Question

A water tunnel is mined through a mudstone rock mass at a depth of 200 m. The rock mass consists of three fracture sets. One set is the bedding planes; these are highly weathered, slightly rough surfaces and are continuous with an orientation of 180/10. The first number is the dip direction which is an azimuth measure from the North. Dip direction is measured 90° clockwise from the strike. The second number is the dip. The second set of fractures in jointing; these joints are slightly weathered, slightly rough, and have an orientation of 185/75. The third set is also jointing; again, the joints are slightly weathered and slightly rough and have an orientation 090/80. The strength of the rock is 55 MPa and the values of RQD and the mean fracture spacing are reported as 60% and 0.4 m respectively. The groundwater condition in a tunnel mined through mudstone at a depth of 200 m is assumed to be damp to wet. Calculated RMR for both water conditions. Use RMR to determine adjusted Rock Mass Class and assess the stability of a 10 m wide excavation mined from east to west. Since the tunnel is being driven from East-West, the azimuth of the tunnel face is 270°. Hint: You need to do determine the adjusted Rock Mass Class for each joint set (3 RMR Values) to determine the CRITICAL classification. Do your best at the sketch relationship between each fracture plane and tunnel face so you can adjust RMR for discontinuity orientation.

Answer 1

In order to apply the RMR system, five principal parameters are assessed: Intact rock strength, groundwater conditions, RQD, fracture spacing and fracture condition. This provide the basic RMR value for the rock mass. The orientation of the fractures is then accounted for through the use of rating adjustment factors to determine final RMR value. Here, we have three sets of fractures. So, we have to apply the RMR system to each set in turn and hence identify the set that is most critical for this classification.

Step 1:- The strength of intact rock mass is 55 MPa. So, conservative value for the strength rating is 7.

Step 2 :- The water tunnel which is mined is situated at a depth of 200m, and the rock mass is a mudstone. At a depth of 200m, the vertical stress component is = 25*200*10-3 N/mm2 = 5 MPa (Assuming a unit weight of rock of 25 KN/m3), and this stress will probably be sufficient to keep the fractures tightly closed.

Step 3 :-Given in the ques, the groundwater conditions in a tunnel mined through mudstone at a depth of 200m to be assumed between damp to wet. So, rating values ranges from 7 to 10.

Step 4:- Given the average RQD value as 60%. So, the rating value is about 13.

Step 5:- The mean fracture spacing is given as 0.4m, So the corrosponding rating value is 10.

Step 6:- Adding these above four ratings together gives a total rating value of = 7 + (7 to 10) + 13 +10= 37 to 40

Step 7:- Classification using Set 1 :

• These bedding planes are highly weathered, with slightly rough surfaces, and are continuous. The rating values for these specific attributes are 1, 3 and 0 respectively.
• An assessment of aperture can be made on the basis of in-situ stress state at the location of the tunnel mined: a stress of 5 MPa will mean that the aperture is low , and so a reasonable rating will be 5.
• We have no information regarding infilling but, as there is no evidence to suggest the presence of any, so we can assume a rating value of 6.
• The total rating value of this fracture is = 1+3+0+5+6 = 15
• The orientation of the bedding plane is 180/10. The tunnel mined is being driven along the strike. This is considered to result in fair conditions, with a corrosponding rating adjustment value of -5.
• Thus, the overall RMR value of set 1 is = (37 to 40) + 15 = 52 to 55, which is classified as 'Fair rock' . Taking into account the orientation rating adjustment of -5, the RMR value is reduced to 47 to 50. But this does not alter the rock classification.

Step 8:- Classification using Set 2:

• The joints are slightly weathered, slightly rough, and has an orientation of 185/75. We can therefore assign the rating values for these specific attributes as 5, 3 and -12 respectively.
• As the fractures are joints in a mudstone, their persistence will probably be in the range of 1m to 2m. So, the appropriate rating value for this is 2.
• As with set 1, we can assign rating values for the aperture and infilling of 5 and 6 respectively.
• The total rating values for this fracture is = 5+3+2+5+6= 21
• So the overall RMR value = (37 to 40) + 21 = 58 to 61. Again, this is classified as ' Fair rock'. Taking the orientation rating adjustment of -12 into account, the RMR value reduced to 46 to 49.

Step 9:- Classification using Set 3 :

• This joining is also slightly weathered, slightly rough, but has an orientation of 90/80. We can therefore assign the rating values for these specific attributes as 5, 3 and -5 respectively. The orientation rating adjustment is -5 as the strike is perpendicular to the tunnel mined axis, and we are driving against the dip which is classified as 'fair'.
• As the fractures are joints in a mudstone, their persistence will probably be in the range of 1m to 2m. So, the appropriate rating value for this is 2.
• As with set 1, we can assign rating values for the aperture and infilling of 5 and 6 respectively.
• The total rating values for this fracture is = 5+3+2+5+6= 21
• So the overall RMR value = (37 to 40) + 21 = 58 to 61. Again, this is classified as ' Fair rock'. Taking the orientation rating adjustment of -5 into account, the RMR value reduced to 53 to 56.

Step 10:- Overall evaluation:- Set 2 leads to the most critical classification, with a range of probable RMR values of 46 to 49. Using a chart linking RMR and excavation span to stand-up time shows that a 10m wide excavation mined from east to west in such a rock mass would suffer an immediate collapse. So, the engineering design will be needed to incorporate rock stabilization measures i.e. support or reinforcement. In addition, some form of staged excavation mined may also be necessary. After that, a small pilot excavation is formed and then systematically opened out to the full size as the engineering behaviour of the rock mass is improved as the stabilization measures are applied.