Environmental Engineering Reference
In-Depth Information
Figure 5.17.
Rotary drilling rig-non coring, also known as wash boring.
Non-coring bits can penetrate rock and care must be taken to change to coring
methods as soon as resistance to penetration is encountered.
When drilling in sandy soils above the water table and in most soils below the water
table, drilling mud is used to support the hole sides and assist in the recovery of drill
cuttings. Either bentonite or more commonly chemical mud is used to form a cake on
the sides of the hole (
Figure 5.19
)
. By maintaining a head of mud above the water table
the excess pressure supports the sides of the hole. Bentonite mud must remain dispersed
to be efficient. In saline ground-water the bentonite may flocculate and reduce the ability
of the mud to support the sides of the hole.
Some chemical muds can be designed to maintain their viscous nature for a limited time
and revert to the viscosity of water after a few hours. These muds have the advantage
of not affecting the permeability of the material surrounding the hole and are suited to
holes where permeability testing is required.
