Slope stabilisation by use of prestressed anchors on three of India’s hydropower projects in the Himalayas is described by Sanjeev Gupta, B.N. Asthana and Gopal Chauhan
Stability of hill slopes, particularly in the geologically weak rocks of Himalayas, needs careful attention for the safety of power projects. Prestressed anchors play a vital role in resisting destabilisation forces and measures adopted are described in three case studies from India: Nathpa Jhakri project on the river Satluj; the Ichari dam spillway basin at Chibro project (Yamuna Stage II) on the river Tons; and, the Dharasu powerhouse site (Maneri Bhali Stage II) on the river Bhagirathi.
General description on anchors
The key reference point for the use of prestressed anchors on dams was the work undertaken in the 1930s by the French engineer Andre Coyne to help strengthen the Cheurfas masonry dam in Algeria1. Over the 1934-5 period a series of high capacity vertical holding down anchors were installed at the dam.
The general arrangement of ground anchors for rock slope stabilisation (Figure 1) shows that they are typically inserted into boreholes, bonded to competent rock by sufficient grout along a fixed length to provide sufficient anchorage and a free length remains up to the bearing plate at the anchor head on the surface. The main body of the anchor is formed by steel wires, bars or strands and these are held closely together in a fixed pattern with spacers to form the tendon, which is grouted in part in the borehole.
Initially, on tensile loading of a tendon relative to the grout, an adhesion bond is mobilised. Thereafter, upon finite relative displacements as pre-stressing and elastic deformation continues along the free length of the tendon, the adhesion bond is effectively destroyed; resistance is then developed by friction between the tendon and the confining grout in the borehole. This frictional resistance, which is augmented by dilatancy of the grout, may be increased by irregularities of the tendon steel, and in part causes further development of the shear strength of the grout.
During the pulling operation to load a tendon, the reaction forces are taken by shear stresses within the grout. Cracks will develop at particular locations where principal tensile stress exceeds the tensile strength of the grout and hence adequate thickness of grout cover is necessary for particular loads. To minimize the effect of cracks the minimum grout cover (usually >10mm) should be maintained throughout the fixed length of an anchor.
To further ensure sufficient grout cover, there should also be centraliser and spacer systems where multiple strands are employed in the tendon. The main function of the tendon centraliser is to maintain the steel centrally within the grout column in the borehole, whereas the spacers are to separate the individual units of the tendon in such a way that proper grout cover is maintained among and along all lengths. But the size of the spacers must also be optimised. General arrangements of the centraliser and spacer systems are shown in Figure 2 and Figure 3, respectively.
Corrosion protection for anchors needs careful attention. An alkaline environment with a pH in the range 9 to 13 should be provided. Hydrated cements have a pH value of about 12.5 but the penetration of carbon dioxide and sulphur dioxide gases react with the alkali and, hence, significantly reduce alkalinity. With the further permeation at the tendon steel surface, and if both oxygen and water are available, there is likely to be corrosion. However, penetration of the gases and moisture is usually small when the grout is sound and the thickness adequate (10mm-15mm).
Use of prestressed anchors for stabilisation of unstable Himalayan slopes in the area of three hydro-electric projects in India – Nathpa Jhakri, Ichari dam at Chibro (Yamuna Stage II) and Maneri Bhali Stage II – is described, and general lessons are drawn from these case studies.
Case Study 1 – Nathpa Jhakri
A 62.5 m high concrete gravity dam for the 1500MW (6 x 250MW) scheme was to be built but before construction work started a
massive rock slide took place in July 1993 on the right bank of the river Satluj approximately 100m upstream of the site. The rock slide completely blocked the river and resulted in the impoundment of a large body of water.
On the left bank, slides had also been observed along the National Highway from time to time, and as a consequence further caution was necessary, particularly in view of stabilisation problems due to steep left bank slopes having foliation planes dipping towards the river bed. It was also unavoidable to build the road at the level of the top of the dam on the left bank.
Further, minimum rock excavation was necessary to build the dam and intake, and any rock cutting on left bank in those areas was expected to disturb the stability of the slope and possibly result in failure along the foliation planes3. With steep slopes, and foliation planes dipping into the valley, and biotite schist bands approximately 15m-25m below the surface and almost parallel to the exposed sloping surface, it was necessary to install stabilisation measures.
