Definitely the related parameters are as highlighted in Rock Mass Rating (RMR), especially rock joint conditions such roughness and grade of weathering.

Thanks! Actually, I am looking for more specific answers that are directed towards to the strength of rock masses not rating of rock masses.

@Morteza Thanks for your posting for valuable links.

Two parameters are important to determine the strength of rock mass;

1. Cohesion (C)

2. Internal Friction angle (φ)

The linear relation between normal and shear stress was first proposed by Coulomb (1776) and then it was theorized by Otto Mohr in 1900. The final equation is called the Mohr-Coulomb failure criterion and it is written as fallow:

τ = c +σ' tan Φ

Where: c= cohesion; φ= angle of internal friction; σ’ =normal effective stress in the plane of failure; τ= shear strength of rock.

So, we are talking about shear strength, compressive strength and may be tensile strength and the last one is assumed almost equal to zero.

Many of the properties of Portland cement concrete are similar to the

properties of rock.

Uniaxial Compressive strength

Shear strength

Tensile strength ( about 10 % of Compressive Strength )

Rock is generally very strong in compression, but weak in tension,

so Rock is utilized to construct arches so the rock stays in compression.

Historically Rock can be used for tall columns, but very short spans

as horizontal beams because the bottom of the beam is in tension.

Alternatively, you can use rock for tall columns with arches

atop the columns as the Romans did for their aqueducts.

Cohesion and internal friction angles are typically utilized in

soil mechanics especially clay based soil mechanics.

Other Properties to consider are Modulus of Rupture ( Young's Modulus )

Thermal coefficient of expansion ( contraction )

Resistance to weathering ( freeze-thaw and chemical attack )

In Concrete ( alkali-silica re-activity, smoothness versus roughness

of the rock , cleanliness of the surface where the cement bonds, etc. )

A study of the properties of Concrete aggregates will held you to

find what you are looking for, and there are standardized test to

determine properties.

@Michael Concrete properties you are talking about is similar to the properties of intact rocks, not the rock mass. Rock mass and intact rock are two completely different phenomena in rock mechanics.

In my tropical country, the strength or stability of rock mass highly influenced by joint characteristics and grade of weathering.

Dr. Mehmat Sari !

I guess your question is not very clear, that's why you cant get the desired answer. If none of the above mentioned answers is reasonable for you then you should repeat your question in points.

@Javed I am partly getting desired answers but, the problem is here that there should be a clear difference between intact rock and rock masses. We already know that a rock mass is composed of intact rock pieces that are separated by a network of joints. Therefore, its strength is partly depends on intact pieces and mostly joint characteristics.

@ Mehmat: I got your point now ............. you can try shear box test of larger dimensions (for cohesion and internal fraction angle). And it does not matter whether intact or not, you will get the accurate rock strength. You can also use other tests like triaxial etc. but reversal shear box tests are very convenient. Have a good day

Civil engineering perspective: i) type/origin of rock (sed, meth, or ign), ii) quality (level of weathering), iii) for those of good quality, the geometry of discontinuities . It is difficult to recover undisturbed specimens from poor-quality material, so you can try in-situ testing if very necessary. In many cases, presumptive values (conservative estimates based on available data) are sufficient. For good-quality samples, one can try lab tests, but care must be given to the direction of loading. The plane of maximum shear stress and the orientation of discontinuities in-situ will be critically important.

I too appreciate the answer of Mustaqim Nordin.

The Rock Mass Rating (RMR) system of (Bieniawski, 1989) as well as Romana (1985). These two are the systems which are mostly used to determine the stability of rock masses. These classification systems provide the parameters such as

compressive strength of intact rock (Uniaxial compressive strength / Point load test), Rock Quality Designation Index (both core samples as well as joints per unit volume), Spacing of discontinuities, condition of discontinuities (separation, infilling, weathering, discontinuity length, roughness), Groundwater condition, Orientation of discontinuities (joint strike, dip direction, dip amount), and finally method of excavation.

