Earthwall Dam 3

[Steffanus]

My Son-in- Law lives in Zambia and is farming tobacco and grains. On the farm is a medium sized dam built some two years ago. Whilst there recently I was taken on a tour of the farm dams shared by eight farms. The medium dam (Capacity not Known at this stage) has a seepage problem. Apaarently there where problems during construction, ie incorrect core materials, not confirmed. What came to my notice after a rather heavy rain storm, some 200mm over six daysI discovered a spring some 15Mtrs below the down stream side of the dam wall in the LH corner. I fyou are aware of the Teton Dam Disaster in 1976 in Idaho, USA you will see a similar patern but on of course a much smaller scale. "Q" What steps if any can be done without dismatleing the Dam can be taken. Any help will be appreciated

 

[ctmtwilliams]

The term for what you observed is piping. A Google search using the terms Dam and Piping yields provides a lot of information. Piping problems arise from poor design due to a number of potiential factors. No simple answers, and huge liability.

 

[cvg]

If the piping succeeds in removing sufficient quantities of material from the foundation of the dam, you could have a sudden failure.

An emergency action could involve placing buttress material over the downstream toe of the dam. The material should be granular to allow drainage. The purpose is to provide both dead weight and a filter over the toe to prevent the erosion caused by the piping.

 

[ctmtwilliams]

I don’t report to be qualified to design dams, but if I remember correctly from school, some of the factors which cause piping problems include sufficient porosity and permeability to allow water to flow at a rate which causes the erosion. Once the base of the dam is saturated, the hydrostatic pressure from the water column behind the dam causes increased pore pressure at the base of the dam. When the pore pressure reaches the point of supporting the load, the catastrophic failure occurs.

As CVG indicates, the additional weight will help mitigate the pore pressure issue and the additional filtering should help with the porosity/permeability. Additionally, simply lowering the water level will also help reduce some of the pressure at the base.

By the way, I got my geology degree from Idaho State University (located less than an hour away from the Teton Dam) so they tended to dwell on dam failures

 

[cvg]

I would also recommend a monitoring plan until any repairs are made which should include periodic inspections, especially if the rains begin again. Also a plan to notify people living downstream of the dam within any potential flood inundation areas that the dam is threatened and there is a potential for flooding. These people should be prepared to evacuate to higher ground if given the notice.

 

[GeoPaveTraffic]

The advice given above is indeed good; if the problem is through seepage of the dam, which can lead to a piping failure. However, it is possible that this is natural seepage either through the dam or through the abutment. While seepage through a dam CAN lead to problems, all dams have seepage and many have springs near the toe caused by through seepage.

The quantity of seepage should be carefully monitored by installing a weir and measuring the flow. If the quantity is not increasing for consistant pool levels then it is unlikely that significant piping is occuring. In this case the solution may be to just monitor the seepage. If the flow is increasing then action must be taken.

As noted above adding material to the area of the spring is likely the best option. However, the added material must be permeable and properly graded. The goal is to let the water out but not allow any soil to be carried with the water. If this is indeed the case, then a local geotechnical or civil engineer with experience with dams should be called in to look at the problem.

Also, how high is the dam (downstream toe to crest) and how far below the crest is the normal water level?

 

[Steffanus]

Thank you all for your advice. I am a structural eng. with basic civil experience, but was quite concerned when visiting my family back home and saw the dam. A local civil dam eng. came on site but there had been no run off at the time, as the rains increased so did the amount of water at the toe of the dam increase, the water was clear with no obvious evidence of discoloration exist. There is no down stream population of the dam , the river runs into the Kariba dam cachment area but the financial and production loss would be catastrophic. I hope to return within the next few weeks and would like to use your information to kick some sense into the local farming community whom may I add have just developed eight farms from virigin African bush. There is a modern adage which states. There is a crrect way of doing things and a farmers way. Once again thank you all. If you have any more suggestions without dismantling the dam Iwould appreciate them

 

[cvg]

depending upon the size of the farm dam and the size of Kariba dam - a dam breach with sudden release of the reservoir into another dam may have a substantial impact on the downstream dam (Kariba). If that dam has insufficient storage or spillway capacity, then it could overtop / fail. This is commonly called the "Domino Effect". It would be wise to notify the downstream dam owner that you are concerned about the safety of your farm dam - so that he may prepare his contingency plan.

 

[Steffanus]

Thanx, but I do not think it could effect Kariba, it is one of the biggest man made lakes in the world also the water would be diverted initially away from Lake Kariba thro various other river sources so that when it reaches kariba it would just be another miniscule rain storm which the area is renouned for

 

[Steffanus]

Well! I am returning to Zambia shortly and have been asked to supervise the spillway works on behalf of the farmers and to check the seepage/leekage problem. I wish to thank you all for your support and will give you an update on my return. Links indicated here have assisted greatly. (Civil Works is simple hey, lol.)Much Simpler than Structural Eng.;>)