Effect of different drainage systems on bearing resistance of some West Coast, South Island soils

Abstract

Abstract Development of extensive areas of West Coast, South Island wetland soils into pasture has led to unacceptable levels of pugging and loss of pasture production when cattle are introduced following sheep grazing. Since no quantified drainage trials had been carried out previously on the West Coast, a trial was established to test two main drainage systems, subsoiling and “humps and hollows”, with several supplementary treatments including tile drains and gravel-filled slits. The site was on a very gently sloping, weakly dissected, mainly loess-covered glacial outwash terrace in the upper Grey Valley under an annual rainfall of about 2000 mm. Gley podzols (Okarito Series) from loess were the most common soils, with smaller areas of gley soils (Maimai Series) from glacial outwash gravels. Minor areas of organic soils (Kini Series) from peat were significant in the trial area. The site was developed from pakihi vegetation to ryegrass/clover pasture, beginning in 1977. Extensive rush clumps subsequently grew there. The main field measurement made to determine the effect of drainage on bearing capacity was plate-bearing resistance. In addition, water content, dry bulk density, loss on ignition, and thickness of root mat were determined on cores taken near each measurement site. Measurements were made from 1983, before the systems were installed, to 1987. General relationships were found between water content and plate-bearing resistance for each trial plot, and for all sites with Okarito soils. Sites underlain by Kini, Maimai, and Okarito soils with peaty topsoils, had significantly lower plate-bearing resistance compared with Okarito soils with mineral topsoils on major interfluves, regardless of the drainage treatment applied. In the first year after installation, the driest parts of the hollows in the humps and hollows treatment (Okarito subsoil material) had very high bearing resistance. The trial data show that Okarito soils with mineral topsoils will withstand cattle treading without the additional drainage treatments attempted here. Detailed soil survey, with subsequent fencing and management to isolate areas with low bearing resistance, will lead to future higher productivity on such soils.