IS 8764 : 1998Method of determination of point load strength index of rocks

IS 8764:1998 specifies the method of determination of the point-load strength index of rocks — the quick, portable field/lab test that estimates rock strength by breaking an irregular lump or core between two conical platens. It is the workhorse rock-strength index for site investigation of tunnels, rock slopes, rock foundations and quarry/aggregate sources, because it needs no machined specimen and can be run on site.

It is read with the rock-engineering stack:

• IS 13365 Part 1 — Rock Mass Rating (RMR), which uses rock strength as an input
• IS 9143 — uniaxial compressive strength of rock (the definitive, lab-machined test)
• IS 4464 / IS 11315 — core logging / discontinuity description · IS 5878 — tunnelling
• IS 1888 / IS 6403 — bearing capacity (rock-founded foundations)

A specimen (core — axial or diametral — or an irregular lump) is loaded to failure between the conical platens; from the failure load P and the platen separation/equivalent diameter D, the index is:

• Point-load strength index Is = P / D², then corrected to a standard 50 mm size to give the size-corrected index Is(50)
• The classic correlation: UCS ≈ k × Is(50), with k typically ~ 20–25 for many rocks (the multiplier is rock-type dependent and should be calibrated against a few IS 9143 UCS tests for the site)
• Strong anisotropy is captured by testing both parallel and perpendicular to bedding/foliation (the strength-anisotropy index)

Its value is quantity and speed — dozens of tests on recovered core to map strength variation along a tunnel or down a foundation, which a handful of slow UCS tests can't. It is an *index/screening* test, deliberately correlated to, not a replacement for, UCS.

Scenario: characterising rock strength along a tunnel from drill core.

Step 1 — test: run diametral point-load tests on many core pieces; for one, P = 18 kN, core D = 54 mm.

Step 2 — index: Is = P/D² = 18000 / 54² ≈ 6.17 MPa (raw); apply the size correction to Is(50) for the 54 mm core (small correction here) → Is(50) ≈ ~6 MPa.

Step 3 — estimate UCS: UCS ≈ k·Is(50). With a site-calibrated k ≈ 22 → UCS ≈ 22 × 6 ≈ 132 MPa → strong rock.

Step 4 — calibrate: run a few IS 9143 UCS tests on matched core to fix the *actual* k for this rock — don't trust a textbook k blindly.

Step 5 — use: feed the strength (with RQD, joints, water) into the IS 13365 RMR class for support design, reach by reach. The point-load test supplies the *strength density*; UCS anchors the correlation.

1. No size correction. Is is size-dependent — always correct to Is(50) before using or correlating, or strengths from different core sizes aren't comparable.

2. Using a textbook UCS/Is ratio uncalibrated. k varies widely with rock type (weak/porous rocks especially); calibrate against a few site UCS tests, don't assume k = 22–24.

3. Ignoring anisotropy. Foliated/bedded rock is much weaker across the planes — test both orientations or you mis-rate the rock mass.

4. Testing weak/weathered rock with point load. The test is unreliable for very weak, soft or friable rock — fall back to UCS / appropriate soil-rock tests there.

5. Treating the index as the design strength. It is a screening index correlated to UCS — design values come from UCS/rock-mass classification, with the point-load test supplying the spatial coverage.

• IS 13365 Part 1 — Rock Mass Rating (RMR); strength is an input
• IS 9143 — determination of unconfined compressive strength of rock (definitive)
• IS 4464 — presentation of borehole/core logs · IS 11315 (parts) — rock discontinuity description
• IS 5878 (parts) — construction of tunnels · IS 13063 — rock-slope stability
• IS 6403 — bearing capacity (foundations on/in rock)
• IS 2720 — soil tests (the soil counterpart suite)

IS 8764:1998 is reaffirmed and methodologically aligned with the international ISRM point-load test, so it interoperates with global practice. Its enduring value in Indian site investigation is coverage: a portable rig lets you run many strength estimates on recovered core or rock lumps and build a *strength profile* along a tunnel or down a foundation that a few slow IS 9143 UCS tests never could.

The non-negotiable practitioner discipline is size-correct to Is(50), and calibrate k against a small set of site UCS tests — the single biggest source of bad rock-strength estimates is a blindly-assumed UCS/Is ratio applied to a rock type it doesn't suit (especially weak, weathered or anisotropic rock). Used as a *calibrated index for spatial coverage*, feeding IS 13365 RMR, it is one of the most cost-effective tools in rock-site characterisation; used as a standalone design strength, it is a hazard.