The geological section at the dam site is shown in Figure 4.
After careful investigations and analyses, a comprehensive plan to stabilising the slopes was worked out and implemented.
The concept plan included drainage and dressing of overburden material, construction of retaining walls and, most importantly, anchoring of the potential slide rock mass with rock bolts and prestressed cable anchors of different capacity, viz. 40t,100t and 200t.
From the computations based on potential failure plane of rock mass that required to be stabilised, it was found that destabilising force was very high and could not be tackled by installing only rock bolts and dressing of the slopes at feasible locations. Therefore, it became necessary to provide prestressed cable anchors of higher load capacities along with other measures.
Prestressed anchors on left bank
The left bank was analysed for stabilisation from 15m upstream of the dam axis to 105m downstream. The proposed road cut, along with the proposed excavation profile of dam, was marked on the geological sections.
The sliding mass, based on the failure plane, was analysed considering the effect of resisting forces, disturbing forces, seismic forces, uplift pressure, angle of internal friction and cohesion along the failure plane. The anchorage forces at different sections were computed for a factor of safety of 1.1 for non-seismic case and 1.0 for seismic case. The design adopted the critical of two cases to determine the capacity, number of rows and spacing of the anchors4.
In left bank dam area and in the intake area, it was decided to install 166 and 274 prestressed cable anchors, respectively, each of 200t capacity.
The project is owned by Satluj Jal Vidyut Nigam Ltd (SJVN, which was formerly Nathpa Jhakri Power Corp Ltd), which was established in 1988 as a joint venture of the Government of India and Government of Himachal Pradesh.
Case Study 2 – Ichari dam, Chibro project (Yamuna Stage II)
The installed capacity of underground powerhouse at the Chibro run-of-river scheme is 240MW (4 x 60MW). The project was completed in 1975 and in 2001 its ownership was taken over by Uttarakhand Jal Vidyut Nigam Ltd (UJVN), a wholly owned corporation of the Government of Uttarakhand.
The Ichari dam on the project is a 59.25m high concrete gravity diversion structure on the river Tons, a major tributary of the Yamuna. In this case, prestressed anchors were required for stability of the left training wall of the spillway basin.
Left Training Wall
The bed and left bank rock consists of interbedded quartzitic slates and slates. The soft, thinly bedded slates are close together and in some parts are less than 10mm apart. The rocks dip at 10º-15º towards the river and above approximately El. 618m consist of many shear zones and glide cracks.
Due to the adverse geological features, it was decided to provide a training wall tied with prestressed anchors. The alternative was to construct a full gravity section, which would have required much excavation that could, it was anticipated, destabilise the slopes.
Proposal for Prestressed Anchors
The design was to provide a thin section of RCC training wall (the bottom width being 4m) tied to the rock with grouted prestressed anchors, making the section safe against overturning and sliding5.
Four anchors were to be installed, each of 50t capacity and spaced at 2m intervals at an inclination of approximately 60º to back face of the wall. The design called for the anchors to be 12.5m long with a fixed length of 5m. The proposal is shown in Figure 5.
Proposal of Positive Pre-Tensioned Anchors
Later, however, it was decided to replace the initial design proposal for grouted prestressed anchors with positive prestressed anchors, which were similar to those installed in the underground powerhouse and other cavities on the project.
In the case of pre-tensioned anchors, only one end is accessible. However, in the case of positive prestressed anchors, there is access to both ends of a tendon. The positive prestressed anchor can be loaded from both ends (Figure 6).
At Ichari dam, considerations when deciding on the potential for providing positive prestressed anchors, which is a highly skilled job, were6:
• the availability of an additional face in the rock mass for
anchorage in an excavated tunnel for the third flushing conduit;
• installing of positive anchors reduced the uncertainty of good peformance, as both the ends are approachable and the bearing plates fixed as required.
Case Study 3 – Dharasu powerhouse, Maneri Bhali Stage II
Maneri Bhalli Stage II is a 304MW (4 x 76MW) run-of river scheme harnessing a head of 285m on the river Bhagirathi, which is a tributary of the Ganges, between Uttarkashi and back water of Tehri dam project near Dharasu7. Also owned by UJVN, the project lies downstream of both the 90MW Tiloth scheme and the tailrace channel of the 90MW Maneri Bhali-I project.
