Saturday, November 3, 2012
Construction Grading Jobs and Soils Work
Construction grading jobs and soils work: these are arguably the phases of construction work that can cost the most per hour with the greatest risk of surprises.
Large pieces of land grading equipment with their heavy equipment operators cost in the $100’s per hour to run. The basis of the grading work is the evaluation of the soils conditions derived from scattered samples. You really don’t know what you will find when you start uncovering the ground. No other work can proceed until the ground is ready for construction and if surprises are found – you have no choice but to rectify them immediately.
The long term success of your custom home’s foundation and structure begins with proper bearing soil. Your home foundation is engineered to the type and bearing capacity of your soil. There are many types of soils, based on their clay to sand ratio, amount and type of rock and degree of stability. Most top soil is unsuitable for home foundations and can be easily compacted allowing for little bearing foundation bearing capacity. Most home sites, even if the grade elevations are not changing, will have the top few feet removed and replaced. This replacement can be a reinstallation, under controlled conditions, creating an even density and bearing throughout the build area.
If a home site is underlayed with formational soils (very firm, old ground) the weathered soils above will still need to be replaced and keyed into this firm layer. There are times when the depth of home footings can extend into this layer and the need for removal and recompaction is greatly diminished. In the southern California, coastal regions, this usually only occurs when a house basement is constructed with the home. The depth of the home foundation under the basement can easily be set deep into the underlying soil formations.
In locations where stable soil is located very deep on the site, coastal hillside or cliff side lots, we will drill down to reach the formational layer and extend the house foundation down with caissons. Caissons are vertical tubes of reinforced concrete typically extending three times the depth of the loose unstable top soils, into the formations below. This is a more costly foundation system for most homes but the only alternative when stable soils lie too deep to reach with traditional grading. The other use for a house caisson system is to shore up the side of an excavation against a nearby property line. This is to avoid the chance of the grading cut collapsing.
The surprises can come when unexpected soil conditions exist. The Soils Report can only assure us of conditions where the samples were taken. I have been on a project where an old stream bed lay between two sampling sites. The soil in this area had to be removed to a depth of sixteen feet and replaced. An expensive surprise but work that had to be done to ensure a home that could last many years with a solid and well founded home foundation system.
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