Adequacy of Geotechnical Report - Pile Bridge Foundations

[NLEng1]

Hi Everyone,

I have been requested by a contractor to provide a sub-structure design for a small design-build bridge replacement project in Atlantic Canada. The tender package included a geotechnical report which was tailored towards shallow foundations; however, the client has requested that bored concrete piles be used instead. The geotechnical report is fairly generic with borehole logs (13m deep), SPT values, and soil layer classification. The boreholes primarily indicate a sandy gravel mixture with few cobbles and boulders (ie, glacial till)without encountering bedrock. I am concerned that critical data and interpretation has been omitted such as: shaft friction, end bearing resistance, adhesion freezing forces (ad-freeze), etc. Not to mention recommendations from a qualified geotechnical engineer on minimum embedment length, number of piles, spacing, construction method (dry, casing, slurry, etc.), etc.

I am a civil/structural engineer with a good understanding of geotechnical, and I am able to deduce the much of the required information from the SPT-N values based on Meyerhof, soil descriptions, etc. But regardless, I feel that this information should come from a geotechnical report.

FYI, the Canadian Highway Bridge Design Code (6.7.3) re-iterates many of the same items which should be included in a geotechnical report.

Initially, the contractor agreed that they have had similar concerns from other consultants. However, my resistance to provide them a design based on the current report has resulted in them seeking out another consultant who will provide a design based on the current report.

I am very curious on what others would do in a similar situation?

Thanks!

 

[BigHarvey]

You mention that originally the bridge should have been supported by shallow footings. The owner wants a piled structure and the reason is certainly for protection against scouring. The only important information, which should be part of the report, is the possible depth affected by scour.

 

[Mad Mike]

It happens all the time- geotechnical services tailored toward a concept that later changes. The original geotechnical consultant should be re-commissioned to provide an assessment of the bored pile option now being requested.

Are you not in a position to request this re-assessment? Usually a small fee that goes a long way!

All the best,
Mike

 

[oldestguy]

Sounds like the geotech writing the report was not given any specific as to what the site may need, such as resistance to scour, possibly a utility trench being dug near the bridge, etc. Since that person is most familiar with the site and the borings, he (she) should be asked for more alternative recommendations, including piles. I see no reason for getting another geotech involved. While you are at it indicate he contractor would like such and such piles, etc. and what would be the comments for them? If the geotech recommended footings due to least cost, mention that higher cost for more security is OK. Chances are a professional should be able to do this at no charge. Gotta keep the clients happy should be the attitude.