During the period of 1985, a high-pressure steel pipe of Bath County Pumped Storage Power Station was detained. Although the drainage curtain was carefully arranged in the steel pipe area, it did not play the expected role due to the layered structure of sandstone. Buckling failure accidents of high-pressure steel pipes in hydropower stations at home and abroad often occur under the action of external water pressure. There are also many examples of hydraulic fracturing in high-pressure hydraulic tunnels. The frequent collapse accidents of hydraulic tunnels and other tunnel projects are mostly caused by the adverse effects of rock fissure water.
Landslides are frequent natural disasters. Larger natural landslides are mostly landslides in rock mass. 1963, the volume of rock mass landslide in the reservoir area near the left bank of Vajont arch dam in Italy reached 250 million m3, which was the largest landslide ever recorded in terms of scale, sliding speed and disaster degree. From 65438 to 1960s, rock mechanics, especially rock hydraulics, was still in the primary stage. It was not estimated that landslides would cause thousands of deaths, so effective treatment and forecasting measures could not be taken. In April 2000, a landslide of about 300 million cubic meters occurred in the granite mountain on the left bank of Zangbo River in Gong Yi, Tibet. According to analysis, landslides are caused by melting snow and water infiltrating into the mountain. There are high rock slopes caused by manual excavation in hydropower station sites, highways and railways. Many artificial rock slopes are affected by rainfall, construction water and domestic water, resulting in different degrees of loss. Many projects have taken comprehensive control measures mainly based on drainage, which effectively prevented landslides.
To sum up, many engineering accidents are related to rock hydraulics. This paper only illustrates the importance and urgency of studying, learning and mastering rock hydraulics with several examples of major engineering accidents.
Rupture of Marbasque Arch Dam in France
2. 1 Introduction of Marbasque Arch Dam The Marbasque hyperbolic arch dam is located on the Raylan River in the south of France, with a dam height of 66m and a total storage capacity of 5 1 10,000 m3. The elevation of the dam crest is102.55m, and the arc length at the top is 223m ... The thickness of the dam gradually changes from 1.5m at the top to 6.76m at the center bottom, which is a hyperbolic thin arch dam. There is a gravity thrust pier with wing wall on the left bank, which is 22m long and 6.50m thick. The maximum height from concrete to foundation surface is 1 1m, and the excavation depth is 6.5m A spillway without gate control is set in the middle of the dam crest. The dam foundation is gneiss with schistosity dip angle of 30 ~ 50, which inclines to the downstream right bank. The middle of the larger rock is full of mylonite. There are two main faults in the dam site. One fault is F 1, nearly east-west, with an inclination of 45, and inclines upstream. The fault zone is filled with clay-bearing breccia and is 80 cm wide. The other is F2 in the north-south direction, which inclines to the left bank with an inclination of 70 ~ 80 (Figure 1).
Figure 1 Main Geological Structure of Marbasa Arch Dam
Fig. 2 hydrograph of Marbasa arch dam reservoir
2.2 The Marbasa Arch Dam was completed and put into storage at the end of 1954. The water level in the reservoir rises slowly. Five years later, by the middle of1959165438+1October, the water level of the reservoir reached 95.2m At this time, water flowed out of the rock at 20m downstream of the dam site and 80m elevation. Due to a rainstorm, the reservoir water surged to 100m on the morning of February 2, 65438 (Figure 2). In the afternoon, engineers visited the dam to study how to prevent the adverse effects of water seepage. As no abnormality was found in the dam, it was decided to open the gate at 6 pm to lower the water level of the reservoir. No vibration was found after the gate was opened. Managers checked the dam repeatedly at night and found nothing unusual. They left the dam at 2 1. At 2 1: 20, the dam burst suddenly, when the water level of the reservoir was 100. 12m. According to several witnesses of the disaster at the downstream of the dam 1.5km, they first felt the violent vibration of the dam, then heard a sudden loud noise similar to the roar of animals, and then felt a strong air fluctuation. Finally, they saw a huge water wall rushing along the valley, and at the same time, the power supply was interrupted. After the flood left the canyon, the flow rate still reached 20km/h, and the town of Frehus at downstream 12km was partially destroyed, with 42 1 person killed and 30 billion francs in property losses. The next morning, it was found that the dam had been washed away, only the right bank near the foundation had residual arch dam, and some dam blocks at the downstream 1.5km were washed away, and the rock mass at the left bank dam foundation was washed out of the deep ditch.
2.3 Investigation and Analysis after Dam Break 1959 The major disaster caused by the dam break of Marbasa Arch Dam shocked the engineering community, and there was no precedent for dam break before. The accident occurred in dam construction, especially arch dam construction, which is the most advanced country in the world. The dam was designed by Coyne, the most prestigious design master. It was the highest dam in the dam-break record at that time; The dam break destroyed the city of Freijus and caused great disaster in the richest Mediterranean region. This accident shows that any type of arch dam, including the one considered as the safest, will be destroyed (Serafim, 1987). The accident of Marbasa arch dam shows that little was known about the flow law of water in rock mass at that time. This painful lesson greatly promoted the development of rock mechanics, especially rock hydraulics. This paper will quote the published literature and analyze the mechanism of the accident from the perspective of rock hydraulics.
