ROCK PARAMETERS AND CLASSIFICATION SCHEMES BASIC AND TUTORIALS

The purpose of rock mass classification is to establish the quality of a particular rock mass (or part of a rock mass) by assigning rating values to a set of rock parameters. Webster’s dictionary defines ’classification’ as “the act of classifying or forming into a class or classes, so as to bring together those beings or things which most resemble each other, and to separate those that differ”.

This definition immediately highlights two main issues in rock mass classification: the purpose of the classification has to be established and the method of classification has to be commensurate with the purpose.

For example, if we only used the uniaxial compressive strength of the intact rock and the fracture frequency of the rock mass, we could generate a rock mass classification scheme for characterizing sections of rock in a tunnel as shown in Table 12.1.


Table 12.1 Illustrative simple rock mass classification scheme
Parameter Ratings, R
Uniaxial compressive strength, a,
Fracture frequency, h Ifh54/m, R = 1 If h 4/m, R =2
If a, 3 100 MPa, R = A If a, < 100 Ma, R = B


On the basis of this scheme, all rock masses must then be one of the categories, AI, A2, B1, B2. We could call this a Rock Index and assign the words ’Good’ to AI, ’Fair’ to A2 and BI, and ’Poor’ to B2. But what is the purpose of this classification?

Perhaps, the Rock Index would indicate the excavatability and stability of the rock masses in each category. If so, is the classification the best one for that purpose?

There are four main steps in the development of any rock mass classification scheme:
1. decide on the objective of the rock mass classification scheme;
2. decide on the parameters to be used, their ranges and ratings;
3. decide on the algebra to be used for the rock index (e.g. do we simply select values from a table, do we add rating values together, do we multiply ratings together, or something else?); and
4. calibrate the rock index value against the objective.

The advantage of using a rock mass classification scheme is that it is a simple and effective way of representing rock mass quality and of encapsulating precedent practice. The disadvantage is that one cannot use it for a different objective or in significantly new circumstances.

The rock mass classifications that have been developed to date follow this basic approach, but include more parameters and use a greater number of classes than the simple 'good', 'fair', 'poor' example we gave above.

For example, by adding a third parameter to the classification given in Table 12.1, 'thickness of the layers', and using more rating values (Vervoort and de Wit, 1997'), a useful rock index for rock dredging has been developed. By judicious choice of the relevant parameters, such rock mass classification schemes can be a powerful tool for rock engineering.

The two main classification systems, Rock Mass Rating and Tunnelling Quality Index (XMR and Q), have both been widely applied and there is now a large database of projects where they have been used as the main indicator of rock stabilization requirements in rock tunnelling.

The systems provide a coherent method of using precedent practice experience and can now be linked to numerical analysis approaches.

With all schemes, the key issues are the objective of the classification system, choice of the optimal parameters, assigning numerical ratings to parameter values, the algebraic manipulation of the parameter ratings, and drawing conclusions from the mean and variation of the overall rock quality index values.

ROCKS USED IN CONSTRUCTION STRENGTH CLASSES BASICS AND TUTORIALS

ROCKS USED IN CONSTRUCTION STRENGTH CLASSES BASIC INFORMATION
What Are The Strength Classes Of Rocks Used In Construction?


Based on the scale effects and geological conditions discussed in the previous sections, it can be seen that sliding surfaces can form either along discontinuity surfaces, or through the rock mass. The importance of the classification is that in essentially all slope stability analysis it is necessary to use the shear strength properties of either the discontinuities or of the rock mass, and there are different procedures for determining the strength properties as follows:

• Discontinuity shear strength can be measured in the field and the laboratory.
• Rock mass shear strength is determined by empirical methods involving either back analysis of slopes cut in similar geological conditions, or by calculation involving rock strength indices.

