WORK: SLOPE STABILIZATION/ TOE STABILITY PROTECTION 

PRODUCTS: Double Corrosion Protection Passive Cable Anchors

Introduction

A 192m high Lakhwad Dam with its 900MW underground power station and 86m high Vyasi Dam situated 5km downstream of the Lakhwad with its 120MW power station at Hathiyari. Our project site was located at the Hathiyari Powerhouse station. The project location comprises of thick overburden of fully disintegrated and small blocks of fragmented mudstone and sandstone rock. Slope profile was a very steep and challenging terrain that passes through NH. Geomorphology, unstable geological conditions required stabilization work for the protection of dam powerhouse and national highway.   

Geological Setup 

The geological structure of Doon valley (Dehradun) is characterized by two major faults, crustal and fractures along which rock slabs of mountain mass have been uplifted and moved southward. The Doon valley and Shiwalik range, which are classified into the lower, the middle, and the upper Siwaliks. It is composed of grey, salts, and peeper textured poorly to moderately sorted, fine to coarse-grained multistoried sandstones with a subordinate amount of mudstone. Multistory sandstone complex of Middle Shiwalik exhibits vertical faces variation from sandstone –mudstone to sandstone to mudstone –conglomerate. The multistoried sandstone’s very characteristics feature is the repetitive occurrence of selective cementation layers of varying thickness and sharp contacts with the under and overlying friable sandstone. 

The Shiwalik belt consists of sandstone and conglomerates, which are often soft and friable. Bands of clay are interceded with this sandstone and conglomerates, giving cohesion to the soil and thereby improving its physical qualities. The soil resulting from the Shiwalik sandstone is a sandy loam, with sometimes a large proportion of clay. In higher regions with steep slopes, the soil is very shallow and dry. Consequently, the vegetation as a whole is more xerophytic. It is, however, deep on northerly aspects and in lower slopes. The best soil occurs on the lowest slopes, where the proportion of clay is large, and the drainage is adequate. But such conditions do not prevail over large areas.

Fig 2: Executing work in steep slope terrain at site. 

Site problem

The main disturbing factor was the presence of weak rock formations of high rise hilly terrain. Topsoil or rock comprises highly weathered mudstone, sandstone including sand layer sandwich between two highly weathered formations and effect of rainfall liquefied the layers, thereby creating large mass movements. The increase of Pore water pressure during monsoon season created the loss in shear strength of the sandwich soil layer and particularly along the weak zone. Constant seepage unbounded the stratified particles resulted in the sinking of overlying loose particles and created an increase in dilation under normal stress conditions: this ultimately created settlement and mass movement problems.

Solution 

Spar Geo Infra Pvt. Ltd. had been awarded the work for the execution of the installation of high capacity Cable Anchors at Hathiari Power Station region. Geo Spar did the complete work on cable anchors that were designed as per BS 8081, and the ground anchors were subjected to proving test, suitability test, and on-site acceptance test.

Preparation of approach & Platform for drilling of Cable Anchors

The presence of 4-5m unstable overburden caused a major hindrance for the preparation of stable surface required for drilling of holes. Stabilization works were done at different elevations for the whole unstable profile. At EL+587.5m, NH was passing stabilized with a 6m high cladding wall with cable anchors, likewise level below this up to EL+560m also protected with the same design section as the level above the geotechnical design analysis. Elevations 550.4m and 535m stabilized with 4m height of cladding wall and double corrosion passive cable anchors. The slope is protected using a 311 number of DCP Anchors at different elevations in a total stretch length of 365m. 

Fig 3: Platform preparation for installation of anchors.  Fig 4: Execution works at different elevations of site. 

Fractured rocks conditions poised drilling difficulties

During drilling, fractured and fragile rocks were encountered at various drill depth intervals, which were always a threat for DTH hammer and bits getting stuck.

Fig 5: Typical view of manpower were engaged to installation of the cable anchor. 

Passive Cable Anchors was big & bulky

The cable anchors of 900kN capacity with the length of 15m to 30 meters were weighing 450kgs each, and there was no way to mechanically carry these anchors to the designated locations; means to different elevations from fabrication yard.

