Are All the Good Lots Going, Going, Gone?
Making Way for WAFFLEMAT Foundations
With most "good lots" gone, residential home builders are regularly faced with a major challenge: expanding construction activities into areas previously considered less desirable (e.g., hillsides, shallow water tables, highly expansive or collapsible soil).
Traditionally, one of the following three foundation approaches have been incorporated by home builders in these situations:
- Post-tension (P/T) with slabs on grade
- Drilled pier and grade beam with raised floor assembly
- Driven pile with both slab and raised floor when over bay mud
Unfortunately, each of these approaches brings a series of individual and combined limitations, including increased material costs, extended building cycle and the need to mitigate present/future risk and warranty issues.
As a result, builders have started to integrate alternative foundation types.
The WAFFLEMAT foundation system application for expansive soils has gained great interest for many home builders as an alternative, particularly to the P/T slab-on-grade used in a "marginal soil" environment.
The WAFFLEMAT system has voids underneath (formed with the 8.5" high WAFFLEBOX), allowing the expansive soil to move, as opposed to the P/T slab that rests entirely on the grade and responds in turn to soil expansion. Even if water was to seep underneath, the voids provide space for the soil to expand and contract, thus avoiding the uplift stress that, in excess, may cause the P/T slab to fail (crack).
On the Bay Meadows project, our office turned to the WAFFLEMAT foundation system when P/T slab-on-grade was analytically proven to be incapable of providing satisfactory edge-and-center deflection requirements, as well as exceeding the geotechnical engineer's allowable bearing pressures.
WAFFLEBOXES are made from 100% recycled reprocessed polypropylene (plastic), a "green" product.
Nominee for '07 Air Quality Management Award in Greenhouse Gas Emissions Reduction category, due to 20% mitigation of concrete use (eliminating 5 to 9 tons of CO 2 emissions) per residential foundation.
WAFFLEMAT has been implemented in 7 million square feet of residential units or approximately 4,000 homes in Northern California, which equates to 20 to 36 tons of CO 2 emissions eliminated over the past 10 years.
The polypropylene material used is tough and impact resistant, ensuring that the WAFFLEBOXES will hold up well in the construction environment, including being strong enough to support the weight of the concrete workers during the installation and the pouring process. It is a highly stable and chemically nonreactive resin that, when covered with concrete, will last decades before any degradation. It is impervious to water as well as water vapor, thus providing an effective moisture barrier.
A fairly good rule of thumb for calculating the number of WAFFLEBOXES required for a given floor is to divide the total square footage of the foundation by 4 to 4.5. For example, for a 2,000 square footprint, the approximate range of required WAFFLEBOXES would be 444 to 500 boxes.
The amount of time required to install WAFFLEBOXES is determined by several factors; however, on average 75 WAFFLEBOXES are typically installed per hour.
For over 10 years, WAFFLEBOXES have been manufactured by Kennerley-Spratling, Inc., in San Leandro, CA, ksplastics.com
July 2007 Builder Architect Edition Issue