Senior Geotechnical Engineer • Published: June 2026 • 11 min read
Standard Penetration Testing (SPT) & Geotechnical Analysis in Itahari & Sunsari
The subsoil of Sunsari and Morang districts is formed by alluvial flood deposits from the Koshi and Budhi river basins. Stratification commonly consists of fine silty sand, soft plastic clay, and intermittent gravel lenses, paired with shallow groundwater tables (often 1.5m to 3.5m below surface).
Under NBC 105:2025 and municipal building bylaws, buildings above three stories or commercial structures mandate a certified subsoil geotechnical investigation including Standard Penetration Tests (SPT).
1. SPT Methodology (IS 2131)
The test is conducted using a 63.5 kg hammer falling freely from 750 mm height onto an anvil attached to drill rods. The number of blows required to drive the split-spoon sampler three successive 150 mm increments is recorded. The sum of blows for the last two increments represents the observed SPT $N$-value.
Mandatory N-Value Corrections:
- Overburden Pressure Correction ($C_N$): Corrects for confining stress at increasing depth (Liao and Whitman / Peck formulation).
- Dilatancy Correction ($N''$): Applied when saturated fine sand/silt with $N' > 15$ develops negative pore pressures ($N'' = 15 + 0.5(N' - 15)$).
2. Typical Bearing Capacities in Sunsari & Morang
| Location | Predominant Subsoil | Average Corrected $N_60$ | Net Allowable Bearing Capacity (q_net) |
|---|---|---|---|
| Itahari (Urban Core) | Medium dense silty sand | 12 – 18 | 120 – 160 kN/m² |
| Dharan (Foothills) | Gravel-boulder with sandy matrix | > 30 | 220 – 300 kN/m² |
| Biratnagar (South) | Soft clayey silt (High water table) | 6 – 11 | 80 – 110 kN/m² (Raft required) |
Standards & Testing Norms:
- IS 2131:1981 — Method for Standard Penetration Test for Soils
- IS 1892:1979 — Code of Practice for Subsurface Investigation for Foundations
- IS 1904:1986 — Design and Construction of Foundations in Soils: General Requirements
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