Stressing of Cable Anchors 

Speed of work being the essence of the project, it was required to carry out the cable anchors at very fast pace. But the stressing of these cable anchors was not possible until grout achieved a strength of 35mpa. Stressing of these installed cable anchors at such height and space constraint was again a challenge. Difficulties were faced for the installation of bearing plates and stressing jacks for stressing activity. The challenge was overcome by using hydraulic cranes of suitable capacity for the job. 

Cable anchor Installation Process

A pre-stressed grouted cable anchor is a structural element installed in soil or rock, which is used to transmit an applied tensile load into the ground. Pre stressed ground anchors act against the potential slip surface’s thrust and increase the normal stress on the potential slip surface. Both of these actions contribute to an increase in the stability of the slopes. Anchored slopes and landslide stabilization systems are designed to restrain forces associated with unstable ground masses. 

Fig 6: Typical view of tensioning of cable anchor

Process of Prefabrication and Transportation

Number of strands combined together to form anchor on which various accessories such as sheathings for free length, injection pipe to provide grout around anchor periphery, centralizer, etc. Depending upon the type of application and environmental conditions accordingly, corrosion protection measures are used for anchors to increase its effectiveness. 

Drilling

Generally, the hole diameter varies b/w 50 to 200 mm, which is commenced using drill tools. Drill direction and angle is selected based on stresses generated from structure and ground interaction. 

Fig 7: Typical view of cable anchor drilling process at site location. 

Installation of Anchors and Grouting 

Designed anchor diameter are placed in the drilled hole. Before placing anchors, the drill hole is flushed with compressed air to remove any loose material inside the hole. After placing anchors, grouting is done to provide a bond between ground, surrounding strata, and anchors. Grout application also stabilizes the drill hole and fills the void spaces, if any.

Stressing/ Tensioning 

Stressing of pre-stressed anchors carried out at 1 to 7 days after injection of grout depending on the type of ground and the grouting compound used and its designated strength achievement.  

 Protection

Appropriate material like grease, wax, and cement grout is used to provide corrosion protection. The metal parts such as anchor head are also treated against corrosion protection materials according to environmental conditions. 

Fig 8: Complete installation of ground double corrosion protection passive cable anchors at site. 

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The Border Roads Organization (BRO) works for the development of road network infrastructure in India’s border areas. After the formation of BRO on 7 May 1960, the organization is responsible for protecting and developing the border roads in the north and northeastern regions.

Atal (Rohtang) Tunnel is a highway tunnel constructed under the mentorship of Border Road Organization. The Rohtang Pass region provides fascinating lush beauty, famous for its mountain pass, with EL+ 13,058 ft, located on the eastern Pir Panjal Range. During most of the months, the pass is covered with heavy snowfall and blizzards, making the route inaccessible. To provide better road connectivity and reduce travel time, a tunnel of around 9.02 km long breaks between the south and north portal. The Rohtang Tunnel has been planned and designed to provide all-weather route connectivity to Leh and Lahaul Spiti valleys in Himachal Pradesh.

Geological Setup

The area around the proposed tunnel is situated in higher Himalayas across the PirPanjal range and Rohtang ridge (striking NW-SE direction), which marks the divide between the drainage system of River Chandra in the North and River Beas in the south.

The project location is situated in an NW-SE trending series of thrusts. It folds collectively called the Rohtang Axial Zone, representing a major tectonic divide, separating the Permian- Mesozoic Tandi basin to the SW from the Lahul-Spiti-Kinnaur Phanerozoic basin to the NE. Two dominant rock units lie within the region. A thick sequence of Schist and Phyllite (Salkhala Group) in the Central and South West areas of the project site. The soil in the study region is characterized as brown hill soils (over sandstones and shales), podsolic or sub-montane, and the glacial and eternal snow types, which are not fully developed as they are found in snow-covered areas.

The soil is generally thin but deep only in valleys. Sheet and gully erosion caused due to snowmelt is the predominant phenomenon in the region due to which the top layer of soil over large areas gets washed away. The texture of soil samples collected at site location such as sandy-loam, loam texture, and Muck from Rohtang tunnel, which consists of broken rocks, is sandy and falls under the class Loamy sand.

