Development of backwater of Pijiazhai karst spring by beam pressure regulation

3.4. 1. 1 development technical conditions

Pijiazhai spring is located in the valley at the bottom edge of the northeast karst basin in western Shandong Province. The valleys are banded in the north-south direction, flat and open, and the valleys are high in the north and low in the south, all of which are paddy fields and lotus ponds. The Azuma Tani, west and north of the trough are surrounded by mountains, and the valley slope is about 25 ~ 45. The elevation of western and northern mountainous areas is 1830 ~ 1900 m, and depressions and sinkholes are developed. The landform is a peak-cluster depression, and the elevation of the eastern mountain is 1900 ~ 2300 m, which is a karst mountain landform at the edge of the basin.

The flow rate of spring water in Pijiazhai is1072.75 ~1957.5 L/s. At present, only a part of the flow rate (about 200L/s) is used for agricultural irrigation in luxi county nitrogenous fertilizer plant, tobacco redrying plant and other factories and central towns, while most of the spring water flows into medium and large rivers along streams. Due to poor drainage, the downstream section of the spring mouth often suffers from waterlogging in rainy season. Dongdagou, about 300m away from the spring on the east side of the valley, was built in 1956, with a total length of 13km. It is planned to irrigate 4000 mu of farmland, and the diversion source is the dam core spring with the flow rate of 250 ~ 400 L/s. At present, only 3.0km of ditches are light ditches built with mortar on three sides, and the rest are earth ditches. Due to years of disrepair, the ditch collapsed, the ditch bed silted up and the canal leaked seriously. In addition, the spring flow in the center of the dam is small. At present, only a small amount of ditch water reaches Xiaolongdian village, and the actual irrigation area is about 500 mu. During the dry season, more than 20,000 mu of farmland in Lvliang Village, Shenshupo and Daxing Fort under Xiaolongdian Village are irrigated by electric pumps in the middle reaches.

3.4. 1. 1. 1 water source profile

Pijiazhai Daquan is located in the north of Pijiazhai village, 2.5 kilometers away from the county seat. The coordinates of the spring mouth are103 47' 20 "east longitude, 24 32' north latitude16" and the elevation of the spring mouth is1711m. There are mainly two karst springs in the spring area, dam core spring and Pijiazhai spring. The spring area is long and distributed in the northeast, with an area of 1 15km2. The sandstone and mudstone of Gejiu Formation are the water-resisting boundary in the east, the underground watershed of Xiaogaopo-Daqishan and Arugudong underground river system in the west, and the watershed of Xiaojiang River Basin in Luxi and Jiulongjiang River Basin in Luoping in Shizong Cai Yun area in the north.

3.4. 1. 1.2 Characteristics of water-bearing medium

The spring area is a monoclinic structure inclined to the west, and faults are not developed. The lithologic combination of Gejiu Formation, the main aquifer group, is medium-thick layered limestone, dolomite limestone, thin limestone and argillaceous thin limestone. There are three groups of high-angle joints and fractures, the strike is 60 ~ 80, 300 ~ 320 and 340 ~ 350 respectively, and the spacing is generally 30 ~ 80 cm. Karst development is strong but uneven, and the degree of karst development is obviously controlled by lithology, joints and cracks. The depressions, funnels and sinkholes are beaded, and the distribution density is high along the thick pure limestone stratum. The karst development in Baishui-Cai Yun area of spring water recharge area is relatively uniform, and the upper strong-medium karst development zone is generally less than 120m, which is dominated by dissolved pores and fractures with good connectivity, forming a network vein structure; The karst development below 120m is weak, mainly small dissolved pores, with poor connectivity. In the central area of Baishaba in recharge area and runoff area, karst develops strongly within the depth range of 50 ~ 1 10m, but the uniformity is poor, and the thickness of vadose zone is generally greater than 30m. The vertical karst forms are mainly tiankeng and vertical shaft, forming an uneven gap pipe structure. According to the statistics of seven boreholes, the hole opening rate of boreholes is 57.50008080005, and the hole height is generally 1 ~ 5.5m, with the maximum of 16. 10m. Clay filling-semi-filling, cave elevation 1723 ~ 1782m. Below 1 100 m, karst development is weak, mainly karst cracks and pores. According to the analysis of exploration data, the karst development in the runoff discharge area from the dam core to Pijiazhai is quite different horizontally and vertically. At the valley bottom on the east side of Pijiazhai, the karst strong-medium development zone is generally 25-40m thick, mainly composed of dissolved cracks and pipelines, and the deepest part can reach 100m. However, the karst development on the downstream side of Daquan is weak, and the thickness of strong-medium karst development zone is less than 10m, which is mainly karst fractures and pores.

