Engineering Geology

E n g i n e e r i n g G e o l o g y
0.53 and 0.65 mm, respectively. Generally, larger pores were found in the Upper Chalk and in the
southern area. In southern England the median pore diameter in the Lower Chalk was 0.22 mm,
a feature attributed, in part, to a high marl content.
The permeability of chalk is governed by its discontinuity pattern rather than by intergranular
flow. As can be seen from the values given in Table 5.33, chalk has a high porosity but when
the values are compared with intergranular permeability the relationship is poor. The values
of primary permeability obtained by Bell et al. (1999) are more or less the same as those
found by Ineson (1962). Ineson quoted a range between 0.1 ¥ 10 -10 and 25 ¥ 10 -9 m s -1 . The
values provided by Bell et al. (1999) are shown in Table 5.33. The reason for the low primary
permeability is the small size of the pores and, more particularly, that of the interconnecting
throat areas. Price (1987) showed from mercury porosimeter testing that the median throat
diameter typically was less than 1 mm, and that throat diameters are smaller in chalks from
the north of England than in those from the south. Due to the operation of capillary and
molecular forces, drainage of the “larger” pores (according to Price, their median diameters
are approximately 5 mm) via such throats will not occur unless a suction on the order of 30 m
head of water (approximately 300 kPa) is applied. Since gravitational drainage represents a
suction of about 10 m, chalk has a very high specific retention.
The unconfined compressive strength of chalk ranges from moderately weak (much of the
Upper Chalk) to moderately strong (much of the Lower Chalk of Yorkshire and the Middle Chalk
of Norfolk). However, the unconfined compressive strength of chalk undergoes a marked
reduction when it is saturated (Bell et al., 1999). For instance, the Upper Chalk from Kent may
suffer a loss on saturation amounting to approximately 70%. Chalk compresses elastically up
to a critical pressure, the apparent preconsolidation pressure. Marked breakdown and
substantial consolidation occurs at higher pressures. The coefficients of consolidation, c v ,
and volume compressibility, m v , are around 1135 m 2 a -1 and 0.019 m 2 MN -1 , respectively.
The Upper Chalk from Kent is particularly deformable, a typical value of Young’s modulus
being 5 ¥ 10 3 MPa. In fact, it exhibits elastic–plastic deformation, with perhaps incipient
creep prior to failure. The deformation properties of chalk in the field depend on its hardness,
and the spacing, tightness and orientation of its discontinuities. The values of Young’s
modulus also are influenced by the amount of weathering the chalk has undergone (Ward
et al., 1968).
Discontinuities are the fundamental factors governing the mass permeability of chalk. Chalk
also is subject to dissolution along discontinuities. However, subterranean solution features
generally tend not to develop in chalk since it is usually softer than limestone and, hence,
so collapses as solution occurs. Nevertheless, solution pipes and swallow holes are present
in chalk.
270