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Geotechnical Drilling for Highway & Bridge Foundation Projects in the US

By Meta Drill f 16, 2026

Why Highway and Bridge Investigations Need a Different Plan 

Transportation projects are long, narrow, and full of transitions. Conditions can change between landforms, across a floodplain, at the edge of historic fills, or where a roadway passes from a cut into an embankment. Bridges concentrate high loads at a relatively small number of substructures, while approach fills spread new loads across a much wider footprint. 

The field program also has to work around live traffic, limited right-of-way, utilities, waterways, rail corridors, wetlands, steep slopes, and neighboring properties. 

My view is that the best investigations are designed backward from the decisions the engineer must make. 

Build a Ground Model Before the Rig Arrives 

Drilling should test a preliminary model, not create one from nothing. Start with available geology, topography, historic aerial imagery, LiDAR, water-well records, nearby boring logs, as-built plans, old bridge records, and previous roadway reports. Review the proposed profile, substructure locations, cut-and-fill limits, wall alignments, drainage features, and known utility corridors. 

Walk the site with the civil, structural, hydraulic, environmental, and geotechnical leads when practical. 

The reconnaissance should divide the corridor into geotechnical units. 

FHWA’s subsurface investigation program encourages using drilling alongside in-situ testing and geophysics to establish appropriate soil and rock parameters. 

Put Borings Where the Structure Actually Loads the Ground 

Bridge borings normally follow the substructure layout. Each abutment and pier needs coverage appropriate to its width, load, foundation concept, and subsurface variability. If a pier sits in a channel, the location should also support the hydraulic and scour model. Where rock elevation changes sharply, a second boring or a geophysical line can be far more valuable than pushing one hole much deeper. 

FHWA Federal Lands guidance provides a useful planning reference: at least one boring for a pier or abutment under 100 feet wide and at least two for those over 100 feet, with added borings in erratic conditions. 

Set Boring Depth From the Failure Mechanism 

“Drill to 100 feet” is not a technical scope. A depth should be linked to the zone that influences bearing, settlement, axial resistance, lateral response, scour exposure, slope stability, or construction. 

For a spread footing, the boring must extend through the stress influence zone and any compressible layer that could contribute meaningful settlement. For a pile group, exploration should continue below the anticipated tip far enough to confirm the bearing stratum and identify weaker material beneath it. 

FHWA Federal Lands examples call for deep-foundation borings in soil to extend at least 20 feet below the estimated tip or twice the maximum pile-group dimension, whichever is greater. 

Match the Drilling Method to the Evidence Needed 

No single method is best across an entire highway corridor. The rig and tooling should recover the samples the engineer needs while keeping the borehole stable and production reasonable. 

Hollow-stem auger 

Hollow-stem auger drilling is common in soils because it advances without circulating drilling fluid and provides a protected path for SPT or tube sampling. It works well in many cohesive and granular formations above severe cobbles or hard layers. 

Sampling and Testing: Protect the Design Data 

The drilling log should capture more than soil names and SPT blows. Record method, tooling, casing, sampler, hammer system, penetration rate, losses, heave, groundwater, recovery, refusal, drilling response, and anything unusual. Coordinates and ground elevations need survey-quality control appropriate to the project. 

  • SPT samples at planned intervals and at significant stratum changes 

  • Thin-wall tube or other relatively undisturbed samples in cohesive soil 

  • Continuous core in rock, with recovery and RQD 

Scour Changes the Foundation Condition 

For bridges over water, the geotechnical, hydraulic, and structural teams must work from the same assumed scour elevations. Scour is not simply another load. It removes supporting material around a foundation and changes the ground profile used for axial and lateral resistance. 

That has two direct consequences for drilling. First, borings need to extend below the estimated scour depth. Second, the log must distinguish channel deposits, armoring, erodible soil, rock, and layers that could control contraction or local scour behavior. 

FHWA’s scour guidance stresses coordination across hydraulics, geotechnical engineering, and structural design. 

Plan Traffic, Utilities, Access, and Environmental Controls Together 

Field logistics should be resolved while the drilling plan is still flexible. A high-quality boring at the perfect plan location is useless if the crew can’t occupy it safely. 

For work in or beside traffic, coordinate lane closures, shoulder closures, barrier requirements, work hours, ingress, egress, and the rig’s swing or mast envelope. OSHA identifies struck-by, caught-between, electrical, and fall hazards in highway work zones and points users to the MUTCD for traffic-control guidance. 

Before drilling, arrange utility clearance and reconcile markings with records, visible features, and proposed boring coordinates. 

A Pre-Mobilization Checklist 

Before the first geotechnical drilling rig leaves the yard, confirm: 

  1. The current bridge layout, profile, foundation concepts, wall limits, and earthwork geometry are available. 

  1. Every exploration has a stated design purpose, target depth, and stop criterion. 

  1. State DOT, owner, federal-aid, and project-specific criteria have been reconciled. 

  1. Scour, seismic, settlement, slope, and groundwater questions are reflected in the scope. 

  1. The drilling method and sampler match the expected formation and required laboratory testing. 

  1. Traffic control, utilities, overhead hazards, access, railroad, environmental, and right-of-entry needs are cleared. 

Frequently Asked Questions 

How many borings are needed for a highway bridge? 

There is no universal number. Boring quantity depends on the number and width of piers and abutments, foundation type, ground variability, scour, loads, and owner requirements. FHWA Federal Lands guidance gives minimum examples tied to each substructure, but state DOT and project criteria may require more. 

How deep should bridge foundation borings go? 

They should extend below the zone that affects bearing, settlement, axial and lateral resistance, scour, or stability. For deep foundations, that normally means drilling below the estimated tip and confirming the supporting layer. Rock-supported foundations need enough core to characterize the socket or bearing zone and weaker material beneath it. 

Which drilling method works best for bridge investigations? 

It depends on the formation and samples required. Hollow-stem auger is common in many soils, mud rotary helps stabilize deeper water-bearing holes, sonic or casing systems can handle difficult fill and gravel, and wireline coring is used in rock. A project may use more than one method. 

Why are approach embankments investigated separately? 

Approach fills can load a much wider area than the abutment and may sit over soft or compressible soil. Settlement, lateral movement, and downdrag can occur even when the bridge foundation itself is competent. Separate borings and high-quality samples help evaluate those risks. 

What should buyers ask drill rig manufacturers about highway work? 

Ask how the proposed rig will perform each required drilling and sampling method, fit within traffic and access limits, mobilize legally, operate in the expected climate, and receive parts or technical support. Request customer references for comparable transportation projects and ground conditions. 

You can also check: Frost-Line Drilling: Site Investigation Techniques for Cold-Climate Regions 

Better Foundations Start With Better Evidence 

Good drilling doesn’t eliminate geotechnical uncertainty. It makes that uncertainty visible early enough to manage. 

For US highway and bridge projects, the strongest program connects every boring to a structure, load, hazard, or construction decision. It uses the right mix of borings, in-situ tests, geophysics, samples, and monitoring. It also treats access, traffic, utilities, and field quality as part of the engineering—not separate administrative tasks. 

If you’re evaluating Meta Drill geotechnical drilling equipment for transportation work, start with the project profile: expected depths, soil and rock conditions, sampling methods, access envelope, climate, transport limits, and support needs. MetaDrill can help compare rig configurations for bridge foundations, highway earthwork, retaining structures, drainage crossings, and other North American infrastructure investigations. Visit MetaDrill North America to discuss the field requirements behind the specification sheet.