CAPILLARY ACTION A CAUSE

Hokitika Guardian, 17 October 1930, Page 2

CAPILLARY ACTION A CAUSE

J HYDRO ST AT IC ; PRESSURE

• LOOSENING OF THE JOINTS ; EFFECTED.

WELLINGTON, : oct.“ 1 9

In : tbe • course of r his, report. Profes-sor-ijfcornel! ing .ahd 1 "ifEgfily ner the' puisps ;pf t>Up ’ TUP,-’ ture tjint appeared atf/rapuni op; June. 7. ■ The: expansion’ of'• the. rpfk; causing stress and movement was, capaed,. he- says,; by i saturation, pi QP-. ably■ due to capillary action. , j , ‘dp vview, of , the , csipillab: charactevstates “the rupture majr -tye interpreter! in the following : ; T * i anhi?r:r" . “When, tlje .water . was , impounded in tfi® neadrac®,’ a. constant, source, for. f®Vdihk; i»‘to ( 'the cupilUry intersticesof the columnar rpch was created OY*T: a; source .was lifted ■ thirty., feet ,to forty, feet' ; higher than the. water-level of the -^yamp';.that had previously, served as a supply of water tp the rppk.',, • HYDROSTATIC! PRESSURE. "If need hardly h® explained tho under the • njew : amount! of hydrostatic; ' pressure developed in well-defined, joint-planes r or th® columnar rook, where before there had been no such pressure, .and where there had previously been a .topdeqcy to contraction, due to evaporation, taking place-from the ridge, jn this way lOeisejiing of thp joints would ■be effected- ' ' . . ■•! “The lower part of the columnar rqck swelled to an. extent corresponding to the difference between complete saturation and the saturation that \had prevailed previous to the headrace being filled. The upper part and the overlying strata underwent very .little change. Consequently, there was created' a;stress between the lower and the upper strata, tending to eause a break in thfe country. . “Compression and bending’ stresses in the very ’flexible columnar rock behind the intake, and trinPile' 'stresses in the joint-planes in front of it, added a factor of instability to the ground. In this connection it should be noted that the tensile strength oT the rock across the joint-planes is probably very small.' “Any : foreing-in of air into the deeper, strata, which may have been brought about by the capillary action of, water soaking into the ground, would also aontribute a stress tending to cause a break fin the country. . "WHERE TROUBLE STARTED. ‘T'b§Tieve that the' splitting of the ground started at, or down-stream of, tlie sharp corner between the intake and, the spillway, , ,■ “Once a crack had formed, certain internal stresses were automatically relieved. Water immediately filled the crack, developing the full hydrostatic pressure, and widening the fissure until a new' state of equilibrium was established between the hydrostatic pressure on one side and the elastic effect of the combined cantilever and beam action of the ridge on the other side. “Other subsidiary cracks appearing on the surface may be attributed to temporary phenomena due to the shock produced by the formation of the main crack. Whether or not some connection with deeper fissures was created at the ocoasion is uncertain.,

“Evidently the ridge between the headrace and the powerhouse has, during the accident been subjected to a severe test load without failing as n dam. This indicates that the ridge will stand safely in the. future, if it is adequately protected from the influence of destroying a,gouts. “I am convinced that no rupture would have taken place if water under an increased pressure had beer prevented from entering the. grounc in the bottom and sides of the head race! I am equally convinced that no rupture will take place in the future if the cracks which have formed be effectively sealed and adequate* measures, as proposed, 'be taken to prevent the headrace-water from entering the ground. REASSURING EVIDENCE. “The fact that the crack began to close up when the water was lowered in the headrace clearly demonstrates the great elasticity of the rock. “It also gives the reassuring evidence that no internal failure due to crushing of the rock or washout has taken place in the ridge or in the underlying strata.

“The fact that the crack has notcompletelv dosed is to a great extent due to the obstruction produced by material washed into it from the headrace. What remains of the crack indicates that there is still some compression in the columnar rock of the ridge, which will remain as an initial stress in the future, tending tq restoic the ridge to its ordinal position. “Obviously, the ground under the power-house and the structure itself Lave also been subjected to very great stresses, due to fhe movement of the ridgp. The fact that the building nearly regained its old position shows that the foundation is uncrushed and well capable of taking its normal load. Also, it- may be said that the various concrete structural elements have very well withstood the severe condition temporarily imposed upon them through the rock-movement.”