Foundations

A foundation supports and distributes the weight of the entire structure, and loads imposed on it by wind, snow, earthquakes, people and storage etc. Footings on or near slopes may be affected by (and may affect) the slope itself. This is a guide to the geotechnical challenges of residential foundations in BC. 

The foundation is the interface between the structure and the ground, and may involve interaction between the structural engineer and ourselves. We are commonly asked to provide recommendations for bearing capacity and settlement – these being the primary considerations for structural engineers. Sometimes, we are asked to provide the modulus of subgrade reaction, which is not a single value, and is unique to a load’s distributed area.

Figure 1 Typical residential foundation construction

Figure 1 Typical residential foundation construction

It is necessary to assess the subsurface conditions to design, construct, and maintain foundations for buildings, earth structures, highways, roads, and marine installations. As part of the assessment, we would study the site by collecting information on historic data and reviewing climate information. We ask that the client provides the structural loads and settlement criteria of the proposed structure. 

Earthquake Effects 

In consideration of earthquake effects, we look at the risk of liquefaction, which may occur when susceptible soils (e.g. wet sands) are shaken by an earthquake of sufficient magnitude and duration. Where liquefaction is considered a risk, special precautions may need to be taken.

Figure 2 Liquefaction Analysis showing estimated settlement in an earthquake of about 40cm

Core Geotechnical Inc. will provide options for shallow footings (e.g. strip, raft or pad foundations) or deep footings (e.g. bored or driven piles) as appropriate to the particular site under investigation. Our recommendations will be in accordance with contemporary geotechnical practice. 

Foundations on Poor Ground 

Where building is proposed on poor ground, a compromise may need to be made regarding the expectations building performance (settlement), the cost of the foundation structure, and the cost of ground improvement. We can use various techniques to evaluate settlement, and although this is a phenomenon that is historically very difficult for any geotechnical engineer to reliably predict, we can provide estimates in accordance with contemporary practice. Where expected settlements are intolerable, it may be necessary to either provide a piled structure or improve the ground by earthworks, including rock columns, wick drains, or simply an engineered soil raft.

Figure 3 Engineered Soil Raft

Figure 3 Engineered Soil Raft