A milestone for high slope excavation17 September 2001
The permanent shiplock slope works at the Three Gorges project in China have been unprecedented both in scale and difficulty. How to maintain integral stability and restrain deformation with a slope on each side of 170m and 68m vertical walls was a challenge of great risk. Rushu Wang explains how 3600 bundles of pre-stressed cables and 100,000 high strength bolts have stabilised the structure and solved a key technical problem of the Three Gorges project
One of the most striking structures of the Three Gorges is a two-way, five-step permanent shiplock. The maximum slope on each side reaches 170m, and a 68m vertical wall forms the lower section of the shiplock. A high vertical rock pier with a section of 57x68m remains in the middle between two chamber channels. Three longitudinal conveyance tunnels pass through the chamber and 36 shafts have been excavated on both sides of each lock head. The vertical wall has a concrete liner structure, 1.5-2.4m in thickness with high strength anchor bolts.
The characteristics and difficulties of the shiplock slope are:
• It is a steep high slope, deeply cut in the mountain body. It was of a considerable height and complex in structure, extending over a large range. The stress was fully released, indicating unloading and non-homogeneity.
• To meet strict requirements for slope stability, especially for deformation property, the slope of the permanent shiplock had to ensure both integral and local stability. The rheology of the slope had to be strictly controlled to meet the normal operation of the miter gates of the shiplock.
• The shiplock works had a complex construction, and excavating the vertical wall was very difficult in the narrow, deep and steep chamber. How to solve the influence of excavation blasting, minimise the damage to the rock body and ensure construction safety were all issues to be dealt with.
To meet structural requirements, the excavation elevations for the upstream and downstream approach channels had to be 130m and 56m respectively. As the chamber floor of the main section was excavated into the slightly weathered and fresh rock body, the elevations corresponding to the five chambers were 122.6m, 112.95m, 92.2m, 71.45m and 50.7m respectively.
The slope ratio for the permanent shiplock, through integral analysis, was determined according to the principle that the bench slope had to be basically self-stabilising.
The vertical wall was 10-12m, and a 5m wide bench was provided to increase the integral stability of the slope. The height of each bench was around 15m, depending on the structural layout, transportation, and overall stability requirements.
For the second and third chambers, which lay in the middle of the slope at elevations of 215m (on the north slope) and 200m (on the south) respectively, the bench width was increased to 10-15m. A 15m wide platform was provided on the top of each chamber.
On the top of the north slope, a platform was excavated at el 230m from the second chamber to the third gate head and connected with access roads. On the south slope, the top of Tanziling Hill, lying in the middle of the third chamber, was excavated from el 263m to 250m to decrease the maximum slope height and increase slope stability.
Based on a comprehensive analysis on the relationship between the replenishment and drainage of ground water, permeability and the hydrological and geological conditions of the site, the prevention and control strategy was mainly concerned with ground water drainage. The interception, prevention and drain-age of surface water to lower the water table, decrease the permeable pressure and improve the slope stability was supplementary to this. To reduce the infiltration of rainfall and block the hydraulic connection of the slope surface with the groundwater to allow full play to the underground drainage system, a system of peripheral ditches was provided on locations 5m away from the opening line of the slope crest to intercept the surface runoff outside of the slope.
To prevent infiltration of rainfall on the slope, the slope crest within the interception ditches and badly weathered zones of the sloping surface were netted and grouted with12cm thick concrete. Slightly weathered and fresh rock was grouted with 7cm thick plain concrete, and the platform of the wall crest, the platform of the central pier and the whole benches were grouted with 20cm thick concrete slabs.
Slope drainage holes were provided for both inclined and vertical slopes to eliminate residue phreatic pressure behind the protection layer. The holes on the sloping surface were drilled 10° upwards, with distance of 3x3m, 46mm in diameter, and 0.7-0.3m in depth.
