Over the years, the question of whether or not to have a home’s foundation engineered has come up numerous times. Is it worth the extra cost to drill 20-25 feet into the ground to get soil samples and have them analyzed in the lab? Our answer is yes. Your home is too big of an investment not to make sure that the foundation is designed correctly because your home is only as good as the foundation that it is sitting on. At CGM Home Builders, we always suggest having an engineering company design the foundation for your new home. It is extremely important to know what type of soil is under your home, in order to know how much it will expand or contract due to moisture. The following article was provided by Gessner Engineering, in College Station, TX.
The soils in the Brazos Valley, which include sands, gravels, and clays, can vary over relatively short distances. All of these different soil conditions can pose potential construction and foundation design issues. The clays in this area can be highly plastic and exhibit expansive soil.related movement properties. Structures constructed on-grade may experience differential movements due to volumetric changes of the underlying and surrounding soils. Movement of expansive soils is caused by fluctuations in the moisture content of soil particles. Because homogeneous expansive clay soils have very low permeability, fluctuations in the moisture content of the soils might normally be expected to occur over a very long period. However, permeability is increased with geotechnical phenomena such as ground faults, surface fractures due to desiccation of clays, and decomposition of tree roots which cause fissures and cracks that become widely disseminated over time.
Due to the repeated wetting, swelling, drying, and shrinking of the clay as it weathers, the fissures often fill with silt and sand, and create pathways for water that can exacerbate the infiltration process. Water can also easily move through naturally occurring sand strata, sand seams, and micro-cracks in clay soil caused by previous shrinkage. High negative pressures, also known as suction, in clay soils with low water content also increase the tendency for water to be absorbed into the clay.
Environmental factors other than climatic conditions can also affect expansive soils. Water extraction by trees and other vegetation, a process known as transpiration, can cause soil shrinkage. Swelling can be a result of water infiltration into the soil from lawn irrigation systems, broken water pipes, flooded and leaking utility trenches, poor drainage, or leaking swimming pools, or it can be a result of slow moisture replenishment and equalization after the removal of a tree. The combined effect and variability of all of these possibilities make it difficult to accurately predict expansive soil ground movements.
Foundation movements are considered problematic only if they result in negative phenomena that detrimentally affect the performance or appearance of the building. The negative phenomena are considered to be structural if the load carrying capacity of the superstructure or foundation elements are affected, or are considered to be cosmetic if only the appearance of the exterior cladding or interior wall, floor, or ceiling finishes are affected. Foundation movements can also affect the serviceability of the building, such as the opening or closing of doors. This type of movement typically occurs because of differential movements between various parts of the building. Differential movements often lead to high internal stresses in building components and subsequent cracking and separating of exterior cladding systems such as brick, cement-board panels, or in the interior finishes such as gypsum drywall panels, wood paneling, and flooring.
By having a Geotechnical Subsurface Investigation performed before design and construction of the building, the owner will be able to better prepare for the conditions on the site. Engineers and contractors can then work together with the owner to develop the appropriate engineered foundation system for their project.