Timber Bulkhead Design 1

[bgarbasz]

I am currently designing a tied back timber bulkhead to serve as a bridge abutment. The bulkhead is tied back near the top and there are two rows of timber walers. The timber walers are sandwiched between the piles and the timber sheeting planks. I have not had any luck finding design examples that deal with the design of timber bulkheads with multiple rows of walers. My problem lies in determining the distribution of the loads on the walers from the lateral earth pressure. Due to the flexibility of the bulkhead wall the lower waler cannot be treated as a fixed support and I was wondering how others have approached this type of design. Does anyone know of any design examples or has experience in designing timber bulkheads with multiple rows of walers?

 

[PEinc]

Please give more information. Do you have soldier beams (piles)? How many rows of tiebacks are there? Is the bulkhead temporary or permanent? Are the sheeting planks vertical between wales or horizontal between soldier beams? Is there water in front of this bulkhead? How high is the bulkhead? Is the bulkhead supporting a surcharge from the abutment footing or is the abutment pile supported?

 

[PEinc]

If this bulkhead is for a highway bridge, you probably should be following the requirements of the state DOT, AASHTO, and FHWA for anchored retaining walls.

 

[bgarbasz]

Here are the particulars about the wall:

1. 12" timber piles spaced at 4'-0" on centers. Every other pile is tied back to a drag pile.
2. Timber sheeting planks are vertical 2"x8" sheets in two layers (ship lapped)
3. The bulkhead is permanent since it will act as a bridge abutment.
4. There is water in front of the bulkhead but it is located within a stream that runs dry during summer months.
5. The distance from the top of the wall to the stream bed is approximately 11 feet.
6. The superstructure (glulam deck panels) rest on a timber pile cap which is the only vertical loading on the piles.
7. Walers are 6"x8" timber planks and spaced at 3'-6" on center. There are two rows of walers.

 

[DRC1]

Assuming you have cohesionless material behind the bulkhead, for two rows of walers your pressure would be
p= .65ka gama H^2 where gama is the unit weight and H is the retained height. The wales would support the laggingat the two points. If the height is 11 feet and the wales are at say 4 ft and 5 ft, Take a unit width of 1 foot, compute p and the compute reactions for an 11 ft beam with supports at 4 ft and 8 feet. These are your unit loads for the wale.

 

[bgarbasz]

DRC1, I considered treating this as a braced cut but since the only the top waler is tied back the flexibility of the wall should allow for active earth pressure to develop. I was considering to distribute the active earth pressure load to the walers in somewhat the same manner as you describe utilizing the lateral pressure for a braced cut. I was thinking of utilizing the equivalent beam method and determine a pseudo reaction at the point of zero pressure and the reactions at the walers. From this I could also determine the moment in the sheet piles and then determine the required depth of sheet pile embedment to develop the pseudo reaction. Does this approach seem rational??? Any input would be greatly appreciated. Thanks for all the responses thus far. I am a structures person with just enough geotechnical knowledge to get myself in trouble!