Why the Port Hills slips are showing

Press, Volume CXV, Issue 33933, 28 August 1975, Page 1

Why the Port Hills slips are showing

The unsual number of slips on the Port Hills in recent weeks has prompted a Christchurch soil - engineering technician, Mr J. F. Johnson, to explain what has been causing them.

Mr Johnson says that although periods of heavy, continuous rain, which has been frequent lately, are one obvious cause, there are others, associated with the structure of the Port Hills and their relation to the Southern Alps. Generally speaking, the Port Hills are composed of layers of hard lava flow and soft volcanic ash lying inclined outwards from the highest point. The harder, more durable

rock remains in the form of prominent, exposed buttresses. Over this framework a layer of dust blown from the Southern Aips covers the rock in sheltered positions with hundreds of feet of silt, but with a much thinner layer in exposed places. This wind-blown silt, or loess, is uniform in grain size, not unlike beach sand. Because of this uniformity spaces are formed between the particles in much the same fashion as are the spaces between plums in a basket. The spaces make up about 35 per cent of the volume of silt. Notwithstanding this loose construction, wind-blown silt is very strong in its natural state, the particles being tied together by the elements.

On hillsides exposed to the north-west, the silt tends to be coarser, and on the leeward side finer, the lighter material carrying further in the wind. Water action also puts the finer particles in the bottoms of valleys, gullies, and depressions. The coarser silts have about 10 per cent of fineclay particles mixed with them. The silt with the greater amount of clay becomes denser and more impervious to water, while the coarse silt absorbs water like a sponge. In summer, when the rainfall is low, deposits of silt open into cracks 20mm wide and a metre or so deep. In the damper autumn the cracks close, but in unseasonally heavy rain, water runs

into the cracks and permeates the void area in the ‘coarse silt, making it a slurry.

The wet area progresses until cut off by a denser laver of silt, or rock, underneath — usually about two metres below the surface. As it moves downhill, the pressure increases with the weight of water behind it until it boils out on the surface and tons of slurry flow down the hillside. What remains is a subcutaneous cavern, tunnel, under-runner, or Maori hole, as it is known locally. The last name might come from ample evidence that moas were trapped in the slurry, providing easy feed for the Maori. Today, lambs are lost in it. Under-runners usually link up to form gullies, which if blocked by a rock, will de-

viate and become unpredictable in course. During the last heavy rain caverns rapidly filled with water and followed old and new tunnels, and then burst out in new areas. CONTROL MEASURES Mr Johnson believes that proper control of this behaviour depends on tree, gorse, and broom planting, or developing complete, rather than ribbon, areas of houses. The housing should provide cut-off reading and drainage. He says that in recent housing development in Heathcote County, cut-off drains behind sections have been made mandatory. "Most property owners around the foot of the hills pay $2O to $3O a year to the Christchurch Drainage Board Tor hypothetical land drain-

age, and a smaller amount to the North Canterbury Catchment Board for the same purpose.

"There appears to be no co-operation among local bodies for the control of stormwater. Many ratepayers were referred by their councils during the recent storm to the Drainage Board, to the Catchment Board, and back to the council. Mr Johnson said when he was once consulted about foundations for road construction he had suggested grading the road to slope down to the river to relieve local flooding caused by runoff from the hills. Both the National Roads Board and the county council engineers had said: “That’s the Drainage Board’s worry.”