Rockfill Dam and Anchors

[wollundry5]

Our company is a major electricity supplier located in Australia. One of our dams is a 93 m high Concrete Face Rockfill Dam. The slope of the downstream face of the dam is 1V : 1.3 H (38 degrees). The dam was constructed during the early 1980's and now the spillway has been deemed to have insufficient discharge capacity.

One of the options to increase the capacity of the spillway is to cover the face with Roller Compacted Concrete (RCC) constructed in steps and allow water to spill over this. The RCC will need to be anchored to the rockfill. When the this secondary spillway is in use the estimated uplift forces will be in the order of 10 - 15 tonnes per square metre of spillway area.

The problem we have is how to drill a hole on face of the dam, how to install the anchor (about 3 m in length) and how to grout the anchor into the rock (the rock fill has no shear strength). Does anybody have any suggestions from past experience about installing anchors / dowels in rock fill

 

[clayR]

How would you classify the rock fill?
Two methods to consider
1) mass stabilization of the upper 3 m of rock fill with grout.
2) Construct anchored/grouted columns with mass sufficent to resist uplift.
Both methods will require special drilling techniques and the ability to control the lateral and vertical migration grout. The selection of grout, such as viscosity modified, low slump or quick set will depend on the nature of the rock fill.

 

[GeoPaveTraffic]

I would be concerned about covering the downstream face of the dam with concrete. Currently any water leaking into the dam can just flow out through the rock fill. Covering the downstream face could effect this drainage. Any changes to the drainage through the dam should be very carefully considered.

 

[letrab]

Wollundry5
Assuming the filters and drains are not affected by the downstream face you can drill through the RCC and rockfill to the foundation. Anchoring post tensioned tendons into the foundation will provide sufficient downforce.

However, the dowstream face is very steep and it may be more economical to provide additional RCC to flatten the slope and provide the additional weight required.

 

[flamby]

wollundry5,

I am not very comfortable with the idea of drilling into disintegrated mass of rockfill and grouting an anchor. The intake of grout may be so huge so as to make it economically prohibitive. The grout will just not stop at upper 3m or in the vicinity of the hole. You may end up grouting the whole damn.

One way may be to thicken your RCC layer so much so as to be stable under its own weight. I understand that you will get very high dimensions, I guess more than 6m deep, but this is the very purpose of RCC technology -- cheap concrete for massive jobs.

 

[cvg]

I agree
RCC is not reinforced. I always design it to be stable under its own weight. The assumption that once cracks form, you will have a series of stacked blocks. This will necessarily complicate your anchoring computations and makes it difficult to analyze. For instance, how will you know that a crack doesn't form at the anchor?

 

[Riggly]

In my recent involvement with RCC, the RCC was overlayed with reinforced concrete. Therefore, the formation of 'stack blocks' could be eliminated once the anchorage is placed above the reinforced concrete.