Problem

The approach of the tunnel portal was almost vertical, which could cause an unstable reach. In addition, the construction of the tunnel passage created destabilization of the slope near the portal. Combination of various problematic factors like vertical profile near the portal, thick overburden created slope instability problems. To construct a tunnel with keeping the region around the portal to be safe, the implementation of a suitable support system was necessary.

Solution 

Spar Geo Infra Pvt. Ltd. was awarded the work to execute the installation of high capacity Cable Anchors at the Atal Tunnel region. Geo Spar did the complete work on cable anchors were designed as per BS 8081, and the ground Anchors were subjected to Proving test, Suitability test, and on-site acceptance test.

Preparation of approach & Platform for drilling of Cable Anchors

As the slope was covered with thick unstable overburden saturated with water seepage due to melting of snow above the surface. This caused a major hindrance for the preparation of the stable surface required for drilling of holes. Ramps were constructed to make a platform for the execution of anchor works.

Fractured rocks conditions poised drilling difficulties

During drilling fractured and fragile rocks were encountered at various intervals, which caused stucking of drill bits.

Water Penetration Test

WPT was carried out for each hole for Cable Anchor, and the holes that failed the WPT test were grouted and redrilled to maintain quality.

Passive Cable Anchors was big & bulky

The cable anchors of 3100kN capacity with the length of 50 meters weighed 850kgs each, and there was no way to mechanically carry these anchors to the designated locations from the fabrication yard. Thirty-five members of manpower were engaged to transport the cable anchors up to the distance of 500 mtrs for homing of these cable anchors on a marshy path.

Limited working period

The area of work was covered with snow for a period of 5-6 months from November to April. Huge snow covers on the uphill slopes were always a thread for Avalanches in the work area. During the early months of starting work, care was taken to avoid misshapes against Avalanches.

Extreme cold climatic conditions

Maintaining skilled manpower and machinery in good health was always a challenge in this region. The manpower was provided with adequate PPE’s to work in such climatic conditions.

Stressing of Cable Anchors

Speed of work being the essence of the project, it was required to carry out the cable anchors at a very fast pace. But the stressing of these cable anchors was not possible before the grout achieved a strength of 35mpa. Stressing of these installed cable anchors at such height and space constraint was again a challenge. Difficulties were faced for the installation of bearing plates and stressing jacks for stressing activity. The challenge was overcome by using hydraulic cranes of suitable capacity for the job.

Cable anchor Installation Process

A pre-stressed grouted cable anchor is a structural element installed in soil or rock used to transmit an applied tensile load into the ground. Pre­ stressed ground anchors act against the potential slip surface’s thrust and increase the normal stress on the potential slip surface. Both of these actions contribute to an increase in the stability of the slopes. Anchored slopes and landslide stabilization systems are designed to restrain forces associated with unstable ground masses.

Protection of Anchors against Corrosion

Double corrosion protection cable anchors were used for permanent anchoring, which has a long service life, and these are generally recommended for aggressive environments. In this system, a corrugated sheathing of high strength HDPE is installed over the anchor’s full length. The annular gap between anchor and HDPE is filled with grout before the installation of the anchor.

Conclusion

Spar Geo Infra Pvt. Ltd. completed the work on time and has become the first company in India to install 3100-kN capacity DCP Rock Cable Anchors (length: 50 Mtrs.) at a nearly vertical slope of 85 degrees at an altitude of 3,100 Mtrs for the stabilization of South Portal of the Atal Tunnel.

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Location: Chungthang Village near Mangan, North Sikkim

Brief of the Problem: Unstable Massive Sliding Material in the Vicinity of Dam Spillway

The Site geology of the dam site area lies in the rock formation of central crystalline; subdivided into Chungthang series, Darjeeling gneiss & Rongli series of Pre-Cambrian age. Chungthang series comprises of quartz, biotite gneiss, biotite schist, calc-silicate and thinner bands of argentiferous – sillimanite heavily micaceous gneissose. Rock units are highly folded; asymmetric, isoclinal in nature with NE dips.

Landslide issue was observed due to excessive stripping and cutting of the left abutment site near the spillway chute. About 1 lac m3 debris slid down the hill after the cutting work at the left abutment of the dam site.

