Engineering and Construction

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      a. The modulus of subgrade reaction can be determined from a plate load test (para 4-6) using a 1- by
1- foot plate


The equation above is valid for clays and assumes no increase in the modulus with depth, which is incorrect,
and may give k1,  which is too large.
For footings or mats on sand:

For a rectangular footing or mat of dimensions of B x mB:

       (  15m5)(10-3)

with a limiting value of ksf = 0.667ksl


      b. ks may be computed as ks = 36qa. (kips per square foot) (10-4)  which has been found to give values about as reliable as any method.  This equation assumes qa (kips per square foot) for a settlement of about 1 inch with a safety factor,

F ≅ 3.  A typical range of values of ks is given in table 3-7.

Table 3- 7.  An Engineering Evaluation of Shales

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      a. Allowable bearing pressures.   Procedures for determining allowable bearing pressures are
presented previously.  In many instances,  the allowable bearing pressure will be governed by the
allowable settlement.  Criteria for determining allowable settlement are discussed previously.  The maximum bearing pressure causing settlement consists of dead load plus normal live load for clays, and dead load plus maximum live loads for sands.  Subsoil profiles should be examined carefully to determine soil strata contributing to settlement.

      b. Footings on cohesive soils.

          (1) If most of the settlement is anticipated to occur in strata beneath the footings to a depth equal to the distance between footings, a settlement analysis should be made assuming the footings are independent
of each other.  Compute settlements for the maximum bearing pressure and for lesser values.  An example of such an analysis is shown in figure 10-1.  If significant settlements can occur in strata below a depth equal to
the distance between footings,  the settlement analysis should consider all footings to determine the settlement at selected footings.  Determine the vertical stresses beneath individual footings from the influence charts presented previously.  The footing size should be selected on the basis of the maximum bearing pressure as a first trial.  Depending on the nature of soil conditions,  it may or may not be possible to proportion footings to equalize settlements.  The possibility of reducing differential settlements by proportioning footing areas can be determined only on the basis of successive settlement analyses.  If the differential settlements between footings are excessive,  change the layout of the foundation,  employ a mat foundation, or use piles.

Figure 10-1.  Example of method for selecting allowable bearing pressure.

          (2) If foundation soils are nonuniform in a horizontal direction, the settlement analysis should be
made for the largest footing, assuming that it will be founded on the most unfavorable soils disclosed by the borings and for the smallest adjacent footing.  Structural design is facilitated if results of settlement analyses are presented in charts (fig 10-1) which relate settlement, footing size, bearing pressures,  and column loads.

Proper footing sizes can be readily determined from such charts when the allowable settlement is known.  After a footing size has been selected,  compute the factor of safety with respect to bearing capacity for dead load plus maximum live load condition.

      c. Footings on cohesionless soils.   The settlement of footings on cohesionless soils is generally small and will take place mostly during construction.  A procedure for proportioning footings on sands to restrict the differential settlement to within tolerable limits for most structures is given in figure 10-2.

      d. Foundation pressures.   Assume a planar distribution of foundation pressure for the structural analysis of a footing.  This assumption is generally conservative.  For eccentrically loaded footings, the distribution of the bearing pressure should be determined by equating the downward load to the total upward bearing pressure and equating the moments of these forces about the center line in accordance with requirements of static equilibrium.  Examples of the bearing pressure distribution beneath footings are shown in figure 10-3.

Figure 10-2.  Proportioning footings on cohesionless soils.

Figure 10-3.  Distribution of bearing pressures.

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General.  
When required footings cover more than half the area beneath a structure, it is often desirable to enlarge and combine the footings to cover the entire area.  This type of foundation is called a raft or mat foundation and may be cheaper than individual footings because of reduced forming costs and simpler excavation procedures.  A mat foundation also may be used to resist hydrostatic pressures or to bridge over
small, soft spots in the soil, provided the mat is adequately reinforced.  Although mat foundations are more difficult and more costly to design than individual spread footings,  they can be used effectively.

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A shallow foundation is often selected when the structural load will not cause excessive settlement or lateral movement of the underlying soil layers. Shallow foundations are also used when there are adequate bearing strata at shallow depth. In general, shallow foundations are more economical to construct than deep foundations. Common types of shallow foundations have been described later.

Important considerations in shallow footing excavations are as follows:

1. Dimensions of footings. The geotechnical engineer will often be required to confirm the dimensions of the footings per the building plans. The depth of the footing should always be referenced from the final grade, which may be different from the grade at the time of the footing observation. Usually it is acceptable if the footings have a width and/or depth that is greater than as indicated on the foundation plans. Footings often end up wider than planned because mechanical equipment is used to excavate the footings.

