Geology Reference
In-Depth Information
Box 3-1 De
nition of rockhead
Care must be taken in using the term rockhead because it can be de
ned in various ways and the wrong
impressionmay be conveyed within a geotechnical design team that things are clear-cut when they are not.
Geological de
nition
Rockhead is de
ned in BS 3618 (BSI, 1964) as
'
the boundary between super
cial deposits (or drift) and
the underlying solid rock
nition is also adopted by the US Department of the Interior
(Thrush et al., 1968). The term solid rock is de
'
and this de
ned, in turn, in Thrush et al., following Challinor (1964)
as
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rock which is both consolidated and in-situ
'
. Solid is also generally used in a geological sense to
describe formations that predate super
cial deposits (Whitten & Brooks, 1972) as in solid vs. drift
maps. Rockhead used in this way, essentially de
nes a geological boundary usually marking an
unconformity. The solid rock shown on a geological map says nothing about its strength or weathering
state, so rockhead does not necessarily mark a boundary between soil and rock in strength terms.
Geotechnical de
nition
The term rockhead or engineering rockhead is often used in geotechnical design to de
ne a boundary
between soil-like material and rock that is stronger and more resistant, whatever the geological condi-
tions. It is also sometimes used more generally
'
as the level at which the engineering parameters of the
ground satisfy the design parameters for a speci
c project
(GEO, 2007).
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Sometimes the geological pro
le is simple (recent soil over rock) and rockhead is readily de
ned, but
often the situation is more complex and care must be taken not to represent a dif
cult and variable
geological condition in over-simpli
ed diagrams that might be misunderstood by designers. Weaker
material or voids below the
uence on
mass strength, compressibility and permeability and have severe effects on constructability, for example,
collapse of pile borings or sudden in
first occurrence of rock in a borehole might have a controlling in
ow of soil into a tunnel.
ne a simple level for rockhead in regions of weathered rock. In the
opinion of Knill (1978), in the case of karstic limestone, rockhead is the geological contact between
in situ limestone and overlying super
It is particularly dif
cult to de
cial deposits (despite often great complexity due to dissolution
features). Similarly, Statham & Baker (1986) de
ne rockhead as the top of in situ limestone (despite the
presence of sediment
The unevenness
of the top-of rock surface (or rock head in British usage) on karstic limestone presents obstacles for the
designer.
-
in
lled voids
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below rockhead
'
). Goodman (1993) comments that
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'
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Unlike
most other rocks, the existence of a solid-appearing outcrop right at the location of a footing or pier does
not guarantee that good rock will occur below the outcrop.
He notes the many potential dif
culties for design and construction and notes that
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The same is true for other rock types with
large corestones sitting on the ground surface, underlain by severely weathered rock, as discussed by
Ruxton & Berry (1957). An example of a landslide that occurred where rockhead was misinterpreted by
the slope designers on the basis of boreholes that terminated 5m in rock, is described in Hencher &
McNicholl (1995).
Attempting to form a ground model of a site based solely on descrip-
tions from boreholes and test results, without recourse to an informed
geological interpretation of the data, would be like trying to put
together a complex jigsaw without having the picture on the box lid.
Geologists are trained to examine rocks and soils at scales of a hand
specimen or a quarry and to draw conclusions on the likely origins and
