Tool to estimate Consistency & Shear Strength of Weathered Rock as per IS 2911-2010

Consistency & Shear Strength of Weathered Rock Tool

Developed By:
Mohan Dangi (Gold Medalist)
& Geotechnical Engineer

This tool estimates the consistency, shear strength, breakability, penetration characteristics, and weathered rock grade using SPT N-value correlations commonly adopted in geotechnical engineering practice.

Reference Correlation Diagram

Enter SPT N Value:

Enter an SPT N-value and click the button.

Correlation Summary Table

SPT N Value	Consistency	Approx. Shear Strength (kN/m²)	Grade
2 - 4	Very Soft	10 - 20	-
4 - 8	Soft	20 - 40	-
8 - 20	Firm	40 - 100	-
20 - 40	Stiff	100 - 200	-
40 - 60	Very Stiff	200 - 400	-
60 - 80	Hard / Very Weak	400 - 600	F
80 - 100	Hard / Very Weak	600 - 1000	E
100 - 200	Weak	1000 - 2000	D
200 - 400	Moderately Weak	2000 - 6000	C
400 - 600	Moderately Strong	6000 - 10000	B
> 600	Strong	> 10000	A

Explanation:

• Lower N-values indicate soft or weak weathered rock strata.
• Higher N-values indicate stronger and denser rock formations.
• Shear strength values are approximate and based on empirical correlations.
• Breakability and penetration characteristics are useful for field identification.
• SPT testing becomes difficult and less reliable for very high N-values (>200).

Reference:

1. IS 2911 (Part 1/Sec 2): 2010 – Design and Construction of Pile Foundations – Bored Cast In-Situ Piles.

2. Standard Penetration Test (SPT) correlations used in geotechnical engineering practice.

3. Correlation Chart – “Consistency and Shear Strength of Weathered Rock”.

Disclaimer:

This calculator is intended only for preliminary estimation and educational purposes. Results are based on empirical correlations and should not be used as a substitute for detailed geotechnical investigation, laboratory testing, or professional engineering judgment. The developer assumes no liability for design or construction decisions made using this tool.