Any one who has spent time in Borrego has likely observed a contractor preparing a site for pouring a foundation pad. Typically, several high capacity “Rain Bird” sprinklers are allowed to run day and night for at least several days with little monitoring. According to the Borrego Water District (BWD):
- The BWD does not systematically collect data on water use for soil compaction in conjunction with new construction; but anecdotal evidence from one contractor who has built a large number of homes in the valley suggests that about 133,500 gallons or approximately 0.41 acre feet of water per home may be a reasonable estimate. The amount of water used, however, may vary significantly depending on wind, vertical and horizontal dimensions of the foundation pad, the contractor doing the work, etc. BWD reports some recent examples where approximately 1.3 acre feet of water have been used to prepare a single foundation.
- This significant
water use is not addressed in BWD’s proposed tiered rate structure.
Using a construction meter at a rate of $2.70 per unit (1 cubic foot or
748 gallons of water), as most builders do, the total cost for 133,500
gallons of water would be approximately $200. Applying the
proposed tiered rate structure for single family homes to this same
usage would raise the average water cost for compaction to approximately
$652, and the cost at the high end, as represented by recent examples,
to approximately $2,345, an amount that may encourage more responsible
use of our groundwater.
According to a structural engineer of many years experience who is familiar with the Borrego valley:
- While all soils need some moisture for optimum compaction, the amount is critical and varies with soil type. Too much water actually prevents compaction by filling the voids between soil particles. When the excess water drains away or evaporates the void remains and the soil continues to settle with untoward results for the structure that rests on it. The optimum amount of water for any given soil type must be determined by a soils engineering lab; but soils in the Borrego valley will generally require less water than soils of finer granularity found elsewhere.
- In addition to applying the appropriate amount of water, soils beneath a foundation pad should be brought to optimum moisture content as determined by a licensed soils geologist in the field, and compacted in layers of not more than 12 inches using a mechanical compactor. For the best outcome, a soils geologist should conduct compaction tests at the site.
It does not appear that contractors in Borrego bother with any of these technical details, preferring instead to simply pour water on a site until it is saturated, or they remember to turn it off, in order to save engineering costs. In so doing, they are wasting huge amounts of water in a misguided and ineffective attempt to compact soil on the cheap. If builders were charged a proper rate for water used at this stage of construction it would provide an incentive to use appropriate methods, improve the structural integrity of buildings in the valley, and save precious groundwater at the same time.
It seems intuitively unlikely that using water in this way and in these amounts would be permitted in other desert states with similar soil types but more enlightened water laws, e.g. Arizona and Nevada. In any case, it should not be permitted in the Borrego valley given our serious and seemingly intractable groundwater problem.
One may wonder why the County of San Diego Department of Planning and Land Use does not require proper compaction procedures for structures placed on 4 feet of fill in an area prone to earthquakes to protect them from damage caused by future settling. In Borrego, however, it is first and foremost a water problem; and the BWD should be the lead agency in solving it. Unfortunately, the BWD board has shown scant interest in doing so.
11/03/07