Engineered Solution: Mitigation Measure for Slope Stabilization

How to cut the area vertically 30 m*400 m horizontally in this critical geology was the biggest question and that’s when Mr. SK Aggarwal (MD, Teesta Urja Limited) came up with the concept of cladding wall with permanent ground anchors.

Energy Infratech calculated various designs of ground anchors like Rock bolt, Self-Drilling Anchor and finally they came up with a design of 1-meter thick cladding wall with permanent active cable anchor. The anchors of a 100-ton capacity of 20-50 meter length at the inclination of 150 with the spacing of 3(H) × 3(V) meters was purposed and was tested for the design loads as per the Standard Codal provisions.

The mechanism behind the stabilization is due to an increase in the shear strength along the failure surface, as a prestressed cable anchor increases the active resistance resulting from the stage of mobilization of sheared mass along the slip line.

Difficulties faced during execution:

Presence of variable strata such 30-35 m thick overburden, boulders created damage to driving operation for the casing. A greater amount of boulder strata created damage to casing shoe. GEO SPAR import some specialized tools to overcome challenges so as to operate and drill through loose and thick overburden, underlying profiles.

The technology used for Execution:

GEO SPAR used robust machines of Atlas Copco – Mustang-5 for site execution operations to tackle the restriction in drilling (15 – 45 m) and driving casing (3-30m).

Achievements:

Stabilization of poor class rock in seismic and region of high pore pressure is very vital. The use of a prestressed cable anchor provides an effective and long term solution.

The current status of the project site is that the 30m vertical cladding wall having prestressed cable anchor with cladding wall survived the 2011 earthquake of 7 Richter magnitude scale with no signs of distress, whose epicenter was near the site. The earthquake caused significant damage to the dam body as well as the spillway.

This incident shows our solution accuracy and our perfect execution of slope stability work.

Geo Spar’s mission to provide fast and efficient services with lasting solutions was achieved.

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Location: Madhya Bharath HEP, Sikkim

What: Brief of the Problem: Global Instability of Road near Dam Area

National Highway 31-A runs between Siliguri and Gangtok. Construction activities for the dam lead to settlement and global failure of road parallel to it. Activities for dam construction include blasting, excavation, submergence of the region; this is in turn leads to the generation of faults/fracture in the surrounding geological region due to blasting and excavation events. This (faults/fractures) may cumulatively go on widen with respect to time. Stress accumulation due to the movement of mass along fault and shear region leads to the issue of global instability.

This is same occurred for the HEP, Sikkim. Where the road was on the verge of global failure. This requires some rigid geotechnical stability measures to address the global instability issue.

How: Engineered Solution: Addressing the Global Instability Issue using Cable Anchors

Since, Stress accumulation due to movement of mass along fault and shear region leads to the issue of global instability. In addition, any seismic event can cause a disastrous event.

So, there was a need to provide the system which confines the zone to restrict lateral movement, which will also add the road strengthening and address the issue of global instability.

To serve the above purpose a system of cable anchors was used. Anchors prestressed at the load of 100-ton. Holes of 125mm diameter were drilled with 30 m depth for the installation of cable anchors. This resulted in to increase in road stability and provided resistance against lateral deformation.

Difficulties Faced during Execution:

Since the strata were extremely difficult in terms of implementation of the stability measure. Mass excavation resulted in collapsible strata. In addition seismic events also added the disturbance in the geological profile of the surrounding mass. So drilling and installation of cable anchors have to be done with extreme care.

Fragile geology created obstruction in maintaining the direction of drilling holes. The presence of the tough geological strata like boulders in anchoring direction led to repeated changing of drill bits.

The technology used for Execution:

Emerging and latest technology of passive ground anchors for the purpose of providing stability to the roadway was used. The implementation of this new technology served not only the purpose of providing stiffness around the excavated mass and road passing through a zone of disturbance but also provide global stability and safe path for vehicle movements.

Achievements:

All issues were overcome after the successful implementation of the cladding wall with cable anchors.

Geo Spar’s mission to provide fast and efficient services with lasting solutions was achieved.

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