11. A massive hillside of rhyolite contains joint planes that dip at an angle of 40° toward a stream valley. A rectangular-shaped rock bounded by other joint planes rests on the dipping joint plane. The block measures 1.5 m (5 ft) high, 4.5 m (15 ft) wide, and 3 m (10 ft) long. The unit weight of the rhyolite is 2.76 Mg/m³ (172 lb/ft³). Assume a dry slope. a. What is the weight of the rock block? b. Assuming that only friction holds the block in place (no cohesion along the joint plane), what is the minimum frictional force needed to resist sliding? c. What coefficient of friction is needed to produce this minimum force? d. What would be an actual coefficient of friction for this rock block, that is, rhyolite sliding against rhyolite? A friction angle is commonly used, the coefficient of friction being equal to the tangent of that angle. Determine that angle of friction.
11. A massive hillside of rhyolite contains joint planes that dip at an angle of 40° toward a stream valley. A rectangular-shaped rock bounded by other joint planes rests on the dipping joint plane. The block measures 1.5 m (5 ft) high, 4.5 m (15 ft) wide, and 3 m (10 ft) long. The unit weight of the rhyolite is 2.76 Mg/m³ (172 lb/ft³). Assume a dry slope. a. What is the weight of the rock block? b. Assuming that only friction holds the block in place (no cohesion along the joint plane), what is the minimum frictional force needed to resist sliding? c. What coefficient of friction is needed to produce this minimum force? d. What would be an actual coefficient of friction for this rock block, that is, rhyolite sliding against rhyolite? A friction angle is commonly used, the coefficient of friction being equal to the tangent of that angle. Determine that angle of friction.
Chapter2: Loads On Structures
Section: Chapter Questions
Problem 1P
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Question
11
Transcribed Image Text:(FS) of 3 relative to compre
coal in the pillars? Assume an unconfined com-
pressive strength of 2.7 MPa (3000 psi) for the coal
and use the equation
FS=
rock strength (1-fraction removed)
rock load
Hint: Use a unit weight of rock = 2.56 Mg/m³ (160
lb/ft3). If the answer is no, what is the FS?
c. What percent of the coal can be removed at a
depth of 343 m (800 ft) using an FS = 3 and the
same compressive strength as in part (b)?
11. A massive hillside of rhyolite contains joint planes
that dip at an angle of 40° toward a stream valley.
A rectangular-shaped rock bounded by other joint
planes rests on the dipping joint plane. The block
measures 1.5 m (5 ft) high, 4.5 m (15 ft) wide, and
3 m (10 ft) long. The unit weight of the rhyolite is
2.76 Mg/m³ (172 lb/ft³). Assume a dry slope.
a. What is the weight of the rock block?
b. Assuming that only friction holds the block in place
(no cohesion along the joint plane), what is the
minimum frictional force needed to resist sliding?
c. What coefficient of friction is needed to produce
this minimum force?
d. What would be an actual coefficient of friction
for this rock block, that is, rhyolite sliding against
rhyolite? A friction angle is commonly used, the
coefficient of friction being equal to the tangent of
that angle. Determine that angle of friction.
12. From the following rocks prepare a list in order of
their decreasing unconfined compressive strength.
a. W
S
b. H
14. a.
b.
C.
15. Re
O
P
th
16. R
u
it
b
17. F
t
H
8
i
L
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