Civil engineering dam paper 1 project overview
Baishuihe Reservoir is located at the source of the first tributary of Baisuo River. Baishuihe Reservoir was built in 1977, designed by the former county hydropower bureau and undertaken by local mass organizations. According to the data recorded in the Database of Small Reservoirs in Qiannan Prefecture, the catchment area above the dam site is 1.7km2, the maximum dam height is 12m, and the total storage capacity is 50× 104m3. The main task of the project is farmland irrigation, and the flood control function is relatively small (Ⅱ) After investigation, the karst in the area where the reservoir is located is developed, and there are two large karst caves exposed at the end of the reservoir. Combining the surface and underground basins, after this review, the catchment area above the dam site of the reservoir is 2.3km2, the length of the main channel is 3. 17km, and the weighted average gradient is 34.82‰.
2 dam status quality evaluation
2. 1 dam foundation quality evaluation
According to the geological survey in the dam area, the stratum in the dam area is timely sandstone (P 1q 1) in the first member of Qixia Formation of Lower Permian, with black shale and thin limestone. Occurrence of rock strata in the dam site area: 28 1 ∠ 9, which inclines to the downstream right bank and is a transverse valley. The rocks in the dam area are timely sandstone and black shale mixed with thin limestone, and bedrock is exposed at the downstream of spillway and dam foot, which is grayish yellow timely sandstone and black shale mixed with thin limestone. There are no major defects in the engineering geological conditions of bedrock in the dam site area. The main engineering geological problem is the uneven foundation caused by differential weathering. Due to the early construction time of this project, under the historical background at that time, the slope deposits and completely weathered rocks may not be completely cleaned during the construction and excavation, and the weak parts such as interlayer, weathered zone and joint fissure dense zone that may leak may not be properly overbreak and backfilled, resulting in poor contact and formation of leakage channels; There is no anti-seepage treatment in dam foundation grouting, which makes the dam foundation and abutment leak. According to the field investigation and interviews with local residents, there are obvious water damage areas under the left abutment bank slope and the prism at the foot of the dam; There is an obvious leakage channel at the right abutment, and the leakage is about 5 ~10l/s. ..
2.2 dam engineering quality evaluation
The dam is an earth dam, with the maximum dam height 12m, dam top length 132m, dam top width 7.5m, dam bottom width 66.22m, dam top elevation 1293.8m, and dam bottom elevation1281.8m. The upstream dam surface is protected by dry block stones, and the downstream dam surface is protected by turf. According to the data, Baishuihe Reservoir was completed in June 1977. Through on-site investigation and interview, since the operation of the project, there have been leakage problems in dam foundation and shoulders. It is speculated that due to the early construction of the dam and the restriction of economic and technical conditions at that time, the dam foundation was not thoroughly cleaned and the dam foundation (shoulder) was not treated with seepage control. The dam material is mainly yellow clay mixed with weathered gravel. The dam filling materials are local materials, which are limestone, sand shale weathered clay mixed with gravel on the nearby hillside, and the filling quality is poor. According to the field investigation of dam filling soil, the coarse particle content of dam filling soil is about 50%, and the coarse particle size is between 0.2 ~ 10cm, with large particle size change and poor soil quality. When building a dam, the construction machinery is scarce, the engineering technical force is insufficient, the rolling quality is poor, and the quality of the dam filling soil can not meet the requirements. According to the quality of fill, it is estimated that the permeability coefficient of dam body is between 7.5×10-4 and 8.5×10-4 cm/s, which is greater than1×10-4 cm/s.
3 dam reinforcement design
3. 1 dam leakage treatment
Since the operation of Baishuihe Reservoir Project in Changshun County, there have been problems of dam foundation and abutment leakage. It is speculated that due to the early construction time and the restriction of economic and technical conditions at that time, the dam foundation was not cleaned thoroughly, and the dam foundation (abutment) was not treated with seepage control, so the construction quality was poor. In view of the above situation, the dam seepage control treatment of this project adopts geomembrane laid on the upstream dam surface combined with curtain grouting treatment; For the leakage channel in the contact zone of the right abutment, the original masonry retaining wall is demolished, and the leakage channel is filled with buried stone concrete first, and then combined with the seepage control treatment of the dam body and dam foundation, geomembrane laying and curtain grouting are used for seepage control treatment.
3.2 Design of Breakwater Wall
3.2. 1 Dam crest elevation recheck
According to the conclusion of safety evaluation, it can be seen that the spillway discharge capacity and dam flood control capacity can not meet the requirements of the code under the current conditions of the dam. In this design, the overflow weir is reformed. After the spillway is widened, the discharge capacity of the spillway meets the requirements of the specification. The existing dam crest elevation of the reservoir is1293.80m, which is higher than the highest static water level of the reservoir. After considering the corresponding safe superelevation value, the designed flood level of the reservoir is 1294.50m, and the checked flood level is 1294.43m, both of which are higher than the current dam crest elevation, and the dam crest does not meet the superelevation requirements.
