case 2: Tunnel bore stable, tunnel heading not stable
12.1 Rock mechanics for Karadj dam
The Karadj dam is located on the Karadj river, approximately 25 miles from the city of Teheran (Iran). The project will assure adequate and reliable
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Part Four
Case histories
Engineering geology and rock mechanics are applied sciences, the final object of which is to assist in solving practical engineering problems. How they do this is best illustrated by case histories. Some of them have already been discussed in the chapters on engineering geology and rock mechanics;
others have been mentioned in the three chapters on rock slopes, galleries, tunnels and excavations and on dam foundations. A few additional cases of special interest have been selected to illustrate the many very different lines of approach vital to design problems; each case stresses a few special points.
Chapter 12 examines cases concerning dam foundations and underground works of American design. Additional information is given about tunnel design and testing. Chapters 13 and 14 describe the two most dramatic failures of large engineering structures in recent times, that of the Malpasset dam and the rock slide at Vajont. Chapters 15 and 16 concern the consolida- tion of difficult rock slopes and the construction of three large underground power-stations. These five chapters supplement chapters 5 to 11.
12 Dam foundations and tunnelling
Examples of particular rock engineering problems have been given in the previous chapters. The following details are taken from authoritative public- ations. They supplement the information given in parts two and three. It is interesting to note the methods responsible engineers have, in particular cases, employed to test rock materials and rock masses and how they have interpreted the results.
12.1 Rock mechanics for Karadj dam 12.1.1 Description of the project
The Karadj dam is located on the Karadj river, approximately 25 miles from the city of Teheran (Iran). The project will assure adequate and reliable
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supplies of municipal and industrial water to the capital of Iran. A secondary benefit of the project is hydroelectric peaking capacity.
The drainage area is 764 km2; the average annual run-off at the dam site is estimated at 472 x 106m3; the maximum recorded flow is 350m3/s and the spillway design flood is 1450 m3/s. The live storage capacity is 172 x 106 m3. There is a small regulating capacity of 0-6 X 106 m3 on the downstream side of the dam. The additional annual water supply available because of storage is estimated at 115-6 x 106 m3. The main dam is a double- curvature, thin concrete arch. It has a maximum height of 180 m and a crest length of 390 m. The horizontal circular arches have an approximately constant central angle. The radii of the arch centre lines vary from a minimum of 74 m at an elevation of 1612 m to a maximum of 201 m at elevation 1770 m (roadway level). The arch thickness is 32 m at elevation 1612 m, varying to 7-85 m at the roadway. It has been analysed as a symmetrical structure, with base at elevation 1600 m. Below this, and on the right abut- ment, in the lower portion, massive concrete plugs have been constructed.
These plugs are intended to function as foundation rock, and are not analysed as a portion of the arch structure. The volume of concrete is 350 000 m3. The double-curvature design substantially saves on concrete compared to other arch-dam shapes and is approximately 60% less than the amount needed for a gravity dam type for the same site. The power-station will ultimately have an installed capacity of 120 000 kW. The average annual electrical energy delivered will be 149 x 106 kWh.
The dam was first analysed by the crown cantilever method, Further analyses were made with three and then five cantilevers. The maximum computed stresses are 150 lb/in2 for tension and 1000 lb/in2 for compression.
A large programme of model studies was planned to include tests that would incorporate the effect of the differential in the modulus of elasticity of the foundation rock relative to that of the concrete in the arch dam. However, the field tests performed in galleries in the foundation revealed that the modulus of elasticity was quite close to that of the concrete. Measurements ranged generally from 2 x 106 to 2-5 x 106 lb/in2 while in the main dam it was 3 x 106 lb/in2. The material used to construct both the model of the dam and its foundation was homogeneous plaster. The model studies also answered two economically important design questions: firstly, thrust blocks would be necessary on either abutment; secondly, the opening in the arch dam for the spillway crest piers and gates on the right abutment did not materially affect the stresses in the dam nor the abutment loads. Therefore a thrust beam would not be required across the top of this opening.
A small regulating dam is located 4000 ft downstream of the power plant.
It is an arch gravity concrete dam with a centrally located gated spillway (dam crest at level 1600 m; maximum high water elevation 1610 m).
Rock mechanics for Karadj dam 361 12.1.2 Geology
Waldorf and contributors describe the geology of Karadj dam as follows:
Karadj dam is founded on a diorite sill approximately 360 m thick which was intruded along the bedding planes of the enclosing tuffaceous rocks: the sill strikes nearly parallel with the chord of the arch dam and dips downstream at an angle of approximately 40°. Hydrothermal alteration, which has resulted in kaolinization of the felspars, is found throughout the entire mass of the sill and is particularly evident along fractures spaced approximately 5 metres apart, where zones to a metre wide have been strongly kaolinized and sericitized.
