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Choosing long distance pumping
The finished diameter of HRT at Sainj HEP is 3.85 m with an average lining thickness of 0.25 m. In such smaller diameter tunnel, the vehicle movement and crossing are restricted and carrying out multifarious activities poses severe challenge due to congested working space. The movement of concrete conveying transit mix is often restricted to one-way traffic only. The temperature and humidity and installation of an adequate ventilation system also create challenging working conditions as tunnel length increases.

To overcome these problems, HCC decided to use long distance concrete pumping for the concrete lining work of HRT. The long distance concrete pumping although has several advantages such as reduced cost, improved speed of construction and most importantly enhanced quality of construction, it has serious challenges as a technique. Considering the difficult task at hand, HCC envisaged to pump up to 1.36 km on upstream side and 2.432 km on the downstream side.

Challenges encountered
Pumping of concrete is primarily done for higher productivity, better quality and safer operations. Modern construction is demanding escalating heights (up to 600 m) and longer distances (up to 1.9 km) of concrete placement through pumping. With taller heights and longer distances come the challenges of materials, selection of right pump and pipe configurations, scheme of pipe line, and coordination amongst the team. While implementing one such case of long distance concrete pumping at Sainj HEP, HCC engineers encountered following challenges.

While attempting long distance concrete pumping, selection of concrete pump, layout of concrete conveying pipeline, selection and quality control of concrete materials, dealing with diverse climatic conditions and above safe execution of all this operations are crucial.

Selection of concrete pump: Initially, the pumping was envisaged for horizontal distances up to 2.432 km and for such long distances, the pump selection became critical. After scrutinising various options, Schwing Stetter make SP 4800 pump was selected. The operating pressure of this pump was best suited for the intended distances and pipeline configuration selected by HCC. Depending on the pumping distances and hence the pressures, this pump could be operated on two sides viz. the piston side (for long distances) and the rod side (for short distances). HCC developed a protocol for correlating the pump back-pressure with concrete properties. The engineers ensured the continuity of the long distance pumping through continuous inspections and maintenance during pumping and in idle conditions to avoid any kind of breakdown and consequent time delays as the concrete pump is the heart of the operation.

Pipeline installation: Selecting the right pipe diameter is a critical aspect. The pipe thickness selection is governed by the pump pressure. The challenge then is also to blend this fact with most cost effective solution. Considering the required flow rates, the productivity and pressure considerations, HCC choose a 150 mm internal diameter pipeline. To sustain high pumping pressure, a variable thickness pipeline was selected (10 mm thick near the pump and then 7 mm thick). An important part of pipeline is the clamp and the associated safety, since the working pressure are much higher than conventional and possibly dangerous, if not handled properly. While installing the pipeline for such large, unconventional length a patient mindset is required.

Following were the considerations taken into account for pipeline layout:
I.Efficient for installation and maintenance
II.Cost-effective,
III.Least pressure drops due to bends,
IV.Less wear of pipeline
The installation of pipeline was done on concrete blocks with an objective of providing stability; adequate clamping arrangements were required for jerk-free operations, especially for the close-to-pump pipeline. Complete installation of pipeline for 2.4 km is a laborious and time consuming activity and HCC had to plan and execute the same with great discipline and precision.

During installation and operations, complete and thorough cleaning of pipeline is very essential as even a small chunk of concrete remaining inside the pipeline has the potential for disrupting the pumping operation. HCC had organised rigorous training of the cleaning team for safe and efficient operations.

Keeping cleaning accessories and toolboxes at several locations and distributing the worker teams and supervisors to address issues in several segments helped accelerate the operations. Adequate lighting, ventilation and constant de-watering and cleaning were ensured for effective working.

Concrete materials: The material includes aggregates, cement, fly ash, admixtures and water. For typical hydro project, controlling the variations in the crushed aggregates is the key for controlling the quality and consistency of concrete. Cement and admixtures being manufactured under stringent quality control need only periodic third party testing. Variations in fly ash need to be properly monitored in order to maintain consistency in the supplied concrete. After understanding the potential variations, HCC optimised the aggregate crushing plant for optimal performance while meeting the specific aggregate requirements and ensured the following of a strict QA and QC regime. Subsequently, the key challenge was to establish a set of constructible, robust and responsive concrete mixes meeting the envisaged challenging pumping distances.

While developing concrete mixes for various distances, HCC took the following factors into account:
I.Holistic understanding of the materials,
II. Pump configuration for various distances of concrete piping
III. Diverse climatic conditions,
IV. Required placement rates vis-a-vis available concrete batching and mixing plant, and
V. A critical assessment of skill level of available manpower.
Developing proper interfaces between human and non-human elements of the project formed an essential feature of this operation. After critically studying the materials - equipment - processes - human resources, HCC objectively translated the same into specific concrete parameters, concreting parameters and associated control points. Setting time regulation - the key to stripping: With the help of a specially formulated chemical admixture (BASF SKY8787), the self compacting concrete (SCC) could provide varying retention periods (4-12 hr), however, with similar setting times ranging between 18-24 hr. This was required for maintaining the required time cycle and hence productivity. The setting times were measured in the lab and field, which in-turn was correlated for better control.

Dealing with adverse climatic conditions: The project location in the Himalayan region has varied climatic conditions in different reasons with temperatures ranging from 5?C to 35?C. Such diverse climatic conditions required HCC to develop and keep a vigilant eye and continuously develop and monitor robust, flexible yet responsive concrete mixes. Hence, periodical lab and full scale field trials were performed. HCC?s difficulties increased when the difference between the outside and inside temperature were substantially more, making concreting appear as a serious challenge. One such example is when the external temperature was around 5oC (RH = 25 per cent) and at the pouring point the temperature was 40oC (RH > 95 per cent). To overcome such difficulties, rigorous lab and field tests continuously guided the execution.

Ensuring safety: Though pumps are operated by skilled operators, the entire 2.4 km long pipeline posed a serious safety concern due to high sustained pressure. To ensure safe working conditions inside the tunnel, the pipeline anchoring, clamping, alignment, joints, and bends were double-checked before commencing the operations and also during the operations. Adequate ventilation and lighting were provided inside the tunnel as well as first-aid and medical facilities were provided as regular intervals inside the tunnel.

Team work - the essence
For successful execution of such a challenging task, HCC built a team that exhibited characteristics of seamless cooperation, trust and team work without which delivery of concrete at such distances cannot become a reality. Hence, planning of each activity and the responsibility matrix were defined up to worker gang level. Furthermore, the team was encouraged to participate in open discussions and openly provide solutions to every problem encountered during such implementation.