3.2.2 Design of wave wall
The original dam crest elevation is 1293.80m, and there is no breakwater at the dam crest. According to the recheck results, the elevation of the breakwater crest should be 1294.50m under normal working conditions and 1294.43m under recheck working conditions. In this reinforcement design, a new wave wall was built on the dam crest. The wave wall is 80cm high and 60cm wide, and C 15 concrete is poured, which is also used as the geomembrane reinforced wall at the dam top.
3.3 Design of slope protection for upstream dam face
3.3. 1 Laying
Firstly, the upstream dam slope is reshaped, the existing dry block stones are removed, and the surface floating soil is excavated, and then the composite geomembrane is laid according to the proposed seepage control structure. The impervious structure consists of composite geomembrane, protective layer and slope protection. In order to ensure the normal operation of the filtration and drainage system, the following measures are taken for the surface protection and fixation of the composite geomembrane: to prevent the composite geomembrane from being punctured, 10cm thick fine sand is laid under the composite geomembrane, and then the composite geomembrane is paved after leveling, and then the 10cm thick fine sand is laid, and then the hexagonal geomembrane with 8cm thickness is used. In order to prevent the geomembrane from sliding, a keyway is set on the dam surface. Embedding the composite geomembrane into the concrete on the dam crest; At the foot of the slope, in order to prevent the geomembrane from cracking, the composite geomembrane is extended and folded back to make a pressure pillow, which is buried in the concrete of the tooth wall. The selected composite geomembrane is two pieces of cloth and one piece of membrane. The selection of composite geomembrane specifications is related to the smoothness of the underlying cushion, the allowable tensile stress of the material, the elastic modulus of the material, the maximum water head within the laying range and the maximum particle size of the covering layer. In addition to the strength required by water pressure, the design of geomembrane thickness should also consider the use conditions such as exposure, buried pressure, climate and service life. The design thickness and actual thickness should be determined according to relevant national standards. The laying of composite geomembrane is the key to the construction of this project, and the quality and performance of geomembrane are related to the anti-seepage effect. The composite geomembrane entering the site must have the certificate of approval, performance and characteristic indicators and instructions provided by the manufacturer. After the composite geomembrane enters the site, the composite geomembrane shall be randomly selected and the relevant units shall be entrusted to re-examine its performance indicators, and the construction can only be started after the re-examination results are all qualified.
Design of cutoff wall
The cutoff wall is made of C 15 concrete and designed as a rectangular structure. The closure wall is divided into riverbed part and bank slope part. The width of the cutoff wall in the riverbed section is 2m, and the minimum height is not less than 2.5m The specific height is determined according to the actual topographic and geological conditions. The bottom of the cutoff wall must be not less than 0.5m deep into the bedrock. In the unfavorable geological section with deep bedrock, the bottom of the cutoff wall must also be located on the sedimentary soil.
3.4 Treatment of leakage channel in contact zone on the right bank of dam area
Aiming at the problem of leakage passage in the contact zone of the right abutment of the dam area, the original masonry retaining wall was demolished, and the leakage passage was filled with buried stone concrete first, and then combined with the seepage control treatment of the dam body and dam foundation, geomembrane laying and curtain grouting were used for seepage control treatment. The stone burying rate of stone burying concrete is 15%. During construction, a layer of concrete should be paved first, and then vibrated and compacted until the stones sink into the concrete. Stones are not allowed before pouring concrete. The size of buried stones should not be greater than 1/3 of the minimum size of concrete blocks, and they should be hard, fresh, free from weathering and cracks, with a saturated compressive strength greater than 200kg/cm2 and clean.
3.5 Slope protection design of downstream dam face
The downstream dam slope ratio of Baishuihe Reservoir is 1∶2.5, and the dam slope is protected by turf, which causes local collapse and overgrown weeds, and the drainage ditch on the dam surface is seriously blocked, which has a great impact on the downstream dam surface monitoring. In this reinforcement design, the slope of the downstream dam slope is trimmed to cultivate turf; The slope drainage ditch shall be dredged, and the downstream dam toe shall be provided with dry masonry footguards with section size of 1.0× 1.5m (width× height).
4 conclusion
Aiming at the leakage problem of dam body and dam foundation, geomembrane is used for dam surface seepage control, and curtain grouting is used for dam foundation and abutment seepage control. In view of the problem of leakage channel in the contact zone of the right abutment of the dam area, the original masonry retaining wall is demolished, and the leakage channel is filled with buried stone concrete first, and then combined with the seepage control treatment of the dam body and dam foundation, geomembrane laying and curtain grouting are used for seepage control treatment; In view of the collapse of local dry block stones on the upstream dam surface, the dry block stones were removed and the hexagonal precast concrete blocks were used for slope protection. In view of the local collapse of the downstream slope, the downstream dam surface is trimmed, and dry stones are built at the foot of the dam to protect the feet and cultivate turf.
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