Shallow Footing Excavations.

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
Excavation of Footings.
FIGURE 16.2 Excavation of footings.

Installation of steel reinforcement in footings. The arrow points to the workers in the footing excavation in the process of installing the steel reinforcement.
Installation of steel reinforcement in footings
FIGURE 16.3 Installation of steel reinforcement in footings. The arrow points to the workers
in the footing excavation in the process of installing the steel reinforcement.

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.

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