True Bedrock
One of the challenges of installing bedrock wells in the Piedmont (and less so in the Mountains) is finding true, competent bedrock in which to set the well casing. Setting the well casing in a socket in competent bedrock is essential for sealing the monitoring well from influences from the saprolite aquifer. Thus, mistakenly setting a casing in anything other than competent bedrock is costly and time consuming, as the well will have to be abandoned and a replacement well drilled.Finding competent bedrock can be difficult because saprolite often contains fragments or boulders of unweathered rock within its clayey matrix. This is due to differential weathering rates of rock bodies. When one drills into one of these "floating" rock bodies, it can be easy to mistake it for true bedrock.
There is no magic bullet to distinguishing competent bedrock from saprolitic boulders. However, the following is a list of a few helpful guidelines for identifying true bedrock:
- Local experience. When drilling in an area in which you do not have much experience, a couple phone calls to local drillers or others who have a good knowledge of the geology of that area is a great way to assess the likelihood that saprolitic boulders will be present in that area.
- Observe the rock cuttings. Do they look like fresh rock, or do some or all of the mineral grains look weathered. Are the cuttings competent, or are they friable, coming apart easily in your fingers. If the cuttings are not competent, there is a good chance you are not in competent bedrock.
- Drill a 10-foot socket into what you think is competent bedrock. If you all the cuttings look competent, there is a good chance that you are in competent bedrock. Besides, setting the casing in a 10-foot socket is a great start to sealing off the well from influence of shallow groundwater and/or runoff.