How to Write One That Actually Protects You When the Driller Hits “Differing Site Conditions”

Every contractor’s nightmare: the bid assumed 80-ft straight augering in medium clay, the boring logs looked clean, and on day four the drill rig is bouncing on glacial boulders the size of pickup trucks. The owner shrugs and says “changed conditions are your risk.” If the contract had a real Geotechnical Baseline Report instead of a 15-page Data Report and a boilerplate disclaimer, that conversation never happens.

A GBR is the only contract document that legally defines what subsurface conditions are “expected” and therefore priced into your bid. Anything materially worse triggers a Type I Differing Site Condition claim and gets you paid for the extra time and money. Write it wrong and the owner will gut you in arbitration. Write it right and you sleep at night.

The Difference Between a GDR and a GBR (Most People Get This Wrong)

A Geotechnical Data Report (GDR) is just the raw facts: boring logs, lab results, historical data, and a big fat disclaimer that says “use at your own risk.” A Geotechnical Baseline Report (GBR) takes that data and draws a line in the sand: “This is what we all agree the contractor priced.” Baselines must be clear, measurable, and enforceable.

FHWA, ASCE, and the U.S. National Committee on Tunneling Technology all say the same thing: if it’s not in the GBR, it’s not baselined, and the contractor eats the risk.

The Six Deadly Sins That Kill Contractors in Court

1. No GBR at all — just a GDR and “interpret for yourself.”
2. Vague baselines (“boulders may be present”).
3. Baselines hidden in the spec book instead of the contract.
4. Conflicting statements between drawings, GBR, and GDR.
5. Owner-written GBR that baselines the absolute worst-case condition.
6. Failure to update the GBR after Value Engineering or design changes.

Fix those six and you’ve already beaten 90 % of the bad GBRs out there.

The Essential Baseline Statements Every GBR Must Have

Write these in plain English, put them in a standalone section titled “Baselines,” and make every parameter measurable.

1. Soil and Rock Classification “All material above elevation –45 ft is anticipated to be fat clay (CH) with SPT N-values between 4 and 12. Cobbles and boulders larger than 12 inches are not anticipated above elevation –45 ft in more than 5 % of the shaft volume.”
2. Obstructions and Boulders “Boulder frequency shall not exceed one boulder > 3 ft³ per 200 ft³ of excavation. No single boulder shall exceed 15 ft³.”
3. Groundwater “Static groundwater is baselined at elevation +3.5 ft. Inflow into any excavation below elevation –10 ft shall not exceed 50 gpm total from all sources.”
4. Stand-Up Time and Raveling Ground “Clay above elevation –20 ft is anticipated to stand vertical and unsupported for minimum 72 hours.”
5. Drivability and Installation Parameters “Driven steel H-piles are baselined to reach tip elevation with a Delmag D46-32 diesel hammer operating at 80 % efficiency and achieving set of 10 blows per inch at practical refusal.”
6. Karst and Voids “No solution features or voids larger than 2 ft in diameter are anticipated within the pile embedment zone.”

Who Writes the GBR and Who Should Review It?

Best practice (and the one that wins claims): the owner’s geotechnical engineer writes the GBR, but the contractor gets formal review and comment before bid. Anything the contractor flags as unpriceable gets fixed or removed from baseline. Worst practice: owner hands over a sealed GBR with the bid docs and says “take it or leave it.”

The Nuclear Option: Mutual Waiver of DSC Claims

Some owners now insert language that says “the GBR represents the absolute worst conditions anticipated and there shall be no differing site condition claims.” That is legal in most states, but contractors who sign it deserve what they get.

The Contractor’s Checklist Before You Sign the Contract

• Is there a standalone GBR (not just a GDR)?
• Are baselines clear, quantifiable, and measurable?
• Did your estimator actually read every baseline and price the risk accordingly?
• Did you submit written comments during bid if something was unpriceable?
• Is the GBR incorporated into the contract by reference and given contractual precedence over the GDR?

Miss any of those and you just volunteered to eat the risk.

Bottom line: a good GBR is the best insurance policy a contractor never has to file. A bad GBR is a loaded gun pointed at your bonding capacity. Demand one, read it like your livelihood depends on it (because it does), and never bid a job where the owner says “we don’t do baselines.” The mud doesn’t care about your profit margin, but a tight GBR makes the owner care a whole lot.