The Standard Penetration Test is an in-situ dynamic penetration test used to evaluate the geotechnical properties of subsurface soils. It is performed inside a borehole by driving a standardised split-spoon sampler into the soil using a drop hammer of known weight and fall height. The number of hammer blows required to drive the sampler through a defined depth of soil is recorded — this count is called the SPT N-value.

The N-value is a direct indicator of soil resistance and is correlated to key engineering parameters including relative density, angle of internal friction, undrained shear strength, bearing capacity and liquefaction potential. The test simultaneously retrieves a disturbed soil sample from the split-spoon sampler for visual classification and laboratory testing.

In India, the SPT is governed by IS 2131 : 2025 — Method for Standard Penetration Test for Soils (Second Revision), published by the Bureau of Indian Standards. The 2025 revision updated the equipment specifications and procedures to incorporate modern advances and reduce human error, while referencing IS 1892 : 2021 for subsurface investigation and IS 9640 : 2025 for split-spoon sampler specifications.

SPT is conducted inside a drilled borehole at regular depth intervals

Standard SPT equipment includes the split-spoon sampler, drill rods and drop hammer

IS 2131 : 2025 specifies the following equipment for conducting the SPT:

• Drop Hammer: Mass of 63.5 kg (± 1%), dropped freely from a height of 750 mm (± 1%). The standard recognises both donut hammers (cylindrical with central hole) and safety hammers. Automatic trip hammers are preferred in the 2025 revision to eliminate human energy variation.
• Split-Spoon Sampler (as per IS 9640 : 2025): Outer diameter 50.8 mm, inner diameter 35 mm, length 650 mm with a hardened steel cutting shoe at the bottom and a ball check valve at the top to retain the soil sample.
• Drill Rods: AW or BW type rods of minimum 41 mm outer diameter with straight, tight couplings. For depths beyond 10 m, centring spacers must be used at every 10 m to prevent rod whipping and energy loss.
• Drilling Equipment: Capable of producing a clean borehole of 100 to 150 mm diameter using rotary or percussion methods. Drilling fluid (bentonite slurry, specific gravity 1.04–1.08) is used to stabilise borehole walls and maintain a positive hydraulic head.
• Anvil: Placed at the top of the drill rod string to receive hammer blows. Energy transmission efficiency directly affects the recorded N-value.

The SPT is performed at regular depth intervals (typically every 1.5 m or at changes in soil stratum) inside a drilled borehole. The complete step-by-step procedure as per IS 2131 : 2025 is as follows:

1. Advance the Borehole: Drill the borehole to the required test depth using rotary or wash boring. The borehole diameter must be between 100 and 150 mm.
2. Clean the Borehole Bottom: Remove all disturbed and loose material from the bottom of the borehole completely before inserting the sampler. This is critical — any cuttings left behind artificially increase the N-value.
3. Insert the Split-Spoon Sampler: Lower the split-spoon sampler to the bottom of the borehole attached to the drill rod string.
4. Seating Drive (150 mm): Drive the sampler through the first 150 mm using the 63.5 kg hammer dropped from 750 mm. Record the number of blows. This is the seating drive — blows for this phase are NOT counted in the N-value as the soil at the very bottom may be disturbed by drilling.
5. Test Drive — Phase 1 (next 150 mm): Continue driving the sampler for the next 150 mm and count and record all hammer blows (N₁).
6. Test Drive — Phase 2 (final 150 mm): Drive the sampler for the final 150 mm and count and record all hammer blows (N₂).
7. Calculate SPT N-Value: The SPT N-value = N₁ + N₂ (total blows for the last 300 mm of penetration). This is the field N-value before corrections.
8. Refusal: If the sampler refuses to advance (i.e., 50 blows are required for less than 300 mm total penetration, or 10 blows for less than 5 mm in any phase), the test is discontinued and refusal is recorded. This indicates rock or very dense stratum.
9. Retrieve the Sampler: Extract the split-spoon sampler, open it and collect the disturbed soil sample for visual classification, moisture content, Atterberg limits or other laboratory tests.
10. Record and Report: Record test depth, N-values for each drive phase, soil description, groundwater level, and any anomalies observed during testing.

The split-spoon sampler retrieves a disturbed soil sample with every SPT test

When two identical granular soils at different depths are tested, the deeper soil will record a higher N-value simply because of greater confining pressure — not because of greater density. This makes raw N-values from different depths incomparable without correction.

