2.06.2010

Bentonite Injection

In November and December, EWB was focusing on the repair of a few small but potentially dangerous leaks in the earthen embankment. There were three outflow points where water could be seen flowing out on the downstream side of the embankment, and we were able to locate three inflow points on the upstream side of the embankment. After digging test pits at the inflow points, we found pipe-shaped voids, around 1.5"-2.5" in diameter, that we believed traveled straight across the embankment. The voids are believed to either have been old termite tunnels, or left behind from decomposed tree roots.

The proposed method of repair was bentonite injection. Bentonite is a material with very unique properties. It is a naturally occurring material found in the earth - a clay - which can swell up to 16x its original size when it comes in contact with water. This property makes it a useful material for sealing leaks, waterproofing buildings, and many other applications where water flow needs to be stopped.

While bentonite would help us fix the leak, we had a minor problem to overcome - where could it be purchased in Cambodia? We soon found out - nowhere! So, we looked to our closest geographic neighbors - Thailand and Vietnam. Through a material supplier located in Siem Reap, we were able to make a purchase from a Thai supplier. After the purchase, two weeks later, the bags of bentonite powder were delivered to HT's office.

Special Delivery

After researching and consulting with our project mentors, we started experimenting with the bentonite and water to determine what the mix proportions would be that would yield the most desirable end product - something that would plug the "pipes", the voids, completely. The ideal mix would have a low enough viscosity to pass through the 2" water pump, yet have a high enough bentonite concentration to expand and "solidify", thus completely filling the voids. We used a bentonite slurry that was about 11-12% bentonite powder to water, by weight. In this application, measurements did not have to be very precise, so out in the field, proportions were altered slightly as necessary to make the pumping operation continuous and effective.



Left: Bentonite injection technique
Center: Things get real! Dirty, that is
Right: The setup - Filtering water, the mixing barrel, pumps and hoses

With the fluid slurry mixed, we started pumping by inserting a length of typical 1.5" diameter flexible hose as far into the inflow point as possible. Things were going well - messy, wet, dirty, but so far so good.

Meatballs, bottom of photo

Once the voids were full, we had to plug the inflow point to prevent the slurry from draining before it expanded fully. Our solution - meatballs! Well, they looked like meatballs anyway. Excess bentonite that was starting to clog the pump was removed from the mixing barrel, and shaped into balls the same size as the inflow point of the pipe shaped voids. Once we had pumped slurry through the embankment, we quickly jammed 8-10 meatballs right in the void and blocked any slurry from draining out. Now, theoretically, the slurry was inside the embankment, and slowly, the water should have been permeating through the bentonite powder, expanding each particle until the mass of slurry had filled the whole shape of the void.

To finish the repair, a layer of bentonite powder was spread over a 3-meter long strip, covering the three inflow points. This area was then backfilled with clay and protected with a layer of rip rap.

The downstream area has been dry since the bentonite was injected. The real test will be the upcoming rainy season, which will start in May/June. We'll keep you posted!

29 comments:

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