SPT Test Sheet

The Standard Penetration Test (SPT) is the most widely-used in-situ geotechnical test in India + worldwide. The result — the N-value (number of blows for 30 cm penetration of standard split-spoon sampler) — is the input to: - Bearing capacity of foundations (Terzaghi / Meyerhof / Vesic formulas) - Settlement estimates - Pile capacity (skin friction + end bearing) - Soil density / consistency classification - Liquefaction analysis (seismic zones) - Slope stability - Earthwork compaction parameters for fill

Under IS 1892:1979 (Subsurface Investigation Code), SPT is conducted every 1.5 m of bore hole depth OR at every soil-strata change, whichever is earlier. For a typical building project, 5-15 bore holes × 15-30 m deep × 10-20 SPT per bore = 50-300 SPT tests per project.

The SPT Test Sheet captures every individual test: bore hole, depth, soil type, blow counts, refusal, and the calculated N-value. Without disciplined recording: - Bearing capacity calculations become unreliable - Foundation design has insufficient basis - Disputes between geotech consultant + structural engineer - Construction issues when actual ground varies from report

Governed by IS 2131:1981 (SPT Method) + IS 1892:1979 (Subsurface Investigation Code of Practice) + IS 1498 (Soil classification) + CPWD / NHAI Specifications for foundations.

Standard procedure per IS 2131:

Equipment: - Hammer: 63.5 ± 0.5 kg (140 lb) - Free fall height: 760 ± 25 mm (30 inch) - Anvil + guide rod assembly - Split-spoon sampler: - OD 50.8 mm (2 inch) - ID 35 mm (1.375 inch) - Length: 600-750 mm total - Cutting shoe at bottom - Drill rods + lifting equipment - Counter (manual / automatic) to count blows

Test sequence:

Step 1 — Bore hole advanced to test depth - Casing pipe installed if needed - Hole cleaned (no slough at bottom) - Water level stable (for below-water-table tests)

Step 2 — Split-spoon sampler lowered to bottom - Coupled to drill rod - Hammer assembly setup

Step 3 — Driving (penetration in 3 stages of 150 mm each): - First 150 mm — Seating drive: blows recorded (typically 0-15); DISREGARDED in N-value - Next 150 mm — First test interval (N1): blows recorded - Final 150 mm — Second test interval (N2): blows recorded

Step 4 — N-value = N1 + N2 (blows for 30 cm penetration)

Refusal criteria (per IS 2131): - > 50 blows for any 75 mm increment - > 100 blows for full 150 mm - > 10 blows for last 25 mm - Total 100 blows reached without 30 cm penetration

Sample recovery: - Split-spoon retrieved - Soil from middle 150 mm extracted (disturbed sample) - Sealed in plastic / glass jar; labelled - Sent to lab for classification + tests (Atterberg, sieve, etc.)

Corrections to raw N-value:

Overburden correction (Terzaghi-Peck): - N' = N × CN where CN = √(95.76 / σv') - σv' = effective overburden pressure at test depth (kPa) - Brings shallow + deep tests to common basis

Dilatancy correction (for fine soils below water table): - If raw N > 15: N' = 15 + 0.5 × (N - 15) - Accounts for excess pore pressure in fine saturated soils

Energy correction (modern): - Different hammers have different efficiencies (45-95% of theoretical) - N60 = N × (ER/60) where ER = Energy Ratio of hammer - Standardises to 60% efficiency (American practice)

N-value interpretation:

Granular soils (sand, gravel): - N < 4: Very loose; bearing 25 kPa; not foundation grade - N = 4-10: Loose; bearing 75 kPa - N = 10-30: Medium dense; bearing 150-200 kPa - N = 30-50: Dense; bearing 250-400 kPa - N > 50: Very dense; bearing 400+ kPa

Cohesive soils (clay): - N < 2: Very soft (φu < 25 kPa) - N = 2-4: Soft (25-50 kPa) - N = 4-8: Medium stiff (50-100 kPa) - N = 8-15: Stiff (100-200 kPa) - N = 15-30: Very stiff (200-400 kPa) - N > 30: Hard (> 400 kPa)

1. Hammer energy not standardised — donut hammer (40-50% ER) vs safety hammer (60% ER) vs automatic hammer (85-90%); without correction, N-values not comparable.

2. Free fall variable — manual hammer drop; rope friction reduces fall energy; N-values inflated.

3. Borehole not cleaned — slough left at bottom; first few blows seat through slough; N1 inflated.

4. Casing not used in caving soil — sand collapses; sampler in disturbed material; N-value not representative.

5. Water level not maintained — below GWT, water in bore should be higher than GWT; if lower, sand flows up; loose register.

6. Disturbed sample mishandled — sample in middle 150 mm only; if storekeeper takes from cutting shoe, soil identification wrong.

7. First 150 mm not disregarded — seating drive blow count added to N; over-stated.

8. No refusal criteria applied — driller continues beyond > 50 blows; recorded as routine N; should be "refusal".

9. Sampler reused without cleaning — soil from previous depth in sampler; cross-contamination.

10. Drill rod size affects N — long rod sections reduce blow energy (rod weight); compensation factor (η_r) needed for depths > 10 m.

11. Hole diameter wrong — > 200 mm holes reduce confinement; N-values dilated.

12. No corrections applied — raw N used in design without overburden / dilatancy / energy correction; over- or under-design.

13. Tests at < 1.5 m spacing — test affected by previous test disturbance.

14. No water table recorded — GWT depth crucial for buoyancy + dilatancy correction; not measured at SPT depth.

15. Truncated blow counts — driller "rounds" 17 to 15; data integrity lost.

16. N-values plotted incorrectly — depth-N profile drawn wrong; design errors cascading.

Companion formats: - CBR Test Sheet (FMT-GEO-004) — for subgrade strength - Plate Load Test (FMT-GEO-003) — for bearing pressure verification - Lab Test Request (FMT-GEO-005) - Bore Log / Field Sheet — overall log - BBS Footing (FMT-SIT-004) — foundation rebar (downstream)

Codes + references: - IS 2131:1981 — Method for Standard Penetration Test for Soils - IS 1892:1979 — Code of Practice for Subsurface Investigation for Foundations - IS 1498:1970 — Classification + Identification of Soils for General Engineering Purposes - IS 1888:1982 — Method of Load Test on Soils (Plate Load) - IS 1904:1986 — Design + Construction of Foundations in Soils - IS 1080:1985 — Design + Construction of Shallow Foundations - IS 2911 Parts 1-4 — Design + Construction of Pile Foundations - IS 4434:1978 — In-situ Vane Shear Test - IS 11593:1985 — Cone Penetration Test (companion to SPT) - IS 1894:2002 — Criteria for Earthquake Resistant Design (uses N-value for liquefaction) - ASTM D1586 — US Standard Test Method for SPT (very similar) - Terzaghi + Peck Soil Mechanics Bible — original N-value to soil property correlations - NHAI / MoRTH Specifications — foundation design parameters