In Texas the types of structures most often damaged from swelling soil include foundations and walls of residential buildings, light (one- or two-story) commercial buildings and retaining walls. Lightly loaded one- or two-story buildings such as residences are especially vulnerable to damage because these structures are less able to suppress the differential heave of the swelling foundation soil than heavy, multi-story structures.
What Type of Damage Is Caused?
Damage sustained by these structures includes: distortion and cracking of pavements and on-grade floor slabs, cracks in grade beams, walls, and drilled shafts, jammed or misaligned doors and windows, and failure of steel or concrete plinths (or blocks) supporting grade beams.
The magnitude of damages to structures can be extensive. It may impair the usefulness of the structure and detract aesthetically from the environment. Maintenance and repair requirements can be expensive, and the expenses can grossly exceed the original cost of the foundation.
What are the Causes?
The leading cause of foundation heave or settlement in expansive soils is change in soil moisture, which varies due to rain fall, vegetation, irrigation and drainage. Differential heave may be caused by non-uniform changes in soil moisture, variations in thickness and composition of the expansive foundation soil, non-uniform structural loads, and the geometry of the structure.
Doming Heave
Heave of foundations, although often erratic, can occur with an upward, long-term, dome-shaped movement that develops over many years. Movement that follows a reduction of natural evapotranspiration is commonly associated with a doming pattern of greatest heave toward the center of the structure. Evapotranspiration refers to the evaporation of moisture from the ground surface and transpiration of moisture from heavy vegetation into the atmosphere.
The pattern of heave generally causes the external walls in the superstructure to lean outward, resulting in horizontal, vertical, and diagonal fractures with larger cracks near the top. The roof tends to restrain the rotation from vertical differential movements leading to additional horizontal fractures near the roofline at the top of the wall. Semiarid, hot, and dry climates and deep water tables can be more conducive to severe and progressive foundation soil heaves if water becomes available.
Cyclic Heave
A cyclic expansion-contraction related to drainage and the frequency and amount of
rainfall and evapotranspiration may be superimposed on long-term heave near the perimeter of the structure. Localized heaving may occur near water leaks or ponded areas. Down warping from soil shrinkage may develop beneath the perimeter during hot, dry periods or from the desiccating influence of trees and vegetation located adjacent to the structure. These edge effects may extend inward as much as eight to ten feet. They become less significant on well-drained land. Heavy rain periods may cause pending adjacent to the structure with edge lift and reversal of the down warping.
rainfall and evapotranspiration may be superimposed on long-term heave near the perimeter of the structure. Localized heaving may occur near water leaks or ponded areas. Down warping from soil shrinkage may develop beneath the perimeter during hot, dry periods or from the desiccating influence of trees and vegetation located adjacent to the structure. These edge effects may extend inward as much as eight to ten feet. They become less significant on well-drained land. Heavy rain periods may cause pending adjacent to the structure with edge lift and reversal of the down warping.
Edge Heave
Damaging edge or dish-shaped heaving of portions of the perimeter may be observed relatively soon after construction, particularly in semi-arid climates on construction sites with pre-construction vegetation and lack of topographic relief. The removal of vegetation leads to an increase in soil moisture, while the absence of topographic relief leads to ponding. A dish-shaped pattern can also occur beneath foundations because of consolidation, drying out of surface soil from heat sources, or sometimes lowering of the water table. Changes in the water table level in uniform soils beneath uniformly loaded structures may not contribute to differential heave. However, structures on a deep foundation, such as drilled shafts with a slab-on-grade, can be adversely affected by a changing water table or changes in soil moisture if the slab is not isolated from the perimeter grade beams and if internal walls and equipment are not designed to accommodate the slab movement.
Do You Have Cracks in Your Walls?
Have you been told they are cosmetic and can be patched up? Sometimes, covering up or ignoring cracks can lead to very costly problems. Soils in the north Texas area contain highly active clays which exhibit a high degree of expansion when wet and shrinkage when dry.
In Texas, homeowners have long been concerned with movement in their foundations and the subsequent cracks that can appear in walls around doors and windows caused by differential movement. When you find a crack what should you do? The answer is to monitor it over time to see if it opens or closes indicating movement. To do this effectively, you need to set up crack gauges to measure movement of structural cracks in concrete, brick and masonry.
The monitoring process is carried out over a number of weeks (or months) and allows the movement of the home to be measured. The cost of the monitoring survey is $580 and includes the installation of two crack gauges. The gauges are then read at 4 and 8 weeks. Additional readings can be taken over shorter or longer periods at a cost of $125 per visit. At the completion of the monitoring period you will receive a report detailing the findings. If you have cracks in your walls, act now. Costs for major foundation repair can be astronomical! It's always less costly to fix problems when they are small
