Removal of a clay layer in a sand and gravel pit covering larger aggregate resources
Type:
Presented during:
WODCON XVII: "Dredging in a Sensitive Environment" - 2004, Hamburg, Germany
Authors:
M.O. Boor, M. R. Ouwerkerk, V.L. Toet
Abstract: It is not uncommon to see in the office of an aggregate supplier, a map with circles of 25 to 40 miles around their pit, identifying the area that pit (and other pits of the competitors) can economically serve, based on transportation distances and cost. That common practice is challenged: pits get exhausted but are of late not sufficiently being replaced by new ones. While aggregate resources can be abundant, it is becoming increasingly difficult to obtain permits to open new pits, mainly due to environmental restrictions. In the USA, aggregate shortage has in several areas more than doubled the cost, and acceptable transportation distances are rapidly exceeding the historic 25-40 miles circles.
A method to alleviate the shortages of new accessible pits, is to return to older pits for which earlier permits were issued, but which were abandoned years ago because of more difficult and expensive mining conditions. Among those pits are unfortunately quite a number of pits where dredging was applied. Often the reason for abandoning such pits was, that a clay layer was encountered that proved to be to hard a task for the dredges applied.
While the term "clay" covers a broad array of fine deposits, many types of clay can be removed economically with the technologies available today. It is considered possible that many of such pits can be re-activated again, if the mining is properly approached.
As an illustration of a successful clay removal, this paper will describe recent practical experiences of a well established European sand and gravel miner, used to excavate a layer of approximately 20ft/ 6 m of sand using a plain suction dredge. Investigations proved that under that layer of sand, a layer of 3 to 5 m (10 - 15ft) of highly plastic clay covers a 20 to 30 m (65-100ft) deep valuable sand and gravel reserve. (Total ore reserves of this pit are estimated at about 20.000.000 m3 (26 million cubic yards) of which about 98% is contained in the lower layer.) After several attempts using different types of equipment, which were not successful, it was decided to remove the clay layer using a wheel dredger. This paper will describe the experiences of working with the wheel dredge in this particular application. Attention will also be given to the re-handling of the clay.
Keywords: Atterberg limits, clay removal, IGM, sand and gravel, testing, Wheel dredge