Techniques for Excavating a Hole for a Basement

Soil Problems

• When attempting to select the proper technique for excavating a basement hole, one of the primary considerations is the type of soil in which the basement will be constructed. Soil sliding is a common problem faced in basement excavation. It requires proper planning and supervision. Toppling is another common problem that occurs when stress fractures appear in the walls, causing the soil to topple over into the hole.

Protective Support Systems

• Because of the potential problems associated with soil sliding and other forms of collapse, any type of digging technique for excavating a basement must have some type of protective support system in place. These generally involve creating retaining walls, or benching, the sides to avoid collapse. It can also involve creating some type of shield around the area in which workers are performing their duties. Another common technique that relates directly to digging involves sloping the sides of the hold so that they do not form an angle of a horizontal-to-vertical ratio greater than 1.5 to 1.0.

Island Method

• One way to safely dig out the hole for a basement excavation is to use the island method. This involves a partial excavation in which the central core part of the basement is built first in the center. From there, the outer walls of the basement are sloped until they can be properly braced and fully constructed. The outer walls are eventually braced by the completed inner structure until the entire basement is complete.

Top-Down

• The top-down method of construction for basement excavation is one that is often used in large cities or congested areas. In the top-down method, the load-bearing walls of the basement are constructed and the ground layer slab is poured above where the basement will go. The building may even be fully constructed prior to the completion of the basement itself. The basement is not dug out completely until later, when the building and its foundation are fully in place. This method basically involves carefully digging out a portion of the ground below the building.