"The world," said the Oriental sage, "rests on the back of the Great Turtle."
"But what," asked the Western traveler, "does the Great Turtle stand on?"
"On the back of another turtle," was the reply.
"But what does he stand on?"
"A most enlightened and penetrating question," said the sage, "but it's no use, mister, it's turtles all the way down."
The Enlightened Homeowner should keep this tale in mind when considering structural and foundation problems. The house is resting on its foundation, but what is that resting on?
Construction methods, water, and stability of the structure and its foundation, are all interrelated. The structure built without considering these relationships creates the conditions for its own destruction.
An area where disregard of geologic processes can have the most severe consequences is the area of heavy clay. Clay has fine pores and fine grains and holds water tightly. It is not impermeable, as commonly believed, but slowly permeable -- an important distinction. Wet clay will produce a slow, steady seep of water, too slow for water supply, but too fast for a dry basement. Wet spots may not be the only problem; structural damage may occur as well.
The fantastic spires and columns of Bryce Canyon in Utah are not rock; they are clay -- in an arid environment. Clay will stand in sheer verticle cliffs when dry; but when wet, it gains weight, becomes slippery and loses strength and "bearing capacity," which is the ability of the material to support a load without slithering downhill to collapse in a quivering heap on someone else's carport.
Some clays swell severely, generating tremendous pressures. One particularly nasty variety can swell by 200 percent and exert up to 33,000 pounds of pressure per square foot. Its presence in our area is easily recognized by cracked foundations, tilted sidewalks, leaning retaining walls and roads that are masses and layers of patchwork. You can see these effects at Prince George's County at Beltway Plaza and in Cheverly, both built on soils containing expansive clays.
A basement cut into a slope without provisions for removing ground water causes the water table to rise behind the wall. If foundation materials begin to weaken, swell or slide, cracking may occur and more water may enter. At some point, however, the problem may cease to be the mere annoyance of a damp recreation room and become a more urgent problem of saving the house.
Buildings that are cracking and slumping sometimes are subjected to underpinning, a technique in which additional concrete is pinned under the wall to provide a firmer foundation.
Unfortunately, this remedy often treats the wrong "turtle"; the foundation did not fail, the material supporting it failed because it became saturated, weakened and could not support its load. The weight of the additional concrete may actually worsen the situation, causing more serious failure next time.
Many of these problems can be avoided by proper construction techniques. The ideal basement is constructed as follows:
*The bottom of the excavation is covered with four to six inches of gravel. Footings distribute the weight of the walls to the soil.
*The floor is poured over the gravel, abutting the footings, and covered with a sealant.
*Drain tile is laid at the base of the footings.
*The outside of the wall is covered with a thick coat of waterproofing compound.
*The space between the wall and the excavation is filled with coarse gravel, grading upwards to finer gravel, then to sand, and topped with soil graded to slope away from the wall.
Simple, basic and effective. Gravel is very permeable to water and has low capillarity, that is, water does not rise upward through gravel as it does through clay. The gravel base eliminates most capillary water from moving into the concrete floor. Standing water is eliminated by the drain tile placed at the footing.
Tile laid with a slope (i.e., lower at one end of the run than the other) moves trapped water to a safe, lower area or to a sump pump. Waterproofing compound on the outside of the wall forms a barrier to ground water, any stray surface water and water vapor. Gravel backfill against the wall intercepts the ground water that flows down to the drain at the footings before it can reach the wall.
Filling with gravel all the way to the ground surface would collect surface water. So the gravel is topped with a layer of less permeable soil material, sloping away from the house, to allow surface water to run off and, incidentally, to provide a better material for foundation plantings.
What can go wrong?
Concrete may be poured on a too-thin layer of gravel or on bare earth, resulting in a constantly damp floor.
Drain pipe or tile is of no use if it has no particular slope and does not drain in any particular direction, and if no provision has been made for ultimate removal of collected water.
Waterproofing compounds must be used on the outside of the wall, where any water pressure will force the compound into the pores of the building material. Interior waterproofing paints are purely cosmetic. They are useful for brightening up mildew and sometimes good for fooling potential home buyers, but are completely useless for stopping water. Any water simply pushes the paint out of the pores on the way to the interior, leaving blisters, bubbles, or great pealing, drooping sheets of paint hanging from wet walls.
In clayey soils, the worst mistake is a very common one: backfilling with the original clayey material (sometimes mixed with beer cans, mortar chunks, brush and other trash. The holes in drain pipe sometimes are "protected" from clogging by covering with sheets of tarpaper. This further reduces the available drainage area, because water can enter the pipe only at its base, not at the top.
Because clay does not pass water easily, attempting to drain the material by the very limited area of holes in drainpipe or tile is extremely inefficient. Gravel fill, on the other hand, allows drainage of the entire cross section of the wall. Retaining walls are even more sensitive than basements to the effects of water and moving soil because they do not have the weight of a house sitting on top to provide stability.
Proper design for retaining walls is the same as for basements, except for use of waterproofing compound (and floor). A broad footing is as important for retaining walls as for basement walls. The footing distributes the weight of the wall over a wide area, preventing settling and rotation. A relatively heavy wall built on a relatively tiny base (just one brick wide) tends to sink into its foundation just as a heavy human on little pointy heels would if she strolled across the yard on a soggy day.
Unfortunately, it is not always possible to supervise the building process or to be certain of what exactly was done during construction. There are, however, quite a number of clues the buyer can watch for to judge the existence or severity of potential problems.
NEXT: Evaluating and avoiding water problems.