Engineering Rock Mechanics

Questions and answers: geological sefting 25

model which is based on the geological and engineering rock mechanics

principles.

42.9 A cavern (165 m long, 22 m wide and 15 m high) is to be ex-

cavated in chalk strata beneath the sea. The crown of the cavern will

be 35 m below the seabed. What is the main geological information

you would like to have before proceeding with the excavation?

A2.9 The main danger is that water from the sea will enter the cav-
ern. Hence, much of the geological information should be directed to
establishing whether water inflow will be a problem. In a chalk rock
mass, the water will mainly travel through weathered chalk and the
pre-existing fractures, so information on the degree of weathering and
on the fractures and their characteristics is crucial.
This question was stimulated by the construction of the sub-sea cross-
over cavern on the UK side of the England-France Channel Tunnel. To
paraphase Warren and Varley (1995) 7: "In this area there is a normal
geological succession dipping gently northwards and affected by minor
faulting. The cavern is located within carbonate clayey mudstones form-
ing the lowermost part of the Cenomanian succession, namely lower
and basal chalk, the more sandy glauconitic marl and a clay-dominant
material 7 m thick of carbonate mudstone at the top of the Gault clay.
Weathering at the seabed penetrates down through the overlying grey
chalk and into the upper chalk marl strata to within 20 m of the roof.
Formation mass permeabilities are generally low, i.e. to m/s,
although higher permeabilities do exist in the glauconitic marl (owing
to the presence of open joints) and in the upper chalk marl immedi-
ately above the cavern crown. The rock mass quality was fair to good
with sub-horizontal and sub-vertical joints spaced at l/m and greater
than 2/m, respectively (average persistence 2 m). A number of minor
faults were present, usually of an arcuate nature with downthrows less
than 0.5 m and striking 40" to 220" parallel to one of the major joint
directions."
Given this geological information, we might expect the engineering of
the cavern to be difficult. In fact, few problems were encountered during
construction. Water entering the cavern through fractures was limited to
minor seepage with a maximum local inflow of^5 l/min at the crown,
and a cavern inflow of 50 l/min.

42.10 The pre-existing stress state in a rock mass is caused by geo-

logical processes and is often a critical factor for rock engineering.

Why do you think that quantifying the rock stress is important?

A2.10 Quantifymg the rock stress is important because the pre-existing
stress, concentrated around an excavation, can reach the rock strength
and hence cause rock failure. We will be explaining in the next chapter

7Warren C. and Varley l? (1995) Geology, in Engim'ng the Channel Tunnel (C. J.

Kirkland, ed). E and F N Spon, Chapman and Hall, London, p. 334.