Excavating around exisitng piers

[lgstructural]

I am trying to develop pier modification requirements for several exisitng bridges in a floodplain/floodway. I was initially looking at simply replacing the passive pressure lost after excavation. Some of the bridge owners may not allow any "modifications" to the piers, and want to know at what depth can we excavate to without affecting the integrity of the exisitng piers. Even with one foot of excavation I have a 25% reduction in passive pressure. This seems too excessive. I am only considering lateral loading. Does anyone have any suggestions without having to do in-depth structural analysis of the existing bridges? Is the soil above the pile cap generally relied on for passive pressure? Thanks for any help!

 

[escrowe]

Typically, any material above the scour depth (+ some factor)
is neglected for passive resistance and lateral capacity calculations.
You'll have to find out how the pile cap elevation
relates to the scour elevation for each bridge location.
Hopefully there is a clue in the bridge design docs,
but scour can reach considerable depth in loose sediments
(5 to 10 feet or more).

 

[lgstructural]

These bridges were designed and constructed between 1929 and 1931. There is no indication of scour depth analysis in the as-built drawings. Most of the piles are only 22' to 25' in length and embedded 1'-6" into the pile cap. I would expect the scour depth to be deeper than that even at some of the pier locations (in a river channel).

I have started to look at the "failure wedge" and compare that with any expected lateral loads on the exisitng structure (ie: wind loads). Maybe this approach is not as conservative as only relying on the passive??

Thanks for your input escrowe.

 

[cvg]

you will need to do a pier scour analysis, which really is not that difficult. Recommend using the CSU Pier Scour equation which is described in the following:

USDOT, 1995b. Evaluating Scour at Bridges, Hydraulic Engineering Circular No. 18, Federal Highway Administration, FHWA-IP-90-017, 3rd Ed.


7 Richardson, E.V., D.B. Simons, and P.F. Lagasse, 2001, "Highways in the River, Environment,"
FHWA NHI 01-004, Federal Highway Administration, Hydraulic Series No. 6, Washington, D.C.

Hopefully, depth of scour should be above the pier cap. If not, you may want to deepen the pile cap or construct an apron or grade control structure under the bridge to limit the scour depth. Temporary bracing could be used while you excavate.

 

[escrowe]

Well generally, the geometry of the wedge will partly depend on scour depth as well.

Why are pier modifications needed at this time?
Why excavate?

Hopefully the pile caps were designed to transfer the entire lateral thrust from the piers to the piles.
Often (not always!), the entire pile cap can be excavated without significantly diminishing lateral capacity.

In estimating a failure wedge based on the pile/soil movement under lateral load,
you would still be required to estimate the depth of scour;
anything above this level is ignored.

I expect the piles are embedded in a sound bearing layer below any loose bottom sediments.
(Of course, over time the depth to this layer may have increased in some areas,
due to increased scour effects at the pier locations!)

You may need soil borings to determine the nature of the river bottom materials, particularly the thickness, particle size and density of the loose sediment layer, and the depth to (hopefully) rock or some indurate layer.

You can then complete a lateral analysis for the piles
based on pile dimensions, strength of the concrete, and soil characteristics,
but not knowing the scour depth will continue to be a problem.

 

[lgstructural]

We are doing a scour analysis on the proposed bridge pier modifications. I believe there is some data available of exisitng scour at the exisitng bridge piers.

The excavation is for a proposed re-routing of the river and a lake. We are only trying to find out a limit of excavation before bridge pier mods are required. Our initial analysis was only relying on the passive pressure as we are only concerned with lateral forces - we do not know what the piers were designed for and want to stay away from that as well, we will not be adding any additional vertical load. The idea is to give back the lateral resistance of the pile/soil system that is loast after excavation. Thanks for your comments.

 

[escrowe]

Sounds like a good project. Good luck!