Estimation of Allowable Bearing Capacity of Basement Rock

We have a location where a Standard Penetration Test (SPT) has been done up to 22m below the ground level, and additional 8m coring in weathered rock, with "ZERO" Rock Quality Designation (RQD) values, when the Client ordered termination of boring.

How can we estimate the allowable bearing capacity of this rock? Is carrying out Uniaxial Compressive Strength (UCS) test, the only possible method?

Many thanks in advance.

Rock Mechanics Design Deep Foundations

5 Answers

Votes: 2

Abdul Jabbar

You may perform point load tests on the rock material and u can correlate it with Ucs value.   The angle of repose can be consider in calculating allowable bearing capacity.The Angle of Repose because Rock Triaxial instruments are rarely available in the local industry. Further reading is recommended on intermediate Geo-Materials ”IGM

Votes: 3

Done Nikolovski

Bowles (fifth edition, p279) suggests that you can make calculation of the bearing capacity of the ground, according to the same equations that are derived for soil materials, when you have value of RQD<25. Make the best estimation of the parameters of the ground (fi and C), for the given stress state.


Votes: 3

Louai Jannoun

If rqd is zero. Then very poor rock. As engineering judgement i would consider this rock as very dense gravel with low or zero cohesion and high angle of internal friction. 

Votes: 1



All the above recommendations are great. I would advise you to consider the rock formation as a cohesionless soil with 0 cohesion and fi above 35 degrees. You can improve the design by considering it to be an intermediate Geo-Material, as one of the respondents above has commented. You can solve this problem with the "ALLPILE" program, it has a special provision for this kind of material.


Votes: 1

Abrham Hagazi Tegegn

hi it is very difficalt to estimate the rock bearing capacity in UCS if its RQD value is zero hence the very important thing is that geotecnicall oberve the geological formation/orentation of the rock, discontiniuty, fault and other gaps  of the area/pit/ and identify weather it is suitable or saviere then we can considers as soil or weatherd rock and calculate the bearing capacity using the adopted formmulas