OPERATING EXPERIENCE WITH HYDROELECTRIC POWER PLANTS

W~TER-DEVELOPHENT

PROJECTS

AND

EQUIPMENT

AT

MULTILAYER CREEP OF THE SURFACE OF EARTH EMBANKMENTS

D. M. Yashkul'

UDC 627.824.31:627.18

Creep of the earthen slopes of the dam, of f take, and access has not diminished for some time at the Rayakoski hydroelectric plant. The latter was built in 1956 on the Paz River forming the boundary of the M u r m a n s k Oblast, and has become an integral part of the chain of Paz hydroelectric plants. The earth dam is 22 m high, extends for 18 m along the crest, and mates with a concrete dam 0.5 km long from the right bank [1]. An antiseepage element is formed by a metallic sheet-pile diaphragm. The embankments, which are formed f r o m a dry-fill sandy-gravelly soil, are placed with a coarse natural stone on a leveled face. The overall dimensions of the stone slabs are up to 1.5 m across and up to 40 cm thick, and are underlain by a 40-60 cm layer of a gravel preparation. The upstream and downstream faces are sloped at 1.2 and from 1:1.5 to 1:1.3, respectively. The downstream slope is drained by a prism of quarry stone near the base and by a permeable backfill near the plant site. The height of the open faced surface of the downstream slope is 15 m. The concrete trough of a cable duct, which is embedded to the ceiling slabs in the fill soil runs along the e m b a n k m e n t from bottom to top. Beyond the trough, the dam embankment is continued as an e m b a n k m e n t for a highway covered with a protective layer of crushed stone (Fig. 1). Two years after the startup of the hydroelectric plant, settlement of the facing was noted along the thrust embankment. It was uniform along the front, negligible in magnitude, and thereafter exhibited no pronounced development. Washout of the fine filter fraction between the stones of the facing is observed in a limited volume. Moreover, settlement phenomena exist and have continued for more than 30 years on the downstream slope. The discharge of snow melt and rainfall, which runs o f f from the natural slope adjacent to the dam was periodically observed here along the cable tray in the first years of operation. The leakage of water has been accompanied by a shift in individual facing slabs, local washouts, and removal of fine soil fractions f r o m the underlying layer. The slabs of the facing were leveled as early as 1959, i.e., three years after construction. In 1970-1973, the drain pipes were continued and reinforced-concrete columns were installed along the cable tray to intercept and organize the discharge of water (Fig. 1). The appearance of local overriding of the facing on the downstream slope was noted after the plant's sixth year of service. By eliminating all gaps and pressing one against the other, the heavy stone slabs that slide along the rather steep slope began to push their edges upward at individual joints, forming double-inclined vaults. With the passage of time, the n u m b e r of heaved slabs increased gradually, suggesting continuous creep of the facing. In this case, the creep of the raised slabs was markedly lower on the portion of the e m b a n k m e n t with the 1:1.15 slope than on the 1:1.3 segment near the cable tray. No serious breeches in the dam's integrity, such as mass slides, landfalls, s l u m p - t y p e settlements, and failures of the underlying layer were observed. In 1973, the downstream slope of the dam was repaired: the stones in the facing were reset on the leveled bed, and some of the slabs were anchored by metallic rods driven into the e m b a n k m e n t soil. The creep process of the e m b a n k m e n t surface was not curtailed, however, and continues to this day. The slabs are being displaced downward, forming new override arches (Fig. 2). The entire cable tray, which was laid along the

Translated f r o m Gidrotechnicheskoe Stroitel'stvo, No. 6, pp. 49-53, June, 1991.

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Fig. 1. Plan view of downstream side of earth dam at Rayakoski hydroelectric plant: 1) earth dam; 2) concrete dam; 3) hydroelectric plant; 4) site near plant; 5) rock-slab facing; 6) crushed-stone cover; 7) piezometer; 8) inspection well; 9) drain pipe; 10) cable tray; 11) tunnel cable duct.

