RATIONALIZATION AND EXCHANGE

GEODESIC OPERATIONS FOR THE ARCH DAM

OF EXPERIENCE

DURING EXCAVATION

OF THE TRENCH

OF THE CHIRKEY HYDROELECTRIC

PLANT UDC 627.825:528.486

V. P. Petukhov In 1970 the builders of the Chirkey hydroelectric plant delivered the completed foundation trench for the dam with an excellent rating. The merits of the geodesic service, which was performed in the field on the design profile of the trench, were also included in this rating. To ensure the precise fit of the heel of the arch dam in the trench, staking had to be carried out during its excavation with a r m s error of • cm. The trench under the lower surface of the dam was excavated from top to bottom in the canyon rims from the highest horizon. Working drawings on which the coordinates of all contour points were given, and on which the elevations of protective berms and the slopes of embankments were also indicated, served as initial data for actual shaping of the trench profile. The trench profile in the uppermost canyon horizon was shaped by computed distances and angles using the polar method with bearing points of special triangulatlon. Holes 10-15 m deep were drilled through the shaped contour every 0.8 m at a given Incllne, after which a smooth rock slope was obtained by blasting. A temporary turning point was established on the shelf that had been formed after blasting by the method of sectioning or resection (Fig. i). Each point was fixed by a metallic pin which was driven into a hole drilled to a depth of 30-40 cm and extended 8-10 cm above the surface of the rock. The top of the pin was covered with white paint and its center was marked with a black point. In establishing the position of the points, horizontal and vertical angles were measured by Theo-010 or TB-1 theodolltes in a slngle complete set up. Discrepancies in the coordinates of the temporary point, which were obtained from different triangles, did not exceed 1-2 cm in the majority of cases, while elevation discrepancies did not exceed 3 cm; this corresponds to technlcal-levellng accuracy where the difference in elevation between the objective and bearing points is 50 m. The trench contour points were established from a specific temporary point by the polar method and the top and base of the resulting slope was surveyed (Fig. i). The points were established with a TI0 theodolite and RS-50 tape measure. In surveying, distances to the characteristic points at the top of the smooth slope were always measured from the center of the theodolite barrel using a tape measure when surveying the steep slopes (up to i0@). The horizontal distances were computed from the vertical angles measured by surveying at characteristics points on the slope and from the slope distances to these points. To ascertain how much the trench deviated from the design, we compiled a l:200-scale composite contour plot of the trench containing a representation of all projected horizons. The points were plotted on the plan from given design coordinates. Deviations of the actual position of the rock edges, which had been plotted on this plan, from their design position were determined graphically with a mean error not exceeding • cm. This high accuracy was achieved by plotting the survey in plan using a specially built circular protractor with a circle diameter of 25 cm, which was combined with a scale, and by the short distances covered in the survey (not more than 50 m). A method for determining the positions of temporary points using an MSD-I llght-sensitive range finder was perfected in the lower elevations of the trench, where it was possible to report elevations at temporary points with bench marks fixed in the downstream slopes. At site AB, formed by points of special triangulation, point 1 was located in an area convenient to layout and surveylng (Fig. 2). The distances from the speclal-triangulation point 1 was measured with the llght-sensitlve range finder, while its elevation was taken Translated from Gidrotekhnicheskoe Stroitel'stvo, No. 4, pp. 45-47, April, 1978.

402

0018-8220/78/0004-0402507.50

9 1978 Plenum Publishing Corporation

ARCH DAM OF THE CHIRKEY HYDROELECTRIC PLANT 3

403

3

_ ,,, ,,,

~ " '/

~

J,/

/i ' \

/

Fig. 1. Diagram showing l ~ o u t and s u r veying of profile of dam trench, a) Plan; b) section, i) Bearing point; 2) turn~g point; 3) fixed po~ts; 4) profile of block underlying slope; 5) natural profile of canyon.

%

a

X

18

B]

2 J 4 5 6 71

970H{

Fig. 3

Fig. 2 Fig. 2. finder.

Determining position of point with MSD-I light-sensitive range a) Plan; b) section.

Fig. 3.

Diagram showing area determination.

from the nearest benchmark to determine the horizontal component of the measured line. From the horizontal component d and bearing of AB, we computed the coordinates of point l, and also the error m of its position -~

m2

m2AB

where m s is the error generated in measuring the length of the line with the light-sensitive range finder, a is the angle of inclination of the line AB, mH, is the error in the eleva-

404

V.P.

PETUKHOV

tion of point I, mAB is the error in the bearing of AB, and 0 = 20@62'65 ''. When m s = 5 ram, a = 45 @ , d = 50 m, mAB = i0" and m = 6 ram. Experience gained from surveying operations at the Chirkey hydroelectric plant indicates that the demand for development of an instrument combining the quality of a type MSD-I light-sensitive range finder and a type T5 or T2 theodolite has increased markedly. With such an instrument not only the plan position, but also the elevation of the turning points in all horizons of the trench could be determined from triangulation points. A number of other engineering problems could be solved with greater simplicity, operational facility, and accuracy, using this instrument. In addition to these methods of determining the plan position of turning points during construction, we used (where conditions permitted) the method of determining the position of points with folded planes. The simplicity and reliability of this method are some of its advantages. Moreover, a point whose coordinates were known was constantly being reproduced and this made it possible to conduct the necessary computations for the staking operations beforehand. In hydraulic construction, computations of the volume of earth-rock work require determination of the cross-sectional areas. In our opinion, the method used to compute areas at the Chirkey plant is simple and deserves mention. A cross section was divided into trapezoids with the same heights. Since the area of a trapezoid is equal to the product of half the sum of the bases and the height, computation of areas in plan consist in summing the bases with appropriate selection of t h e h e i g h t s of the trapezoids. For the section shown in Fig. 3, e.g., we constructed trapezoids with heights B = i, 1-2, etc., equal to 1 cm. By laying off the lengths of the sections AB/2 + (I--I') + (2--2') + ... + (ii-- II') in succession on the measuring device and determining their overall length (33.8 cm), we found the area of the actual cross section (33.8 m2). This method of area computation is simple and was also used at the Krasnoyarsk hydroelectric plant.