2.3. 1 official analysis of the causes of dam failure The French Ministry of Agriculture, the owner of Marbasque Arch Dam, set up an investigation committee on February 5, 65438. An interim report was submitted a few months later. In August, the final report 1960 was submitted on behalf of the government, and the summer report (Laeger, 1963) of 1962 was released. The main body of the report is only 55 pages with 40 annexes, so * * * forms a three-page report. The Committee entrusted EDF to recheck the dam stress, and the maximum compressive stress was 6. 1MPa, and the compressive safety factor of concrete was 5.3. There is a local tensile stress of 65438±0 MPa in the vault. EDF also checked the independent working conditions of the arch. The gravity pier on the left bank is also checked, which is safe under the action of arch ring alone. A large number of concrete blocks with bedrock were washed away, and no signs of damage were found at the interface between concrete and rock. The quality of concrete is good, and the compressive strength is 33.3 MPa ~ 53.3 MPa. Judging from this, the dam accident is caused by the dam foundation rock. The Committee believes that the pressure caused by seepage under the dam caused the first stage of damage (Jaeger,1979,391).
2.3.2 Discussion on the causes of dam break in dam engineering circles After the publication of the French official final report, it has aroused widespread concern in dam engineering circles. Coyne and Bellier conducted permeability tests on gneiss at the dam foundation of Marpset Arch Dam (Bellier and London, 1976), and obtained the obvious relationship between permeability and stress. According to this relationship, the causes of arch dam accidents are clearly explained, and Lund (1985, 1987) made a report at the international engineering foundation conference and the international seminar on dam accidents. Some important papers were published during the period'); "> Papers and monographs mainly include Jaeger (1963, 1979), Habib (1987), Post and Bonazzi (1987) and serafim (198/kloc-). It has been more than 40 years since the accident of Marbasa arch dam, and the cause of the accident has not yet been fully understood. However, most experts believe that excessive pore water pressure in dam foundation is the main cause of the accident.
2.3.3 Analysis of the flaky structure of Londe (1987) gneiss. The experimental study shows that when the narrow strip load is perpendicular to the foliation, the stress spreads to the deep part of the rock mass in a diffusion way, while when the load is parallel to the foliation, the stress distribution spreads to the deep part of the rock mass in a strip shape but cannot spread (Figure 3). Because of the spatial relative relationship between Marbasa arch dam and gneiss schist, the thrust of left abutment arch is parallel to schist, and the thrust of right abutment arch is perpendicular to schist. After bearing, the stress distribution of the left and right abutment rocks is very different. Due to the existence of fault F 1 on the left side of the dam, from arch support to fault F 1, a high stress rock mass strip is formed on the left abutment. Bernaix Cox conducted laboratory tests on the relationship between permeability and stress of gneiss after the dam break of Marbasa Arch Dam, and found that the relationship between permeability and stress of gneiss was very obvious. This relationship is represented by the index s:
Fig. 3 Stress distribution of vertical plane and parallel treatment under load
S=k- 1/k50
( 1)
Where: k- 1 is the permeability coefficient of rock block when the tensile stress is 0. 1MPa, and k50 is the permeability coefficient of rock block when the compressive stress is 5MPa.
The test shows that the maximum value of S index can reach 200. According to the test results of the relationship between permeability of rocks and stress, under the action of arch dam thrust, the fault from left abutment arch to F 1 actually forms a series of impermeable curtains, which is equivalent to an underground dam. The permeability coefficient of this area is only1100 or less of the surrounding rock. Due to the difference of permeability coefficient between in-band and out-band 100 times, the seepage head around the dam is all consumed in the impervious zone. Therefore, there is a pressure corresponding to the total head upstream of the seepage control zone. The rock mass of the left dam foundation slides along the F 1 fault under the action of full head pressure, resulting in the dam failure (Figure 4).
2.3.4 Analysis of witt ke and Leonards Professor Wittke of Aachen University in West Germany inspected the dam site of Marbasa Arch Dam in the autumn of 1984, and immediately studied the cause of the dam accident. As a visiting scholar at Aachen University, the author participated in this research. Based on the incremental load theory of rock mass seepage, Wittke analyzed the deformation and stress of dam and dam foundation under water pressure, self-weight and seepage load by finite element method. The results show that the rock mass at the heel of arch dam produces tensile stress in the vertical direction of foliation, and the foliation there produces tensile cracks. Reservoir water enters the fracture and splits the fracture into lower faults, forming full head pressure in the fracture, making the left abutment to F? 1 The fault rock mass is unstable (Figure 5), resulting in dam failure.
Fig. 4 Londe's explanation of Marbasa arch dam failure.
Fig. 5 Vitca's explanation for the failure of Marbasa Arch Dam.
The failure analysis of Marbasa arch dam in Figure 4 and Figure 5 is consistent in form, but the starting point is different. There are many structural planes in rock mass, such as joints, cracks, schistosity, bedding and faults, and the water mainly moves along these structural planes. For most rocks, the permeability of rock blocks is usually negligible. From this point of view, Vitca's analysis of the dam-break reasons of Marbasa Arch Dam is the most practical. Serafim and Vitca have basically the same views.
2.4 Summarize the disaster caused by the dam break of Marbasa Arch Dam. The analysis and study of this accident has deepened the understanding of rock mechanics in the engineering field and promoted the development of rock hydraulics, and now it has become an important branch of rock mechanics. Obviously, the formation of rock hydraulics is of great significance to the development of science and the safety of engineering. 1987 At the international seminar on dam break held at Purdue University, Chairman G.A.Leonards made a comment in his concluding speech: ".................................................................................................................................. ……"