When it comes to coping with expansive clay soil, not all residential foundations are created equal. How the soil affects your foundation will depend on its type and the measures you take to protect it from damage. These measures keep the moisture content of the soil near your foundation constant. This entails watering the soil during droughts, ensuring proper drainage during heavy rains, and preventing tree root growth near your foundation.
It’s important to keep in mind that the primary reason for foundation problems is the soil that the structure rests on rather than any innate flaw in the foundation itself. The expansive clay soil in Texas is one of the most unstable soil types in the country. It expands when it soaks up moisture and shrinks when it dries. This kind of movement will severely test any structure. However, local construction methods do affect your foundation’s sensitivity to the movement of expansive soil. How construction methods affect two common foundation types in Texas is discussed next:
How well a slab foundation bears up to unstable clay soil depends on the construction method used when it was built. If you buy an old home with a slab foundation built in the late 1950s or early 1960s, the concrete slab likely has little or no steel reinforcement. The idea back then was that the slab would simply float on the soil. However, the slab’s inadequate reinforcement meant it lacked the strength to cope with differential soil movement. Differential soil movement happens when the soil beneath the foundation shrinks or expands in differing amounts. This causes the slab to buckle and crack.
Slab foundations built since then use varying amounts of steel reinforcement with wider and deeper perimeter beams. Rebar steel is commonly used because of its great tensile strength which makes up for concrete’s relative weakness under tensile loading. Another method of steel reinforcement called post-tensioning, uses stretched cables held in tension within the concrete. These hold the slab in compression which makes the slab’s concrete stronger and more resistant to stress.
Some of the newer slab designs rest on beams that in turn are held by piers extending deep underground. The slab and beams are held above ground level which isolates them from the clay soil movements while the piers rest either on bedrock or highly stable soil. Of the slab foundations discussed, this design works best for avoiding problems with expansive clay soil.
Pier and Beam Foundations
Prior to the start of the slab foundation’s popularity in the 1960s, homes were commonly built on pier and beam foundations. With this method, the floor of the house is elevated above the ground on beams that are supported on wooden posts or concrete piers. The perimeter of the floor is often supported by concrete beams as well. This creates a crawl space beneath the house which allows ready access to plumbing, electrical conduits, and HVAC ducting.
How well a pier and beam foundation holds up to expansive clay soil movement depends on the piers’ depth beneath the soil. Some so-called pier and beam foundations have posts that simply rest on the soil surface using ‘feet’ that spread out the load on to the soil beneath. Other foundations use piers with insufficient depth which still rest on soil that is subject to moisture variations and movement. However, if the piers extend down to stable, load bearing soil or bedrock, they are reliable.
If you have foundation problems, contact us at Perma-Pier Foundation Repair for a free evaluation. We have extensive foundation repair experience in the Dallas, Fort Worth, Houston, and San Antonio Areas.