Question

Mode of instability in rock mass is mostly controlled by the orientation and type of discontinuities. Thus, the selection of stabilization must be made based on the mode of instability. Answer the following question:
a) Name FOUR (4) basic mechanisms of rock slope instability.


b) List the THREE (3) main parameters that control the stability of a    jointed rock slope.

It is a common practice to install rock bolt together with cement grout. For an unstable slope face why such practice is highly recommended. What is the purpose of cement grout in rock bolt installation?

Answer 1

1)

The four basic mechanisms of rock slope instability: (a) circular slip; (b) plane sliding; (c) wedge sliding; and (d) toppling

2) Rock joint parameters such as persistence, spacing and shear strength are the factors which significantly affects the stability of a jointed rock.

The laboratory investigation showed that the bolt capacity depends basically on the mechanical properties ofgrouting materials which can be changed by water to cement ratio, mixing time, additives, and curing time.
Increasing the bolt diameter and length increases the
bolt bearing capacity. However, this increase is limited
to the ultimate tensile strength ofthe bolt materials.
Mechanical properties ofgrouting materials have an
important role on the bolt bearing capacity. It is offered
that the optimum water to cement ratio must be 0.34–
0.4 and the mortar have to be well mixed before poured
into drill hole. Improving the mechanical properties of
the grouting material increases the bolt bearing capacity
logarithmically. The best relationship was observed
between grout shear strength and bolt bond strength.
Increasing the curing time increases the bolt bond
strength. Bolt bond strength of19 kgycm in first day, 2
77 kgycm in 7 days and 86 kgycm in 35 days was 2 2
determined respectively. The results show that bolt bond
strength increases quickly in first 7 days and then the
increase goes up slowly.
Bond failure in the pull-out test occurred between the
bolt and cement grout, ofwhich the mechanical behav-
iour is observed by shear spring.
This explains the development ofbolt bond strength
and the failure at the bolt–grout interface considering
that the bond strength is created as a result ofshear
strength between bolt and grout. This means that any
change at the grout strength causes to the changing of
bolt capacity. The failure mechanism in a pull-out test
was studied in order to clarify the bond effect of rock
bolt. Thus one main bond effect was explained from
bond strength of rockbrock .