The Edge - Q3 2009 - (Page 26)

PERIMETER POOL DECKING | (Third in a 3-part series) by Neil Anderson Frost Heave: How to Avoid It This is the last of our 3-part series on the main soil-related problems for pool decks. We have already talked about loose fill and expansive clay soil in the previous 2009 editions of The Edge; now let’s talk about frost heave and how to avoid it. First, let’s understand the mechanism that causes it. There are two types of movement associated with freezing soil. The first is simply the expansion of a frozen soil mass. This is probably the most common, but the potential swelling associated with it is limited. The second is what is traditionally called frost heave. This is probably not as common for deck movement, but the potential heave or movement from it is virtually unlimited, resulting in significant damage. MOVEMENT FROM FREEZING SOIL MASS In-place, soil is made up of three things: solid mineral particles, moisture or water and air. The average in-place soil has about 33 percent void space between the soil grains. Typically, this void space is fi lled with water and air. If the soil is allowed to become fully saturated, then the water fi lls all of the void space and it holds 33 percent of water by volume. When water freezes, it experiences a 9 percent volume increase. In the soil mass, this volume increase only applies to the 33 percent by volume water. Consequently, the maximum swell that 12 inches of frozen saturated soil can experience is only 3 percent, or 3/8 inch. For 24 inches of soil, this would be 3/4 inch. Th is freezing and slight swelling will not have enough pressure to crack the wall of a well-built pool shell, but it will cause slight heaving of surrounding decking. If, however, the soil is not saturated, then any freezing water in the soil matrix will simply expand into the air voids within the soil, with no change in volume. Volume increase, or “heaving” of frozen soil can only occur if the soil is 100 percent saturated. So the key to preventing this is simply good drainage that keeps water from ponding on or around your decking. Soil will only become saturated if you have standing or ponded water and there is nowhere for it to drain. Soil will not become saturated from the water “wicking up” from below. 26 www.nespapool.org http://www.nespapool.org

Table of Contents for the Digital Edition of The Edge - Q3 2009

The Edge - Q3 2009
Contents
The President’s Message
Commercial Pools Struggle to Comply with Virginia Graeme Baker Act
Energy Crisis = Opportunity
Is There a Solar Energy Business in Your Future?
Project Focus
Five Sources of Error in Water Testing and What to do About Them
Perimeter Pool Decking
Pressure Testing to Detect and Locate Leaks
Index to Advertisers

The Edge - Q3 2009

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