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Design & Construction of SCB Backfill Slurry Wall

Design and Construction of Soil – Cement – Bentonite (SCB) backfill slurry wall along the earthen dam at Iranamadu Reservoir in Kilinochchi for the 1st time in Sri Lankan Construction Industry

Soil-Cement-Bentonite (SCB) slurry walls have been used with increasing frequency in recent years in United States, Canada and European countries to provide barriers to the lateral flow of groundwater or seepage in situations where the strength of a soil-bentonite wall would be inadequate to carry foundation loads. However, this technique has been used in less frequency in the Asian region up to date. The addition of cement to the backfill blend allows the backfill to set and form a more rigid system that can support additional overlying loads which the earthen dams undergo.

The Iranamadu Tank which is located at Kilinochchi in Sri Lanka (the length of the earthen dam is around 2750 m) and it has not been rehabilitated in a technically sound manner during last few decades due to many constrains. Having identified the requirement of rehabilitation of the tank, the relevant authorities initiated a project to strengthen and augment the Iranamadu Tank.
Installation of SCB wall is one of the key work items under the project which implemented for the first time in Sri Lanka and Access Engineering was entrusted to handle this work. The width of SCB wall was designed as 600 mm and depths was varied from 4 m to 6 m depending on the location and specially the location of the existing puddle core (extremely compacted clayey layer). The construction work included the design of backfill mix conforming to a specified criteria based on the uniaxial compressive strength (UCS) and permeability for the 7 and 28 days, respectively.

The main challenge was to design the SCB backfill mixture maintaining its permeability in the range of 10-7 cm/s. Based on extensive lab tests, the mix proportions was designed as 94 %, 2.5 % and 3.5 %, soil, bentonite and cement, respectively which met the specified requirements.

Initially, a borehole investigation was conducted to identify and locate the existing core paddle. Once it is located the excavation was commenced stage wise. The trench was filled with bentonite slurry to the depth of 60 cm to the top of the surface to avoid any collapse of sides due to deep excavation. Approximately 30 – 40 m section was excavated during each working day. Backfill mixture was transported using the three cube tippers to the excavated trench and placed with the assistance of skid steer loaders. Once the backfill was done, the top surface was covered by 30 cm thickness soil layer.

Separate tanks were set up to prepare bentonite and cement slurries at mixing site. Required quantity of soil was placed on a platform and required quantities of bentonite and cement slurries were pumped on to the soil heap while mixing them using an excavator. Around 20 Cu.m. of backfill mixture was prepared at a time and around 1 ½ – 2 hours needed to obtain a homogeneous mixture. Placing was done within a time of 3 hours to maintain the workability.

Many practical difficulties were encountered in both design and construction aspects during the operation. Mix design was initiated based on available literature and finalization of the mix design required a considerable period of time. A strict quality control was necessary to maintain the desired properties & quality of the mix.

Mixing the backfill material is more challengeable in large scale and will be again time consuming if necessary steps are not being taken. As a result, the productivity and efficiency can be lowered drastically and creating unfavourable condition at the site in terms of finance, time and quality. Mixing time and the way excavator moves are very important aspects. If mixing time passes more than 2 hr, it is unlikely to get a homogeneous and good slump mixture. Another significant aspect is to select the soil quantity for a batch to mix. Higher quantities can caused to extended time to missing and non-homogenous mixture and lower quantity can cause the productivity issues.

Author : Praneeth Wickramarachchi

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