Fig. 2. Facing of downstream slope of dam.

slope, is creeping. Fracture of the concrete walls of the tray, their sideward shifting, and sliding by 20 cm occurred in the lower section adjacent to the underground cable gallery. In 1981, the support in the mass of the gallery was concreted to the combined tray. The angle in the tray at the bend in the upper section was cut o f f and straightened to prevent rupture of the cables that had become tensioned (Fig. 1). In many cases, creep of the slope caused the rupture of asbestos-cement drain pipes located along the tray. This would suggest the appearance of sand in the drain well. On exposure, it was noted that the ruptured seam had opened to 8 cm. The rupture was located in the upper section of the slope. In this same area, the transverse action of the creeping facing stones and soil is manifested especially perceptibly on the upper sections of the tray, which support the 1:2 slope. The top of both walls of the tray, which are coupled to the support by wooden roof slabs, is displaced downward along the slope. Resting on the ground in this case, the downstream wall has broken away from the bottom and shattered the lower edge inside the tray, compressing the cable. To preserve the cables, the walls of the tray have been temporarily pressed out by threaded supports. Work on restoring the drainage and reducing the pressure against the walls of the tray is underway. A piezometer installed beyond the tray and drainage on the edge of the tray is also confirming the effects of the creeping soil. The 17-mm diameter metallic thin-wall tube of this piezometer was found sheared o f f at a depth of 1.7 m in 1981. In 1988, the piezometer was once again exposed to eliminate a bend that was making it difficult to lower 369

Fig. 3. Of f take of Rayakoski hydroelectric plant. Transverse section at station 5 + 50: 1) left-bank dike; 2) stone slabs of facing; 3) retaining wall; 4) steel post-anchors.

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the conductor with contacts to measure the water level in the tube. In the area of the piezometer, the curve of the depression lay at a depth of 5.5 m from the surface of the ground. In connection with the short expanse of the dam, regular geodesic monitoring of its settlements is not being conducted, and no elevation marks have been installed. A lowering of the crest in a 0.5-1.0 m strip adjacent to the edge of the downstream slope, and opening of the joint between a horizontal tier of slabs along the edge of the crest and slabs lying on the slope was noted by visual observations. A 1989 control survey indicated that the settlement of soil along the edge of the slope falls within the range of 25 cm, and is 50 cm only on the angular projection, whereas it does not exceed 5 cm over the basic area of the crest. The 9 5 0 - m long offtake for the Rayakoski hydroelectric plant occurs in rock and morainic soils. The rock embankments are sloped at 10:1, and the morainic at 1:1.5. The low slopes of soft soil above the water in the initial segment of the o f f take are leveled by a layer of crushed stone and partially faced. The roof of the rock on a downstream segment is gradually subsiding, raising the height of the soil slopes of the depression along the right bank and the earth fill of the dike along the left. Here, the soft soil is also protected b y slabs of natural stone 0.4 m thick along a sandy-gravelly preparation 0:6-1.2 m thick. The length of the faced segment is 600 m. The m a x i m u m height of the slope covered by the stone slabs is 13 m. A bench 2-4 m wide along which a retaining wall ranging f r o m 0.5 to 6 m is concreted here and there or a palisade of steel posts that anchor the edge lumps of rock is created has been carried away along the edge of the rock slope (Fig. 3).

370

Fig. 5. Enclosure at site of oil-storage facility, deformed by creeping slope.

The deformation of the faced slope of the of f take was manifested more suddenly than on the dam. Displacements of the strip of slabs and s l u m p - t y p e settlement of the edge of the slope in the area of station 5 + 50 were noted as early as the year after the startup of the hydroelectric plant. The piling of creeping slabs of the facing along the underwater rock bench was exposed during diver inspection of this segment. All stones were secured on the bench by a rod enclosure, and the bottom of the channel was clean. Settlements along the edge over the entire expanse f r o m station 4 + 50 to station 5 + 70 and local settlements in other segments of both the right bank and also the l e f t - b a n k dike were noted visually in 1958 after just one year. Creep of the facing with local s l u m p - t y p e settlements and heaving did not attenuate in subsequent years. Different versions were put forth as causes in different years: settlement of the morainic fill in the segments of the deepened rock depression in the sides (based on filling of the zone of removal of fissured rock and incision for a temporary construction crossover in the channel); the surface flow of water during intense snow melt or movement of melt water beneath the stone slabs; the discharge of ground water in the sandy-gravelly bed layer; sharp fluctuations in the water level in the channel during the plant's rapidly changing operating conditions; the effect of the breakaway action by ice frozen on as the water level in the channel drops (hanging ice floes along the banks may reach 3 m wide and 1.5 m thick); additional shearing loads due to the snow layer; and, frost heaving of the natural morainic soil. An upland trench between stations 4 + 30 and 6 + 00 was opened in 1961 along the right bank of the channel for the intake of surface melt water. An embedded tubular drain for the discharge of ground water was built here in 1969 between stations 3 + 40 and 5 + 40. These measures did not, however, reflect on the condition of the slope - - the rate of its settlement did not change. The secondary nature of the influence exerted by both the ground or surface water, and also the flow in the channel also demonstrate the presence of the creep and overriding of the stone slabs at the initial stations, where the 2 - 3 - m high faced slope does not contact the bank slopes, is located above the rock bench, which is submerged here, and is not subjected to open water. Since 1958, personnel of the chain have taken episodic instrument measurements of slope settlements at station 5 + 50 on the basis of temporary marks. The data that have been retained suggest: the magnitude of the annual settlements has fluctuated f r o m 5 to 21 m m , and has reached 46 m m at one of the nine points. Qualified and regular observations have been made since 1965 after installation of a permanent grid of 30 control marks over the entire expanse of the faced right-bank slope. The average settlement rate at half of the control points exceeds 7 m m per year, and attains 29 m m at control mark No. 93. The m a x i m u m recorded annual settlement was 62 m m at control m a r k No. 92 in 1966. The total settlement after 23 years of observations (in 1988) ranges from 50 to 200 m m for 20 marks, and f r o m 200 to 350 m m for six marks. On two control marks (Nos. 92 and 93), which are located at station 5 + 50, the total settlement is 602 and 669 ram, respectively. The results of observations since 1965 371

