Geology and ground models - Practical Engineering Geology - page 112

10

3

7.5

5

x

2

Most likely strength
φ′ = 36

2.5 , c

′ = 5 kPa °

1

Dry

30 35 40

Friction angle

Figure B3-5.6 Results from numerical analysis. Each point along the lines for the various trial piezometric
surfaces gives a FoS of 1.0. From testing and empirical data, the most likely field strength is φ = 36 degrees
and c´= 5 kPa, which would indicate that the likely perched water pressure was somewhere between levels 2
and 3 at the time of failure (Hencher (1983b), Hencher & Martin (1985)).

An example of a simple ground model prepared for the design of a real
project is given in Figure 3.63. When designing a tunnel one needs to
predict the ground conditions along the route so that one can decide
what tunnelling method needs to be adopted, as discussed in detail in

Urban area
Risk of dewatering, settlement of
buildings and direct damage to
foundations

Tunnel under hill
Potential high in situ stresses
and squeezing ground
May be high water pressure if
connected along faults

Urban area
Risk of damage to
major slope

240.00
220.00
200.00
180.00
160.00
140.00
120.00
100.00
80.00
60.00

Weathered
rock

Ventilation
shaft

Generally good quality rock
extrapolated possible faults

40.00

20.00
0.00
-20.00
-40.00

CHAIN AGE
(m)

Rock Qua lity

GOOD
FAIR
POOR

Figure 3.63 Ground model for tunnel. Predicted rock mass characteristics can be used to estimate
the amounts of reinforcement, such as rock bolts and shotcrete, that will be required. It will also be
used during construction, as part of the risk control, probing ahead, as necessary, to establish zones
of hazardous ground. As illustrated in Chapter 7, tunnelling remains a risky endeavour because very
rarely are ground investigations adequate for characterising the ground along their length.