A simple cover of rock is what we constructed at the 24 UMTRA piles. Over the radioactive wastes and radon barrier was a layer of rock. Many thought this ugly. Personally I think a pile of rock standing proud in the landscape is beautiful: not natural, but so robust and textured that it impresses the mind and eye.
Rock was used to control, nay eliminate erosion, for 1,000 years and more. The rock was placed to resist the forces of erosion by waters rushing down the cover. The rock type was selected to resist decay for as long a period as we had courage to predict.
We designed the rock cover for the flow of water from the probable maximum precipitation. The details of the calculation of the required rock gradation are best studied by reading the example calculation at this link.
At its simplest the method involves determining the flow rate of water down the cover and then sizing the rock so that the force of the flowing water cannot dislodge or move individual rock pieces.
The durability of the rock was a major issue. We went to local cemeteries and studied the weathering of the tombstones for data on the response of different local rocks to heat, cold, chemical attack, and time.
The thickness of the rock layer was determined by the average size of the rock. We deemed it best to make the layer at least 1.5 time the maximum size of rock particle used. That way the layer did not consist of a single rock piece.
The Filter Layer
The rock gradation generally required for a cover is such that the voids are large. The rock could be placed on an underlying layer of soil of clay, silt, or sand. The issue is that when water from precipitation flows rapidly down through the rock and encounters the soil , the water flows laterally through the rock and over the soil. If the flow is rapid enough, the soil will be eroded. This quickly leads to sloughing of the rock and surface irregularities.
To preclude erosion of the underlying soil by water flow laterally through the rock cover, we placed a filter layer of sand between the soil and rock. Carefully chosen using standard filter criteria, the sand layer inhibits erosion.
The Long-Term Performance
The Shiprock, New Mexico UMTRA pile is sometimes referred to as the Shiprock National Forest. The cover is a standard rock cover sand over the clay of the radon barrier. Seeds blown in by the wind quickly pass down through the rock and settled atop the sand. The sand is moist—a source of water in an otherwise dry desert. The seeds soon germinate and force their stems up through the rock. Their roots quickly penetrate the sand to find nutrients in the clay of the radon barrier. The result is an abundant vegetation stand in the desert.
We did not think of or anticipate this when we built the cover. It all came as a surprise. Yet I am told there is no detriment to the function of the pile. It is still erosion resistant and there is no exceedance of radon emanation. Maybe we created a long-term stable cover in spite of our misdesign.
Advantages and Disadvantages
We may thus note these lesson learnt:
- Rock covers have the advantage of being erosion resistant and of requiring little maintenance.
- Some say their disadvantages include poor aesthetics and no contribution to infiltration reduction.
- And you must be careful that they do not become the focus of growth of invasive vegetation.