3.4. 1. 1.3 recharge, runoff and discharge characteristics of karst water

The recharge route of Pijiazhai Daquan is long, and the recharge runoff area is exposed karst areas such as Baishui karst trough and valley in the upper reaches of the basin and peak-cluster depression on the edge of the basin. The relative height difference between the recharge area and the Daquan outlet is about 70 ~100m ... The Cai Yun-Baishui recharge runoff area mainly receives infiltration recharge from atmospheric precipitation and leakage recharge from surface rivers. In Baishuitang Reservoir-dam core area, besides the concentrated recharge of karst water by atmospheric precipitation through depressions, sinkholes and vertical shafts at the bottom of funnels, the seepage recharge of Baishuitang Reservoir is a stable and important recharge source for the dam core area and Pijiazhai.

The general flow direction of karst water in Pijiazhai Daquan is Cai Yun-Baishui-Baishuitang Reservoir-dam core-Pijiazhai. Karst water-bearing space in Cai Yun-Baishui area is dominated by dissolved pores and fractures, and the water-bearing capacity is relatively uniform. The karst water level is shallow, generally less than 20m, and the runoff is slow, and the karst water level is deep to the south. In the process of collecting runoff into the basin, part of karst water is discharged in the form of spring water in Xiaomengze and Xiaowunaibai, and the other part is discharged in the direction of dam core and Pijiazhai through cracks and pipelines. Between Baishuitang Reservoir and the dam center, karst water has a strong cyclic alternation in the process of recharge and runoff, and the hydraulic gradient is large, from 1.43% to 1.62%. Apparent velocity 144.82 ~ 176.45 m/h, and the buried depth of karst water level is generally greater than 30m.

The water level elevation of karst water in Baishuitang reservoir area is about 1760m, and the supplied karst water generally flows from northeast to southwest. When the karst water is transported to the dam core and Pijiazhai, the water level of the karst water gradually becomes shallow, and it is under pressure because of the overlying weak permeable soil layer. Part of it is discharged in the form of rising springs, mainly in the dam center and Pijiazhai springs, and the other part flows laterally under the karst aquifer covered by Quaternary in the basin to participate in the basin.

3.4. 1. 1.4 engineering geological conditions of quankou

Through geophysical exploration and drilling, the overburden near Daquan is Quaternary clay, with bedrock exposed in the west and thick soil layer in the east, with a total thickness of 2.0 ~15.2 m. The foundation soil structure of the whole site is complex and the uniformity is poor, and it can be divided into four layers from top to bottom:

(1) plain fill with a thickness of 0.5 ~ 2m and loose structure.

(2) Silty clay, maroon, hard plastic, lenticular, with a thickness of 0 ~ 2. 1m and moderate compressibility, fk= 140kPa.

(3) Clay, brownish gray, brownish yellow, soft plastic ~ plastic, 0.8~9.6m thick, with powder lens, 0.5 ~1.9m thick, with high compressibility, fk=60 kPa.

(4) Clay, maroon, plastic-hard plastic, with thickness of 0 ~ 7. 1m, medium compressibility, fk= 180 kPa.

