Temporary Sheet Pile Support

[mtm161]

I am designing a temporary sheetpile wall for a bridge abutment excavation adjacent to a small river. The PZ35 sheet must support 9.6 Ft. of potential flood waters and 6 Ft. of soil below that (Total of 16 FT. above the dredge line). The Sheet is supported by a W-beam waler at 10 Ft. below the top of sheeting. The load is then transfered by struts to concrete blocks located at the bottom of the abutment excavation. That is where I am having a problem.

The blocks are at the bottom of the 1.5:1 cut slope and thus support soil rising at that slope from the top of the block. I assume that the strut force transfered to the blocks induces passive pressure into the soil behind the block. The Kp coefficient estimated from AASHTO LRFD Bridge manual is approx. 42 [Figure 3.11.5.4-2 Based on U.S. Navy 1982(a)]. The resulting passive pressure force based on a 7.5' soil height is 73.9 k/Ft, several times larger than the strut force of 140 kips (23.3 K/Ft for a 6 ft. wide block). An overturning check about the "toe", i.e. the strut side of the block, shows that the passive force will overturn the block. This doesn't seem correct.
Any suggestions??

 

[GeoPaveTraffic]

You need to remember that passive pressure developes when an object, such as the toe block, pushes into the soil. When the soil is pushing on the block, then active or at rest pressures control.

One thing I don't understand from your post is the 7.5 foot soil height. What are the dimensions of the block?

 

[DRC1]

Are you using the 73k/ft in your overturning check? If so that may be the problem. The soil will only develop passive pressure to resist the applied load up to a limit of 73 k/ft. Assuming you are using log spiral charts with no other applied loads, the load on the footing blocks will be triangular with the resultant at the 1/3 point.
Two things to check are the stability of the block under active pressure before any strut load is applied, and the stability of the embankment behind the block. I am assuming the block will need to resist the full active load on the block prior to placing and loading the strut.The other item is the stability of the 1.5:1 slope, especically if you are counting on it for passive resistance. You may want to look to see what happens if you cut it back to 2:1.
Good Luck!

 

[PEinc]

I really do not like the idea of designing a one-sided cofferdam, retaining river flood waters, supported by inclined raker braces and concrete heel blocks. There are too many potential problems. Such as, will the soil behind the heel blocks wash or erode away if the river water overtops the sheet pile wall?

I also do not like the fact that the braced wale will be near OG but 10 feet below the top of sheet pile. This sounds a little too top heavy or floppy for my tastes. I realize you have predominately triangular loads but I think the wale is too low. I would be more comfortable with horizontal or nearly horizontal cross braces to an opposite sheet pile wall. If the bracing is located correctly, it could possibly be removed after the abutment footing is poured and backfilled. Then you would have no interference between the braces and the new construction.

What is the normal river level? Is it necessary to design the cofferdam for so much flood water? What is the worst thing that can happen if the cofferdam floods during a storm? How often does the water rise so high? Where is bedrock? Must the cofferdam be braced? How good, bad, and permeable are the soils?

Will you have a problem controlling ground water infiltration at subgrade if you have a single wall cofferdam? Will piping, boiling, or heave be a problem?

 

[mtm161]

DRC1: thanks for the reminder...of course, the calculated passive force is a limit.

PEinc: The low wale elevation is for the second construction phase where the sheet will be braced off of the abutment footing so that the stem could be built. We are trying to limit the angle of the strut to be as close to horizontal as possible. The idea of using additional sheeting behind the abutment with the support struts between them is an option that has been abandoned (for now)because the contractor does not want to purchase the additional sheeting. Indeed the top of wall elevation is absurdly high, but is is what is shown on the signed structure plans so it is what I have to design for. The designer likely used that elevation (likely the 5 or 10 year flood elev.) without realizing the actual extent of the forces it would result in.

Thank You all!

 

[PEinc]

mtm161,

Just because something possibly stupid is shown on a contract drawings does not mean that is what you should design. You will be responsible if it is a bad arrangement. Your job is to design something that will work well and is economical.

What is the normal water level????? If the water rises 10 feet, you would be better providing an ark rather than a poorly detailed one-sided cofferdam. How high is the water with an average storm? Maybe that would be a more appropriate water level to design for.

Your time of exposure until the footing is poured and backfilled is short. Again, what's the worst that can happen if a lower sheet pile wall is overtopped - especially after the footing is backfilled? The contractor could also pour the footing against the sheet pile wall(s) with an appropriate (plywood?) bond breaker so that the sheets can be pulled, if allowed.

Your own words are "the top of wall elevation is absurdly high." I would not do it. Also, it sounds to me like the contractor is not very experienced with cofferdams - correct?

 

[Focht3]

I generally agree with [blue]PEinc[/blue] but have a word of caution: spring is here. Depending on when the structure will be built - and where it is located - a rapid snow melt could cause a significant rise in the river level. The "absurdly high" design water level may not be absurd at all...

I distrust the raker and thrust block approach for cofferdams - you have no redundancy in the system, and nothing to brace against should the footing/raker system begin to fail. It's susceptible to erosion. Etc., etc., etc. A cantilever system, or one with counterforts, would be preferred over the existing concept - if you simply don't want opposing sheets to brace against.

And the waler is too low...



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

 

[PEinc]

You never know for sure what the highest water level will be that you encounter. You have to pick a design water level that you can live with. If or when the water rises above the top of the sheets, you are out of work until the water recedes. It's the contractor's risk. He should provide his input in picking the maximum water level design elevation (top of sheets). If you leave the top of sheeting 2 or 5 or 10 feet above normal water level, you had better design the cofferdam for water at the top of sheets because it could happen. Also, never just use a water level listed on the contract drawings unless you have verified its accuracy. I've frequently seen water levels several feet higher than what was listed on the contract drawings prepared by someone else.

Focht3, thanks for reinforcing my position on the raker braces, heel blocks, and low waler in the proposed single-wall cofferdam. You are correct that a seasonal water level could be higher than the usual level. Again, this is something to discuss with the contractor.