By using all these parameters as input to find out the stability of a concern rock mass.

These rating systems are applicable to determine the engineering properties such as

Average Stand-up Time,

Cohesion and Angle of Internal Friction,

Modulus of Deformation,

Allowable Bearing Pressure,

Shear Strength of Jointed Rock Masses and

Estimation of Support Pressure

Thank you

@Mustafa and Veerappan Thank you for your valuable contributions.

U go by using RMR or by Q-System.

To add to the comments above - stress conditions/confining pressures are also important, eg. In a shaley rockmass at depth or with high stress concentrations this rockmass may become plastic. The RMR or Q system will not represent the rockmass correctly. Oppositely in low stress blocky rock environments simple geometry - taking into account stress and frictional properties acting on the discontinuities - can suffice. In my opinion these are the most important concepts which govern the strength of a rockmass. RMR or the Q system link these two bounding limits - remaining cognizant of the many great empirical relationships associated with the RMR (or Hoek's GSI) and the Q system.

To determine the importance of parameters in rock mass strength a sensitivity analysis must be carried out on desired field of study. the order can differ according to discontinuity distribution in the field.

but just in case I can tell you the importance of parameters in a study I had done. Here is a part of conclusion in my paper:

"Results showed that parameters sensitivity in addition to case conditions in a specific project, depend on formation of plastic zone around cavern. rc and parameters of Hoek–Brown criterion have no sensitivity in elastic state and gradually affect the stability when plastic area occurs in rock mass surrounding the cavern. Also in elastic state two most important parameters in stability analysis are coefficient of lateral stress and modulus of deformation respectively and in plastic state the main parameters concerning cavern stability are modulus of deformation and coefficient of lateral stress respectively. The presented table showed the priority of all parameters in elastic and plastic states."

You can find the paper on my profile

Good Luck

@Efthymios Thanks a lot for your valuable contributions.

What about GSI system ? It is simplifier than other ones i think..

Degree of weathering

Fracture density

Size effect

schistosity

Ground water

These are the main five paramteters that influence on the rock mass strength.

The most important parameters are listed in classifications systems like RSR, RMR, Q, RMi and GSI. To start with, you can assume that the intact rock strength will be the upper bound. Now consider the effect of jointing / fracturing. Frequency of joints, their orientations and their surface characteristics will be the most important parameters. Further, the likely failure mode will be important, which will depend on the type of structure (tunnel, foundation or slope) kinematics and freedom for dilation. Effect of water pressure also needs to be considered. Finally the effect of confinement (linear or non-linear) will be important. There are number of strength criteria available in literature.

I personally feel that some amount of field testing should always be involved if you are working for a project. It will generally not be feasible to stress the rock in the field upto failure, however we can stress it upto certain stress level (or one can monitor displacements). This will give you modulus of deformation of the mass, and the effect of geological features will reflect through this modulus. There exists a definite correlation between the strength reduction and the modulus reduction of a rock due to presence of joints. These correlations can be used with confidence to get the strength of the mass under unconfined state and then suitable strength criteria can be used to consider the effect of confinement.

For further discussion I shall be happy to share with you the work done on this aspect by us. Pl feel free to contact at [email protected] ; [email protected];

Good luck

Mahendra Singh

IIT Roorkee, INDIA

All rock mass classification systems are subjective and give general quality knowledge. Therefore, one must be carefull to uses rock mass quality values while estimating the rock mass strength, which is dependent on the degree of weathering, jointing intensity (fracture density), schistosity condition, ground water condition and size effect.

The most important parameter influencing the strength of rock mass is, by my opinion, the stress state- if the same rock mass is under uniaxial compression stress it has surpassingly lower strength than if it is in state of triaxial loading. And then of course there are all parameters considered in different classification systems, where you can easily see which one influences the rock mass quality (and thereby its strength) the most, by the number of points assigned to each parameter- in RMR classification (1989) for example the joint conditions have 30 points (the most), spacing of joints and RQD 20, strength of intact rock and groundwater conditions 15.

