What's the best way to size a shallow foundation?

1 Soil investigation

The first step to size a shallow foundation is to conduct a soil investigation. This involves collecting soil samples, performing laboratory tests, and measuring the in-situ soil parameters. The soil investigation provides valuable information about the soil type, strength, compressibility, bearing capacity, and groundwater level. These factors affect the choice and design of the foundation type, shape, size, and depth.

2 Foundation type

The next step is to select the appropriate foundation type based on the soil conditions and the structural requirements. There are two main types of shallow foundations: spread footings and mat foundations. Spread footings are isolated foundations that support individual columns or walls. Mat foundations are continuous foundations that cover the entire area of the structure. The choice of foundation type depends on the load distribution, the soil bearing capacity, the settlement tolerance, and the cost.

3 Foundation shape

The shape of the foundation affects its efficiency and economy. The most common shapes for shallow foundations are square, rectangular, circular, and trapezoidal. The shape of the foundation should match the shape of the column or wall it supports, as well as the load eccentricity and direction. The shape of the foundation also influences the stress distribution and the bearing capacity of the soil.

4 Foundation size

The size of the foundation is determined by the load intensity and the allowable bearing pressure of the soil. The load intensity is the ratio of the total load to the area of the foundation. The allowable bearing pressure is the maximum pressure that the soil can sustain without excessive settlement or shear failure. The size of the foundation should be large enough to keep the load intensity below the allowable bearing pressure. The size of the foundation can be calculated using empirical formulas, charts, or software.

5 Foundation depth

The depth of the foundation is the distance from the ground surface to the bottom of the foundation. The depth of the foundation affects its stability and performance. The depth of the foundation should be sufficient to avoid frost action, scouring, erosion, and uplift forces. The depth of the foundation should also be adequate to ensure a uniform stress distribution and a safe factor of safety against bearing failure. The depth of the foundation can be estimated using empirical rules, such as the rule of thumb that the depth should be at least equal to the width of the foundation.

6 Foundation design

The final step is to design the foundation according to the relevant codes and standards. The design of the foundation involves checking the strength, stability, and serviceability of the foundation and the soil. The design of the foundation also involves detailing the reinforcement, joints, and drainage systems of the foundation. The design of the foundation should be verified using analytical methods, numerical models, or field tests.

7 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?