Disintigrating basalt dykes

Many basalt dykes, apparently quite sound when first exposed, later disintegrate. The dykes affected are fine-grained olivine-analcite-basalts of probable Tertiary age. The most severely affected rocks develop light grey pea-size spots connected by cracks, and break down to fine powder which frets away progressively from walls and roof. Slightly affected rock develops widely spaced cracks which cause slabs and blocks of hard basalt to loosen and fall. Disintegration may become apparent within a few hours, days or weeks. In the worst cases saturation with water immediately causes very rapid disintegration.

Joints and faults

These are by far the most important defects, because of the extent of their influence on rock stability. Joints and faults are planes along which the rock mass has already failed in the past under the action of geological forces. The strength of the rock mass is now governed by the joints and faults rather than by the strength of the rock material. Failure around underground openings tends to occur by slabs and blocks of rock, quite strong in themselves, becoming detached from the main mass and falling. The rocks in most places in the Snowy Mountains are extensively jointed. Joint systems consisting of two or three intersecting sets of joints occur throughout entire rock units, dividing them effectively into aggregations of slabs and blocks. The strength of the jointed rock depends on the characteristics of the sets of joints; the spacing and persistence of the joints in each set, whether the joints are closed or open, and whether they are clean or coated with clay or filled with cementing secondary minerals like chlorite.

Joints and faults (continued)

The Snowy Mountains region is also intersected by a great number of faults. In the case of joints, failure has occurred without displacement of the rock on either side of the fractures. In the case of faults, shear failure has been followed by sliding of the rock along the failure plane, usually with crushing and shearing of the adjacent rock, forming a fault zone of some width. In the Snowy Mountains most fault zones consist of weak crushed and sheared rock containing clay both as distinct seams and interspersed through the fragmented rock. Faults frequently branch and the rock in the vicinity of faults is often more closely jointed and less compact than elsewhere.

Faults with zones from a fraction of an inch to several inches in width are very common. There are also many larger faults. Faults are extreme weaknesses and the greatest difficulties encountered underground have been due to them. However, they are concentrated local defects persisting long distances but restricted to one plane or a two dimensional zone of limited width in contrast to joint systems which persist in three dimensions throughout large volumes of rock.

Damage by blasting

The rock forming the outer skin of excavations is always damaged by blasting to some extent. Where the rock mass is unjointed or jointed but very compact, the walls and roof of the excavation consist of freshly fractured rock surfaces. Parallel fractures frequently occur beneath the surface forming more or less detached slabs of rock. Many of these are removed by barring down immediately following blasting and later by the safety miner during periodical inspections.