| OCR Text |
Show Surface avalanches: Yo UP t 0 3° 0 kg/ m3 Ground avalanches ( Bodenlawinen): Yo UP t 0 400 kg/ m3 Since avalanche paths frequently deviate from the steepest fall line, protective structures and obstructions must have sufficient lateral rigidity. Construction elements with large fracture strengths or large mass best resist the local impact stresses of debris entrained in the avalanche snow, rock fall, or rolling snow. Pieces of debris become carried along in an avalanche as soon as the thrust force of the avalanche snow, taking into consideration the upward pressure, is sufficient to repeatedly overturn the fragment or stone. Avalanche snow of density Y and velocity v overturns a prismatic stone of density Y i, ground area a • b, and height c as soon as v2 > 2g ( b2/ c) [ ( Y 1/ Y>- J where b is the width, i. e., the smallest dimension of the ground area. The stone is raised by the upward pressure as soon as v2 > 2gc [( T j / Y )- l] where c is the smallest dimension of the piece. The work expended in entrain-ment of debris can be neglected as a rule, i . e . , the velocity of the fragments can be set equal to the flow velocity of the avalanche. The EMPA Report No. 136 contains data [ 18] concerning the impact requirements of construction which also can serve to estimate the effect of rock fall. The velocity of a rock rolling down the slope or of a snow ball rolling down, from equating kinetic and potential energy, is: v = 0.845 ( 2 g H*)*- in which H* is the total height of fall measured vertically. Sliding can occur if tan * > 3.5p,. The velocity is independent of the size and material of the ball rolling down. The maximum impact pressure of a snow ball of diameter 0 and density Y can be estimated from the momentum equation for loss in velocity over intervals: P dt = ( it 02/ 4) ( Y / g) v dtAv and since dt can be chosen as small as desired: pmax ~ ( * 0 2 / 4 ) ( Y/ g) v2 By way of example a maximum impact of 6 to 7 t occurs when a snow ball with a 0 ~ 1 m strikes the obstruction described in Section IV, 2 g, from a distance of 50 m on a 60 - 80% slope. C. Wedge shaped bulkheads constructed directly in front of the structure to be protected are most effective against all kinds of avalanches; their height is selected tall enough and their wedge angle 2p small enough that the damming 58 |
