Foundation for a Jetty-- 2nd Request

[microseismic]

I posted this question previously and received one comment. I am posting it a second time in case it has been overlooked or forgotten. I am looking for suggestions for a foundation for a jetty extending out perpendicularly from the shore 100 feet into a lake.

There is a small stream of highway drainage/runoff coming into the lake in a direction that will intersect the jetty at a 90 degree angle. The steam is very slow flowing unless there is a big rainfall. The stream is approximately 200 feet away from the jetty. I was told that during severe weather the stream shoots deposits out into the boat lane and the lane must be dredged. The jetty will help to contain the deposits so that it can be easier to control. The lake bottom is located at a depth below the water surface of approximately 2 feet along the shore to about 8 feet at the tip of the proposed jetty.

Originally, the jetty was proposed to be constructed of concrete blocks along the sides and front. The blocks are 2 x 3 x 4 feet and made from concrete. The jetty will extend about 2 feet above the surface of the water. The blocks will be placed along the sides of the jetty and compacted fill will be placed in the center. The surface of the jetty will be bituminous concrete (asphalt). The jetty will extend perpendicular from the shoreline of the lake.

The subsurface investigation made it difficult to design the jetty as originally planed. The investigation shows the lake bottom subsurface is comprised a four to twenty-two foot thick deposit of natural organic silt/peat overlying loose to medium dense deposits of natural glacial outwash. Natural glacial till was encountered in one test boring at a depth below the lakebed of 26 feet. The thick layer of loose organic silt along with the underwater construction limits the type of foundations available for the construction of the jetty.

I would appreciate any suggestions for the type of foundation and the construction methods. Cost is not a problem at the moment. One option that we are considering is pressure injected footings (PIF), with outside piles placed in a pier type arrangement to control lateral movement. However, even if we were to go with PIFs, there still remains the issue of how to construct the side walls of the jetty.

I would appreciate any suggestions you may have and thank you in advance for the advice.

MS

 

[UcfSE]

You should include a reference to the former thread so that members can read what has been said so far.

See the text right below the thread title at the top. Copy and paste the "threadxxx-xxxxxx" exactly as it is from the old thread into the post when you do this and it will provide a link to the old thread.

 

[microseismic]

Sorry,
The Previous thread is
thread256-205539

MS

 

[civilperson]

Does your client own the entire lake, or are others being impacted by the proposed jetty? As to foundation design, how could settlement detract from the final product, (if still above water at final elevation)? Or why worry about it? If cost is not an issue, why not dredge surrounding lake bottom with the fill going into a silt fence area approximating the jetty dimensions? Fill to a sufficient height above the wanted elevation and wait for consolidation equal to 90% of predicted amount. Rip rap is cost effective or use articulated shore armour for erosion protection over a layer of 2" sized crushed stone.

 

[microseismic]

Thank you for the suggestion. The client has the permission for the jetty--others are not being impacted. The finished product should be a stable jetty with near vertical sides and a paved top. The idea was to use the area enclosed by the jetty as a sort of settlement basin. A backhoe can then ride on the top of the jetty for easy access to removing the accumulated settlement. Again, I appreciate the suggestion. Let me see if I understand it fully. Are you suggesting that they dredge the lake bottom through the 20 ft silt layer and then replace it with granular fill or are you suggesting placing stone on the present lake bottom and using the weight to surcharge the area to achieve the desired consolidation?

MS

 

[civilperson]

A contradicting pair of parameters was given in the last post: "near vertical sides" and a "stable jetty". To achieve both; a retaining wall type structure either reinforced concrete or mass gravity type wall at the perimeter is required. My suggestion was to fill on the existing silt layer with dredged material, (fill could also be silt), from the adjacent lake bottom. Build the jetty with an excess of height as a surcharge fill and wait for the consolidation to occur. If the consolidation must be accelerated, install a series of wick drains. A stable slope on the perimeter will probably be 5:1 slope or flatter. My design was only for the jetty proper, no designed change in lake bottom other than depth.

 

[csd72]

Have you checked out the coastal engineering forum, this is more up their alley.

 

[microseismic]

Thanks for the advice,

No one expected to hit the thick layer of silt, so it definitely throws a monkey wrench into the workings. If the jetty is to work as desired, it needs near vertical walls.

I'll will try a duplicate post on the coastal engineering forum. I could use all the advice and suggestions that I can get.

MS

 

[civilperson]

For near vertical walls, try continuous sheet piling around the perimeter. Fill the interior with whatever is cheapest, imported fill or dedged lake material.

 

[microseismic]

Yes,
We were considering sheet piling from the beginning. It is the thick layer of silt that has us concerned.

MS