(Institute of Hydrogeology Engineering Geology Technology and Method, China Geological Survey, Baoding, Hebei, 07 105 1)

The prevention and control project of Lianziya dangerous rock mass in the Three Gorges of the Yangtze River started in 1995 and was completed in August 1999. After the stress adjustment in the construction stage and after completion, the dangerous rock mass gradually tends to a new stability, the safety of the dangerous rock mass has been greatly improved, and the effect of the prevention and control project is becoming more and more obvious. Through the analysis and comparison of monitoring data before and after the prevention and control of Lianziya dangerous rock mass, this paper evaluates the stability of the dangerous rock mass, predicts the deformation trend of the dangerous rock mass, and makes a preliminary evaluation on the treatment effect of the project.

Keywords:: effect evaluation of Lianziya dangerous rock mass prevention project

1 overview

Geological survey 1. 1

Lianziya dangerous rock mass in the Three Gorges of the Yangtze River is located in Quyuan Town (formerly Xintan Town), Zigui County, Hubei Province, and together with Huangya old landslide and Xintan landslide area, it constitutes the hidden danger area of Xiling Gorge in the Yangtze River. The dangerous rock at the northern end of Lianziya dangerous rock mass is over 0/00 meters high, overlooking the Yangtze River. Generally, it is distributed in the north-south direction, oblique to the Yangtze River at 60 ~ 700, high in the south and low in the north, wide in the north and narrow in the south. The cliff top inclines to the northwest with a slope angle of 20 ~ 30, and the distribution elevation decreases from 500m south to Beihe 180m. The dangerous rock mass is composed of limestone of Qixia Formation of Lower Permian sandwiched with several layers of thin limestone and shale, and Maanshan Formation coal seam with its lower thickness of1.6 ~ 4.2m.. There are more than 30 wide and large cracks in the dangerous rock. The mountain is cut into three dangerous rock areas with different sizes, the first area is the t0-t6 coal seam section; ⅱ area for T7 coal seam; ⅲ area for coal seam section T8-T 12.

1.2 project overview

The prevention and control project of Lianziya dangerous rock mass began in 1994+ 10. The whole system is mainly composed of the surface drainage project at the junction of T0-T 12, the load-bearing anti-sliding hub project at the junction of T8-T 12, and the "50,000 square" and "7,000 square" anchor cables. The key point of prevention and control is the dangerous rock at the junction of T8-T 12 (2.5 million m2). Two major projects, the load-bearing anti-sliding project and the anchor cable project, started in May 1995, and were completed in August 1997 and August/999 respectively, marking the end of the construction of the dangerous rock mass prevention project in August 1999, and then fully transferred to the monitoring stage of the prevention and control project effect.

1.3 overview of monitoring system

The monitoring system of Lianziya dangerous rock mass was gradually established in 1970s. By the end of the prevention and control project, a three-dimensional monitoring system with various monitoring methods and automatic data collection and processing has been formed, including:

30 absolute displacement monitoring points on the rock surface (1) (earth deformation);

(2) The relative displacement of cracks is automatically monitored at 26 points and 39 points;

(3) Three-point automatic monitoring of horizontal hole multi-point displacement meter 1 1 point;

(4) 9 monitoring points of prestressed anchor cable dynamometer;

(5) Stress monitoring point 41bearing key rock mass against sliding;

(6) Deep displacement monitoring of five rock masses (borehole inclinometer);

(7) The central processing room is located in 1, which can collect and process monitoring data 24 hours a day. At present, the above monitoring equipment is operating normally.

Figure 1 Layout of Crack Distribution and Load-bearing Anti-sliding Project of Lianziya Dangerous Rock Mass

1. Load-bearing antiskid key; 2. Surface cracks; 3. adit entrance; 4. Deep displacement monitoring drilling

2 dangerous rock deformation before construction

2. 1t8-T9 coal seam section

According to the monitoring data of 1978 ~ 1994, before the treatment of dangerous rock mass, the rock mass at the cliff top creeps towards the NW direction, that is, it generally moves along the stratum. Among them, the east moves horizontally to N 17 W 1.2mm/a and sinks by 0.9mm/a; The horizontal displacement of the central and western parts of the surface to the NW direction is 0.7 ~ 2.5 mm/year, and the subsidence is 0.4 ~ 0.9 mm/year; The rock mass on the south side of T9 cliff joint is displaced to the northeast with a horizontal displacement of 2.3mm/a (see table 1).

Table 1 Average displacement table of Lianziya T8-T9 jointed rock mass treatment in last year.

2.2 T9-T 1 1 coal seam section

For a long time, the rocks in T9-T 1 1 joint have been moving in NNW-NNE direction, with uneven peristalsis. According to the absolute displacement monitoring data of 1978 ~ 1994, the cliff top in the east is displaced to NW at the rate of 1.4 ~ 1.7 mm, and the rock mass on the cliff top in the central and western regions is displaced to NW at the rate of1.6 ~1.9 mm.

