IS 2131 : 1981Method for standard penetration test for soils

IS 2131 specifies the Standard Penetration Test (SPT) — the most-used in-situ geotechnical test for both granular and cohesive soils. A standard split-spoon sampler is driven into the soil at the bottom of a borehole using a 63.5 kg hammer falling 750 mm; the N-value is the number of blows required to drive the sampler the last 300 mm of a 450 mm penetration.

Use SPT for: - Granular soils — primary characterisation tool. N-value correlates to relative density, friction angle, allowable bearing pressure. - Cohesive soils — secondary characterisation. N-value correlates to consistency, undrained strength c_u (less reliable than direct UCS). - Pile design — depth-N profiles drive pile capacity calculations (IS 2911 Part 1 / Part 4). - Liquefaction assessment — corrected (N₁)₆₀ values are the input for Seed & Idriss simplified liquefaction triggering analysis (IS 1893 Part 1 Annex F). - Allowable bearing pressure for shallow foundations on sand (IS 6403:1981). - Settlement estimation for raft and footings on sand (Burland-Burbidge, Schmertmann methods).

Run every 1.5 m down a borehole, plus at every soil change. The N-vs-depth plot is the headline output of any geotechnical investigation.

1. Drill borehole to test depth using rotary drill or auger; clean the bottom (no slurry / cuttings). 2. Lower split-spoon sampler to the bottom (sampler OD 50.8 mm, ID 35 mm, length 600 mm). 3. Drive sampler with 63.5 kg hammer falling 750 mm (manual rope-and-pulley OR automatic trip hammer). 4. Count blows for each 150 mm of penetration for total 450 mm: - First 150 mm = seating drive (NOT counted in N) - Second 150 mm = N₁ (counted) - Third 150 mm = N₂ (counted) - N = N₁ + N₂ (penetration resistance for last 300 mm) 5. Refusal criteria — record as 'refusal' if: - N₁ + N₂ > 50 blows for second + third increments combined - More than 100 blows in any 150 mm increment - Sampler doesn't advance for 10 successive blows 6. Recover sample from split-spoon for visual classification and lab tests (moisture, gradation, plasticity). 7. Record on borelog: depth, N₁, N₂, N value, 'recovery' (length of soil retained vs 450 mm driven), soil description.

N-value vs relative density (granular soils, Terzaghi-Peck):

| N (blows/300 mm) | Relative density | Description | |---|---|---| | 0-4 | < 15 % | Very loose | | 4-10 | 15-35 % | Loose | | 10-30 | 35-65 % | Medium dense | | 30-50 | 65-85 % | Dense | | > 50 | > 85 % | Very dense |

N-value vs friction angle (sand, Peck-Hanson-Thornburn): - N = 5: φ ≈ 28° - N = 10: φ ≈ 30° - N = 20: φ ≈ 32-34° - N = 30: φ ≈ 35-36° - N = 40: φ ≈ 37-39° - N = 50+: φ ≈ 40°+

N-value vs cohesive consistency (Terzaghi-Peck):

| N | q_u (kPa) | Consistency | |---|---|---| | < 2 | < 25 | Very soft | | 2-4 | 25-50 | Soft | | 4-8 | 50-100 | Medium | | 8-15 | 100-200 | Stiff | | 15-30 | 200-400 | Very stiff | | > 30 | > 400 | Hard |

For cohesive soils the rough rule is q_u ≈ 12 N (kPa) and c_u = q_u / 2 ≈ 6 N (kPa). Always verify with a direct UCS (IS 2720 Part 10) — SPT-N for clays has 30-50 % scatter.

Corrections to N (apply in this order): 1. Hammer energy correction to 60 % efficiency: N₆₀ = N × (E_actual / 60). Manual rope-and-pulley typically delivers 45-55 %; automatic trip hammer 65-75 %. Most Indian sites use rope-and-pulley → multiply field N by ~0.75 to get N₆₀. 2. Overburden correction (Liao-Whitman): (N₁)₆₀ = N₆₀ × √(P_a / σ'_v0), where P_a = 100 kPa atmospheric. Capped at 1.7. 3. Dilatancy correction (saturated dense fine sand, original Terzaghi): if N > 15, use N' = 15 + 0.5(N − 15). Often skipped in modern practice.

