Definition - What does Chert mean?
A chert is a sedimentary rock that occurs in nature as layered deposits, concretionary masses or nodules. It's a variety of chalcedony and is formed when dissolved silica is transported into soft sediments, and forms microcrystals of silicon dioxide. A large number of these microcrystals grow and form nodules of irregular shape and sometimes grow large enough to form a continuous layer.
Chert is thought to be one of the hardest rock known to geologists. Sometimes trenchless construction techniques such as horizontal directional drilling (HDD) may encounter chert which can be difficult for the drilling equipment. Knowing the geology of the pipeline route can help contractors to choose the right drilling bit for the subsurface rock condition.
Trenchlesspedia explains Chert
Chert can be encountered in a variety of subsurface operations such as oil and gas exploration, oil drilling, HDD for pipelines and also during tunneling operations using tunnel boring machines (TBM). Extremely hard carbonate lithologies have been drilled using polycrystalline diamond compact (PDC) bits or tungsten carbide insert (TCI) roller-cone bits. However the presence of chert in these formations can raise a very challenging scenario for the drill bits because of slow rate of penetration (ROP) and costly bit changing trips.
Geotechnical and soil investigation is a good way to ensure that the geology of the area is known prior to selecting bits for the drilling process.