The karst of the underlying aquifer is developed, but the uniformity is poor. The permeability of rock mass north of Quankou is generally greater than 50Lu, and the maximum is 4200 Lu. Generally, the permeability of rock mass above 20m to the east of Quankou is 20 ~ 657Lu, that below 30m to the south of Quankou is 7 ~ 80Lu, and that above 30m is 25 ~ 4 10Lu. The contact zone between soil layer and bedrock has weak soil and strong water permeability.

3.4. 1. 1.5 spring flow dynamics

Pijiazhai Daquan 199 1 ~ 1992 long-term observed flow 1093.4 ~ 2 170L/s, long-term observed flow in 2004 ~ 20051072.75 ~. For the seepage regulation of Baishuitang Reservoir, the dynamic range of flow is not large, and the spring flow is stable (Table 3-5). In the Pijiazhai spring system, the karst water level in the recharge runoff area of Baishuitang Reservoir-dam core wall is buried deeply, generally 20 ~ 50m, and the maximum amplitude can reach 70m according to the investigation. Karst water level in Baxing-Pijiazhai area of the river basin is shallow, with many pressures, and the annual variation is less than 2.5m

Table 3-5 Dynamic Characteristics of Pijiazhai Spring Flow in Xiaojiang River Basin, Luxi

3.4. 1.2 experimental situation

The exploration and design work was carried out on the basis of fully collecting and utilizing the existing data, and mainly arranged1:5,000 remote sensing interpretation,1:50,000 regional hydrogeological mapping,1:0,000 engineering planning area hydrogeological mapping, geological radar, high-density electrical profile detection, pipeline tracing test, backwater test and karst. Among them, the field hydrogeological test is an indispensable means to investigate the pipeline flow system of karst caves, which can provide the most accurate hydrogeological information. The main test work and understanding are as follows:

tracer test

This typical study collects and uses existing test records. The launching point of the test is the tiankeng of Baishuitang Reservoir in the upper reaches of Pijiazhai Spring, and the receiving points are Baxin Spring, Pijiazhai Spring, Arufa Spring, Arugudong Underground River and Crazy Longtan Spring. The inlet velocity of the plunge pool is 82.2L/s,16000 kg of salt is added at 9: 00 on June 26th, 992,16:5438+04:00 on June 28th, 992.

Among the five receiving points, the Cl- content in Baxin Spring and Pijiazhai Spring is abnormal. Elevation of launching point 1783. 17m, horizontal distance from dam center of 4200m, height difference of 68. 17m, horizontal distance from Pijiazhai of 5470m and height difference of 78. 17m.

The Cl- background value of Pijiazhai Daquan is 1.69mg/L, and the abnormal time is from June 22nd 15: 45 to June 25th 15, lasting for 79 hours and 30 minutes, including June 22nd 2 1: 30 and June 23rd. The process curve of Cl- content is steep and shows a single peak. The Cl- content rises to the peak rapidly, and then decreases slowly for a long time, indicating that there is a relatively single karst channel from Luoshui Cave to Pijiazhai Daquan, and the karst system is large in scale, and the water-bearing medium is mainly a karst pipeline with strong water conductivity. After calculation, the hydraulic ratio is reduced to 1.43% ~ 1.62%, and the velocity is144.82 ~176.45 m/h.

Pijiazhai Daquan (flow rate 1584L/s) and Baxin Daquan (flow rate 427L/s), the salt recovery amount of the two receiving points is 4893 kg, and the recovery rate is 8 1.6%, of which the salt recovery amount of Pijiazhai Daquan is 4293 kg. It shows that Pijiazhai Spring is the main discharge point, but Pijiazhai Spring and Baxin Spring are not the only discharge points, and there are some undercurrents or other discharge ways.

3.4. 1.2.2 backwater test

In most cases, the characteristics of karst water level are studied by pumping test and borehole water level observation data [7][8], while the literature [9][ 10] studies the characteristics of underground river water level by blocking and discharging test. However, it has not been reported to study the dynamic change characteristics of water level in Daquan and its vicinity by backwater test.

