1. Basic information
Tiandi Reservoir is located in the north of Shanghe Village, Peifeng Town, Yongding County, upstream of Fuxi tributary and about 7km downstream of Tiannong Reservoir. It is an important small reservoir, mainly used for irrigation and flood control. The rainwater harvesting area above the dam site of the reservoir is 5.8km2, the normal water level of the reservoir is 60 1.80m, and the corresponding storage capacity is 840,000m3.
The reservoir hub project consists of main dam, spillway, water conveyance culvert, power station behind the dam and other buildings. The dam is a homogeneous earth dam with a height of 26.0 meters and a crest length of 152.50 meters ... The dam body is filled with coarse-grained residual slope soil, with gravel sand accounting for about 40% and clay accounting for 15%. The bedrock is strongly weathered sandstone with developed cracks, most of which are larger than the allowable opening. In the design and construction of the dam, there is no concrete floor, and the dam is directly filled on the bedrock with developed cracks. In addition, the fill is loose, and the dry density is 1.42 ~ 1.46g/cm3. After the reservoir impoundment, a large area of loose water flooded on the back slope of the dam, and the contact zone of the dam foundation was washed away. Therefore, during the operation of the reservoir, the water level is always controlled, which hinders the normal water storage. Paper reference network.
2. Anti-seepage reinforcement design
The characteristics of the dam are short, short and loose, and the dam body and contact zone have been seriously damaged by seepage. In view of the above characteristics, the following design schemes are compared.
2. 1 split grouting anti-seepage technology: split grouting, two rows of grouting holes are arranged near the dam axis, with the row spacing of1.5m. The holes are arranged in a plum blossom shape and constructed in two sequences. Hole spacing: 4m in the river section and 2.5m in the bank slope section. The hole depth is 0.5m for bedrock, the grouting orifice pressure is 0 ~ 0.5 MPa, the average curtain thickness is 50 ~ 200 mm (cumulative), and the cumulative thickness of the dam crest is not less than 30 mm .. The main drainage holes are arranged along the dam axis to establish a continuous impervious curtain; The auxiliary row holes are arranged along the upstream of the main row holes. The seepage stability and deformation stability of dam body are solved by fracturing, squeezing, seepage, filling and wetting consolidation of pressure mud. The construction period is 90 days and the estimated budget is 280,000 yuan.
Advantages: simple equipment, short construction period, low cost and stable and reliable quality.
Disadvantages are: high technical requirements and the need for professional team construction.
2.2 Concrete cutoff wall: it is arranged along the dam axis, with a wall thickness of 40cm, a construction period of 1.20 days and an estimated budget of 880,000 yuan. Paper reference network.
Advantages: stable and reliable quality.
Disadvantages: high cost, long construction period, huge equipment, high deployment cost and small engineering quantity, which is unacceptable to the construction team.
2.3 High-pressure jet grouting pile (jet grouting pile): the distance between bored piles is 1.0m, the pile diameter is 0.7m, and the high-pressure jet grouting pile between piles is estimated to be1/.2 million yuan.
Advantages: stable and reliable quality, easy to mobilize local construction team.
Disadvantages are: high cost and long construction period.
Scheme 1 split grouting (1): the scheme adopted in dam reinforcement design. Through the coupling analysis of dam deformation and failure mechanism, this is the best scheme among several technical schemes. Because splitting grouting can supplement the soil with insufficient principal stress through grouting pressure p=△p+r'h'. Where △p is the orifice pressure at the top of grouting pipe in kg/cm2；; ; R'h': refers to a certain high mud column pressure kg/cm2 from the grouting nozzle to the dam body. P grouting plays an important role in the splitting of dam and the compression, filling, seepage and stress adjustment of soil on both sides. With the increase of grouting times, the grouting pressure P has the above-mentioned effects on the soil on both sides of the slurry vein for many times, and at the same time, the dam soil rebounds to compress the slurry after stopping grouting for many times, which promotes the drainage consolidation and hardening of the slurry vein. Split grouting process is a process of stress adjustment and stress redistribution in different stress fields of dam body. Paper reference network.
Based on the above analysis, the biggest advantage of this scheme is that it solves the problems of seepage stability and deformation stability encountered by the dam and restores the seepage stability and deformation stability of the dam; Low cost; The quality is stable and reliable.