There are three well-marked joint sets. The most prominent of these strikes roughly parallel with the strike of the sill and dips downstream at an angle of approximately 45°. Spacing of the joints ranges from 0-5 metres to 3 metres. These joints contain calcite and zeolite fillings ranging from a film to 5 cm width.
Orientation of the three major joint sets is, in general, not adverse to the stability of the abutments under the arch dam loadings and there is no evidence of planes of weakness oriented in a direction that would permit sliding of any part of the abutments.
The characteristic figures for the diorite area are as follows: sound diorite E = 5 000 000 lb/in2 (356 000 kg/cm2) from laboratory tests on sound rock samples; altered diorite E = 1 620 000 lb/in2 (114 000 kg/cm2); compressive strength a = 4700 lb/in2 ( ~ 320 kg/cm2).
12.1.3 The test programme
The test programme was established by Waldorf et al. (1963) as follows: a circular loading plate 760 mm in diameter, loaded by two hydraulic jacks was used. Total force exerted by the jacks was 400 tonnes. This gave an average surface loading of 1140 lb/in2 ( ~ 80 kg/cm2). The rock surface or loading plane on which the loading plate rested had minimum lateral dimen- sions of 5 to 5-5 plate diameters. The purpose was to assure that there would be no appreciable effects of the side walls of the test chambers on the rock deflections. Where practicable, the plate loads would be approximately in the direction of the thrust of the dam. The test chambers had to be as near as possible to the plane of the abutment. Three chambers were excavated in each abutment at the elevations 1620, 1680 and 1730 m.
Description of test scheme. The thrust of the hydraulic jack ram was trans- mitted by a spherical head to the loading plate which rested against the flat test surface. The gauging system consisted of a number of dials fastened by adjustable arms and cross members to two pipe supports which spanned the full test surface and with a distance between each of five plate diameters.
Gauges were positioned to measure movements of the loading plate at four points and at a number of points on the rock surface along the axis passing through the centre of the plate. Measurements were made during both the loading and unloading phases of the cycle. Rock displacements outside the
plate would be approximately 10% of the plate displacement, requiring a minimum accuracy of plate displacement measurements of 0-02 mm. The accuracy of the gauges used was actually greater. Close control of the temperature was required.
There were four tests on the normally jointed diorite rock which constituted the bulk of the foundation. Two were on altered rock which appeared in zones and lenses, one of which had close fracturing (chambers 2A and 6).
A single test was on a wedge of rock formed by a prominent joint intersecting the test face at a flat angle. The massive igneous rock (four tests) was inter- sected by a system of joints which divided it into blocks generally under a half metre in linear dimensions. Joints were usually tight but were occasionally filled with zeolites and calcite or both. The total thickness of such materials formed only a minute part of the rock. From the evidence of drill cores and drifts the maximum thickness of a zone or lens of altered rock appeared to be under four metres. Accordingly, in the analysis of the effect on foundation modulus a thickness of four metres was assumed for such a zone.
Altogether thirty-seven tests of various kinds were made on seven sites.
There were two temperature tests, four plate deflection tests and thirty-one rock modulus tests. A minor correction was made to the plate displacement measurements to account for the compression of the mortar pad on which the plate rested. The mortar pad was normally 50 mm thick and was made of one to one cement-sand mortar. Assuming the modulus of the mortar to be 250 000 kg/cm2 (3-5 x 106 lb/in2) the full load compression would be 0-016 mm for the 760 mm plate and 0-044 mm for the 450 mm plate.
12.1.4 Main results of the tests and measurements: discussion The corrected effective modulus values, as determined from plate deflections are set out in table 12.1.
Table 12.1 Effective modulus from plate displacement (in kg/cm2)
Chamber 1 2 3 4 5 Average
Unconnected modulus
158 500 178 000 127 800 135 500 131000 146 300
Corrected modulus
156 500 174 500 127 800 132 300 127 800 143 800
Elevation (m) 1620 1680 1730 1620 1680
A separate analysis was made for the tests in chambers 2A and 6 where rock was altered, and these results are summarized in table 12.2.
Rock mechanics for Karadj dam 363 Table 12.2 Effect on modulus of major discontinuities in rock mass
Types of discontinuity Plate diameter
Measured displacement Corrected displacement Apparent modulus Scale: protoype/test plate Modulus of altered zone Modulus of unaltered rock Eeft for prototype with
discontinuity
Chamber 2A (altered, fractured
rock) 450 mm
1-070 mm 1096 mm 76 500 kg/cm2
51 67 400 kg/cm2 163 700 kg/cm2 136 300