with respect to certain points are presented in diagrams (Fig. 4). Considering the foregoing observations, we can assume that the settlement at control marks 92 and 93 may exceed 1 m for the entire 32-year operating period. It should be noted that the ascent of individual marks as a result of the override of the face or slip of the creeping soil was also observed during certain leveling cycles. No check was made of the lateral displacement of the control points. The access to the assembly area of the hydroelectric plant was built along the right bank of the of f take and emerges at an elevation of 71.75 m, i.e., 5 m below the access at the dam. An oil-storage facility with a building and tanks is located at the site 50 m f r o m the powerhouse along the access. The slope of the depression adjacent to the road and retaining walls, which enclose the area, is inclined at 1:1.5 and is topped by a 0.3-m layer of crushed stone over the morainic soil. The height of the e m b a n k m e n t reaches 10 m. The height of the retaining walls is approximately 1.5 m.

Creep of the protective layer of the slope near the road was noted in the seventh year after the start of operation as a disturbance in the evenness of the graded surface and the formation of "wrinkles" - - wavelike bulges - - in the crushed-stone layer along the slope. Subsequently, tongues of creeping crushed stone filled the drainage ditch of the access to the hydroelectric plant, and rose above the retaining walls at the site of the oil-storage facility. The drains and base of the walls were repeatedly cleaned of stone that had fallen f r o m the slope. The integrity of the retaining walls was disturbed by the creeping slope: cracks and displacement of concrete sections appeared along the walls. In 1981, the retaining walls were topped with a 0.5 m concrete band and strengthened with counterforts. In the enclosure of the oil-storage facility, the extreme section was exposed in the curve of the slope, which circles the site. The section was built in the form of a triangular reinforced-concrete panel, the short leg of which is set on a pedestal, and the hypotenuse, which is below the stiffener, is supported on the slope with a 0.3-0.4-m depression in the ground for crushed stone. The thickness of the panel is 10 cm, the height is 2.9 m, and the length along the upper stiffener is 7 m. As a result of creep along the slope of crushed stone and underlying soil, the panel has been bent into an arc, and its top has deviated by almost 2 m from the line of the enclosure. Cracks have appeared on the panel, and the concrete support pedestal has deviated from the vertical (Fig. 5). The other dams of the chain have shallower slopes; for these embankments, therefore, creep has not been manifested so distinctly. At the earth dam behind the Yaniskoski hydroelectric plant, the 1:2 upstream slope is faced with slabs of natural stone (the 1:2 downstream slope is protected by gravel fill). The dam was constructed and placed under a head in 1942. By the fall of 1944, however, the reservoir was emptied after blasting of the concrete structures. The earth dam was damaged in three areas. The plant was restored and restarted in 1950. There are no data on slope behavior during the first years. Local subsidence of the a b o v e - w a t e r portion of the upstream slope in various segments in the form of funnels with an area ranging f r o m 1 to 5 m 2 and a depth to 50 cm were noted regularly in the 1950s and 1960s. Slump-type settlement of the stone slabs was caused by the downward creep of fine gravel in the transition layer, whose thickness and grain size were approximately 80 cm and I - 6 cm, respectively. The gravel slides were deposited on the slabs of the slope 1-2 m below the water line. This cannot be the result of back pressure on lowering of the level of the headrace, since the reservoir is virtually steadily maintained at the normal surface level, and infrequent drawdowns are conducted slowly. The maximum rate of descent of the water level in the headrace, which was adopted for all hydroelectric plants in the chain, was 6 c m / h with a restriction of 0.5 m per day on the total drawdown. A single forced accelerated drawdown of 62 cm in 4 hours in 1964 demonstrated the adequate stability of the e m b a n k m e n t components, since there were no negative aftereffects. The most likely cause of removal of gravel from beneath the facing is therefore hypothetically called wave action. The height of the waves in the reservoir may frequently reach 1 m. The presence of individual s l u m p - t y p e settlements of the slope near the crest, which rises 3 m above the normal water level and also single funnels to a depth of 3.5-4 m beneath the water, which were detected by divers, however, repudiate the well-defined determination of the cause of the loss of gravel material and settlement of the facing. Cushion gravel filling with preliminary raising of the stone slabs was carried out repeatedly on the dam's embankment in areas prone to s l u m p - t y p e settlement. In 1963, therefore, the area of repaired surface amounted to 70 m 2. The work was carried out during the period of reservoir drawdown and accumulations of crept gravel were used for the backfill. S l u m p - t y p e settlements were also subsequently noted on the upstream slope, and cushion crushed-stone fills and replacement of fine stone were carried out along the washed out seams between slabs. 372

