Controlling Seepage from Rotten Rock Outcrops

New Zealand Journal of Agriculture, Volume 89, Issue 2, 16 August 1954, Page 157

Controlling Seepage from Rotten Rock Outcrops

By

K. L. MAYO,

Instructor in Agriculture,

Department of Agriculture, Invercargill

SOME hill country in Southland and South Otago consists of bedded rocks, some of which are more resistant to weathering than others. The more-weathered rocks occasionally appear as outcrops of what is commonly called "rotten rock", and local wet areas often occur just below these outcrops. Though these wet areas are not large, they keep an appreciable area of land wet throughout the year, interfere with cultivation, and drown crops. Drainage is therefore worth while, particularly in those areas which can be cultivated. THE fact that these wet areas appear high up on the slopes, where water should be able to flow away easily, puzzles many farmers. The reason for the appearance of wet areas in these positions is that rain-water . which is not immediately taken up by plants or by the soil seeps down through the pervious layers of soil until it meets an impervious layer (see Fig. 1). The rotten rock layers which outcrop on the sidelings are shattered and disjointed and act as a reservoir for this surplus water. It does not pass through this rotten rock, because in most cases this rock layer overlies a tight, impervious clay. As more water collects it has to flow somewhere, and as a result appears as weak springs or seepage at the surface in many places where the rotten rock outcrops. Because the soil immediately below this outcrop is composed of the impervious clay, the water flows over it instead of through it and wet conditions occur. Drainage ’ The method of draining these wet areas is to install a tile drain across the slope along the edge of the rock outcrop and just below it, where the trenches can be dug easily in the clay and will thus act as a cut-off drain

(see Fig. 2). The tile must have fall in it and must be set at such a depth as to be just below the bottom of the rock outcrop; otherwise the water will continue to pass underneath it. The tile drain should empty out into a gully or open ditch. The rock layer may have weathered into soil at the surface, but it will be easy to locate with a 1 soil spear, and as trenches are usually dug from the outfall end first, it will be possible to locate the rock outcrop and then cut or dig the trench along its edge. Because the country has plenty of fall, little difficulty will be experienced in keeping sufficient fall in the drain or in adjusting the fall should the rock outcrop ultimately prove to be in a slightly different position. A seepage area of this type which has been successfully drained by employing the methods outlined is shown in the illustration above.

Machinery Unfortunately there is no suitable machinery readily available to handle this country. Possibly the most suitable machine for this work would be a back-acter bucket, which can be mounted on a 40 h.p. tracklaying tractor. This back-acter bucket is strong enough to withstand the conditions encountered along the edge of this rock outcrop, and when mounted on a tracklaying tractor it can be operated ,on sidelings and can be taken on to rough country. The particular job referred to above was done with a dragline excavator and the bottom was prepared for the tiles by hand. The dragline is not very suitable, because it cannot be taken on to most slopes, it digs a very wide trench, and it requires a great deal of hand work to grade the trench and lay the tiles.