Earth and Water

How Deep to Dig / Striking Water

Previously our discussion was briefly based on elasticity, stress, saturation and unsaturated soil, and effects of the soil when loading and unloading occurs. Our goal is to provide a solid base for our footings and foundation walls.

Clayey type soil and the behaviours associated with it when combined with water. We stated; this type looses strength when the water to soil ratio is increased. Normally when the foundation is dug, the deeper we dig the harder the soil will be from natural compaction, the deeper the depth the denser the soil, the tighter the compaction, the smaller the void spaces in the soil.

A problem may exist, when builders put up a structure. They usually don't consider soil as much in light frame construction, because soil in it's natural state, under normal circumstances, will support the single family unit. As a matter of fact; some location through-out the country, building codes allow the builder to place footers without using reinforcing steel rod, that add strength also increase the tensile strength of the footer, allowing added resistance to upheavel that occurs in frozen soil.

Not to worry, these questions and more, concerning soil and the structure are properly addressed by soil engineers and planners, at the planning and drafting stages.

For example:

Do foundation loads match loadings assumed by soils engineer?
Have earth pressure, including effect of surcharges and sloping fill, been included in the design?
Has correct assumption of use of active or passive forces been applied?
Do all wall elements have adequate resistance through friction, cohesion or passive pressures?
Have hydrostatic forces been applied to the wall, or have adequate steps been taken to remove pressure?
Was weight of soil slabs and superimposed loads included in the design of the footings?
Are elevations of foundation consistent with recommendations in the soil reports?
Has foundation settlement or heave been addressed in the design?
Have horizontal forces, applied to deep foundations, been adequately addressed?
Does pile spacing match requirements for soils report and pile cap design?
Has settlement of large fills, adjacent to rigid members, been assessed?
Can both downward and upward loads be transmitted from pile cap to piles?

So, engineers keep an extensive report, design and soil studies, before we start the build.

Safety factors are stressed with the depth of the dig and the soil type, the sandy soil types will likely need reinforced shoring if the dig is over three or four feet. the backing will prevent caving's protecting workers.

Water Table

In the B horizon, which is the depth of earth just below the A horizon which is normally one inch to three feet deep, and composed mainly of top soil. The lower the water content the greater the load bearing strength, now where the water table is concerned, depending on the area and the region of the country. The water table is the depth at which the soil is completely and permanently saturated. This information is available from building inspectors, or the building department etc. This is good to know. For example say you find the water table is at 18 feet and your basement is 8 feet, the water table will be approximately 10 feet below your basement. With this in mind, if your basement is wet you'll instinctively know that the problem is not a water table problem, but a natural drainage or runn-off. And you'll need to look through other avenues to find the problem. Gutters for example may be broken, downspouts missing or the drainage from the building not adequate or it could be a water-proofing situation, or even, drainage into the soil from a broken or damaged water line from a construction site or city repair.

If in the B horizon the type of soil is a clay type there may exist a hard pan where there is low water seepage and you have a greater surface saturation of water unable to penetrate below the high dense clay level the A horizon soils ( top soil ) is normally saturated and will not take on or soak any further. Therefore allowing water to stand at higher levels during in-climate weather or the winter months when snow is on the ground. Again this is due to improper drainage and needs attention to correct this problem.