Concept:
The coefficient of consolidation is given by
\[
\boxed{
C_v=\frac{k}{m_v\gamma_w}
}
\]
where
\[
k=\text{Coefficient of permeability},
\]
\[
m_v=\text{Coefficient of volume compressibility},
\]
and
\[
\gamma_w=\text{Unit weight of water}.
\]
Thus,
\[
C_v\propto\frac{k}{m_v}.
\]
Step 1: Write the proportional relationship.
Initially,
\[
C_v\propto\frac{k}{m_v}.
\]
After the changes,
\[
k'=2k,
\]
and
\[
m_v'=\frac{m_v}{2}.
\]
Step 2: Calculate the new coefficient of consolidation.
\[
C_v'
=
\frac{2k}{m_v/2}
=
\frac{2k\times2}{m_v}
=
4\frac{k}{m_v}
=
4C_v.
\]
Therefore,
\[
\boxed{C_v'=4C_v.}
\]
Hence, the coefficient of consolidation increases four times.
Therefore, the correct option is
\[
\boxed{(C)\;\text{Increase by 4 times}.}
\]