IS 1892:1993 is the Indian Standard (BIS) for subsurface investigation for foundations. This code provides comprehensive guidelines for subsurface investigations required for the safe design and construction of foundations. It specifies the necessary number, spacing, and depth of boreholes, alongside appropriate methods for soil sampling and groundwater monitoring. Proper adherence ensures an accurate understanding of the subsoil profile and minimizes geotechnical risks during construction.
Lays down the procedure for subsurface investigation for various types of foundations.
Key parameters for planning subsurface investigations, including borehole depth, spacing, field test criteria (SPT, CPT), and sample quality ratios.
| Reference | Value | Clause |
|---|---|---|
| Min. Exploration Depth, Isolated Footing— Measured from the proposed foundation level. | 1.5 x Footing Width | Cl. 4.2.2 |
| Min. Exploration Depth, Raft Foundation | 1.5 x Raft Width | Cl. 4.2.4 |
| Min. Exploration Depth, Pile Group— Measured from the level of the pile tips. | 1.5 x Group Width | Cl. 4.2.5.1 |
| Borehole Spacing, Multi-storey Bldg | 15 m to 30 m | Cl. 4.3.1.2 (a) |
| Borehole Spacing, Highways— For preliminary investigation along the alignment. | 300 m to 500 m | Cl. 4.3.1.2 (c) |
| Borehole Spacing, Small Residential— For buildings up to 2 storeys. | 30 m | Cl. 4.3.1.2 (d) |
| Max. Depth of Trial Pit (Unsupported)— May be extended to 6 m with shoring. | 3 m | Cl. 5.2.1 |
| SPT Hammer Weight | 63.5 kg | Cl. 6.3 (ref IS 2131) |
| SPT Hammer Fall Height | 750 mm | Cl. 6.3 (ref IS 2131) |
| SPT Split Spoon Sampler ID | 35 mm | Cl. 6.3 (ref IS 2131) |
| SPT Split Spoon Sampler OD | 50.8 mm | Cl. 6.3 (ref IS 2131) |
| CPT Cone Base Area | 10 cm² | Cl. 6.5 (ref IS 4968 P3) |
| CPT Rate of Penetration | 2 cm/s | Cl. 6.5 (ref IS 4968 P3) |
| Plate Load Test Pit Width | ≥ 5 x Plate Width | Cl. 6.7 (ref IS 1888) |
| Plate Load Test Seating Load— Equivalent to 70 g/cm². | 7 kPa | Cl. 6.7 (ref IS 1888) |
| Undisturbed Sample Area Ratio (Ar)— For good quality samples from thin-walled samplers. | < 10% | Cl. 7.3.1.1 |
| Undisturbed Sample Inside Clearance (Ci) | 1% to 3% | Cl. 7.3.1.2 |
| Undisturbed Sample Outside Clearance (Co) | 0% to 2% | Cl. 7.3.1.3 |
| Rock Quality Designation (RQD) Def.— L_total = Total length of core run. | Σ(L of core >10cm) / L_total | Cl. 7.4.2 |
| RQD Classification - Excellent Rock— Based on Deere's classification, commonly used with this code. | > 90% | Cl. 7.4.2 |
| Min. GWL Observation Period— After stabilization in borehole. | 24 hours | Cl. 8.1 |
BIM-relevant code. See the BIM Hub for ISO 19650, IFC, and LOD/LOIN frameworks used alongside it.
IS 1892 is the code of practice for subsurface investigation for foundations — the master framework for planning, executing, and reporting geotechnical investigation for any building, infrastructure, or industrial foundation in India. It governs borehole spacing, depth, sampling methods, in-situ testing density, lab test programme, and report content.
Use IS 1892 when: - Planning a soil investigation for any building (residential, commercial, industrial) - Setting investigation density for highway / bridge / dam projects - Reviewing geotechnical reports submitted by consultants - Specifying scope of work for borehole drilling contracts - Audit / dispute resolution on inadequate investigation
The 1993 revision modernised the practice over the 1979 edition with current SPT, sampling, and lab-test references.
