Page 108 - Engineering Rock Mass Classification_ Tunnelling, Foundations and Landslides

80 Engineering Rock Mass Classification

Use of Eq. (7.14) for predicting the squeezing ground condition poses practical
difficulties as the measurement of the in situ stress and determination of the in situ
compressive strength of a rock mass are both time-consuming and expensive.

ISRM classifies squeezing rock/ground condition as follows:

Degree of squeezing sy/qcmass (ISRM) qcmass/(g Á H)
(Barla, 1995)
No squeezing <1.0
Mild squeezing 1.0–2.0 >1.0
Moderate squeezing 2.0–4.0 0.4–1.0
High squeezing >4.0 0.2–0.4
<0.2

This approach may be used reliably depending upon the values of sy and qcmass.

EFFECT OF THICKNESS OF WEAK BAND ON SQUEEZING
GROUND CONDITION

From the 29 km tunnel of the Nathpa-Jhakri project in Himachal Pradesh (H.P.), India,
it is suggested that squeezing does not take place if the thickness of the band of
weak rock mass is less than approximately 2 Á Q0.4 meters. However, more project data
is needed for a better correlation.

SUDDEN FLOODING OF TUNNELS

The inclined beds of impervious rocks (shale, phyllite, schist, etc.) and pervious
rocks (crushed quartzites, sandstone, limestone, fault, etc.) may be found along a tunnel
alignment. The heavy rains/snow charge the beds of pervious rocks with water like an
aquifer. While tunneling through the impervious bed into a pervious bed, seepage
water may suddenly gush out. The authors have studied four similar case histories at
the Chhibro-Khodri, Maneri Bhali, BSL, and Dulhasti hydroelectric projects in the
Himalayas where sudden flood accompanied by a huge out-wash of sand and boulders
occurred ahead of the tunnel face where several shear zones existed. This flooding prob-
lem becomes dangerous where the pervious rock mass is squeezing ground due to the
excessive overburden. In two projects in the Himalayas, the machines and tunnel boring
machines (TBMs) are partly buried (Kadkade, 2007).

Seepage should be monitored near the portal regularly. The discharge of water
should be plotted along the chainage of the face of the tunnel. If the peak discharge is
found to increase with tunneling, it is very likely that sudden flooding of the tunnel
may take place with further tunneling. Consult the international experts before tackling
such situations.

CHIMNEY FORMATION

There may be local thick shear zones dipping toward a tunnel face. The soil/gouge
may fall down rapidly unless it is carefully supported immediately after excavation.
There are chances of formation of a high cavity/chimney along the thick shear zone.
The chimney may be very high in water-charged rock mass. This cavity should be
completely backfilled by lean concrete.