Erroneous Concepts behind NATM

p i
p i
'R
PLASTIC ZONE
ASSUMED BY NATM
(NOT APPROVED)
PLASTICITY THEORY
'R
V 1
PEAK STRENGTH
V
1
V
RESIDUAL STRENGTH
LOSS OF STRENGTH
Fig. 17 Ground response curve according to Fig. 18 Loss of strength in shear and
plasticity theory in comparison with triaxial tests
that of the NATM
Only the ground response curve that is resulting from the theory of plasticity is theoretically
founded, see Fig. 17. Whether or not a loss of strength (strain softening) is taken
into account - as may be observed in a shear or triaxial test - the curve does not turn
upwards.
In a detailed research report [20] on the use of the ground response curves as a design
basis for the NATM, the possibility of a trough-like Pacher curve is not even mentioned.
Also, in the ITA Guidelines [21] there is no mention of trough-like characteristic curves.
In the publications "Finite element analysis of the NATM" [22] and "NATM and finite
elements" [23] nothing is said about either activated rock ring structures or a Pacher
curve.
p [MPa]
v
2.5 (H = 100 m)
2.0
1.5
1.0
0.5
INFLUENCE OF LOOSENING
D = 10 m
H = 5 m
p = 0.1 MPa
' F
10 20 30 40
G [cm]
F
Fig. 19 Influence of loosening in the roof area Fig. 20 Increase of pressure on the
on the ground response curve roof lining due to rock loosening
(erratic,unpredictable event
which does not lend itself to
numerical computation)
D
W
F
p v
G
G F
3
H 1
W
H
11