Downward displacement of the edge of the facing along the slope relative to the edge of the crest was not confirmed by instruments, although it was noted here and there "by eye." Creep of the surface was small, most likely due to the short expanse of the slope -- the upstream bench is located 6 m below the crest, and the slope abuts against the crest of the stone blanket for yet another 3 m with respect to height (the maximum height of the dam is 21.3 m, and the length 1 km). Settlement of the crest during the period from 1965, after installation of permanent benchmarks, through 1988 has been negligible, approaching 2 cm only at individual points. All earth dams in the chain, like the Yaniskoski Dam, are equipped with control marks, benchmarks, and marks strictly along the axis of the crest. Observations for slope settlement have therefore been purely visual. Local slump-type settlements of the sandy-gravelly soil underlying a protective layer of rock fill along the upstream 1:2.25 slope are continuing to develop each year along the crest at the earth dam behind the Khevoskoski hydroelectric plant, which was placed in service in 1970. Slump-type settlements in the form of individual isolated pits up to 20 cm deep and up to 30 cm in size, and also longitudinal cracks that appear periodically (without opening) suggest continual movements of the slope and the unstable state of the contact between the protective fill and transitional layer. The upstream edge of the crest has settled 10-15 cm below its central portion with the spotty formation of clearly expressed steps in the dam sections with heights of both 20 and 6-8 m. The settlements of the control benchmarks located along the axis of the dam do not exceed 4 cm. Short longitudinal cracks also appeared earlier along the downstream edge of the crest, and the total settlement with respect to the axis of the crest is observed visually. The wooden encasing boxes installed at a shallow depth on the surface of the ground are disturbed at individual piezometers along the downstream 1:2 slope of the dams behind the Khevoskoski and Yaniskoski Dams. The displacement of the boxes is caused not only by settlement of the soil, but also by the sliding of snow deposits along the slope. During each winter, snowdrifts, whose depths frequently reach 3 m, are displaced downward along the slope both as a whole, and in individual strata, which collapse with the lips and steps that are formed by snowstorms. The creeping snow mass displaces the boxes, pressing them against the piezometer tubes, crushes the covers, bends the tubes, and changes their top elevation. The elevation of the top of the piezometric tubes at the Khevoskoski Dam has been lowered by 3-7 cm in 18 years. The deep embeddment o f the depression surface on all earth dams in the chain of Paz hydroelectric plants eliminates the effect of the internal seepage of water on the behavior of the upper layer of the slopes. The soil structures of other chains operated by the Kola Area Power-System Administration also provide examples of soil creep that has continued for many years. At the dam behind the Lower Tuloma hydroelectric plant, the creep of the bituminous-shielded upstream slope began prior to filling the reservoir in 1936. In the first year, however, the thrust slope was placed from 1:4 to 1:5 by the addition of soil, Settlements to 2.5 and horizontal displacements to 3 m were noted through systematic observations conducted from 1951 through 1965 [2]. The majority of control points were found below water or demolished. Observations on the new MMA grid have been made since 1965. Additional settlements to 0.4 m have been noted after 20 years. The settlement rate of individual points during the last 5-year cycle of observations is still 1 cm per year. According to diver-inspection data, the upstream slope has a wavy surface. At the Neva-2 hydroelectric plant, which was placed in service in 1934, the 4.4-km long intake channel is faced with concrete slabs 15-25 cm thick along the morainic 1:1.5 and 1:2 slopes. As is presumed, the facing has been disturbed over the entire expanse as a result of frost heaving of the underlying layer and creep of the slope: The concrete slabs have developed cracks, have fractured, and shifted. The channel has been the subject of repair work for some time. The left-bank morainic slope of the of f take to the Kuma hydroelectric plant has been creeping since the time of construction in 1962. Attempts have been made to stabilize its surface fill with cement and drainage. Problems of providing for the overall static stability of slopes are treated in the construction rules and regulations and scientific--technical and educational literature, but insufficient attention has been focused on questions concerning the behavior of their open surface under actual operating conditions. On the whole, the results of observations enable us to s t a t e the following: the slopes of the structures formed from earth materials are subject to settlements along the surface, which significantly exceed the settlements of the mass covered by the embankments; 373

