Engineering Geology

Chapter 9
and minimizes settlement. The extent of consolidation grouting upstream and downstream
of the grout curtain depends on the conditions in the upper zone of the foundation (Kutzner,
1996). Consolidation grouting also improves the contact between concrete and rock, and
makes good any slight loosening of the rock surface due to blasting operations. In addition,
it affords a degree of homogeneity to the foundation that is desirable if differential settlement
and unbalanced stresses are to be avoided. In other words, the grout increases rock stiffness
and attempts to bring Young’s modulus to the required high uniform values. Holes usually are
drilled normal to the foundation surface but they may be orientated to intersect specific features
in certain instances. They are set out on a grid pattern at 3 to 14 m centres, depending on the
nature of the rock. Consolidation grouting must be completed before the construction of
a dam begins.
Casagrande (1961) cast doubts on the need for grout curtains, maintaining that a single-row
grout curtain constructed prior to reservoir filling frequently is inadequate. What is more, he
stated that grouting was useless as far as reducing water pressures was concerned and that
drainage systems were the only efficient method of controlling the piezometric level and
therefore uplift forces along a dam foundation. He further maintained that drainage is the only
efficient treatment available for rock of low hydraulic conductivity that contains fine
fissures. Drainage can control the hydraulic potential on the downstream side of a dam, thus
achieving what is required of a grout curtain except, of course, that drainage does not reduce
the amount of leakage. However, leakage is not of consequence in rock masses where the
hydraulic conductivity is low. In other words, Casagrande contended that for fissured rocks
of low permeability (i.e. less than 5 lugeon units), drainage generally is essential, whereas
grouting constitutes a wasted effort. Conversely, if the permeability is high (in excess of
50 lugeon units), grouting is necessary to control groundwater leakage beneath a dam.
Highways
The location of highways and other routeways is influenced in the first instance by topography.
Embankments, cuttings, tunnels and bridges (viaducts) can be constructed to carry roads
and railroads with acceptable gradients through areas of more difficult terrain. Obviously, the
construction of such structures increases the difficulty, time and cost of building routeways.
Nonetheless, the distance between the centres that routeways connect has to be considered.
Although geological conditions often do not determine the exact location of routeways, they
can have a highly significant influence on their construction.
As highways are linear structures, they often traverse a wide variety of ground conditions along
their length. In addition, the construction of a highway requires the excavation of soils and rocks,
and stable foundations for the highway, as well as construction materials. The ground
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