12.3 Foundation investigations for Morrow Point dam and power-plant and Oroville dam and power-plant
12.3.2 Geological and geophysical investigations
The area occupied by the dam was mapped at a scale of 20 ft to 1 in; the area extending 1000 ft up and downstream from the dam was mapped at a scale of 50 ft to 1 in. 108 diamond core drill holes were made with total footage of approximately 8000 ft. The technique of 'fan hole' drilling was used to investigate the foundation in a direction along the principal lines of thrust from the dam. Most holes were NX size (3-in diameter), but some BX (2|-in diameter) and a few 6-in holes were drilled for special test purposes.
Percolation tests were made in the drill holes by the rubber packer method, using a pressure of 100 lb/in2 (fig. 12.3).
7400T 7300- 7200(1
.2 71 OOf
left abutment exploratory tunnel
6900- lit abutment
exploratory tunnel
drills 6800"
6700- 6600- 6500- -
Fig. 12.3 Morrow Point dam site showing angle drill pattern and exploratory tunnels (after Wallace & Olsen, 1965).
Investigations for Morrow Point dam 371 Five exploration tunnels, three of which were excavated during preliminary investigations and two during construction, were completed. Four dam-site tunnels, two in each abutment, extend about 100 ft into the rock. One tunnel extends along the entire length of the underground power-house near the top of the proposed machine hall excavation (fig. 12.4). The abutment
"exploration tunnels
S\
6900 ft 6850
6830- 6800"
6750-
6700-
Fig. 12.4 Morrow Point underground power-plant. Exploration tunnels and drill holes in the area of the underground power-plant (after Wolf, Brown & Morgan, 1965).
tunnels provided data on the attitude and severity of the surface and stress relief jointing, and were also used as stations for seismic shots and sites for jacking tests. The lower tunnel on the right side was located along a soft biotite schist bed to investigate what was considered to be the weakest rock at the site.
Drill holes were extended from the power-house tunnel in various directions and television examination was conducted in several of them. This explored geological features not seen from the drill core. Numerous seismic field measurements of the elastic modulus of in situ rock were carried out at Morrow Point. For estimating the E modulus the following simplified formula was used:
£ = ( 1 2 v )2 x 9-3 x 10-5 x d.
In this formula E is expressed in pounds per square inch, v is the wave velocity in feet per second and d is the specific gravity in grammes per cubic centimetre. This value E can be corrected for the effect of Poisson's ratio.
The measured wave velocity, v9 was 12,800 ft/sec and the Poisson ratio, v, was 0-2. For quartzite, E was found to be:
£ = 5 - 3 x 106 (with d = 2-70 g/cm3).
According to Dominy & Bellport, the general test procedure with seismic waves can be applied in several different ways.
(a) Using a number of vibration detectors (to a maximum of 12 and 4 explosion points) measurements can be made at different locations along a line to ascertain the depth and wave velocity values in layers at and below the surface.
(b) Measurements of wave speed can be made, using one vibration detector at different depths in a drill hole with successive explosions at the mouth of the hole. This procedure can be reversed, i.e. having the detector at the mouth of the drill hole and detonating explosive charges at different depths within it.
(c) Measurements can also be made between drill holes by using a single drill hole detector and exploding charges at different depths. In some cases a charge in one hole and a detector at the same depth in another may be used to study velocity distribution.
(d) It is also possible to measure wave velocity from the bottom of a drill hole by exploding a charge, the effect of which is registered on a detector located at a distant point in an exploration tunnel. This procedure was employed at the Morrow Point site to ascertain the elastic modulus of the rock in which the penstock tunnels would be excavated.
(e) Wave velocity can be measured between an explosion point at the end of an abutment tunnel and a detector on the surface (fig. 12.5).
(/) To check the depth of blast damage the Bureau of Reclamation measured wave speed and layer depths at some twenty points on the walls of the power-plant access tunnel which had been driven using the 'smooth wall' blasting technique. In 1962 they also checked the validity of the seismic wave velocity measurements over a section of an abutment. In fig. 12.5, showing the right dam abutment, a seismic line F extended from a shot point, 95 ft from the portal of an exploration tunnel, upward through a section of gneiss and schists for a distance of 176 ft to a vibration detector on the surface. The measured seismic velocity along line F was 10 000 ft/s and with a computed density of 2-75 from tests of samples, the seismic elasticity of the rock was determined to be E = 3-3 x 106 lb/in2. A drill hole was later put down along the seismic line F. The elastic moduli of rock cores were measured and found to be E = 0-86 x 106 lb/in2 for biotite schists, E = 1-2 X 106 lb/in2 for mica schists and E = 4-14 x 106 lb/in2 for micaceous quartzite or quartz-mica schists. The apportioned average laboratory value from static tests of cores was calculated to be E = 3-1 x 106 lb/in2; which compared favourably with the seismic value E = 3-3 x 106 lb/in2.
The whole foundations of the dam and underground power-plant site
Investigations for Morrow Point dam 373
7040T
J 1
Fur!1
-II
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— i n-ii 1LL1
— i n
—mi i i r - I H
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geophone
E x 106Basedon 1st cycle of loading at 10001b/in:
£ = 3 . 1 x 106 lb/in2 apportioned average
laboratory value from static tests of core
exploratory tunnel
7020' 7000-
6980- 6960
^ „ 6940- 6920- - 6900-
•S 6880-
"tlO531ftlejPthl 6 8 6° "
shot
3 6840 0+20 0+40 0+60 0+80 1+00
tunnel stations, right abutment
Fig. 12.5 Morrow Point dam. Correlation between seismic measurements of rock elasticity and static laboratory tests (after U.S. Bureau of Reclamation). Rock grouping: (I) biotite schist, average 0-86; (II) mica schist, average 1-20; (III) micaceous quartzite or quartz-mica schist, average 4*14 x 106.
were explored with boreholes and exploratory galleries and covered with a dense net of seismic lines. On the basis of the many seismic tests supple- mented by jacking tests a most detailed geological mapping of the whole area was possible.