Settlements of Compensated Foundations.

     a. The sequence of subsoil heave during excavation and subsequent settlement of a deep foundation is illustrated in figure 11-1(a).  If effective stresses do not change in the subsoils upon the initial excavation, i.e., the soil does not swell due to an increase in water content, and if no plastic flow occurs, then only immediate or elastic rebound from change in stress occurs.  If the structural load is fully compensated, the measured settlement of the foundation would consist only of recompression of the elastic rebound, generally a small quantity, provided subsoils are not disturbed by excavation.

Effect of pore pressure dissipation during excavation and settlement response.
Figure 11-1.  Effect of pore pressure dissipation during excavation and settlement response.


     b. If the negative excess pore pressures set up during excavation "dissipate, " i.e., approach static
values, before sufficient structural load is applied, foundation swell occurs in addition to elastic rebound. (The original effective stresses will decrease.) The foundation load recompresses the soil, and settlement of
the foundation consists of elastic and consolidation components as shown in figure 11-1(b).  Consolidation occurs along the recompression curve until the preconsolidation stress is reached, whereupon it proceeds along the virgin compression curve.  Calculate the foundation heave and subsequent settlement using procedures outlined previously.

     c. If the depth with respect to the type and shear strength of the soil is such that plastic flow occurs, loss of ground may develop around the outside of the excavation with possible settlement damage to structures, roads,  and underground utilities.

     d. The rate and amount of heave may be estimated from the results of one-dimensional
consolidation tests; however, field evidence shows that the rate of heave is usually faster than predicted.  A study of 43 building sites found that the field heave amounted to about one-third the computed heave.

Where excavations are large and are open for substantial time before significant foundation loadings
are applied, the actual heave may be close to the computed heave.  Figure 11-2 is a plot of a series of field results of heave versus excavation depth, in which the heave increases sharply with the depth of excavation.

An example of heave and subsequent settlement calculations for a compensated foundation is shown in figure 5-4.

Excavation rebound versus excavation depth.
Figure 11-2.  Excavation rebound versus excavation depth.

Example of settlement analysis.
Figure 5-4.  Example of settlement analysis.
     e. The yielding of the excavation bottom can be .caused by high artesian water pressures under the
excavation or by a bearing capacity failure resulting from the overburden pressure on the soil outside the
excavation at subgrade elevation.  Artesian pressure can be relieved by cutoffs and dewatering of the underlying aquifer using deep wells.  The pumped water may be put back in the aquifer using recharge wells outside the excavation perimeter to avoid perimeter settlements or to preserve the groundwater table for environmental reasons, but this operation is not simple and should be done only when necessary.

     f. The likelihood of bearing capacity failure exists primarily in clayey soils and should be analyzed as shown later.  A factor of safety, Fs > 2, should ideally be obtained to minimize yielding and possible settlement problems.  A large plastic flow may cause the bottom of the excavation to move upwards with resuiting loss of ground.  To avoid this possibility, investigate-

          (1) Potential for plastic flow, i.e., relationship between shear stress and shear strength.
          (2) Sequence of placing wall bracing.
          (3) Depth of penetration of sheeting below base of excavation.

     g. Two commonly used procedures to control bottom heave are dewatering and sequential excavation
of the final 5 feet or more of soil.  Groundwater lowering increases effective stresses and may reduce heave.

Where subsoil permeabilities are not large,  a deep and economical lowering of the groundwater to minimize heave can sometimes be achieved by an educator-type wellpoint system.  Permitting a controlled rise of the
groundwater level as the building loan is applied acts to reduce effective stresses and counteracts the effect of the added building load.  Sequential excavation is accomplished by removing soil to final grade via a series
of successive trenches.  As each trench is opened, the foundation element is poured before any adjacent trench is opened.  This procedure recognizes the fact that more heave occurs in the later excavation stages than in earlier stages and is frequently used in shales.

     h. The tilting of a compensated foundation can occur if structural loads are not symmetrical or if soil conditions are nonuniform.  Tilting can be estimated from settlement calculations for different locations of the excavation.  Control of tilt is not generally necessary but can be provided by piles or piers, if required.  Bearing capacity is not usually important unless the building is partially compensated and founded on clay. The factor of safety against bearing failure is calculated (see chap 6 for quit) and compared with the final total soil stress using the building load,  qo,  less the excavation stress as follows:

The factor of safety should be between 2.5 and 3.0 for dead load plus normal live load.

     i. Settlement adjacent to excavations depends on the soil type and the excavation support system method employed (chap 14).  With properly installed strutted or anchored excavations in cohesionless soils, settlement will generally be less than 0.5 percent of excavation depth.  Loss of ground due to uncontrolled seepage or densification of loose cohesionless soils will result in larger settlements.

Surface settlements adjacent to open cuts in soft to firm clay will occur because of lateral yielding and movement of soil beneath the bottom of the cut.  Figure 11-3 can be used to estimate the magnitude and extent of settlement.

Probable settlements adjacent to open cuts.
Figure 11-3.  Probable settlements adjacent to open cuts.

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