If you’re designing a boardwalk, you’ve probably got a geotechnical report sitting on your desk. It tells you what’s below the surface and what the soil can support. It won’t tell you what size foundation to use, and it won’t give you one soil capacity number that works for everything. What you need from it depends on the foundation you’re actually designing.
In this episode of Ask a Boardwalk Engineer, John Pyle, PE and PermaTrak engineer Kyle Stanton go through four common boardwalk foundation types and the specific geotech information each one needs.
Who Does What: Geotechnical vs. Structural Engineer
The geotechnical engineer drills borings, collects and tests soil samples, and recommends foundation types that suit the site. The structural engineer takes that information and designs the foundation itself, whether it’s a cast-in-place shallow footing, a driven steel pile or anything in between. Together, that work determines the foundation type, its size and depth, and the spans for the boardwalk beams and piers.
If the report recommends testing during construction, make sure your specs say who’s responsible for it. Otherwise the testing can easily fall through the cracks.
What Each Foundation Type Needs from the Report
Spread footings and precast concrete piers. Look for the allowable bearing pressure and the soil coefficients, like a sliding coefficient. Bearing pressure sizes the footing, and the coefficients let the structural engineer design for lateral capacity. Someone should also confirm in the field that the soils match what the design assumed. Cone penetrometer testing or an inspection by a qualified engineer both work.
Helical piers. Allowable bearing pressure matters less here because helicals behave as a deep foundation. The helical pier designer needs the boring logs with blow counts (N-values) to set the right installation depth. The first thing to ask is whether you have the actual boring logs.
Drilled shafts and auger cast piles. Look for side skin friction, end bearing pressure, or both. Some geotechs let you use both values together, and some prefer you pick one. Relying mostly on skin friction usually means a deeper foundation. Relying on end bearing drives the size of the foundation.
Driven piles. These need their own pile driving criteria, usually from a wave equation (WEAP) analysis, with PDA testing in the field to verify capacity. The analysis is based on a specific hammer and cushion, so if the contractor switches hammers, the driving criteria change too.
|
Foundation Type |
What You Need from the Geotech Report |
|
Spread footings & precast piers |
Allowable bearing pressure + soil coefficients (sliding/lateral) |
|
Helical piers |
Boring logs with blow counts (N-values) |
|
Drilled shafts & auger cast piles |
Side skin friction and/or end bearing pressure |
|
Driven piles |
Pile driving criteria (WEAP analysis), verified with PDA testing |
A Simple Check Before Your Design Is Final
Pull up your plans and note the foundation type you’re showing. Then find the recommendation in your geotech report for that exact foundation type. If it isn’t there, work it out with your geotechnical engineer before you finalize the foundation design.
As Kyle puts it, “Having a geotech report isn’t enough. You’re going to need the right information for the foundation you’re actually using.”
If you’re working on a boardwalk project and aren’t sure your geotech report has what your foundation needs, reach out to our engineering team. We’re happy to help.
