they interact with each other and  with the building foundation.