X6 unbalanced pore pressure head (below GWT). A casing may also be advanced to support the borehole, esp. through sand, shell, or gravel. Casing is usually driven by hammer (300 Ib) after a 5-20 ft drill increment. Bits with bottom discharge jets will disturb the soil to be sampled. Side discharge bits will disturb sides where tests may be performed (pressuremeter, borehole shear). Shape of bit depends on soil type. (Figure from Bowles, 1996) o Percussion Drilling is similar to rotary drilling but uses hammer blows to assist cutting. Use for hard soils, tills, and rock. • Sampling (get a piece of the rock, usually pushed in, usually cylindrical) o Disturbance-Samples may be greatly disturbed depending on method of sampling. Choose method for purpose of sample-visual classification, lab classification, or testing. Disturbance affected by sidewall friction - limit length and provide "upset" (enlarged) inside diameter above cutting shoe. Also depends on soil displacement caused by sampler cutting
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