260 Excavation principles
(3) Adverse ground conditions can cause large blocks to become un-
stable at the face, flooding and overbreak, all of which cause delays.
The further one tunnels, the greater is the chance of encountering
adverse ground conditions.
(4) If a hard rock machine encounters water-bearing fractures, these
will also cause delays. Moreover, the further one tunnels, the greater
is the chance of encountering a fracture discharging more water than
any previously intersected.
(5) 'Unexpected events' such as power cuts, broken conveyors, de-
railments and strikes by the work force will also contribute to a
lowering of the U value, and the chance of these occurring increases
with tunnel length and time.Q15.9 When an underground excavation is made in a rock mass,
there are three primary effects, all three of which result from New-
ton's 3rd law, i.e. for every action, there is an equal and opposite
reaction. Thus, when new underground space is created, the res-
istance of the rock previously occupying the space becomes zero,
forces cannot be transmitted from the rock into the space, and the
water pressure is reduced to atmospheric pressure.
Draw a sketch of these three effects and explain why they cause a
so-called Excavation Disturbed Zone (EDZ).A15.9 The three primary effects of excavation are illustrated in the
sketch below, taken from ERM 1. There are many potential complicating
factors when an excavation is made but these complications can all be
considered within the context of the three primary effects. For example,
there can be 'coupled' effects when the alteration of the rock stresses
opens fractures, which then conduct more water into the excavation. A
major fracture in the excavation sidewall may exacerbate the effects and
it may take some time for all the effects to be fully realized.
The three effects will always occur. Therefore, there will always be an
EDZ around an excavation. The amount by which this zone of inevitable
disturbance is further enlarged will depend on the excavation method.
Thus, there is both an initial inevitable disturbance to the rock mass
and an additional disturbance to the rock mass, the severity of the latter
depending on the excavation method. It is incorrect to say that a TBM
will not disturb the rock mass (because of the inevitable effects, which
could have severe consequences), but a TBM will generally induce less
additional disturbance than blasting.
Two of the effects depend on the size of the excavation. Other factors
remaining the same, the inward rock movement does depend on the
excavation size: if the size is doubled, the magnitude of the inward
rock movement will also tend to be doubled. Also, larger excavations in
the same host rock will have larger amounts of water inflow. However,The word 'Disturbed' is preferable to 'Damaged' for the term EDZ because the rock is
indeed disturbed but not necessarily damaged. Whether there is damage or not depends
on the engineering objective.