Definition - What does Silt mean?
Silt is alluvial soil. It might be sand, clay or even gravel so fine it's indistinguishable from either. It often appears with sand or clay deposits. Its accumulation can dam up streams, can change the course of rivers or form a delta.
Because it has the consistency of a handful of ball bearings, it's often an unstable soil for tunneling or through which to bore and can cause a bore to become mechanically unstable or even collapse.
Trenchlesspedia explains Silt
A bore through silt that goes awry can mean a fractured borehole, a collapsed bore, borehole enlargement or closure. It requires attention to the makeup of the drilling mud used. Likewise, it requires careful attention to the pressure of the mud and requires the driller to be very aware of the drill speed.
Shales are sedimentary rock and consist of clay (<12 percent), fine sand and silt >(80 percent). Drilling through shales requires the use of a silt stabilization emulsion, such as calcium cholride brine, in an oil-based drilling mud.