The mass compressibility of fractured chalk
This thesis is concerned with the mass compressibility of fractured chalk and
its influence on the settlement of shallow foundations. A review of the
literature reveals nineteen case records of load-settlement behaviour from
relatively small diameter « 1m) plate loading tests but only six welldocumented
case records of the behaviour of shallow foundations on chalk.
The plate loading tests indicate that highly fractured near-surface chalk
undergoes yield at relatively low stresses (200 - 400kPa) resulting in a
significant reduction in stiffness. This behaviour contrasts with that observed
in other rock types with similar discontinuity patterns. For chalk it has only
been observed in one case record for a full-scale foundation. Little is
understood about the mechanisms causing yield.
At the time of starting this research, based largely upon the experience
..gained from in-situ loading tests carried out at Mundford, Norfolk (Ward et
aI., 1968), it was known that factors such as fracture spacing and aperture
played an important role in controlling the load-settlement behaviour of
shallow foundations. Little attention was paid to the large variation in intact
properties displayed by the chalk. In this research nine 1.8m diameter plate
loading tests have been carried out by the author on chalks with different
intact mechanical properties and similar discontinuity patterns. These data
are used to evaluate other in-situ tests (such as SPT, surface-wave geophysics
and visual assessment) as means of providing parameters for the prediction
of foundation settlement.
The results of this research indicate that fractured near-surface chalk
undergoes yield within the range of stresses likely to be imposed by shallow
foundations and that the pre-yield stiffness of the rock mass is controlled to a
large extent by the looseness of the fracture-block system, which in tum
appears to be associated with the intact mechanical properties of the rock.
The post yield-stiffness of the rock mass is generally about one tenth of the
pre-yield stiffness and is relatively insensitive to the rock material properties.