Archive for October, 2011

Inspecting a Foundation

by David.

I get calls from people buying a house who want their foundation inspected. Since it’s a major component of any structure and costly to replace, it’s important to know what you’re buying into. I thought I’d share some wisdom on what I’m looking for when I come out for an inspection of an existing foundation.

It first helps to understand what a foundation is meant to do:

  1. Dissipate the gravitational loads (weight) of the building into the soil
  2. Transfer lateral loads from wind or earthquakes into the ground while maintaining the integrity of the structure.

Most existing foundations, even the brick ones, are adequate for supporting the weight of the structure. The issue in highly seismic areas like the Bay Area and most parts of California is determining the adequacy of a foundation in supporting the structure during an earthquake. Here are some of the potential issues with foundations and the things I’ll look for when inspecting:

Foundation quality – Foundations in the late 19th and early 20th century were mainly made of unreinforced masonry (brick) or poor quality/unreinforced concrete. The main deficiency with these foundations is their inability to transfer tensile forces (being pulled on). As a result, there is a tendency for the structure to disengage from the footing during an earthquake.  Under most conditions, brick (or poor/unreinforced concrete) foundations should be retrofitted by pouring fresh concrete on top (capping), along side (encapsulating), or just replacing it.

Settlement – The majority of settlement resulting from poor soil occurs within the first 5 to 10 years. Settlement, although potentially unsightly, is not necessarily a critical structural deficiency. Foundations usually settle as a result of soil consolidation, sometimes a result of wet soil. Some signs of settlement include sloping floors, cracked plaster/stucco, tilted door frames, and cracks in the foundation.

Concrete Cracking – One of the things you can always expect concrete to do is crack. It may crack from movement, settlement, shrinkage, poor placement, bad mixture, insufficient reinforcing, etc… More major cracking is a sign that there has been movement. Cracks in concrete are an indication of experienced tension across the crack line. The tension may be from the causes mentioned above.

Soil Condition – The interaction between the soil and the foundation is very important and it’s good to know the precautions of each kind. Clay soils, found in many areas in the east bay, don’t drain well and are sometimes expansive. Expansive soils expand with water and will create large forces on foundations. It is particularly important to keep water away from these foundations as best as possible. Another example might be the bay mud soils like in the Marina. There is the concern of liquefaction with these soils and they require special considerations.

Sloping Site – Sloping sites have a greater demand on the foundation since there is a component of gravity from the soil acting on the foundation. This force is also amplified during an earthquake. Sloping sites usually require special considerations and potentially a soils report.

More often, foundation problems are a combination of several of the outlined issues.

I hope this helps demystify what engineers are looking for when inspecting a foundation. If you have more questions about your property or project, feel free to give me a call. I am happy to consult with you.