For liquefaction analysis, use (N₁)₆₀ — the cleaned-up value.

• IS 1892:1979 — site investigation procedure (governs how SPT fits into the broader investigation).
• IS 2720 Part 4:1985 — grain-size analysis on the SPT recovery sample.
• IS 2720 Part 10:1991 — UCS (cross-check on cohesive soils).
• IS 1498:1970 — soil classification.
• IS 6403:1981 — bearing capacity (uses N or c_u).
• IS 1080:1985 — shallow foundations design.
• IS 2911 Parts 1-4 — pile foundation design (depth-N profile drives capacity).
• IS 1893 Part 1:2016 Annex F — liquefaction analysis using (N₁)₆₀.
• IS 8009 Part 1:1976 — settlement of shallow foundations (Burland-Burbidge uses N).
• IS 1888:1982 — plate load test (more accurate than SPT but expensive).

1. Reporting raw N without hammer-energy correction. The same soil tested with a 50 % efficient rope hammer vs 70 % automatic hammer gives N values 30 % apart. Always cite the hammer type and the corrected N₆₀ in the report. 2. No water-table or buoyancy correction in submerged sand. Below water table, effective overburden is reduced; same field N corresponds to a *higher* relative density in submerged condition. Apply overburden correction. 3. Drilling-fluid-inflow disturbance. If borehole water level drops below ground water during drilling, soil at the bottom heaves — N reads artificially low. Maintain borehole water level ≥ ground water at all times. 4. Skipping the seating drive. The first 150 mm is contaminated by drilling-disturbed material. NEVER include it in N. Beware older logs that report 'N for 450 mm penetration' instead of last 300 mm. 5. Counting refusal as N = 100 (or 50). If you hit refusal (≥ 50 in second-and-third combined), report it as 'refusal' or 'R/350 mm' — not as a number. Using N = 100 in a correlation forces a meaningless result. 6. Using Terzaghi-Peck cohesive correlations for plastic clays without UCS verification. For PI > 30, the q_u ≈ 12 N rule has 50 %+ error. Always verify with Part 10 UCS or triaxial. 7. Not differentiating energy-corrected N₆₀ from in-situ N₆₀'(N₁)₆₀. Liquefaction analysis uses *both* corrections; bearing capacity uses just energy correction. Report each clearly so downstream user can apply the right one. 8. One sounding to characterise a site. SPT scatter is 20-30 %. Run 3-5 boreholes minimum on any project; for tall buildings or major infrastructure, on a grid with ≤ 30 m spacing. 9. Sampler shoe wear. A worn / bent split-spoon shoe penetrates more easily, dropping N. Inspect the shoe at the start of every job and replace if eroded.

SPT cadence: - Every 1.5 m down the borehole (continuous in critical strata) - At every visual change of stratum - Above and below the founding level for shallow foundations - Continuous through any zone where pile shaft friction or bearing matters

Drilling and testing crew: - Borings to maintain proper diameter (≥ 60 mm), use mud-rotary or hollow-stem auger as appropriate - Hammer type and efficiency calibrated annually - Samples sealed and transported to lab for moisture + gradation + Atterberg + UCS

Use of N-profile in design: 1. Plot N vs depth for each borehole (the headline output) 2. Identify founding stratum — typically continuous N ≥ 30 in granular OR continuous N ≥ 8 in clay for shallow foundations 3. Allowable bearing via empirical correlation (Teng, Bowles, IS 6403): q_a (kPa) ≈ 12.5 × N for B ≤ 1.2 m, less for larger; settlement-controlled value usually governs in sand 4. Pile capacity: shaft friction in cohesive soil = α × c_u × area; in granular soil = K × σ'_v × tan δ × area; end bearing on stiff stratum = N_q × σ'_v × area or N × 40 (kPa) for footings 5. Liquefaction screen (IS 1893 Part 1) — for any saturated sand within 20 m depth in seismic Zones III-V

N-value is rough but ubiquitous. Always pair with at least some lab strength tests for any project beyond minor residential.