Thanks a lot @Sanja. The points assigned to rock mass parameters in RMR system are subject to discussion. Some people say that intact rock strength should not be included in the ratings of RMR and it should be evaluated seperately. As earlier stated by @Mahendra Singh, If we assume the strength of intact rock as a starting point, the strength of a rock mass will be equal to some fraction of intact rock strength depending on the intensity (frequency) of joints and their surface conditions. My question is actually aiming to get an answer how much percentages rock mass parameters will be effective on the reduction of intact rock strength.

Best regards.

As i know Mineralogical composition and Petrographic features (grain size, grain bindings, weathering, micro-cracking) of the rock.

Dear Vijayakumar,

Mineralogical composition and petrographic features in a rock are mostly effective on intact rock strength, however, the strength of a rock mass mostly depends on the macroscopic features such as fractures, joints, bedding planes, faults, etc.

The final product of my question is published as a paper entitled "Incorporation of uncertainty in estimating the rock mass uniaxial strength using a fuzzy inference system" published in the Arabian Journal of Geosciences. You can find the full-text of the paper attached below. Thanks for your contributions.

Dear Mehmet,

I believe the main factor that affected rock mass durability can be categorized as discontinuity network, weathering and infills, rock materials, and rock block geometry. on the other hand, some parameters like roughness help to increase the shear stress that describes comprehensively by Patton (1966).

Bests,

Water, Rain, Snow, freezing, expansion, cracking, and finally landslides.

Look at fence post along a hillside. In a few years they can change from vertical, to leaning to horizontal. If it is steep, it will creep down hill.

If it is solid, it will fracture. Water will turn big rocks into sand, gravel, silt, clay, and minerals. The farther you are from the source, the smaller the particle size, and the closer the gradient will be to horizontal. Rivers tend to maximize their s curves to extend the length, and minimize the gradient.

Look at all the videos on landslides, and failed retaining walls, and look for the common things that they have that are similar. Water, too steep, too tall, too much rain, sand low movements can become fast movements.

I forgot another very important requirement

The Angle between the horizontal, and the vertical. The steeper the angle, the more likely it is to move from the top to the bottom.

The forces inside the Earth want to create mountains. Everything else wants to remove those mountains and fill in the ocean basins. If you think about it, if the Earth was not geologically active, in a few ( many ) million years the Earth would become a smooth sphere covered with water, and topped with a gas layer.

Also if you think about it, the first large lifeforms evolved in the oceans after billions of years of microscopic life forms all evolving in the oceans. This implies that the Earth changed from a barren rock sphere to a rock sphere covered with oceans, to a rock sphere with oceans, and mountain ranges, and later continents with an increasing volume of atmosphere.

Looking Back we were represented by all the smaller and moons in the solar system. Looking forward we will join the few and the mighty, Venus with a thick hot atmosphere, then moving up in sequence, Neptune, Uranus, Saturn, Jupiter and eventually, a proto-star, and the Sun.

The Earth is in an evolutionary transition phase from a warm larger terrestrial planet to smaller gas giants, and on upward. This also means that all the Planets, and the Sun are in transition from smaller volumes to larger volumes. We must remember, The big dogs eat first, and eats the most, so we need to move farther away from the Sun as the Sun will grows vastly faster than all of the puny planets.

Mars, for better or worse, here we come .

For those interested, there are numerous You Tube Videos on landslides.

They always seem to start out slowly with a few small pieces moving, then

progress at an ever increasing pace.

The major factors which can affect the rock strength are it's minerology and geological parameters. Direction of joint, joint frequency, nature of joint fillings affect the rock strength

Dear Followers of the Question,

I asked the same question to ChatGPT. Here is the answer:

As an AI language model, I do not have personal opinions; however, I can provide some information about the most important parameters effective on the strength of rock masses.