Tremie method of pier placement and distance between piers.

[hines86]

I have a project with all around bad soil conditions, high water table. I need a 75" diameter concrete pier to handle the bearing pressure. I currently have it at a depth of 18' going deeper does not get me much in terms of friction resistance, so the larger bearing area is driving the design. The distance between the piers is 12'-0" - so 1.92D. I am specifying Tremie method of placement. From what I understand from the process they drill the hole then place the casing and rebar. Please correct me if I am wrong. From what I gather behind the philosophy associated with pier spacing the less compactable soil needs the higher 3D spacing because of the overflow into adjacent soil causing lift pressure. Since they will be drilling I will not have as high of a concern over compaction of the soft clay/silt/sand as it will be removed/ compacted. So I feel that the 2D spacing that I have will be sufficient. Can some of the veteran engineers please weigh in on this thought process?

 

[SlideRuleEra]

The 3D spacing is important for driven displacement friction piling for the reasons you stated. Your description sounds like the piers will be point bearing. Properties of the soil that the piers bear on are more important than spacing. If the supporting soil is good enough, 2D spacing should be ok.

A properly used tremie should be fine. Casing and rebar are placed before concrete.

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[EngineeringEric]

if you are relying on bearing not friction, then the piers are almost acting like really deep spread footings down to suitable soil...2d spacing would be sufficient.

As an option, would ground improvement (rammed aggregate piers or vibro piers or similar) be a possible solution to your problems? don't need to answer but it may be worth exploring or asking your geotech if you are not certain.

 

[wannabeSE]

Why don't you call the geotechnical engineer for the project. They know the specifics of the soil for the project.