12.3 Foundation investigations for Morrow Point dam and power-plant and Oroville dam and power-plant
12.3.3 Laboratory and field tests
Petrographic analysis of rock specimens classified rocks into five types (fig. 12.6). They were given the numbers I, II, III, IV and V, rock number III being the more frequent. Type I is biotite, the weakest rock type. Type II is a mica schist with well-developed but irregular foliation and some jointing.
Type III is a micaceous quartzite with somewhat less developed foliation but a more prominent system of macro-jointing.
Shear and sliding friction tests. Originally it was planned to conduct all the tests in situ in the underground power-plant exploratory tunnel near the dam, but the contractor's work schedule delayed access to the tunnel. Therefore,
100%,
FELDSPAR and accessory minerals
Mineralogical' classification
MICACEOUS QUARTZITE or QUARTZ-MICA SCHIST Structural
(textural) classification
Supplemental classification
Rock type—
or group
BIOTITE SCHIST
I
MICA SCHIST i OUARTZITE
.-AUGEN GNEISS increasing size of eyes
becoming progressively coarser grained- --increasing foliation-
LU
LU
o
AUGEN GNEISS QTZT. & BIOTITE SCHIST
Fig. 12.6 Morrow Point dam area rock classification relationship (after U.S.
Bureau of Reclamation).
only two shear tests and ten sliding friction tests were performed in the field, while five shear tests and forty-nine sliding friction tests were conducted in the Denver laboratory of the Bureau of Reclamation, utilizing the large specimens which retained the in situ joints and cracks. Included in the laboratory tests were one shear and six sliding friction tests conducted on a concrete block cast on type II rock. The equipment and procedure simulated that used in the field tests. Results from direct shear tests were compared with those obtained from triaxial laboratory tests on NX cores.
Rock specimens were prepared for the laboratory from large blocks trimmed
Investigations for Morrow Point dam 375 into cubes of about 30 in by line drilling with a diamond core drill. These cubes were encased in concrete about 36 in in size. Next a 15 x 15 x 8-in- high projection was cut with a 36-in diamond saw. Rocks were orientated so that foliation of the shear specimen was parallel to the shearing load. A steel frame was placed around the upper block. A 5 000 000-lb-capacity machine was used for the tests. Two hand-operated hydraulic pumps each capable of producing a pressure of 10 000 lb/in2 were used to activate the normal and the inclined jacks.
As explained in section 6.3.1, Serafim, in Portugal, employed two failure criteria, one based on maximum shear stress and the other on inversion of the vertical displacement (see fig. 6.16). In the Portuguese tests, the shear stress at inversion was near, but less than, the maximum shear stress reached;
in the tests for Morrow Point dam, the inversion was not so pronounced, being one-half, or less, of the maximum shear stress. This may have been due to the difference in rock types and the fact that the loads in the Bureau tests were considerably larger. The Portuguese granite was weathered, maximum shear ranging from 48 to 159 lb/in2, while the rocks tested by the Bureau reached 917 to 1511 lb/in2.
Immediately after a block was sheared, a series of sliding friction tests were conducted using normal loads of 800, 600, 400 and nearly 0 lb/in2 corresponding to normal loads of 180 000, 135 000, 90 000 and 01b. (The actual area of contact was less than the theoretical 16 x 16 = 256 in2.) Tests were carried out in situ and in the laboratory, in the forward direction and in reverse, on dry and on moist surfaces. The angles of friction varied from <f> = 27° to over <f> = 40° (tan <f> = 0-51 to 0-84).
Laboratory tests on foundation rock cores. The extensive programme just described was supplemented by tests on rock cores. Approximately 250 specimens of foundation rock cores selected from 10 dam site drill holes were tested to determine the main physical properties of the foundation:
absorption porosity, specific gravity, modulus of elasticity, E9 Poisson's ratio, v, and the tensile, compressive and triaxial strength. Most of the relevant tests were conducted along classical lines. Mohr circles and Mohr envelopes were traced for the typical rock types I to V.
Interesting experiments were carried out to determine the modulus of elasticity on 6-in cores with axial hole. A steel chamber was constructed so that hydraulic oil pressure could be applied to the outside cylindrical surface of a 6-in-diameter sample of rock containing an EX hole. The outside of the core was wrapped in plastic to prevent oil penetrating into the rock samples.
The borehole gauge was inserted into the EX hole; deformations versus hydraulic pressures were observed. The modulus of the core was computed by use of the equation:
Ez= l&d p (D2 - d2) d
where D = outside diameter of the core, d = inside diameter of the core, p = external pressure, d = change in inside diameter.
Field-jacking tests of foundation rock. Load was applied vertically, then horizontally, to the rock by two jacks, 3 ft apart, each having a capacity of 200 tons. Loads of 200, 400 and 600 lb/in2 were applied to the rock for a period of approximately one week, maintained by automatic control. Three gauges were installed to measure vertical deformations at each point caused by the various test loads. These consisted of a rock deformation gauge which incorporated a special 'Carlson joint meter', under each of the two jacking loads to take measurements through approximately 15 ft of founda- tion rock (fig. 6.6). In addition one tunnel diameter gauge was placed midway between the jacking loads.
Residual rock strains. These were measured with the borehole gauge method and with Wittmore strain gauges. The equations required to reduce the strains to stresses in plane stress conditions for a 45° rosette of gauges (Merrill & Peterson, 1961) measured in the borehole are:
Id
i - u2y + (u2 - c/3)2]1/2
2d^2 and
+ 2U2 - U± - U3
tan 2d± = — — >
where S and T = the principal perpendicular inherent stresses, Ul9 U2 and U3 = the measured deformations across the diameters 45° apart, E = the modulus of elasticity of the 'homogenous rock', d — the diameter of the EX hole and 0 = the angle from S to U± measured counter clockwise.