Field and Laboratory Tests in Rock Mechanics by L. G. Alexander
The factors are determined for each site for a depth behind the surface about 1/3 the depth of the jack, as the stress given by the jack test is assumed to be more representative of the stress at this depth than at the surface, if there is a variation.
The equations may be solved by the method of least squares. However, when in each group of sites the corresponding factors are of similar magnitude (within ± 15% for the present sites) there is no significant error in taking averages of the equations in each group.
We obtain,
V . . . . . 1,700 = 1.9v − 0.6t,
T . . . . . 2,400 = 2.0t − 0.8v,
L . . . . . 1,400 = 0.9l + μ(0.9v − 1.6t).
These yield v = 1500, t = 1800, l = 1900 p.s.i. A significant result is that the values obtained are high compared with the corresponding vertical and horizontal gravity stresses, computed from the overburden.
With a view to explaining the high values, Worotnicki (S.M.A. internal report) has suggested that the river valley in the vicinity of the exploratory tunnel may be regarded as a V-notch, which concentrates both the vertical and horizontal stresses. The results of a photoelastic study are:
a) Vertical stress:
| Vertical gravity stress, (for an averaged overburden estimated at 700 ft.) |
840 p.s.i. |
| Increase from valley notch concentration, 20% |
170 ,, |
| Component from concentration and deflection of horizontal stress |
130 ,, |
| Total theoretical estimate | 1,140 p.s.i. |
| Total computed from tests |
1,500 p.s.i. |
(b) Horizontal stress:
The horizontal gravity stress is taken to be
that originating under the surrounding
mountainous country of high elevation
(4,000 ft. average elevation) at the level of
the test tunnel (1,785 ft,). Added to this
is assumed a tectonic stress, of unknown
magnitude and origin. The combined
stress is assumed to be concentrated under
the valley. The result is :
| Horizontal gravity stress | 600 p.s.i. (i) |
| Additional or tectonic stress assumed | 670 ,, (ii) |
| Increase from valley notch concentration, 50% (of (1) + (ii)) | 630 ,, |
| Total computed from tests | 1,900 p.s.i.
|
6.— Validity of Photoelastic Method of Analysis.
As the effect of jointing on the “elastic” modulus of rock “en masse”, and the possible disturbance to the surface stresses caused by blasting, are imperfectly understood at present, the following supporting evidence for adoption of the elastic hypothesis for analysing the stress pattern around an excavation is given:
(a) Field tests have generally given higher stresses than have been expected from the overburden. This does not support the hypothesis of a shattered stress-reducing zone around the excavations (in granitic rock at depths of 650-1,100 ft.).
(b) Tests at the abutment corners of Tumut 2 Power Station excavation, measured during the early phase of excavation, showed high stresses (averaging 6,600 p.s.i.) as predicted by photoelastic tests. Further, as the excavation was deepened, the downstream sites gave an 11% increase, whilst on the upstream side there was a decrease of 25%. These are consistent with the change predicted by photoelastic tests for the effect of a primary shear stress field of 200-300 p.s.i.
