GEOMECHANICS During an excavation for a tunnel, 250 m below ground level a highl
ID: 110700 • Letter: G
Question
GEOMECHANICS
During an excavation for a tunnel, 250 m below ground level a highly fractured siltstone rock mass with two major joint sets and many random fractures is encountered. The RQD from the rock cores is 40% and the average UCS is 70 MPa. The joints with average spacing of 75 mm are rather continuous with high persistence with apertures of 3-5 mm, and they are filled with silty clay. The joint surface walls are highly weathered, undulating, and slickensided. There is some water inflow into the tunnel estimated as 15 L/min per 10 m of tunnel length, with some outwash of joint fillings. Estimate RMR, and Q. Assume a unit weight of 28 kN/m^3 for the rock mass.Explanation / Answer
Answer:
RMR are rock mass rating which is sum of rating of following six parameters to classify the strength of rock;
1) Uniaxial compressive strength (UCS)
2) Rock quality designation (RQD)
3) Spacing of discontinuity
4) Condition of discontinuity
5) Groundwater conditions
6) Orientation of discontinuity
For the given question, the value of above parameters are given and we can compare with standard table to rate this parameters.
UCS is given 70MPa, rating would be 7
RQD is given 40%, rating would be 8
Spacing of discontinuity is given 75mm, rating would be 8
Condition of discontinuity is given 3 to 5mm, rating would be 10
Groundwater conditions is given 15L/min, rating would be 7
Orientation of discontinuity is given none means 0.
Now, the mean rating of RMR classification would be 40
And according to the description (21-40 rating comes under the class number IV), the rock would be consider as poor rock.
Q refers to rock tunneling quality index and value ranges between 0.001 to 1000, which can be calculated by following formula;
Q = (RQD/Jn) (Jr/Ja) Jw/SRF) -------------equation 1
where, RQD= 40
Jn= Joint set number= 2
Jr= Joint roughness factor= 3 ,(because slightly rough)
Ja= joint alteration = 1 ,(because highly weathered)
Jw= joint water inflow= 2( because of soft filling)
SRF= stress reduction factor= 2.5 (according to the conditions of joints)
Now put the values in equation 1
we get: Q= (40/2) (3/1) (2/2.5)
Q= 20*3* 0.8
Q= 48
Related Questions
Navigate
Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.