Standard Penetration Test (SPT): Procedure, N-Value Corrections & Bearing Capacity

The Standard Penetration Test (SPT) is the most widely used in-situ soil test in the world. Despite its empirical nature and many limitations, no other test combines simplicity, cost-effectiveness, and the wealth of available correlations that SPT provides. This guide covers the complete test procedure per IS 2131, all N-value corrections, and the most reliable correlations for engineering design.

Purpose of SPT

• Assess soil resistance and consistency
• Estimate bearing capacity and pile capacity
• Assess liquefaction potential
• Determine relative density of sands
• Identify soil variability with depth
• Mandatory requirement for most IS codes (IS 1892, IS 8009, IS 2911)

SPT Equipment (IS 2131:1981)

• Split-spoon sampler: 50.8 mm OD, 34.9 mm ID, 650 mm long drive tube + sampler head
• Driving hammer: 63.5 kg mass, 762 mm free-fall height
• Drill rods: A or AW size, minimum 3 m sections
• Anvil and cathead assembly
• Rotary boring rig

SPT Test Procedure (IS 2131:1981)

1. Advance borehole to test depth by wash boring, rotary drilling, or continuous sampling
2. Clean the bottom of borehole carefully — do not disturb the soil ahead
3. Lower the split-spoon sampler to the bottom
4. Seat the sampler with 150 mm seating drive (not counted)
5. Drive the sampler through 300 mm in three 150 mm increments, recording blows for each increment
6. SPT N value = blows for 2nd + 3rd 150 mm increments (total 300 mm penetration)
7. If N exceeds 50 before 300 mm, record as N>50 or "refusal" and note actual penetration
8. Retrieve sampler, collect disturbed sample for lab testing (classification, moisture content)
9. Repeat at 1.5 m depth intervals (or at every change in stratum)

SPT N-Value Corrections

The raw N value must be corrected to a reference energy ratio (60% of theoretical free-fall energy, N₆₀) and to a standard overburden stress of 100 kPa (N_1,60). The corrected value N_1,60 is used in all bearing capacity and liquefaction correlations.

1. Energy Ratio Correction (C_E)

2. Borehole Diameter Correction (C_B)

3. Rod Length Correction (C_R)

4. Sampler Correction (C_S)

• Standard split spoon with liner: C_S = 1.00
• Split spoon without liner (common in India): C_S = 1.10–1.30

Computing N₆₀ and N_1,60:

N60 = N × (CE × CB × CR × CS) / 0.60
N1,60 = CN × N60
CN = (Pa/σ'v0)^0.5 ≤ 2.0  (Liao & Whitman, 1986)
Where Pa = 100 kPa, σ'v0 = effective overburden stress

Bearing Capacity from SPT — IS 6403:1981

IS 6403 provides Terzaghi's equation for net safe bearing capacity. For sandy soils, SPT-based estimation using Teng (1962) formula:

qnet, safe = Cw × [3.6 N² B + 5.4 (100 + N²)] (kN/m²)  for B ≤ 1.2 m
qnet, safe = Cw × [2.1 N² (B + 0.3/B)² + 7.2 (100 + N²)] for B > 1.2 m

Where C_w = 1.0 (GWT deep), 0.5 (GWT at foundation level); B = footing width (m).

SPT N-Value Interpretation

Liquefaction Assessment Using SPT

Liquefaction susceptibility of saturated sandy soils during earthquakes is assessed using the Cyclic Resistance Ratio (CRR) vs Cyclic Stress Ratio (CSR) method (Seed & Idriss, IS 1893 Part 1 Annex F):

• CRR from N_1,60 value (chart from IS 1893)
• CSR = 0.65 × (σ_v/σ'_v) × a_max/g × r_d
• Factor of Safety = CRR/CSR; FS < 1.1 → liquefaction likely

Limitations of SPT

• Highly dependent on operator technique and equipment
• Unreliable in gravels (overestimates N) and very soft clays (use vane shear test instead)
• Does not provide continuous data — point measurements only
• Energy actually delivered to sampler varies ±30% even with good equipment
• Always supplement with laboratory testing (grain size, Atterberg limits, consolidation)

Frequently Asked Questions