As a further illustration of the effects of geology on shear strength, relative strength parameters for three types of discontinuity and two types of rock mass are shown on the Mohr diagram. The slope of these lines represents the friction angle, and the intercept with the shear stress axis represents the cohesion

A description of these conditions on Figure 4.7 is as follows:

Curve 1 Infilled discontinuity: If the infilling is a weak clay or fault gouge, the infilling friction angle (φinf ) is likely to be low, but there may be some cohesion if the infilling is undisturbed.

Alternatively, if the infilling is a strong calcite for example, which produces a healed surface, then the cohesive strength may be significant.

Curve 2 Smooth discontinuity: A smooth, clean discontinuity will have zero cohesion, and the friction angle will be that of the rock surfaces (φr). The friction angle of rock is related to the grain size, and is generally lower in fine grained rocks than in coarse-grained rocks.

Curve 3 Rough discontinuity: Clean, rough discontinuity surfaces will have zero cohesion, and the friction angle will be made up of two components. First, the rock material friction angle (φr), and second, a component (i) related to the roughness (asperities) of the surface and the ratio between the rock strength and the normal stress.

As the normal stress increases, the asperities are progressively sheared off and the total friction angle diminishes.

Curve 4 Fractured rock mass: The shear strength of a fractured rock mass, in which the sliding surface lies partially on discontinuity surfaces and partially passes through intact rock, can be expressed as a curved envelope. At low normal stresses where there is little confinement of the fractured rock and the individual fragments may move and rotate, the cohesion is low but the friction angle is high.

At higher normal stresses, crushing of the rock fragments begins to take place with the result that the friction angle diminishes. The shape of the strength envelope is related to the degree of fracturing, and the strength of the intact rock.

Curve 5 Weak intact rock: Rocks that are composed of fine grained material that has a low friction angle. However, because it contains no discontinuities, the cohesion can be higher than that of a strong intact rock that is closely fractured. The range of shear strength conditions that may be encountered in rock slopes clearly demonstrates the importance of examining both the characteristics of the discontinuities and the rock strength during the site investigation.

COMMON BUILDING STONES USED IN CIVIL ENGINEERING CONSTRUCTION BASICS AND TUTORIALS

STONES USED IN CIVIL ENGINEERING CONSTRUCTION
What Are the Types of Stones Used in Civil Engineering Construction?


The following are the some of commonly used stones:

(i) Basalt and trap (ii) Granite
(iii) Sand stone (iv) Slate
(v) Laterite (vi) Marble
(vii) Gneiss (viii) Quartzite.

Their qualities and uses are explained below:

(i) Basalt and Trap:
The structure is medium to fine grained and compact. Their colour varies from dark gray to black. Fractures and joints are common. Their weight varies from 18 kN/m3 to 29 kN/m3. The compressive strength varies from 200 to 350 N/mm2. These are igneous rocks. They are used as road metals, aggregates for concrete. They are also used for rubble masonry works for bridge piers, river walls and dams. They are used as pavement.

(ii) Granite:
Granites are also igneous rocks. The colour varies from light gray to pink. The structure is crystalline, fine to coarse grained. They take polish well. They are hard durable. Specific gravity is from 2.6 to 2.7 and compressive strength is 100 to 250 N/mm2. They are used primarily for bridge piers, river walls, and for dams. They are used as kerbs and pedestals. The use of granite for monumental and institutional buildings is common. Polished granites are used as table tops, cladding for columns and wall. They are used as coarse aggregates in concrete.

(iii) Sand stone: These are sedimentary rocks, and hence stratified. They consist of quartz and feldspar. They are found in various colours like white, grey, red, buff, brown, yellow and even dark gray. The specific gravity varies from 1.85 to 2.7 and compressive strength varies from 20 to 170 N/mm2. Its porosity varies from 5 to 25 per cent. Weathering of rocks renders it unsuitable as building stone. It is desirable to use sand stones with silica cement for heavy structures, if necessary. They are used for masonry work, for dams, bridge piers and river walls.

