Environmental Engineering Reference
In-Depth Information
FIGURE 6.64
Aerial oblique of San Andreas Lake, formed in the trough of the San Andreas fault, a strike-slip fault (San
Pedro, California).
6.6.2
Tensile Straining
Condition
Tensile strainings occurs at low residual stresses, which are insufficient to cause bursting.
Significant deflections can occur in river valley and excavation walls in some relatively
flat-lying, massive sedimentary rocks, and intensely folded and massive rock bodies.
Examples
A quarry floor in limestone in Ontario suddenly experienced a heave of 8 ft (2.5 m)
(Coates, 1964).
The walls of a 30-m-deep excavation in the Fordham gneiss in New York City (see
Figure 6.46)
expanded to cause rock bolt failures (Ward, 1972). Residual stresses were
determined to have been as much as 10 times overburden stresses.
In a Canadian National Railways Tunnel (Mason, 1968), pressures substantially higher
than overburden pressures, of the order of 80 tsf, were measured in the tunnel lining.
Displacements due to stress relief essentially reached an equilibrium within 8 to 30 days,
but creep displacements continued over a period of several years.
At the Snowy Mountains project in Australia (Moye, 1964), flatjack tests indicated that
lateral stresses in granite were about 2.6 times vertical stresses. Measured lateral stresses
ranged from 40 tsf to a high of over 200 tsf.
6.6.3
Rock Bursts
Significant Factors
Rock bursts are sudden explosive separations of slabs, often weighing as much as several
hundred pounds or more, from the walls or ceilings of underground openings. They are
