Causes of Foundation Movement
Your homes foundation is designed to transfer the weight of the structure and contents to the underlying soil or rock. On Vancouver Island most foundations constructed today are slab-on-grade; however, pier and beam foundations are still used in certain situations and many older homes have pier and beam foundation systems.
Swelling and shrinking of expansive soils underlying your homes foundation are the primary causes of foundation movement throughout the south. Expansive clay soils swell when soil moisture levels increase and shrink when moisture levels decrease resulting in lifting or settlement of your home’s foundation.
The second most frequent cause of foundation movement is consolidation of improperly compacted soil or rock fill, which can cause portions of the foundation to settle.
Because your homes foundation transfers the weight of the structure and contents to the underlying soil, your foundation moves when the soils move. If the foundation moves uniformly and does not deflect or become unlevel the foundation is performing as designed. If some parts of the foundation move more than others then the foundation is undergoing differential movement. It is this differential movement that damages your foundation and the cosmetic finishes throughout your home.
Limiting Differential Movement
Because soil moisture variations cause swelling and shrinking of the soils supporting your foundation anything you can do to minimize those moisture variations will also minimize foundation movement. The three most common examples are:
1) Water your lawn and the areas adjacent to your foundation uniformly during dry periods.
2) Make sure that surface water cannot pond against the perimeter of the foundation.
3) Large trees or extensive plantings of shrubs in close proximity to the foundation can result in drying of the soils under the perimeter of the foundation. If existing trees or shrubs are affecting the stability of your foundation, a barrier trench can often be installed between the trees and foundation.
Minimizing foundation movement normally involves underpinning the foundation with deep piers, correcting poor drainage or both. Deep piers are designed to extend below the active surface soils to areas less affected by season moisture variations. Piers transfer the foundation loads to those deeper, less active soils. The vast majority of registered professional engineers specify deep drilled, steel reinforced, cast-in-place concrete piers. This is the same pier design used in the construction of commercial multi-story buildings and highway overpasses.
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