The writer explains that when a tunnel does not go deep underground, ____ .

In the case of shallow tunnels or in urban areas it is often
possible. by means of carefully sited boreholes, to gain an
idea as to the nature of the ground and water conditions.
Under high mountains boring becomes expensive so
reliance has to be placed upon geological interpretations.
As strata can vary so much, surprises are often met with
and techniques sometimes have to change in a single
tunnel. In the Severn railway tunnel (4 mls 628 yd long,
completed in 1886) great quantities of water were
unexpectedly encountered and are still being pumped out.