The Constructor

Properties of Rocks for Engineering and Construction Purposes

Properties of Rocks for Engineering Purposes

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In the nature there are so many types of rocks are available which are used in construction and engineering purposes. But when it comes to usage all of them having different properties and parameters. Mainly for engineering purposes rocks need to be classified. In 1974 Z.T.Bieniawski proposed the Geomechanics classification system of rocks based on the Rock Mass Rating (RMR) value of rock.

Properties of Rocks for Engineering Purposes:

The Rock Mass Rating (RMR) value depends upon the rock quality and its value vary from 0 to 100. The RMR is derived from 6 parameters given below.
  1. Strength of rock
  2. Drill core quality
  3. Joint spacing
  4. Joint characteristics
  5. Ground water conditions
  6. Orientation of joints

Compressive Strength of rock:

The RMR value depends up on the unconfined compressive strength (qu) of the rock and it can be determined from laboratory compression test on a prepared rock specimen. Or otherwise find out the approximate value of compression strength from point load test on intact pieces of drill core. Use the following equation to find qu.

qu = 25 Is

Where Is = point load strength. The following table gives RMR value of rock depending upon the point load strength (Is) and unconfined strength (qu). The increments vary from 15 to 0.
Point load index (Is) (MPa) Unconfined compressive strength (qu) (MPa) Rock mass rating increments
>8 >200 15
4-8 100-200 12
2-4 50-100 7
1-2 25-50 4
------ 10-25 2
------ 3-10 1
------ < 3 0
When RMR value increment is less than 3 (for last 3 rows), then do not use this rock for engineering purposes.

Drill core quality:

The drill core quality is related to rock quality designation (RQD) which is expressed in terms of %. The RQD of a rock is determined from the percentage recovery of core in lengths greater than 10 cm for NX cores (57.2 mm diameter). And for other types, the percentage recovery is determined in lengths greater than twice the corresponding diameter. The RMR value for drill core quality depend upon the RQD value is shown in below table. The increment vary from 20 to 3.
RQD (%) RMR increment
91-100 20
76-90 17
51-75 13
25-50 8
<25 3

Joint spacing:

Spacing of joints should also decides the rock mass rating value. The rating increment should also reflects the joint set which is most critical for a particular application. If the rock mass has fewer sets of joints, the rock mass rating is increased. The below table will gives the rating increment for different joint spacing’s (m) and the increments may vary from 30 to 5.
Joint spacing (m) RMR increment
>3 30
1-3 25
0.3-1 20
0.005-0.3 10
<0.005 5

Joint characteristics:

Along with the joint spacing, the characteristics of joints are also influence the RMR increment value. That is the condition of a joint set which is most critical for a particular application should be considered for the rating. And RMR for joint characteristics also depends upon the roughness of the joint. Below table gives the rating increments depending upon the joint characteristics and these vary from 25 to 0.
S.No Joint description RMR increment
1 Very rough surfaces of limited extent, hard wall rock 25
2 Slightly rough surface, aperture < 1mm, hard wall rock 20
3 Slightly rough surface, aperture < 1mm, soft wall rock 12
4 Smooth surface, aperture of 1-5mm, joints extending more than several meters 6
5 Open joints filled with more than 5mm of gauge, joints extending more than several meters 0

Groundwater conditions:

Ground water conditions can influence the rock mass behavior to a large extent because of more dependency of rock condition on ground water. The rock is assigned the category dry, moist, water under moderate pressure or severe water problems and the rating increments vary from 10 to 0. If there is an exploratory adit or pilot tunnel then the RMR increment should also depends on inflow per 10m length. It also depends on joint water pressure divided by major principal stress.
S.No General condition Inflow per 10m tunnel length (lit/min) Joint water pressure divided by major principal stress RMR increment
1 Completely dry None 0 10
2 Moist 25 0.0-0.2 7
3 Water under moderate pressure 25-125 0.2-0.5 4
4 Severe water problems 125 0.5 7

Orientation of joints:

Orientation of joints is the other parameter that considered for engineering purposes especially for specific applications in tunneling, mining and foundations. If the orientation of joints is very favorable for the work under consideration, no points are subtracted from the sum. But, if they are unfavorable, then negative ratings are to be applied which are given in the below table. The rating increments for tunnels are from 0 to 12 and for foundations 0 to 25.
Orientation of joints condition RMR increment for tunnels RMR increment for foundations
Very favorable 0 0
Favorable -2 -2
Fair -5 -7
Unfavorable -10 -15
Very unfavorable -12 -25
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