3. Grouting construction
3. 1 Construction measures:
In view of the dam, the following measures have been taken in grouting technology: during and after the construction of the dam, a lot of settlement and deformation occurred, so that the soil area at the top of the dam and under the bank slope dam is in the longitudinal tensile zone (3) along the dam axis, and the small principal stress plane is perpendicular to the dam axis. By densely arranging holes and increasing grouting pressure, the small principal stress surface can be adjusted to 90 degrees, which can only be completed after the third recharge. In the dam section with bank slope less than 5m, the small principal stress surface along the longitudinal direction is not obvious, and the following principles have been mastered in the construction:
3. 1. 1 dense holes: two rows of plum blossom holes, with the final hole spacing of 2.5;
3. 1.2 Increase the drilling depth: pass through the dam body from the dam top and enter the bedrock by 0.5m
3. 1.3 Slurry is diluted first and then thickened: dilute slurry is poured in the stress adjustment stage, with the specific gravity of 1.3g/cm3, and the specific gravity of curtain construction stage is greater than1.4g/cm3;
3. 1.4 Increase the grouting pressure as much as possible;
3. 1.5 Increase the number of refilling, not less than 6 times; Extend the grouting time, and the interval shall not be less than 3 days.
3.2 Construction control:
In order to improve the grouting effect and ensure the stability of the dam body during grouting construction, the following controls are implemented on the process and construction technical indicators:
3.2. 1 Before and after each grouting, the horizontal displacement of any measuring point on the dam slope shall not exceed 2cm. After the dam body starts to rebound, the next backfilling is allowed;
3.2.2 In the stress adjustment stage, the specific gravity of mud is 1.3 ~ 1.4 g/cm3, and in the impervious curtain construction stage, it is1.4 ~1.5g/cm3;
3.2.3 The grouting technology at the bottom of the hole is adopted to delay the splitting of the dam crest as much as possible. Generally speaking, before the second recharge, the dam crest is not allowed to split. The crack width at the top of the dam is not more than 2.0cm at a time, and the dam body begins to rebound after the irrigation is stopped, and before the irrigation;
3.2.4 Grouting pressure (grouting nozzle pressure): the opening pressure is not limited, and the operating pressure is maintained at about zero pressure. When negative pressure occurs, the specific gravity of mud can be increased to above 1.4 g/cm3;
3.2.5 Every recharge interval is 3-5 days;
3.2.6 Primary grouting quantity: 0.5 ~ 1.0 m3/m/ time for the first time and about 1.0 m3/m/ time for the second time. The total grouting amount shall meet the design requirements for the thickness of the dike-the average design thickness is about 0.20m (the dry bulk density of the dike is1.6g/cm3, based on the total amount of injected soil);
3.2.7 Final grouting standard: transverse and oblique cracks will no longer appear at the dam crest, negative pressure will no longer appear at the grouting nozzle after zero pressure is maintained for 5 days, and the thickness of mud vein will reach the design value;
3.2.8 Hole sealing: the longitudinal cracks on the dam crest are connected with mud veins, and the mud sinks by 0.3m after dehydration, and is filled with dry soil after hardening;
4. Grouting effect analysis (4)
Because the dam was built and raised twice in 1957 and 1977, the dam structure has certain particularity, which leads to the complex stress field of the dam body and great difficulty in grouting. With the increase of stress adjustment stage, the grouting quantity and time limit far exceed the design. Due to the loose filling outside the right wall of the spillway, the mountains at both ends of the dam are steep, and a large number of concentrated slurry leakage points appear on the side and left and right bank slopes of the dam near the spillway. After grouting is completed, all the above phenomena disappear and the stability of the dam body is guaranteed.
4. 1 seepage change analysis
The fracturing grouting of the earth dam of this reservoir lasted for 90 days, and 620m3 of clay was put into it. According to the dry bulk density after consolidation and hardening of mud veins, the average thickness of each main mud vein is 16.7cm. Through the excavation verification of the earth dam after grouting, it is found that the distribution of mud veins in the dam body is that all fine mud veins are parallel to the dam axis within 3m below the dam top, and there are 1 ~ 2 main mud veins below. At the height of 1/2, the thickness is about 30cm, and the permeability coefficient of the dam vein is k = i×10-7cm/s. After the appearance of the dam vein, the permeability gradient of the dam soil is greatly reduced, and the permeability gradient of the dam body is increased from 0.07 before grouting to 0.22 after grouting. The swamp and open flow behind the dam have disappeared, the escape point of the saturation line falls below the river bed elevation, and the surface seepage is zero. During grouting, there are 12 grouting points in front of the dam and 18 grouting points behind the dam. The grouting point is basically the same as the dispersed soaking point when the reservoir water level is high. It shows that the continuously closed mud wall curtain that has been split along the dam axis has been connected with the leakage channel, and the leakage channel has been blocked by mud. Because the pressure of the mud column is greater than the pressure of the water column at the high water level of the reservoir, the seepage control gradient produced by the mud column pressure is much greater than the hydraulic gradient produced at the highest water level in the original seepage channel and some soil areas with loose dam soil and low seepage control gradient. Theoretical analysis and high water level test after grouting this year prove that the seepage curtain of mud wall formed by split grouting and the squeezing, infiltration and filling of mud to dam soil on both sides of slurry vein ensure the seepage stability of earth dam.
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