Engineering Geology

Chapter 5
when peat possesses high mineral residues. The specific gravity of peat ranges from as low as
1.1 up to about 1.8, again being influenced by the content of mineral matter. Due to its extremely
low submerged density, which may be between 15 and 35 kg m -3 , peat is prone to rotational failure
or failure by spreading, particularly under the action of horizontal seepage forces.
In an undrained bog, the unconfined compressive strength is negligible, the peat possessing
a consistency approximating to that of a liquid. The strength is increased by drainage to
values between 20 and 30 kPa and the modulus of elasticity to between 100 and 140 kPa.
When loaded, peat deposits undergo high deformations but the modulus of deformation
tends to increase with increasing load. If peat is very fibrous, it appears to suffer indefinite
deformation without planes of failure developing. On the other hand, failure planes nearly
always form in dense amorphous granular peat.
If the organic content of a soil exceeds 20% by weight, consolidation becomes increasingly
dominated by the behaviour of the organic material (Berry and Poskitt, 1972). For example,
on loading, peat undergoes a decrease in permeability of several orders of magnitude.
Moreover, residual pore water pressure affects primary consolidation, and considerable secondary
consolidation further complicates settlement prediction.
Differential and excessive settlement is the principal problem confronting the engineer working
on peaty soil (Berry et al., 1985). When a load is applied to peat, settlement occurs
because of the low lateral resistance offered by the adjacent unloaded peat. Serious shearing
stresses are induced even by moderate loads. Worse still, should the loads exceed a
given minimum, then settlement may be accompanied by creep, lateral spreading, or in
extreme cases, by rotational slip and upheaval of adjacent ground. At any given time, the
total settlement in peat due to loading involves settlement with and without volume change.
Settlement without volume change is more serious for it can give rise to the types of failure
mentioned. What is more, it does not enhance the strength of peat.
Description of Rocks and Rock Masses
Description is the initial step in an engineering assessment of rocks and rock masses.
Therefore, it should be both uniform and consistent in order to gain acceptance. The data collected
regarding rocks and rock masses should be recorded on data sheets for subsequent
processing. A data sheet for the description of rock masses and another for discontinuity
surveys are shown in Figures 5.15 and 2.17, respectively.
The complete specification of a rock mass requires descriptive information on the nature and
distribution in space of both the materials that constitute the mass (rock, water and air-filled voids)
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