(1) backwater test layout: the natural spring mouth of karst spring is closed, and the overflow mouth is set to make it overflow. There is a 16 water level observation well near the spring mouth, with a depth of 30 ~ 50m. Before backwater, the water level of the spring mouth and observation well was measured once. During backwater, observe continuously for 3 times every 10 minute and 5 times every 30 minutes, and then observe 1 time at 1 h until it is stable. The backwater started on February 24th, 2004 10 and ended on February 25th, 2004 10. The observation lasted for 24 h, and it was measured again after three days, and the water level of each observation point remained stable. Limited by the height of cofferdam, the maximum water level rise height is 0.66 meters in spring backwater.

(2) Analysis of water inflow test results: After the backwater from Pijiazhai karst spring, the water levels of 65, 438+065 and 438+0 in the nearby 65, 438+06 observation wells respond, and the rising range and response time of water levels are very anisotropic. (1) The spring water level rises the most. The spring mouth is in a state of self-flow discharge, and the maximum rising stability of the water level is 0.6 1m, and the rising stability in other areas is 0.02~0.56m, which are all smaller than the spring mouth, forming an irregular ring centered on the spring mouth, with a relatively large rising range from north to south and a relatively small rising range from east to west (Figure 3-20). The mechanism of water conduction and drainage is like a sprinkler and a connected water pipe system. It also shows that Daquan water pipeline is buried deeply and the water pressure is higher than the surrounding solution system. Due to the weak permeability of karst aquifer around the water pipeline and the outlet of Daquan, the diffusion of water pressure in the spring mouth is limited, which makes it rise and gush out, forming a confined rising spring, which has the potential condition of beam backwater. (2) The difference of water level rise is great, and the anisotropy is obvious. For example, the distance between ZK 10 in the northeast and ZK8 in the southwest is 85m, and the water level rises by 0. 14m and 0.06m respectively, while the water level of ZK9 in the southeast only rises by 0.02m. The upstream responds to the change of backwater pressure in the spring mouth faster than the downstream. Generally, when the backwater of the spring rises, the upstream observation well generally responds after 2 minutes. When the spring water becomes turbid, the water in ZK 1, ZK2 and ZK6 also becomes turbid, which also shows its good connectivity. The response speed of the downstream water level rise is slow, and it takes about 60 minutes to respond slowly. For example, the response time of ZK3 hole upstream and ZK9 hole downstream of 100m is 2 10/0min, while the response time of ZK 1 hole upstream of 50m and ZK 12 hole downstream of 75m is 2min and 60min respectively, which are obviously different. When the spring water becomes turbid, the gravity holes such as ZK8, ZK9, ZK 12 in the downstream always remain turbid, which also shows that their connectivity is poor. This is mainly due to the difference of karst development degree. According to the high-density resistivity method and drilling data, the karst development in the upstream and downstream of Daquan is quite different. The karst in the upper reaches and eastern covering areas is developed strongly, and the core is broken, with RQD less than 40%, mainly distributed in cracks, pipelines and caves. The porosity of drilling is 76.9%, the porosity is 4.7m/ 100m, and the cave elevation is 17 10. The karst development in the lower reaches of Daquan River is weak, and the core is complete, with RQD of 63% ~ 77%, mainly dissolved fractures, partially filled with clay, with porosity of 66.7%, porosity of1.2m100m, and cave elevation of1700 ~1. The downstream of the spring mouth is like a natural front dam, which is the hydrogeological prerequisite for the establishment of the beam backwater project.

Fig. 3-20 Schematic Diagram of Isohydrographic Line of Pijiazhai Daquan in Xiaojiang River Basin of Western Shandong.

