In most instances contractors must cope with the soil that is available; they do not purchase soil specifically suited to the particular job. Fortunately, they can make simple observations and test to guard against potential difficulties. The best simple investigation of the soil is to drill a number of holes on the site with a hand auger. An old ship's auger such as used by carpenters works fine. If the auger is carefully removed it will bring with it samples of the sill. These can be studied in the hand. If the holes themselves are carefully watched some idea can also be obtained of the local groundwater conditions. Where a more through investigation is needed test pits can be excavated. Even if such pits are deep it is often cheaper to dig them than to bring in special boring equipment. In sand and gravel the only consideration is the slope, or angle of repose, at which the soil will naturally come to rest on the sides of the excavation. For ordinary sand and gravel it is always at lest 30 degrees. More serious considerations arise in excavating silts and clays. In fine-grained damp silts and clays, faces that are almost vertical usually will appear to be safe when actually they are unstable. The old construction practice of immediately covering exposed faces of fine-grained sites with tarpaulins to keep them from losing stability by drying out is effective for only a short time. In loose sand or gravel the cribbing must be driven into the ground before excavating. Sheet metal piling or wood piles with sheathing attached may be used for the cribbing. Trouble from backfilling of trenches can be avoided by bringing the backfilled soil to the same density and moisture content that it had before ti was removed. With sand and gravel this is easily done by replacing it in the trench in thin layers and properly compacting and vibrating it as placed.