HIGH ALTITUDE & MICROPILE BRIDGEFOUNDATION

BORDER ROADS ORGANIZATION (BRO), INDIA 

INDIA'S PILOT PROJECT OF BRIDGE AT ELEVATION OF 14,650 FT ABOVE M.S.L 

PRODUCTS : Tam Grouted Micro piles 




Introduction

Construction of foundation for bridge foundation is a challenging task in difficult terrain like the high altitudes of Leh-Ladakh. In order to ensure coordination and expeditious execution of road projects, the BRO (Border Roads Organisation) maintains operations in twenty-one states, one UT (Andaman and Nicobar Islands), and neighbouring countries. BRO operates and maintains over 32,885 kilometres of roads and about 12,200 meters of permanent bridges in the country.

One of such bridge construction was operated under BRO and was awarded to design and execute the foundation for bridge structure at difficult terrain of Leh-Ladakh. The bridge is considered the best suitable method of connectivity to cross over the obstruction, such as Natural River channels, valleys, etc. There is one important road that leads to the China border, which crosses the Shyok River having a channel width of around 500 meters. There was an urgent requirement of class 70 R Bridge to move across this river. Geographically, the terrain is located at a very high altitude, and oxygen is deficient in the atmosphere. Temperature touches -30 degrees centigrade, and consequently, snow-bound areas, including frozen river beds, are frequently encountered. A number of glacial deposits lead to the worst environmental condition, allowing hardly three to four working hours daily. Due to site remoteness, limited working period, need of special piling rigs for piles, lack of experience of the pneumatic sinking of wells, connectivity across the river forced to develop a new technique for bridge foundation particularly suited to this area. The river flows through the difficult terrain of Ladakh valley, and this area is prone to 'Glacial Lake Outburst Floods.' These constraints demand the construction of a permanent structure for inland road connectivity across the Shyok River. The bridge piers constantly remain in the threat of bed erosion due to the turbulence effect around the pier periphery. A new technique was proposed to transfer of superstructure load to the desired firm bed level using vertical and battered micropiles.

High altitude mountainous terrain poses maximum difficulty at the site. Due to difficult approaches, limited accessibility, larger time for deployment of resources, and distant movement across high mountain passes, which ultimately affect the logistics and communication, hampering both the sustenance and maintenance efforts. The construction site was located across River Shyok at approximately 14,650 feet above mean sea level, and the approach to this location was connected by a single road axis 300 Km away from the nearest habitat of Leh city.



Strata and Site Profile

The site conditions comprise of varying thicknesses of overburdened glacial soil with cobbles and boulders. It has a predominantly huge quaternary deposit. The general strata comprised of sand mixed with boulders of size 40mm to 200mm. For only one abutment and one pier below the 10m, there was the presence of rock strata remaining, having almost river sand with gravel, soil classification of SW- well-graded sands, gravelly sand, little or no fines and GP- poorly graded gravels, gravel-sand mixtures, little or no fines. The entire site soil condition was the river-borne material (RBM) with glaciers (ice).

Solution

Spar Geo Infra Pvt. Ltd. was awarded the design and execution of bridge foundation on Shyok River at Leh-Ladakh region. Micropile pile cap bridge foundation was adopted based on the circumstances at the location.

Site and executing difficulties:

Construction of foundation of bridges in remote hilly terrain, in adverse climatic conditions, was very difficult due to the limited working period and unavailability of transportation of machinery and equipment as in Leh-Ladakh region. In such a situation, regular pile and well foundation did not seem feasible. The study was taken up to carry out experimental design for bridge and ascertain the applicability of micro-piles as bridge foundation under such adverse conditions. For the installation of micropiles, drilling boreholes faced huge obstacles at this site. The methodology included field and lab tests for soil and sub-soil investigation, vertical and lateral load tests on micro-piles.

The micro piles have been designed for load consideration of dead load, live load, snow load, and seismic load due to a vertical load of 40 Tons and a horizontal load of 1.25Tons. The soil investigation of boreholes has been explored to a depth ranging from 24 m to 40 m. Borehole data was taken, and the subsoil strata were analyzed. The data provided inputs that the top layers consist of sandy gravel to gravelly sand with high SPT N values. This type of strata was found to be adequate for supporting micro piles as well as permeation grouting and TAM grouting, i.e., replacement of pore water with cement slurry.