IS 2131 specifies the overburden correction for cohesionless soils (sands, gravels) as:

The correction factor is read from the chart in IS 2131 plotting effective vertical overburden pressure (in kN/m²) against the correction multiplier. At shallow depths the correction factor exceeds 1.0 (increases raw N), while at greater depths it is less than 1.0 (reduces raw N). No overburden correction is applied to cohesive soils (clays) as per Peck et al. (1974).

1. Safe Bearing Capacity (SBC)

The corrected SPT N-value is the primary input for calculating the safe bearing capacity of soils for shallow foundations (strip, isolated, raft). As per IS 6403 : 1981, SBC is determined using bearing capacity factors (Nc, Nq, Nγ) derived from the angle of internal friction (φ), which is itself correlated to the N” value. A minimum factor of safety of 2.5 to 3.0 is applied over the ultimate bearing capacity.

2. Pile Foundation Design

For deep foundations (piles) as per IS 2911 (Parts 1–4), SPT N-values are used to estimate both end bearing capacity at the pile tip and skin friction along the pile shaft. The variation of N-values with depth also guides engineers in deciding the founding depth of piles — typically where N-values consistently exceed 30–50 for granular soils.

3. Settlement Estimation

For granular soils, theoretical settlement analysis using N-values follows Terzaghi & Peck, Meyerhof and Burland & Burbidge methods. Settlement governs foundation design in many Indian soil conditions, particularly in loose to medium dense sands where bearing capacity may be adequate but settlements could be unacceptable.

4. Liquefaction Assessment

In seismic zones (as per IS 1893 Part 1 : 2016), SPT N-values are the primary parameter for assessing the liquefaction potential of saturated sandy soils. Zones II to V in India require liquefaction checks, and soils with corrected N” below a threshold value (typically 15–20 for fine sands) are considered liquefiable and require ground improvement or deep foundations.

SPT N-values directly inform the type and depth of foundation selected

5. Type of Foundation Selection

The SPT borehole profile, showing N-value variation with depth, is the primary tool engineers use to decide which type of foundation is suitable for a site:

• N < 10 at shallow depth: Soft ground — raft or pile foundation recommended
• N = 10–30 at 1–3 m depth: Isolated or strip footings may be suitable
• N consistently > 30: Good bearing stratum — shallow foundation viable
• Variable N with depth: Detailed pile design required; founding depth below weak layers

SPT reports are prepared and reviewed by NABL registered technical signatories

A technically correct SPT N-value depends as much on correct field procedure as it does on the soil itself. IS 2131 : 2025 emphasises the following critical precautions:

• Clean the borehole thoroughly before every test — even a thin layer of loose cuttings at the bottom will give a falsely low N-value for the seating drive and affect overall results.
• Maintain exact hammer drop height of 750 mm. Even a 5% variation in drop height will produce a proportionally erroneous N-value. Use of automatic trip hammers (recommended in IS 2131 : 2025) eliminates this error.
• Use straight, undamaged drill rods. Bent or damaged rods absorb energy through rod whip, transferring less energy to the sampler and inflating the N-value.
• Inspect the split-spoon sampler before each test. A deformed barrel, worn cutting shoe or blocked check valve gives unreliable results and a compromised sample.
• Maintain groundwater level in the borehole at or above the in-situ groundwater table at all times. A drop in borehole water level below the groundwater table causes heave and disturbance at the bottom, leading to unreliable N-values.
• Record groundwater depth accurately. Groundwater location directly affects the overburden and dilatancy corrections.
• Never rush the test. The standard specifies a maximum rate of approximately 30 blows per minute. Faster rates alter drainage conditions and affect results in fine soils.

While SPT is always recommended for any significant structure, certain categories of projects practically require SPT-based soil investigation reports in India:

• Multi-storey buildings (G+3 and above) in Delhi, where DDA and municipal authorities require soil investigation reports
• Bridge and highway projects — NHAI, MoRTH and state PWD specifications mandate SPT-based geotechnical reports
• Industrial structures, towers and chimneys where differential settlement could be catastrophic
• Pile foundation designs — IS 2911 explicitly requires SPT N-value profiles for pile capacity calculations
• Sites in seismic zones III, IV & V — IS 1893 requires liquefaction assessment based on SPT N-values
• Sites near waterbodies or with high groundwater — where liquefaction and heave risks must be quantified
• Government and CPWD projects — NABL accredited SPT reports are a standard tender requirement

SPT is mandatory for pile foundation and seismic zone projects as per IS codes