(iv) Slate: 
These are metamorphic rocks. They are composed of quartz, mica and clay minerals. The structure is fine grained. They split along the planes of original bedding easily. The colour varies from dark gray, greenish gray, purple gray to black. The specific gravity is 2.6 to 2.7. Compressive strength varies from 100 to 200 N/mm2. They are used as roofing tiles, slabs, pavements etc.

(v) Laterite:
It is a metamorphic rock. It is having porous and sponges structure. It contains high percentage of iron oxide. Its colour may be brownish, red, yellow, brown and grey. Its specific gravity is 1.85 and compressive strength varies from 1.9 to 2.3 N/mm2. It can be easily quarried in blocks. With seasoning it gains strength. When used as building stone, its outer surface should be plastered.

(vi) Marble:
This is a metamorphic rock. It can take good polish. It is available in different pleasing colours like white and pink. Its specific gravity is 2.65 and compressive strength is 70–75 N/mm2. It is used for facing and ornamental works. It is used for columns, flooring, steps etc.

(vii) Gneiss:
It is a metamorphic rock. It is having fine to coarse grains. Alternative dark and white bands are common. Light grey, pink, purple, greenish gray and dark grey coloured varieties are available. These stones are not preferred because of deleterious constituents present in it. They may be used in minor constructions. However hard varieties may be used for buildings. The specific gravity varies from 2.5 to 3.0 and crushing strength varies from 50 to 200 N/mm2.

(viii) Quartzite:
Quartzites are metamorphic rocks. The structure is fine to coarse grained and often granular and branded. They are available in different colours like white, gray, yellowish. Quartz is the chief constituent with feldspar and mica in small quantities. The specific gravity varies from 2.55 to 2.65. Crushing strength varies from 50 to 300 N/mm2. They are used as building blocks and slabs. They are also used as aggregates for concrete.

REQUIREMENT OF GOOD BUILDING STONES IN CIVIL ENGINEERING WORKS BASICS AND TUTORIALS

GOOD BUILDING STONES USED IN CIVIL ENGINEERING WORKS
What Are The Requirements for Good Stones Used In Civil Engineering?


The following are the requirements of good building stones:
(i) Strength: The stone should be able to resist the load coming on it. Ordinarilly this is not of primary concern since all stones are having good strength. However in case of large structure, it may be necessary to check the strength.

(ii) Durability: Stones selected should be capable of resisting adverse effects of natural forces like wind, rain and heat.

(iii) Hardness: The stone used in floors and pavements should be able to resist abrasive forces caused by movement of men and materials over them.

(iv) Toughness: Building stones should be tough enough to sustain stresses developed due to vibrations. The vibrations may be due to the machinery mounted over them or due to the loads moving over them. The stone aggregates used in the road constructions should be tough.

(v) Specific Gravity: Heavier variety of stones should be used for the construction of dams, retaining walls, docks and harbours. The specific gravity of good building stone is between 2.4 and 2.8.

(vi) Porosity and Absorption: Building stone should not be porous. If it is porous rain water enters into the pour and reacts with stone and crumbles it. In higher altitudes, the freezing of water in pores takes place and it results into the disintegration of the stone.

(vii) Dressing: Giving required shape to the stone is called dressing. It should be easy to dress so that the cost of dressing is reduced. However the care should be taken so that, this is not be at the cost of the required strength and the durability.

(viii) Appearance: In case of the stones to be used for face works, where appearance is a primary requirement, its colour and ability to receive polish is an important factor.(ix) Seasoning: Good stones should be free from the quarry sap. Laterite stones should not be used for 6 to 12 months after quarrying. They are allowed to get rid of quarry sap by the action of nature. This process of removing quarry sap is called seasoning.

(x) Cost: Cost is an important consideration in selecting a building material. Proximity of the quarry to building site brings down the cost of transportation and hence the cost of stones comes down.

However it may be noted that not a single stone can satisfy all the requirements of a good building stones, since one requirement may contradict another. For example, strength and durability requirement contradicts ease of dressing requirement.

Hence it is necessary that site engineer looks into the properties required for the intended work and selects the stone.