India's mountainous and hilly areas, the Himalayas, and the Western Ghats are marked by highly diversified geology, rugged slopes, and adverse climatic conditions. Construction of roads in these areas demands a strong perception of the subsurface conditions to maintain safety, stability, and durability.
Geotechnical surveys are the foundation of hill road construction and design, as discussed in IRC:SP:19-2001 "Manual for Survey, Investigation and Preparation of Road Projects in Hill Roads" and related IS codes (IS 2720 series, IS 1498, IS 12070, etc.). The Ministry of Road Transport and Highways (MoRTH Specifications, Section 300 & 700) also requires site investigation to be done properly before hill road work.
This paper gives a comprehensive account of the aims, methods, tests, and equipment used for geotechnical investigations in hill road works in India.
Geotechnical surveys are the foundation of hill road construction and design, as discussed in IRC:SP:19-2001 "Manual for Survey, Investigation and Preparation of Road Projects in Hill Roads" and related IS codes (IS 2720 series, IS 1498, IS 12070, etc.). The Ministry of Road Transport and Highways (MoRTH Specifications, Section 300 & 700) also requires site investigation to be done properly before hill road work.
This paper gives a comprehensive account of the aims, methods, tests, and equipment used for geotechnical investigations in hill road works in India.
Aims of Geotechnical Investigation in Hill Roads construction in India
The primary reasons for geotechnical investigations according to IRC and IS codes are:- Determining the soil and rock parameters for foundation, slope stability, and pavement design.
- Evaluating groundwater conditions and their impact on slope stability.
- Determining landslide susceptibility zones and weak horizons.
- Design input for hill road retention structures, breast walls, and culverts.
- Selection of appropriate sources of construction materials (soils, aggregates, rock).
- Assessment of erosion vulnerability and drainage requirements.
Relevant International Standards and Guidelines for Hill Road Construction
Geotechnical work on hill road projects is regulated through the following standards and manuals :- IRC:SP:19-2001 – Manual for Survey, Investigation and Preparation of Road Projects in Hill Roads.
- IRC:52-2019 – Guidelines for the Alignment Survey and Geometric Design of Hill Roads.
- IS 1498:1970 – Soils for general engineering purposes, classification and identification.
- IS 2720 (different parts) – Tests on Soils (shear strength, permeability, compaction, etc.).
- IS 12070:1987 – Code of practice for the design and construction of shallow foundation in soils (for guidance on slope foundations).
- IS 14496 (Part 2):1998 – Guidelines for preparation of landslide-hazard zonation maps.
- MoRTH Road and Bridge Works Specifications, Section 300 & 700 – Earthworks and foundations requirements.
Different Phases of Geotechnical Investigation during hill road construction
Reconnaissance Survey
- Study of topographic sheets, geological maps, remote sensing information.
- Recognition of landslide-risk zones, unstable slopes, and drainage patterns.
- Broad sketching of rock outcrops, joint planes, bedding planes, and fault zones.
Preliminary Investigation
- Trial pits and boreholes in strategic locations along the route.
- Collecting disturbed and undisturbed samples of soil.
- First classification tests (Atterberg limits, grain size distribution according to IS 2720 Parts 4, 5, 6).
- Slope angle measurements and initial stability test.
Detailed Investigation
- Seismic refraction and resistivity investigation (optional).
- Systematic boreholes at regular spacings (usually 30–100 m spacings; actual value to be confirmed).
- In-situ tests: Standard Penetration Test (SPT, IS 2131), Plate Load Test (IS 1888), Field Density Test (IS 2720 Part 28).
- Rock coring for classification based on RQD (Rock Quality Designation) according to IS 11315.
- Monitoring of groundwater table.
- Testing of samples in the laboratory.
Special Investigation
- Landslide hazard mapping (according to IS 14496 Part 2).
- Geophysical techniques: seismic refraction, resistivity survey (IS 1892:1979 "Code of Practice for Subsurface Investigation for Foundations").
- Instrumentation for monitoring slopes (piezometers, inclinometers).
Laboratory and Field Tests to Strengthen Hill Road Construction Projects in India
Soil Classification Tests To Test Hill Area Soil
- Grain Size Distribution – IS 2720 (Part 4).
- Atterberg Limits – IS 2720 (Part 5).
- Specific Gravity – IS 2720 (Part 3).
Soil Strength Tests
- Unconfined Compression Test (UCS) – IS 2720 (Part 10).
- Direct Shear Test – IS 2720 (Part 13).
- Triaxial Shear Test – IS 2720 (Part 11).
- CBR (California Bearing Ratio) – IS 2720 (Part 16), as input for pavement design.
Soil Consolidation and Permeability Tests
- Consolidation Test – IS 2720 (Part 15).
- Permeability Test (Falling Head/Constant Head) – IS 2720 (Part 17).
Other Field Tests
- Standard Penetration Test (SPT) – IS 2131.
- Plate Load Test – IS 1888, applicable for foundation recommendations.
- Field Density Test – IS 2720 (Part 28), by core cutter or sand replacement method.
Rock Tests
- Point Load Strength Test – IS 8764.
- Slake Durability Test – (reference: IS 10082).
- Water Absorption and Porosity of Rocks – IS 1124.
Landslide and Slope Stability Investigation
Mapping and Hazard Assessment
- IS 14496 (Part 2):1998 specifies standards for landslide hazard zonation mapping in mountainous regions.
- Slope instability is examined by taking into account:
- Lithology and weathering.
- Joint spacing, orientation, and shear strength.
- Groundwater conditions.
Slope Stability Analysis Methods
- Limit Equilibrium Methods (to be cross-checked with specific codes).
- Input from direct shear and triaxial tests employed for factor of safety estimation.
Instrumentation used in Hill Road Construction
- Inclinometers – to observe slope movement.
- Piezometers – to observe pore water pressure.
- Settlement indicators – to monitor subsidence in unstable areas.
Exploration for Retaining and Breast Walls
For retaining and breast wall design in hill roads:- Earth pressure parameters from laboratory tests of shear strength.
- Backfill soil properties in accordance with IS 2720 series.
- Drainage considerations are required as per MoRTH Section 2500 (Drainage).
Groundwater and Drainage Studies
Groundwater contributes enormously to slope stability. According to IRC:SP:19-2001:- Springs and seepage lines have to be mapped.
- Piezometer readings influence design of subsurface drainage.
- Good design of cross-drainage structures is crucial for long-term stability.
Investigations of Material Sources
- Borrow pits and quarries must be checked for suitability as per MoRTH Section 100 & 300.
- Aggregate tests (IS 2386 series – crushing value, impact value, abrasion value).
- Bitumen tests in case of black-topping proposals (IS 1201–1220 series).
Reporting and Documentation
According to IRC:SP:19-2001, a geotechnical report must contain:- Project description and alignment details.
- Geological and geotechnical maps.
- Field and laboratory test results.
- Classification of soil and rocks.
- Slope, foundation, and retaining structure design recommendations.
- Recommended monitoring and maintenance.
Special Challenges Facing Hill Roads
- Irregular and steep topography making site access difficult.
- Rock weathering and severe erosion rates.
- Unstable rainfall patterns inducing slope failures.
- Earthquake-prone areas in the Himalayas (Seismic Zones IV & V according to IS 1893:2016).