1- spring; 2-Drilling and numbering; 3— isohead line (m above sea level); 4-Isogram of Head Rise (m)

3.4. 1.2.3 technical scheme

Pijiazhai spring has a large hydraulic gradient and has hydrodynamic and geological conditions to raise the head. According to the hydrogeological conditions of Pijiazhai Spring and the local water demand, the suitable mining method is to make full use of favorable hydrogeological conditions, carry out underground grouting around the spring point to form an anti-seepage beam curtain, build a horseshoe-shaped beam surge shaft with the bottom connected with the anti-seepage beam curtain on the ground, raise the water level to the level of gravity flow in Dongdagou, and then introduce water into Dongdagou through pipelines to realize gravity flow diversion development (Figure 3-2 1 The technical scheme of development engineering includes underground anti-seepage beam curtain.

(1) Underground seepage-proof beam curtain: around Pijiazhai Spring, according to the characteristics and degree of karst development, the seepage-proof beam curtain is injected into the carbonate aquifer to form a suspended seepage-proof beam curtain, which is connected with the deep weak karst development zone and distributed in a horseshoe-shaped plane. The purpose is to inhibit the flow of karst water and prevent its diffusion speed from accelerating after the water level rises; Avoid the increase of potential erosion intensity and protect the stability of the overlying soil layer. Among them, the southwest grouting depth is 30m, the east grouting depth is 15m, and the north is the main inflow direction of karst water, and the grouting depth is less than 8m, which can not only hinder the inflow of big springs, but also achieve the expected purpose.

Fig. 3-2 1 Schematic Diagram of Pijiazhai Large Spring Beam Flow Regulation and Backwater Project in Xiaojiang River Basin, Luxi.

1- water-conducting karst cave pipeline; 2- Loose soil cover; 3— Karst water flow direction; 4- original spring water level; 5 —— Water level after backwater of the project.

A total of 90 grouting holes were completed, with the hole depth of 8 ~ 30.00 m, total footage 1854m, cement 260.65t and sand19.94 t. The unit injection rate is above 20kg/m to 1000kg/m, and the average unit injection rate is 202.6kg/m, of which 29.2 are 20-50kg/m, 50-100-5000.

(2) Above-ground beam surge tank: According to the topographical conditions, the above-ground beam surge tank is designed as a horseshoe with a maximum side length of 55m, a semicircular radius of 27.5m, a height of 4.6m and a volume of 10255m3. The beam surge shaft is connected with the seepage-proof beam curtain to form an integral beam-type water return bucket, which plays the role of backwater and water level adjustment. The two gates on the water tank can adjust the water level and divide water as needed. The bearing capacity of the foundation soil in Quankou is low and varies greatly, and the uneven settlement and staggered joints of the masonry pool wall designed in the early stage are serious. Then, according to the engineering geological characteristics of the foundation, clay dam is used to replace and reinforce the pool wall, and some wooden stakes and concrete precast piles are used to reinforce the silt and soft soil foundation around the sluice, which effectively solves the above problems and makes the project play a stable role.

(3) Water conveyance pipeline project: In order not to occupy farmland, the water conveyance pipeline is set as an embedded inverted siphon with a length of 3 12m, with prestressed concrete pipes with a diameter of 0.8m and a diameter of 0.8m, with steel pipes at the inlet, valves at the inlet and gates at the outlet. The inverted siphon is buried underground 1.5m, and the bottom is all padded with gravel mortar except the silt foundation section.

In addition to the backwater development of Pijiazhai Spring, the dam spring was also jointly diverted. The big spring in the center of the dam is exposed at the foot of the valley source slope at the edge of the basin, and it is distributed in the north-south direction, located at the northeast of Pijiazhai big spring 1.5km. The long-term observation flow of the big spring in the center of the dam is 250 ~ 400 L/s, and the spring flow is dynamic and stable. The development plan is to clean the outlet of Daquan, enclose it with mortar masonry, and then build a mortar masonry three-sided CD valley with a width of 2m and a depth of 2.5m along the Azuma Tani foot of the trough to divert water, so as to realize the purpose of self-flowing water diversion development.