the increased settlements, displacements, and creep of the slopes occur both on the thrust structures, and also on the thrust-free fills or depressions from the upstream and downstream sides of the dams, in the dry and underwater sections of the structures, and for protective embankment coverings of different soils, height, and steepness; when the slope is placed at 1:2 and greater, no bulging of the stone facing has been noted, although creep manifestations are observed. There is no direct relationship between bulging and the steepness of the slope: the centers of deformation of the facing are smaller in the section sloped at 1:1.15 than on the 1:1.5 slope; in the first years o f service, movement of the slope may have gone visually undetected, and could be ascertained only by specialized observations. For this purpose, it is necessary to call for the installation of surface control marks along the critical slopes; the creep may be protracted for many years and decades, and its rate varies, as a rule, in steps and slowly; the basic causes of the creep is the natural tendency of the slopes to flatten, rheologic processes, and the influence exerted by such external factors as wetting by atmospheric precipitation and melt water, choking of the surface layer with silt and composting organic material, fluctuations in the level of the open and ground water, temperature deformations, the condensation of moisture on the lower surface of the stones, freezing and thawing, loading due to snow masses and ice, wave impacts, floating ice and logs, the effects of blasting and thunder, vibration induced by moving transport and working mechanisms, etc.; external factors change to some extent the physicomechanical characteristics or stress state of the soil from the surface. The discontinuity and periodicity of their effect, and also the variability of the combinations with time contribute to the increase in the duration of creep phenomena on the surface of the slopes; external forces act not only on the properties of the soil along the surface, but also on the condition of the contact between the soil particles and the bottom of the facing or support of some structures on the slope. The coefficient of friction between the concrete (stone) and soft soil varies during the year, decreasing by six times, for example, during wetting, altering, accordingly, the stability of objects on the inclined surface of the slope; creep is characteristic not only for the facing and the protective or transition cushion fill, but also for the nearsurface soil of the slope with attenuation, most likely, at a depth of 1.5-2 m, i.e., in the layer where the majority of external effects are actually felt. The properties of the deeper soils are less subject to variation; and, the stated hypothetical conclusions concerning observations on the slopes of structures are in agreement with data derived from study of the "secular reworking" of natural slopes [4]. Design work on embankments and their beds, on the edge of the crest and the benches of any engineering structures and construction projects, including the facing components, utility runs, and control-measurement installations should take into account the tendency of the surface of earth embankments to increased settlements and prolonged creep. LITERATURE CITED I.

2.

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4.

374

D. M. Yashkur, "Earth dams of the hydroelectric plants in the Paz chain and some results of their operating experience," Gidrotekh. Stroit., No. 11, (1980). M. I. Zarkhi, "Operation of water-development works at hydroelectric plants operated by the Kola Area PowerSystem Administration," in: Operation of Water-Development Works at Hydroelectric Plants [in Russian], l~nergiya, Moscow (1977). Manual on Hydraulic Engineering [in Russian], Gosstroiizdat, Moscow (1955). N. N. Maslov, Conditions for Stability of Slopes and Embankments in Water-Power Construction [in Russian], Gosenergoizdat, Moscow (1955).