2. Bearing conditions. The bearing conditions exposed in the footings should be checked with the conditions anticipated during the design of the project. If the bearing soil or rock is substantially weaker than that assumed during the design phase, the footings might need to be deepened. The footings may also have to be deepened or the footing design revised if unanticipated conditions are encountered, such as uncompacted fill, loose soil, expansive material, or unstable soils. A metal probe can be used to locate loose soil at the bottom of the footing excavation.

During the excavation, the bottom of the footings often become disturbed, creating a loose soil zone. This disturbance occurs during the actual excavation, especially when mechanical equipment is used such as shown in Fig. 16.2. The bottom of the excavation can also be disturbed when workers descend into the footings in order to install the steel reinforcement, such as shown in Fig. 16.3.Also, debris such as loose soil or rock fragments may be inadvertently knocked into the footing trench after completion of the excavation. Even a thin zone of disturbed and loose soil can lead to settlement that is greatly in excess of calculated values.

Excavation of Footings.

Installation of steel reinforcement in footings

It is important that undisturbed soil (i.e., natural ground) or adequately compacted soil be present at the bottom of the footing excavations and that the footings be cleaned of all loose debris prior to placement of concrete. Any loose rocks at the bottom of the footing excavation should also be removed and the holes filled with concrete (during placement of concrete for the footings).

3. Groundwater conditions. The presence of groundwater can impact bearing conditions. For example, groundwater in a footing excavation may cause the side of the hole to cave or loose slough to accumulate at the bottom of the footing. The groundwater table may need to be lowered in order to cleanout any loose debris at the bottom of the footing.

If the footings should become flooded, such as from a heavy rainstorm, then loose debris can be washed into the excavations. This loose or soft soil will have to be removed prior to placing the concrete for the footings. Water can also soften the soil located at the bottom of the excavations. For example, clayey soil may absorb water and swell, producing a layer of very soft and compressible material. An example of such a condition is shown in Fig. 16.4, where the footing excavation has become flooded during a heavy rainstorm. Note also in Fig. 16.4 that the steel reinforcement has been prefabricated at ground surface and once the footings are dry and clean, the prefabricated steel reinforcement sections will be lifted and lowered into the footing trench.

FIGURE 16.4 Footing excavations. Note the flooded condition of the footing trenches.

4. Steel reinforcement inspection. Sometimes the geotechnical engineer may be required to inspect the type and location of steel reinforcement in the footings.

5. Local building department requirements. In many cases, field observations to confirm bearing conditions and check foundation dimensions will be required by the local building department. In addition, a memo or letter indicating the outcome of the observations must be prepared by the engineer to satisfy the local building department. Building departments often refer to these types of reports as final inspection reports. An example of such a report for the construction of a foundation is as follows:

Footing Inspection: The footings at the site have been inspected and are generally in conformance with the approved building plans. Additionally, the footings have been approved for installing steel reinforcement and the soil conditions are substantially in conformance with those observed during the subsurface exploration. Furthermore, the footing excavations extend to the proper depth and bearing strata. Care should be taken to keep all loose soil and debris out of the footing excavations prior to placement of concrete.

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These types of shallow foundations are probably the most common types of building foundations.

Figure 5.1 shows various types of shallow foundations.

Figure 5.2 shows various types of mat foundations. Based on economic considerations, mat foundations are often constructed for the following reasons (NAVFAC DM-7.2, 1982):

1. Large individual footings. A mat foundation is often constructed when the sum of individual footing areas exceeds about one-half of the total foundation area.

2. Cavities or compressible lenses. A mat foundation can be used when the subsurface exploration indicates that there will be unequal settlement caused by small cavities or compressible lenses below the foundation. A mat foundation would tend to span over the small cavities or weak lenses and create a more uniform settlement condition.

3. Shallow settlements. A mat foundation can be recommended when shallow settlements predominate and the mat foundation would minimize differential settlements.

4. Unequal distribution of loads. For some structures, there can be a large difference in building loads acting on different areas of the foundation. Conventional spread footings could be subjected to excessive differential settlement, but a mat foundation would tend to distribute the unequal building loads and reduce the differential settlements.

5. Hydrostatic uplift. When the foundation will be subjected to hydrostatic uplift due to a high groundwater table, a mat foundation could be used to resist the uplift forces.