In total, 14 drainage tunnels in seven layers were provided in the mountain body on each side with a horizontal distance of 30-45m. The elevations of these tunnels were, from top to bottom 200m, 170-175m, 152-157m, 130-135m, 110-125m, 89-94m, and 70-72m respectively. In general, the cross-section was 2.5x3m, while that concurrent for anchoring construction was 3x3.5m. For ventilation and equipment handling during construction and operation, two shafts of 3m in diameter were provided on each side, connected to the drainage tunnels of each layer by access branches. The floor of the drainage tunnel close to the rock body was provided with 0.25x0.3m drainage ditches.
The slightly weathered and fresh rock was impermeable, slow to drain and difficult to dewater. So drainage holes had to be drilled in all the drainage tunnels that had been excavated in this material.
The sections of drainage holes built through the geological defects and strongly weathered rock body were provided with inner protection in the form of plastic pipes wrapped with industrial filter cloth to prevent them from collapse.
Pre-stressed cables were mainly used to consolidate large unstable blocks on the slope, to restrain the further development and deterioration of damaged zones due to unloading and blasting, and to improve the stress deformation conditions and stability of the rock bodies.
To consolidate unstable blocks,1000kN and 3000kN pre-stress end cables were used, mainly in the lock chamber and the sloping section above the top of the chamber wall. To improve the stress deformation of the upper slope, a total of 226 1000kN cables and 1684 3000kN cables were arranged on two side slopes of the second and third chambers, the highest of the slopes.
Compared with the systematic cable of 40-60m, the block consolidation cable was generally 25-50m long, depending on the embedment depth of the sliding blocks.
Systematic bolts for weathered zones of the slope surface were arranged in conjunction with netted concrete facing. The bolts for the vertical slope section, including both sides of the central pier, were controlled by lined wall stability, except for an anchorage depth of three to five rows of bolts on the top of the vertical wall which was controlled by slope consolidation support. So, class five carefully rolled indented bars were used for this section. On the contact face of the concrete to the bed rock is a 30 – 50cm free segment with anchor bolts extending 5cm into the concrete lined wall. Some 100,000 bolts of high strength were provided for the vertical section. Common indented bars were used for interlock bolts. Five rows were arranged from 1m below the top of the wall, so, about 7,000 systematic interlock bolts were installed during construction.
Random bolts with a cohesive common mortar were used to treat minor unstable blocks outcropping from time to time or for other temporary treatment during construction.
Analysis results indicated a very good integral slope stability. In the highest slope section, the safety coefficient of stability was at least 2.9 when in non-earthquake condition. Design requirements could be met by an increase of safety coefficient by non-pre-stressed anchor cables in the vertical section without an earthquake. Pre-stressed cables in the vertical section during construction could increase the safety coefficient by 10%. For an earthquake measuring seven on the Richter scale, considering the amplification of force, the safety coefficient of stability decreases by some 15%, but remains over 1.5.
Spaced blocks means geometrical movable blocks of fault-fault (vein) combinations and outcropping on the slope. Since two faults or veins in reverse directions seldom develop simultaneously in the same segment, only a few of spaced blocks were found in the technical design stage; most of them located in the vertical section with a volume of several hundreds to several thousands cubic metres and embedded in depths of 6-48m. Most of spaced blocks were stable.
Semi-spaced blocks have intersecting faults with cracks and outcropping on the slope. At the technical design stage, assuming that persistent cracks in each favourable set were10m and based on the faults outcropping on the slope, the semi-spaced blocks were searched and identified in the light of the most unfavourable combination. Analysis indicated that most of them could not meet the stability requirements.
Random blocks include geometrically movable crack-to-crack combinations and outcropping on the slope. The length of cracks was generally 2-10m, although a few were more than 10m, and most developed with steep dip angles on the steep slope. The blocks of this kind were mostly long and thin with a small volume and embedded within 8m. Stability analysis indicated that most random blocks were poor in stability.
According to analysis, plastic zones mainly occurred on the shoulder of each bench section and over a large range of the vertical sections of the left and right sides and the central pier. Taking the vertical slope as representative, the values adopted were:
•The residue strength of rock as the rock mechanic parameter in the plastic damaged zones.
•The width of the platform on the top of the vertical slope, say, 15m for the crest width of slope, and 40m for the height.
•20% of the double of the conductivity after excavation unloading for the crack conductivity.
•7.5m for the depth considering anchorage to prevent crack propagation caused by unloading at the slope crest.
