Road and Highway Geotechnical Investigation: What NHAI and State PWD Projects Require

India is in the middle of one of the largest road construction programmes in its history. NHAI’s ambitious Bharatmala Pariyojana is building tens of thousands of kilometres of national highways. State governments are upgrading state highways and expressways. And in the Delhi NCR region specifically, signature infrastructure projects — the Dwarka Expressway, the Delhi-Mumbai Expressway (NH-148N), the Urban Extension Road (UER II), the FNG Expressway, and multiple elevated corridors — are reshaping the region’s connectivity.

Every one of these projects begins with a geotechnical investigation. But road and highway geotechnical investigation is a specialised discipline that is different in important ways from building foundation investigation. It covers a linear alignment, addresses a different set of engineering problems (pavement design, embankments, cuttings, bridges, culverts), and must comply with specific IRC and NHAI standards rather than building codes.

This blog explains what a highway geotechnical investigation involves, what NHAI and State PWD projects require, and how Terratech Engineers delivers compliant investigation services for road and infrastructure projects across Delhi NCR and beyond.

Why Highway Geotechnical Investigation Is Different

A building geotechnical investigation is essentially a point investigation: you study the ground at a specific location and at depth below that location. A highway geotechnical investigation is a corridor investigation: you must characterise the ground along an alignment that may be tens or hundreds of kilometres long, at varying depths depending on what is being built at each point.

The engineering questions are also different. For a highway project, the geotechnical investigation must answer:

• What is the subgrade soil strength (CBR — California Bearing Ratio) for pavement design?
• Are the embankment materials on site suitable for use as fill, or must imported fill be used?
• Are there any soft compressible soils along the alignment that will cause unacceptable embankment settlement or instability?
• Are there any slope stability problems in proposed cut sections?
• What are the foundation conditions at bridge, culvert, and structure locations?
• Is there a risk of pavement failure from expansive or frost-susceptible soils?
• Are there any underground hazards (mine workings, karst cavities, buried utilities) along the alignment?

Key Components of a Highway Geotechnical Investigation

Desk Study and Aerial Survey

A highway geotechnical investigation begins with a desk study of existing geological maps, soil survey maps, satellite imagery, and historical land use data along the alignment. Drone aerial survey of the entire corridor provides the topographic base data and identifies visible ground features — old landslides, waterlogging, existing embankments — that should guide the field investigation programme.

Subsurface Exploration Along the Alignment

IS 1892 and IRC:SP:105 specify the minimum investigation requirements for highway projects. Boreholes or trial pits are located at regular intervals along the alignment, at bridge and culvert locations (where deeper investigation is needed), at proposed cut and fill slopes, and at any locations where the desk study has flagged potential problems.

Typical spacing per IRC:SP:105: boreholes at 250 m intervals in uniform terrain, 50 m intervals in variable terrain or near structures. Depth: typically 3 to 6 m below subgrade for pavement design, and structure-specific depths for bridges and major culverts (often 20–45 m).

CBR Testing for Pavement Design

The California Bearing Ratio (CBR) is the fundamental soil parameter for flexible pavement design in India (IRC:37). It measures the resistance of the subgrade soil to penetration, expressed as a percentage of the resistance of standard crushed stone. CBR can be measured in the field (in-situ CBR) or in the laboratory on compacted samples at the design moisture content and density.

For NHAI projects, laboratory soaked CBR tests are typically required for all distinct subgrade soil types encountered, at the design compaction level and worst-case moisture condition. The design CBR is usually taken as the 90th percentile (i.e., only 10% of test values are below this) to account for natural variability.

Proctor Compaction Tests for Earthwork Specification

All earthwork in a highway project — embankment fill, subgrade preparation, back fill behind structures — requires specification of compaction standards. The Modified Proctor test (IS 2720 Part 8 / IRC:36) gives the maximum dry density and optimum moisture content for each material. Field compaction is then checked against 95 to 98% of this maximum dry density during construction quality control.

Expansive Soil Identification

Black cotton soils (expansive clays) are encountered in several areas within the Delhi NCR periphery, particularly in parts of Haryana and Rajasthan. These soils swell when wet and shrink when dry, causing severe pavement distress if not treated. Atterberg limits, free swell index (IS 2720 Part 40), and swell pressure tests identify expansive soils and determine the treatment needed (lime or cement stabilisation, replacement with non-expansive material, or isolating the pavement from the expansive subgrade with a granular capillary cut-off).

Embankment Stability Analysis

For high embankments (above 6 metres) or embankments over soft ground, a slope stability analysis is required using the shear strength parameters of the embankment fill and the foundation soil. This is particularly important along the Yamuna floodplain sections of NCR expressways, where soft alluvial soils under a high embankment can fail by circular sliding if the embankment is raised too quickly.

Bridge and Structure Investigation

Bridge foundations require the most thorough investigation on a highway project. At each bridge location, a minimum of 4 boreholes (one per abutment and one per major pier location) are required, drilled to the anticipated pile tip depth plus at least 10 pile diameters below. Full laboratory testing including bearing capacity analysis, settlement prediction, and liquefaction assessment is required.

NHAI-Specific Requirements for Geotechnical Reports

For NHAI projects under the Bharatmala programme or NH development contracts, geotechnical investigation must comply with:

• IRC:SP:105 — Guidelines for Geotechnical Investigation for Highways
• IRC:78 — Standard Specifications and Code of Practice for Road Bridges (Foundation and Substructure)
• IRC:37 — Guidelines for the Design of Flexible Pavements
• IRC:36 — Recommended Practice for the Construction of Earth Embankments for Road Works
• IS 1892, IS 2131, IS 2720 (various parts) for field and laboratory testing
• MORTH (Ministry of Road Transport and Highways) Specifications for Road and Bridge Works — Section 2000 for earthwork

NHAI project contractors and DPR consultants who submit geotechnical reports not compliant with these standards face rejection at the approval stage and significant contract delays. Reports must be signed by a qualified geotechnical engineer and supported by NABL-accredited laboratory test certificates.

State PWD Requirements in Delhi NCR States

State PWD projects in Uttar Pradesh, Haryana, and Delhi have their own investigation standards, broadly aligned with IRC codes but with state-specific administrative requirements:

• Uttar Pradesh PWD: investigations must comply with IRC:SP:105 and UP standard specifications; investigation reports for major structures require approval by the Chief Engineer (Designs) or their designated technical committee
• Haryana PWD and HSVP: similar IRC compliance requirements; projects in Gurugram often require additional hydrogeological data given the complex groundwater conditions in the Aravalli zone
• Delhi PWD and DMRC: urban highway and metro infrastructure projects require investigation compliant with both IRC standards and MoRT&H specifications, with additional requirements for survey of existing underground utilities along the alignment

Terratech Engineers: Highway and Infrastructure Geotechnical Investigation

Terratech Engineers provides complete geotechnical investigation services for road, highway, and infrastructure projects across Delhi NCR and the wider UP-Haryana region. Our services include corridor borehole drilling, CBR testing, Proctor compaction testing, bridge structure investigation, embankment stability analysis, and preparation of IRC and NHAI-compliant geotechnical investigation reports.

We work with DPR consultants, EPC contractors, project management consultants, and government departments to deliver investigation programmes that are properly scoped, efficiently executed, and produce reports that pass technical scrutiny at approval stage.