The powerhouse was built near the river bank at a location where the terrain was El. 860m-El. 892m, being cut into the rock slope to a deepest foundation level El. 809m. Geology in the powerhouse site comprises phyllite and greywacke up to El. 855m and above was river borne material (RBM). The first stage of works consisted of removal of RBM down to the rock head, and thereafter the rock excavation in the powerhouse pit to a depth of approximately 40m took the works to El. 815m.
At this stage, the design required the upstream boundary of the powerhouse to be shifted towards the hill, which resulted in steep slopes up to El. 855m. Dressing of the slopes was not possible to improve stability.
Unfortunately, before any other stabilisation works could be undertaken, there was a heavy rock slide nearest to Unit 4 at one end of the powerhouse in July 1985 after heavy rains. Initial stabilisation works involved stone pitching along with 4m-5m long grouted ground anchors at 2m-3m centres with reinforcement by shotcrete with mesh. However, the strengthening measures were not sufficient and it was decided to use prestressed anchors (Figure 6).
Geology, Design Parameters and Criteria for prestressed anchors
At the location of Unit 4 the geology comprised mainly phyllite that was a thinly foliated rock mass of poorer grade quality with properties matching a soil mass. Stability analysis was performed using slip circle criteria as normally adopted for soil. The prestressed anchors were to have their grouted fixed length beyond the slip circle.
For analysis the values of cohesion (c) and angle of internal friction (f) of material were adopted as 0.60 kg/cm2 and 36º, which were found by conducting in-situ tests under saturated conditions. The bond strength between steel and concrete was assumed as 6 kg/cm2 and bond strength between phyllite and concrete was taken as 2.75 kg/cm2. A factor of safety greater than 1.5 was ensured under normal loading condition and more than 1.0 in seismic condition.
There is predominance of greywacke on the slopes leading up from Units 1-3 of the powerhouse. Fresh greywacke rock is hard and moderately to highly jointed, the cracks spaced from a few mm to approximately 300mm and filled mostly with clayey material. The c and f values were tested under saturated conditions and were found as 1.30 kg/cm2 and 41º, respectively. The slopes were tested by wedge method. Adequate factor of safety of wedge was ensured assuming that sliding is resisted by friction only.
Slope Stabilisation Measures
Prestressed anchors of capacities varying from 70tonnes-100tonnes were installed on the hill slope facing the powerhouse in the area where penstocks approach the machine hall (Figure 7). The length of the anchors installed varied from 39m-56m as they had to extend beyond the slip circle nearest Unit 4, were spaced at 2.4m centres, and the 100m diameter boreholes also had different inclinations. The fixed length varied from 8.75m-16m beyond the slip circle.
In the greywackes nearest Units1-3, the anchors had 100t capacity and varied in length from 14m-19m, including the fixed length of 8m. The anchors were spaced at 2.6m centres both ways with upward slope of 5º with the horizontal.
Behaviour of slopes
The slope stabilising measures adopted at the Nathpa Jhakri and Ichari dam sites have so far performed satisfactorily without damage. In the case of Dharasu powerhouse, following the slope stabilisation works through prestressed anchors (installed during 1987-91), the area between the hill face and the upstream wall of powerhouse had lean concrete placed and used to site transformers.
The following are the findings8:
1) General slope protection measures, such as placing wire crates or constructing retaining walls/breast walls, including drainage arrangement, rock bolts, etc., are generally found to be inadequate to counteract major destabilisation forces. Prestressed anchors have been found to be more reliable, as shown in the case studies.
2) Where possible, use positive prestressed anchors which usually becomes possible where a conduit exists for other purposes.
3) For effective transference of load in the fixed length portion of a prestressed anchor, minimum grout cover (usually>10mm) should be ensured among strands and betweem them and the borehole wall. For multi-strand tendons of longer lengths, centraliser and spacer systems should be used.
Sanjeev Gupta, B.N. Asthana, Gopal Chauhan are attached to the Department of Water Resources Development and Management (WRD&M), Indian Institute of Technology (IIT), Roorkee, India, as graduate, Guest Faculty and Emeritus Fellow, respectively.
1. Barley, A. D., Windsor, C. R. (2000). Recent advances in ground anchor and ground reinforcement technology with reference to the development of the art, GEO 2000 International Conference on Geotechnical and Geotechnical Engineering, Melbourne, Australia, 19-24 November 2000.