•Total water head for water load in the tensile crack, below which was distributed in the triangular form.
Based on these, the safety coefficient was 2.7, higher than the design required value.
After excavation of the slopes, unloading caused rebounded deformation on the right and left slopes: the left slope moved upwards to the right and the right to the left. In addition, the central pier was related with the difference of the heights of the right and left slopes; the horizontal displacement of the lower slope was bigger than that of the higher slope. The maximum horizontal displacement was 20-28cm in the completely and strongly weathered zones of both slopes, and 17-26 and 18-28mm for the vertical slope crests of the left and right slopes. The maximum vertical displacement was: 43cm in the badly weathered zones of both slopes, 16 and 26mm for the vertical slope crests of the left and right slopes respectively, and 48mm for the crest of the central pier.
A two-dimensional numerical analysis by generalised Kelvin models showed that after excavation, displacement condition changed slightly compared with construction period due to the viscous displacement caused by rheologic behaviour of rock directing to the air face of excavation: the horizontal displacement increases while the vertical displacement decreased. but the total displacement increased. After completion of excavation, the horizontal rheologic displacement develops on the top of the vertical slope. The rheologic behaviour of rock was weakened in a small amount, belonging to stable rheology, most of the deformation occurs during construction.
Rheology was not the major issue for the granite rock of the shiplock, however, the structure of the gate head was quite sensitive to the deformation in rock wall. Require-ments stated that displacement should not exceed 5mm after gate installation. Therefore, long-term deformation was the key.
Scientists undertook in-house and field rheologic tests to determine the rheologic parameters of the rock and long-term deformation analysis on monitoring data. The results indicated that the long-term deformation could meet the normal operation of the miter gates of permanent shiplock.
The major purposes of monitoring control for the shiplock were to: verify the preliminary slope studies and design results; provide a basis for dynamic feedback design; and supervise the long-term operation of the slope.
Control was systematic and random during construction. Some 1500 instruments were embedded, mainly for external and internal deformation monitoring of slope rock, initial geostress and geostress evaluation monitoring, bolt stress monitoring, monitoring over the loose areas of slope, seepage monitoring, blasting vibration monitoring, as well as concrete stress, strain, temperature, joints, re-bar stress monitoring and hydraulic monitoring.
Dynamic design is the basic principle complied for the permanent shiplock slope works and also one of the approaches of technical guarantee under implementation. The dynamic design included:
• Arranging two rows of interlock bolts close to the shoulder of each bench to treat the very loose areas formed by excavation unloading and blasting on the location.
• Rearranging the systematic bolts on the slope according to geological data obtained in construction and in the light of crack concentration and loose rock areas.
• Treating more than 580 blocks on stability analysis. The large unstable blocks were consolidated by pre-stressed cables, the minor ones by common mortar bolts. Pre-stressed bolts were seldom used due to the complexity of the construction.
The present situation
By September 2000, the displacement measurements were: in the north slope, a maximum displacement of 44mm to chamber centreline and a maximum horizontal displacement 32mm on the crest of the vertical wall; in the south slope, a maximum accumulative horizontal displacement of 57mm to the chamber centreline and a maximum displacement 47mm on the crest of vertical wall.
There was less annual vertical displacement on the measurement locations, the accumulative vertical displacement was within +/-6mm; the maximum accumulative displacement actually measured 37mm from the crest of central pier to north chamber and 19mm to south chamber; the vertical displacement rebounded in the rock body on the crest of the central pier, with a measured maximum rebound of 24mm. These data were basically consistent with the designed values.
Monitoring instruments, embedded in the permanent shiplock slope, along with a lot of deformation monitoring data during construction, indicated that the permanent shiplock slopes and the central pier were integrally stable.
Since the chamber channel excavation had been completed by April 1999, the displacement rate per month in the rock bodies of the two slopes and the central pier started to decrease and tended in a way of convergence, indicating the deformation of the rock body has been gradually stable. The practice of safety monitoring indicated that the monitoring results have already been exerted a great influence on the dynamic optimal design and construction decision.
The shiplock is scheduled to become operational in 2003.