For most structures, the foundations are established below the surface of the ground. Therefore, they cannot be constructed until the soil or rock above the base level of the foundation has been excavated.
Ordinarily, it is not the function of the foundation engineer to select the equipment for excavating at a given site or to design the bracing if any required. These are considered to be within the province of the contractor. It is generally the engineer’s duty, however, to approve or disapprove the construction procedure proposed by the builder and to check the design of the bracing. In exercising these functions, the engineer is properly concerned only with the ability of the proposed procedures to permit satisfaction construction of the structure as designed, he should not ordinary presume to dictate the manner in which the construction should be carried out.
On large or complicated jobs, it may be impossible to prepare a design without also deciding upon the method of construction. When the design and the construction procedure are related so closely that they must be considered as a unit, the engineer is obliged to specify the construction method and possibly to design the bracing.
Open Excavations With Unsupported Slopes
Shallow excavations can be made without supporting the surrounding material if there is adequate space to establish slopes at which the material can stand. The steepness of the slopes is a function of the type and character of the soil or rock., the climate and weather condition, the depth of the excavation, and the length of time the excavation must remain open. As a rule, construction slopes are made as steep as the material will permit because the occurrence of a few small slides is generally not serious. The cost of time removing the material affected by the slider may be considerably less than that of the additional excavation required to provide flatter slopes.
The steepest slopes that can be used in a given locality are best determined by experience. Most sands contain a small amount of cementing material or display a small amount of apparent cohesion because of the moisture they contain. This cementation or cohesion can not safely be dependent on the stability of permanently exposed slopes, but it is ordinarily utilized for the periods during which excavation stands open.
The maximum slope at which a clay soil can stand is a function of the depth of the cut and of the shearing resisting of the clay. If the clay is soft below the base level of the excavation, flat slopes may be required to avoid a rise of the bottom. Furthermore, stiff or hard clay commonly possess or develop cracks near the ground surface. If these cracks become filled with water, the hydrostatic pressure greatly reduces the factor of safety of the slope is likely to decrease with time. For these reasons, bracing is often used to support the sides of excavation in clay, even though the clay would stand briefly to the necessary height without lateral support. You might also be interested in our article on building foundation construction process.
Sheeting and Excavation For Shallow Depth
Many building sites extend to the edges of the property lines or are adjacent to another site on which structures already exist. Under these circumstances, the site of the excavation must be made vertical and must usually be supported. Several methods of bracing are common use.
If the hole is not extended to a depth greater than about 12 ft, it is common practice to drive vertical planks known as sheeting around the boundary of the proposed excavation. The depth to which the sheeting is driven is usually kept near that of the bottom of the hole as excavation progresses. The sheeting is held in place by means of horizontal beams called wales that in turn are commonly supported by horizontal struts extending from side to side of the excavation. The struts are usually of timber, but if the excavation is not more than about 5 ft wide, extensible metal pipes called trench are commonly used. Their usage requires the soil in the base of the excavation to be firm enough to provide adequate support for the inclined members.