Issue link: https://read.dmtmag.com/i/506676
MAY 2015 8 THE JOURNAL BY GEORGE PORTER SERVICE & SET-UP Sand Over the years I have seen sand play a large role in the proper installation of a home. First, what is sand? Well, there are many types of sand and they are used for different things. The term sand is a very general class of soil; it is like say- ing "rock". Webster defines sand as, "a loose granular material resulting from the disintegra- tion of rocks." While Mr. Webster usually knows what he is talking about; in this case he may want to expand his definition a bit. If you go to the Florida Keys you will find quite a bit of sand but no rocks. You may see something that looks like rock, but it is called coquina. It is composed of coral and shells that became a solid mass about a million years ago. When the waves beat on it, it crumbles and grinds on itself and makes sand. How much the rocks/coral get beat up by water helps determine how fine the sand gets. The grain sizes and the type sands mix de- termine their load bearing capacity. In order to better understand sand lets talk about really big grain sizes like gravel. Gravel is rock on the way to becoming sand; it just needs to be beat up some more for a few centuries. If you take a load of ¾ inch gravel and spread it on the ground to make a road it will pretty quickly spread itself apart where the tires go and you will have two dirt tracks in the otherwise gravel road. Gravel that is all the same size will roll on itself and move around away from the pressure. Pro- fessional road builders will always order stone with a "binder." The binder for ¾ inch gravel could be rock dust or any very small stone that will kind of fill in the gaps between the big stones and lock it all in place. Have you ever noticed that after the two ruts form in the gravel road they seem to kind of stay put if there is any base under them? The "lock" for the stone is the dirt under it. When it mixes it tightens up if the soil is not too organic. Soils with organic material in them never lock up because the soft organics just mush out of the way. Trying to make a road on topsoil could cost you a fortune in stone and you would never have a decent road. The stone would just disappear and you would always need another layer, probably for decades. Let's get back to sand. If you look in all the manuals for HUD Code homes you will find a chart that gives the load bearing capacity for soils. When you find sand they all say 1500 Pounds per Square Foot (or PSF). This means that sand can hold 1500 lbs. on a 12-inch by 12-inch square of itself and it won't sink or give way under the weight. (According to the chart) Do you know where NASCAR racing started? Daytona Beach Fla. Do you know where they raced? On the beach! If you have ever been surf fishing in a four wheel drive you know that you have to let lots of air out of the tires (down to about 15 psi) and going fast would not be some- thing you would like to do. So the sand at Day- tona is very different from most beaches. So is the sand like surface at the Bonneville Salt Flats. These soils have a binder in them and you don't sink in like most other sands. So in truth, all sands do not have the same load bearing capac- ity; the chart uses a worst case scenario. If you think you have Daytona Beach type sand in Mis- souri you need to get an engineer to say so. Sands do have one thing in common every- where you find them…. they drain like crazy. Pour a bunch of water on sand and it will disap- pear as fast as it hits the stuff. (Assuming that the water table is not at the surface) In many manuals today you will see that if you dig out a footing and fill it with sand that you can stack the pad and supports right on top of the sand and it is considered frost free. Well…. maybe not in all cases. If you dig a hole for the footing in "well drained" soil then yes, it will work. But, what if it is dense heavy stuff like greasy red clay? The kind you could make a brick out of if you wanted to? This stuff doesn't drain! When you make a hole in it you are really making a bucket in the ground. Guess what happens if you fill a bucket up with sand; add water and freeze it? It swells up of course, because it is a big chunk of ice. Even if it doesn't freeze and the water just satu- rates the sand. Can you guess what happens to the load bearing capacity? It gets way below 1500 PSF and the home sinks out of level. Recently I got to look at a home that had a basement. ("Had" being a clue here) When they dug the hole for the basement they saved the dirt. (Heavy dense dirt) They built the tall walls for the basement complete with lots of drainage tiles all around the foundation. They were not going to have a wet basement for sure. When all this was done and the home was in- stalled they put the original dirt back around the walls. The basement walls collapsed under the home during a big storm and the people in the home were lucky to be alive. Sand would have changed everything here. Backfilling with a soil that allowed the water to reach the drain tiles would have been a good thing! Using sand is an example of what we find in manuals today but there is some advice that is missing in all manuals that should be on every first page. "Besides faithfully following the direc- tions in this manual it may be necessary from time to time that the installer also be allowed to think and apply good reasoning." George Porter is a consultant to the manufactured housing industry. His Company is Manufactured Housing Resources, P.O. Box 863, Rehoboth Beach, DE 19971, (302) 645 5552, Web: www.george- porter.com Some of his services are both in person and On-line training for certification in many states plus expert witness and investigation for the industry. T J