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Case study

Planning and execution of an open cut deep excavation under critical site conditions

Date
05 February 2019

This explains how detailed open excavation analysis, dewatering and practical operation strategies were used to overcome challenging site conditions for a deep excavation.

Planning and execution of an open cut deep excavation under critical site conditions
Excavation on a building site

Project Details

Location: NTPC Coal Handling Plant, Tanda, Uttar Pradesh, North India

Value: £34m (312 Crores INR rupees)

Date of completion: 30/04/2019

Duration: 40 months (total project)

Client: National Thermal Power Corporation Limited (NTPC)

Contractor: MMH, L&T Construction

Project manager: Matta Sattanaryan

Challenge summary: Maintaining dry conditions in deep excavation work with a high ground water table (GWT) is always an execution challenge. Also, this particular site had dumped aggregates on two edges of the excavation site and a surface raw water plant reservoir about four times the size of the excavation at one edge of the excavation site.

Challenge solution: The excavation works and their access routes were planned based on investigation of the site-specific constraints. The excavation slope stability was investigated by collecting soil bore log data , then the slope was modelled through finite element analysis. Both multiple well point and deep well systems with adequate dewatering capacities were established as per the plan. The client team (both site and engineering) had to be convinced of the suitability of the excavation plan. Risk assessment and continuous excavation monitoring helped to locate early signs of cracks, fissures and seepage, and proactive measures were taken accordingly to ensure the stability of the excavation work.

Excavating for a wagon tippler structure

NTPC is an Indian state-owned electricity generation company. At Tanda, Uttar Pradesh, NTPC is carrying out the expansion of a 2x660MW thermal power plant. As part of coal handling facility, a wagon tippler complex is planned for emptying the loaded wagons by tipping them. The Wagon Tippler complex comprises of three wagon tippler structures with conveyor tunnels and transfer point structures.

For the multiple wagon tippler structures, we excavated to a depth of 26m for a plan area of 29000sqm. This excavation work in predominantly cohesion-less multi-layered soil was the most difficult work, and critical to get right, in the entire coal handling plant package.

Site inspection to determine constraints

Site inspection of the proposed wagon tippler area before construction revealed overburdened pre-existing heaps of aggregate of about 7m height and a large over ground reservoir, both in close proximity to the excavation area. The available soil bore log report suggested the ground water table was 3.5m below the energy grade line (EGL).

Avoiding reservoir leakage into the water table

The nearby over ground plant reservoir with an old lining was a major safety concern - there was a risk that if excavation occurred too close to this, the reservoir base would break and the water would escape from the reservoir. The client insisted on carrying out sheet piling to protect the excavation. However, cantilever sheet piling was not technically feasible for the 26m depth. Also, with an excavation width of over 100m, a large quantity of steel would be required for multitier lateral bracing of the sheet pile. Further, this would have had tremendous impact on the entire construction planning and schedule.

The client was apprised of the adverse effect of the overburdened excavation slopes and immediate removal was planned. Decade-old drawings of the surface reservoir were collected for ascertaining the composition of the lining. The water level in plant drains whose invert levels were below the reservoir base level were observed and no signs of leakage from the reservoir were found. Further, we explained to the client that the proposed dewatering arrangements would not cause any water seepage or migration of silt particles below the reservoir base. This was possible because the top level of the existing GWT was below the reservoir base and there was a sandwiched clay layer at the top of the stratified soil layers, which would prevent silt particle migration.

Further strategies were used in the excavation work plan:

  • Independent borehole drilling up to a 36m depth was carried out to ascertain the soil profile to determine the right dewatering arrangements
  • Permeability tests were carried out to ascertain the coefficient of permeability values of various soil layers to be used for dewatering calculations
  • A water meter was installed to check the water level during each stage of excavation
  • The excavation encountered one full monsoon season in which the excavation bottom level was purposefully not extended below 8m. Surface drains were planned separately to suitably discharge any possible rainwater coming inside the excavation pit by surface run-off.
  • In order to keep the excavation area dry, the groundwater was discharged at a specific location away from the excavation pit so that there was no seepage into the excavation pit from the discharge point. Before each stage of excavation, the water level was monitored by piezometer.
  • During excavation, the movement of the loaded dumpers from the excavation base to the top was difficult considering the ground conditions. Steel plates were laid along the route to facilitate easy vehicle movement.

Draining the deep well system

Prior to excavation, a deep-well system at 10m intervals was set at the periphery of the excavation pit. The deep wells were installed below the water table to a depth of 36m along the periphery of the excavation pit. The well comprised of a well screen (a perforated portion of casing pipe) and casing (an unperforated section of casing pipe) installed centrally inside a borehole. The boreholes were formed by the rotary method. The annulus between the screen and borehole wall was filled with granular filter media to form a filter pack. When the wells acted together, the interaction of the cones of drawdown created by each well resulted in groundwater lowering over a wide area. The discharge from the deep well was dewatered using electrically operated submersible pumps placed at the bottom of deep wells. The outlets of the pumps were connected to a common header which led the discharged water to a collection tank. From the water collection tank, the water was emptied by a 20HP pump to a nearby drain. The dewatering system was designed for a maximum groundwater inflow of 200 l/s (litres per second) from the excavation pit.

Stages of excavation

The excavation was carried out in six stages using predefined slopes and berms. The sequential excavation stages were modelled in Plaxis software under drained conditions to arrive at the optimum excavation slope. The excavation slope and berm selected was made stable for the surcharge load of reservoir embankment. Considering the size of excavation and the nature of the soil, a dual dewatering system was used in which both deep well dewatering and a multistage well point arrangement (with three tiers) helped to keep the base of excavation dry. The final level of the GWT was planned to be at least 1m below the deepest point of excavation.

Well point drainage

The well points were installed during the third stage of excavation. The spacing was limited to 1.5m. Well point discharge was calculated as 0.5 l/s per well, and the number of well points per stage was calculated as 120 for the entire excavation, giving a total discharge of 60 l/s. Two 20HP pumps were used during this, and extra standby pumps (also 20HP) were available but unused. For the optimisation of well points, during stages four and five, we reinstalled the well points used in the third stage.

Conclusions:

To achieve a cost-effective and user-friendly deep excavation scheme, the essential parameters include:

  • Meticulous planning
  • Detailed risk assessment
  • Temporary works engineering
  • Constant progress monitoring during execution
  • Sssessing potential risks and taking proactive measures as mentioned above.

Engineering analysis led to the optimisation of excavation slopes and berms, thus resulting in the reduction of open excavation quantity by 55%, which allowed a cost saving amounting to £43.2m (4.9 Crores INR rupees at time of writing).

  • Paramartha Som, Senior level professional (CEng MICE UK) - Construction specialist