Standard investigation phases: 1. Reconnaissance — desk study, geological maps, walk-over 2. Preliminary investigation — limited boreholes for site characterisation 3. Detailed investigation — full borehole programme + lab tests 4. Verification — additional boreholes / tests if anomalies found 5. Report — comprehensive geotechnical report with foundation recommendation
Investigation density (Clause 5):
| Project type | Borehole spacing | Boreholes per project (typical) | |---|---|---| | Light residential (< 3 storeys) | 1 per 250 m² OR per 30 m | 1-2 | | Multi-storey residential / commercial | 1 per 100-150 m² | 3-6 | | Industrial / heavy load | 1 per 50-100 m² | 5-10 | | Highway (2-lane) | every 300-500 m + every soil change | per project length | | Highway (4-lane / expressway) | every 100-200 m | per project length | | Bridge | 1 per pier + 1 per abutment + 1 per approach | 3-10 per bridge | | Tank / silo | minimum 2-3 boreholes | per facility |
Borehole depth (Clause 6): - Minimum 1.5 × footing width below founding level - For footings: minimum 5-8 m below natural ground (low-rise); 10-15 m (multi-storey) - For piles: ≥ 1.5 × pile length OR to refusal - For tall buildings: ≥ 30 m - For dams / large tanks: ≥ 1.5 × structure height - Until: refusal in rock; OR 5 m of competent stratum below founding
SPT cadence: - Every 1.5 m down the borehole (continuous in critical strata) - At every soil change - Per IS 2131:1981
Sampling: - Disturbed: SPT split-spoon at every 1.5 m - Undisturbed: Shelby tube every 3 m in cohesive layers OR per critical stratum - Rock cores: NX size in rock-bearing strata
Lab tests per stratum (typical): - Index: moisture, gradation (IS 2720 Part 4), Atterberg (Part 5), specific gravity, classification (IS 1498) - Strength: UCS (Part 10), triaxial UU/CU - Compressibility: consolidation - CBR (Part 17) for pavement subgrade
Investigation report content: - Site location + layout - Boring + sampling programme - Soil profile per borehole + cross-section - Lab test results - Allowable bearing pressure (IS 6403:1981) - Settlement estimate (IS 8009) - Foundation recommendation (shallow / pile / raft) - Construction-stage advisories
1. Single borehole for entire project. Soil heterogeneity ignored; design optimistic. Multiple boreholes per IS 1892 spacing. 2. Inadequate borehole depth. Stops at first competent stratum without verifying ≥ 5 m of it. Continue to design depth. 3. No cross-sections. Soil profile per BH only; lateral variation invisible. Plot cross-sections between BHs. 4. No undisturbed sampling in cohesive layers. UCS / triaxial impossible without good sample. Specify Shelby tube sampling. 5. Lab tests only on top stratum. Lower strata characterised by SPT only; strength uncertain. Lab test on every major stratum. 6. No water-table observation. Affects bearing + settlement + buoyancy. Record water level during + 24 hr after boring. 7. No SPT energy correction noted. Per IS 2131 recommendation; missing makes design incomparable across sites. 8. Report missing foundation recommendation. Just data; designer must interpret. Recommendation needed. 9. Old report (> 5 years) reused. Soil conditions can change (water table, settlement). Fresh investigation for new project. 10. Boreholes outside building footprint. Soil there ≠ soil under building. BHs within or at edge of footprint.
Project investigation cascade:
1. Project brief — building type, loads, layout. 2. Reconnaissance — desk + walk-over. 3. Investigation plan (IS 1892:1993) — BH count, depth, sampling, in-situ tests. 4. Field work — drilling, SPT, sampling, water table observation. 5. Lab testing — index + strength + consolidation per stratum. 6. Synthesis: - Soil profile + cross-sections - Bearing capacity per IS 6403 - Settlement per IS 8009 - Foundation type recommendation 7. Report — comprehensive geotechnical report. 8. Foundation design — by structural engineer using report. 9. Construction monitoring — settlement plates, inclinometers.
IS 1892 sets the technical baseline; the quality of investigation directly determines the safety + economy of foundation design. Skimping on investigation is the cheapest false economy in construction — saving ₹1 lakh on investigation can cost ₹50 lakh in remedial work.