The design of micropile foundation and substructure are based on the successful results of the tests, i.e., the safe load capacity achieved in these tests. Based on in-situ test results, confirmation for using micro piles was accorded. In the future, micro-piles can prove to be a better solution in where the river bed is laden with boulders and other similar difficult situations.

Micropile installation Process

The micropile casing generally has a diameter in the range of 3 to 10 inches. The casing is advanced to the design depth using a drilling technique. Reinforcing steel, typically an all-thread bar is inserted into the micropile casing. High-strength cement grout is then pumped into the casing. The casing may extend to the full depth or end above the bond zone, with the reinforcing bar extending to the full depth.

This technique has been used to support most types of structures. Micropile drill rigs allow installation in restricted access and low headroom interiors, allowing facility upgrades with minimal disruption to normal operations.

Micropile technology with one or more of our other ground modification techniques to meet unique or complex project requirements cost-effectively and efficiently. A group of micropiles formed to create the pile cap to be capable of high load transfer from structures. A line of micropiles spanned by wooden lagging can be ideal for excavation walls in low headroom and other confined areas. Post-grouting (i.e., TAM grouting) within the bond length can increase frictional forces with surrounding soils and achieve greater capacity. Micropiles can serve to “stitch” the soil together within predicted shear zones to enhance mass stability. In liquefiable profiles, micropiles can transfer loads to competent bearing strata to follow seismic design requirements.

TAM grouted micropile

TAM grouted micropile is constructed by drilling a borehole, placing steel reinforcement, and grouting the hole having a diameter around 300 mm. These are installed where access is restrictive and in all soil types and ground conditions. In this method, grout is placed for a complete length (40m below ground level the soil strata river born material) using a double packer. Primary grout is injected via preplaced sleeved grout pipe with 20 Mpa. The non-return value of sleeved pipe prevents the grout return ensuring effective grout.

Tam grouting having various benefits, such as injections can be carried out in various stages, and different mixtures can be injected for each grouting stage. This permits better penetration of the fine voids after the big ones have been closed. More permeable soil layers can be sealed first regardless of the order of injection level, which prevents the loss of high-cost, low-viscosity grouts.

Testing of piles

To verify the efficacy of the design of the piles on ground, 22 test piles were installed. At each site, two test piles were installed. In this, 11 were Vertical piles, and another 11 were Raker/Batter piles at an angle of 20 degrees. Out of this, three vertical piles were tested for vertical loading with a load of 80 Tons. Eight vertical piles were tested with a load of 2.5 Tons. In addition, three numbers in a group of two Piles (one vertical and one Raker pile) were tested for a load of 22 Tons. After load testing, it was observed that the behaviour of the piles was exceptionally good in terms of the settlement. The settlement was much lower than the permissible settlement.

Considering that as per IS 2911, the maximum slope of the raker pile can be one horizontal to six verticals. Accordingly, the raker piles slope was restricted to 10 degrees. The casted pile is left for a period until it attains a strength of 75% of the compressive strength desired. Thereafter, load testing is carried out to check its settlement as per IS Code 2911- Part IV. Load testing of the pile is shown in Figure. Vertical and lateral load capacity test was performed.

1.   Vertical Load Test Result: Maximum Settlement of 8.9 mm at test load of 80 tons.

2.   Lateral Load Test Result: Maximum Displacement of 3.25mm at test load of 2.5 tons.

3.   Lateral Load Test Result for group of one raker pile and vertical pile: Maximum Displacement of 3.57 mm at test load of 22 tons.


Finalization of designs for piers and abutments

The side slope of the embankment was protected with a 0.6m thick stone pitching. From the field test of piles, the design for each pier has been finalized with a pile group of 49 piles. Out of these 49 piles, 25 piles at the center are vertical piles and at the outer 24 piles are of raker piles. The raker piles designed at an angle of 10 degrees and the corner four raker piles at an angle of 45 degrees to the pile cap axis plan.

The abutments were designed with 69 piles. Out of these 69 piles, 25 piles at the center were vertical piles, and at the outer 44 piles were rakered piles. The raker piles were designed at an angle of 10 degrees and the corner four raker piles at an angle of 45 degrees to the pile cap axis plan.



Spar Geo Infra Pvt. Ltd.

hrmanager@spargrp.com

http://www.spargrp.com