Table 2 Average displacement scale of Lianziya T9-T 1 1 jointed rock mass treatment in the previous year

2.3 inch 7000 square inch sliding fuselage

For a long time, the "7000 square" surface sliding body has been sliding in NW direction with R402 as the sliding surface. According to the monitoring data at S7 point, before 1995, the soft layer along R402 gradually shifted to N30°~ 45° W, with a rate of 34.36mm/a and a slip angle of 30, which was basically consistent with the occurrence of rock strata (the dip angle of rock strata was 27 ~ 35).

2.4 "50,000 square" rock mass

From 1978 to 1995, the point on the cliff top G moves in the direction of N20 W, the speed is 1.5mm/a, the subsidence is 0.7mm/a, and F/H= 1/0.47. It shows that the deformation feature of "50,000 square meters" before treatment is creeping along the rock stratum, accompanied by subsidence.

2.5 Lightning strike stone sliding body

At the end of 1978 ~ 1995, the lightning strike sliding body moves towards NW at the speed of 1.6 ~ 2.0 mm/a (T80 1 and T802).

It can be seen that before the project construction, the rock mass above the precipice and the "7,000 square" and "thunderbolt" sliding bodies at the T8-T 12 junction mainly slide along the NW direction, while the rock mass below the precipice moves towards the Yangtze River.

3. Deformation of dangerous rock mass after engineering construction

3. 1t8-T9 coal seam section

According to the monitoring data of 1997 ~ 2003 (see table 3), after the treatment of dangerous rock mass, the horizontal displacement of the eastern part of rock mass cliff top in T8-T9 joint section decreased from 2.5mm/a before treatment to 2.0mm/a(t 8 1 point) in 2003, and the settlement decreased from 0.9mm/a before treatment to 2003. The horizontal displacement in the western region decreased from 0.7 ~ 1.8 mm/a before treatment to 0.6 ~ 1. 1 mm/a in 2003, and the settlement decreased from 0 ~ 0.4 mm/a before treatment to 0 ~ 0.2 mm/a in 2003 (T82, T82). The deformation direction changes from NW direction before treatment to NE direction; The rock mass on the south side of T9 joint under cliff moves from NNE to SW direction, and the horizontal displacement decreases from 2.3mm/a before treatment to 0.8 ~1.7 mm/a in 2003 (T9x1,a low point).

The deformation of rock mass tends to be stable (see Figure 2, Figure 3 and Figure 4), which shows that the prevention project has achieved results.

Fig. 2 Annual variation-time curve of T8 1 in T8-T9 coal seam section.

The monitoring data of relative displacement (see Table 4) also shows that the deformation of dangerous rock mass tends to be relatively stable after stress adjustment.

Fig. 3 Annual change-time curve of T83 in T8-T9 coal seam section

Fig. 4 Annual change-time curve of T82 in T8-T9 coal seam section

Table 3 Annual change table of absolute displacement monitoring points before and after rock mass treatment in T8-T9 joint section

Table 4 Annual Variation Table of Relative Displacement of Rock Mass in T8-T9 Joint Section

3.2 T9-T 1 1 coal seam section

According to the monitoring data of absolute displacement for many years, the rock block in T9-T 1 1 fracture section has been moving in NNW—NNE direction before treatment, and its peristalsis is uneven. The monitoring data of absolute displacement after treatment (see Table 5) shows that the horizontal displacement of rock mass at the cliff top in this fault section is reduced from 1.4 ~ 1.9 mm/a before treatment to 2000 mm/a. The settlement decreased from 0.5 ~ 0.8 mm/a before treatment to 0. 1 ~ 0.5 mm/a in 2003, and the deformation direction was basically NNE-NE-NS. The rock mass under the cliff is displaced from nearly N to NNE and NE, and the displacement is reduced from 1.8 ~ 2.0 mm/a before treatment to1.3 ~1.7 mm/a in 2003 (point B, T9x2).

Fig. 5 annual change-time curve of T9-t11coal seam point B.

Table 5 T9-t 1 1 Annual Change Table of Absolute Displacement Monitoring Points before and after Treatment of Cracked Rock Mass

After rock mass treatment in this fracture area, the displacement, deformation and settlement gradually decrease, which is lower than the average displacement rate for many years and smaller than the point-by-point error, and the deformation trend has been basically stable (see Figure 5 and Figure 6), indicating that the displacement and deformation of rock mass are not obvious, and the prevention project has achieved results.

Fig. 6 Annual change-time curve of point F of T9-T11coal seam section.

3.3 inch 7000 square inch sliding fuselage

For a long time, the "7000 square" surface sliding body has been sliding in NW direction with R402 as the sliding surface. According to the absolute displacement monitoring data (see Table 6), after the reinforcement of the "7000m3" sliding mass anchorage project, the rock mass is displaced along the direction of anchor cable tension, and then the displacement in this direction gradually decreases, from 4.9mm/a before treatment to 65438 0.3 mm/a in 2003 (S7 point), and the deformation trend (see Figure 7) is basically close to a relatively stable state. This shows that the prevention and control project is effective.

