Site investigation for road construction

Site investigation for a road finds the ground conditions along the alignment so that the road is safe, economical and stable. It is done in stages.

1. Reconnaissance and desk study

• Collect topographic maps, geological maps, aerial photographs, satellite images and earlier reports.
• Walk over possible routes to compare them and avoid landslides, steep slopes, marshes and fault zones.

2. Preliminary survey

• Detailed engineering geological mapping of the corridor: rock types, structures, weathering, slope angles, springs, existing landslides.
• Aerial photograph interpretation for lineaments, drainage and unstable areas.
• A few test pits and shallow boreholes at key locations.

3. Detailed investigation of the chosen alignment

• Test pits and trenches every 200 to 500 m or at changes in soil type to find soil type and depth of rock; sample for laboratory tests (grain size, Atterberg limits, CBR, compaction).
• Boreholes (auger/rotary) at bridge sites, high embankments, deep cuttings and landslide areas to get strata, groundwater and rock quality.
• Geophysical methods: electrical resistivity and seismic refraction for depth of bedrock, water table and buried slide surfaces.
• In-situ tests: SPT, plate load test, field CBR.
• Slope stability study by kinematic analysis of joints and bedding relative to the cut face.

4. Material survey

• Locate quarries for stone and aggregate, borrow pits for fill, and sand and gravel sources; test for quality and quantity.

5. Reporting

• Prepare strip maps, longitudinal geological sections, and recommendations on cut slopes, retaining structures, drainage and bioengineering.

6. Construction stage

• Inspect cuttings and foundations, and revise treatment if conditions differ.