CAUSE OF SLIPS

New Zealand Herald, Volume LXXV, Issue 23123, 23 August 1938, Page 13

CAUSE OF SLIPS

THAMES COAST ROAD SEISMIC MOVEMENT THEORY OF ENGINEER [by telegraph—owx correspondent] THAMES, Monday 'Die continued series of slips on the Thames Coast Road is considered by Mr. J. H. Adams, civil engineer, of Thames, to be due to seismic movement. Mr. Adams, who for six years was Thames County engineer and supervisor, has boon making a close study of tho question. The slips cannot ho traced to abnormal weather conditions, although onco they have commenced to fall, the rain has no doubt helped to increase their volume, he states. A good many slips have come down on slopes which have never been touched for roadforming purposes and their movement is somewhat of a mystery. There is a distinct fault plane, from two chains to 10 chains back from the sea front and the tendency is for the seaward portion to lift. During this lift, which may be from lin. to 6in., and the subsequent return to normal, the steep, sloping batter on the landward side becomes shattered and rolls down on to the road. There are intrusive rocks all the way down the coast, but one particular instance is at a point .'SO chains south of the Hawke's Bay Bridge, about halfway between Tapu and Waikawau. In this case the apparent slip commenced at the south end in sedimentary rock. It worked northward and the solid intrusive mass became completely shattered. The rock showed movement so strong that it cracked and the cracks gradually opened, causing the fractured pieces to fall off. In one instance the working foreman, who was interested in tho phenomenon, saw tho whole mass lift and fall back again. At Puru, further south, a point was cut to make a filling for widening the road and about 3000 tons were lifted, exposing an intrusive rock coming through. This was just before last Christmas. On January 15 the rock lifted and fell back into place and this brought down about 6000 yards of spoil on to the road and extending six chains up the hillside. Mr. Adams states that the bare rock fall showing at intervals along the coast is the result of an upward movement, not a downthrow. He considers that the falls of rock at present causing trouble in the Karangahake Gorge are caused by the same movement.