Table 6 Annual change scale of "7,000 cubic meters" sliding mass displacement before and after treatment

Essays on Geological Disaster Investigation and Monitoring Techniques and Methods

Fig. 7 Annual variation-time curve of "7000m3" sliding mass at S7.

The analysis of the monitoring data of relative displacement of "7000 cubic meters" landslide after treatment (see Table 7) shows that the deformation of rock mass tends to be stable, indicating that the prevention project has achieved results.

Table 7 Annual change scale of relative displacement monitoring points after treatment of "7,000 cubic meters" sliding mass

3.4 "50,000 square" rock mass

The "50,000-square" dangerous rock mass experienced the process of bedding slip in NW direction (before construction), movement in SE direction, and then slow displacement in SE direction and SW direction, and the displacement gradually stabilized from large to dangerous rock mass (see Table 8). After the construction of the anchor cable project, the "50,000 square" rock mass is displaced in a direction conducive to the stability of the rock mass, and the deformation gradually tends to be stable. Take the G point at the cliff top as an example. The average horizontal displacement for many years before treatment was 65,438 0.5 mm/a, in 2003 it was 0.8mm/a, and the settlement before treatment was 0.7 mm/a. In 2003, there was no vertical deformation at this point (see Figure 8). The deformation of other monitoring points is similar to that of G point.

The monitoring of anchor cable dynamometer also reflects the above deformation phenomenon (see Figure 9, Figure 10, Table 9). 1996 and 1997, after the dangerous rock mass is reinforced and locked by the anchor cable, the locking force of the anchor cable gradually decreases (the annual variable of the dynamometer is negative, and the absolute value is getting smaller and smaller), indicating that the dangerous rock mass is displaced in the direction of the anchoring force, and the displacement variation is changed by. During the period of 1999, the annual variable of anchor cable dynamometer is mostly positive, which shows the characteristics of anchor cable holding force, which is consistent with the deformation phenomenon of rock mass shown by displacement monitoring. Through the monitoring data in recent years, the stress of rock mass has been readjusted and tends to a relatively stable state, which shows that the effectiveness of anchoring engineering has been brought into play.

Table 8 Annual change scale of "50,000 square" absolute displacement monitoring point

Fig. 8 Annual variation-time curve of G point of "50,000 square" dangerous rock mass

Fig. 9 Monitoring data-time curve of anchor cable dynamometer in "50,000 square" dangerous rock mass

Figure10 Displacement-time curve of "50,000 cubic meters" anchor cable dynamometer in dangerous rock mass

Table 9 Statistical Table of Annual Variation of Anchor Cable Dynamometer Monitoring

The relative displacement monitoring data (see table 10 and figure 1 1) show that the deformation of dangerous rock mass tends to be relatively stable due to the effectiveness of the prevention and control project after treatment.

Table 10 Annual change table of relative displacement monitoring points of "50,000 square" dangerous rock mass

Figure 1 1 "50,000 square" time-history curve of relative displacement of cracks in dangerous rock mass

3.5 Lightning strike stone sliding body

The displacement of lightning strike stone sliding body decreased from 1.6 ~ 2.0 mm/a before treatment to 0.6 ~ 1.7 mm/a in 2002 after treatment (see table 1 1) (T80 1 and T802 points).

Table 1 1 Annual Change Table of Absolute Displacement Monitoring Points of Lightning Strike Sliding Body (T80 1, T802)

From the analysis of monitoring data, it can be seen that the deformation trend of dangerous rock mass obviously slows down and tends to be relatively stable before and after prevention and control, which shows that the prevention and control project is effective and effectively curbs the deformation of dangerous rock mass in a direction that is not conducive to rock mass stability.

4 Effect evaluation

The above analysis shows that the displacement and deformation of T8-T9 fractured rock mass, T9-T1/kloc-0 fractured rock mass, "7000m3" rock mass, "50000m3" rock mass and lightning fractured rock mass are not obvious after the prevention project is completed. There is no obvious displacement and deformation between blocks. Judging from the deformation trend, after the completion of the prevention and control project of dangerous rock mass, the stress of rock mass is readjusted and the deformation trend tends to be stable gradually. This shows that the prevention and control project is effective.

The comprehensive analysis shows that after the self-control project is completed, the deformation of dangerous rock mass has entered a relatively stable period after deformation adjustment, and the stability of rock mass has been obviously improved. The dangerous rock mass has reached a relatively stable state. The effect of the prevention and control project has been initially reflected.

5 conclusion

After the completion of Lianziya dangerous rock mass prevention project, through the analysis of monitoring data of dangerous rock mass, it can be concluded that the deformation of each joint rock mass of dangerous rock mass is obviously reduced and becomes relatively stable; Deformation of rock mass between cracks tends to be relatively stable. This shows that the prevention and control project has achieved results, the effect of the prevention and control project has been initially reflected, and the dangerous rock mass is in a relatively stable state.

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