Trenching behind sheetpiles 2

[gwynn]

In designing a sheet pile coffer dam I've run into a situation where the piles cannot be driven for the lower third of the excavation (cone penetration is well in excess of 100 blows/foot). The solution being used is to trench behind where the piles are to go, placing the piles and then backfilling. The piles may be driven another foot or two past the trench depth, depending on whether or not any penetration can be achieved.

While it is the excavated material being used to fill the rtench in, it won't have the same degree of compaction. My current method of attack is to take the failure plane of the fill back to to where it intersects the far edge of the trench. Above this line I'll use friction values for the fill material, and below it use values for the in place material.

As this is not a situation I've run into before (usually I see mostly fill material or stuff that has the consistency of hair gel), I'd appreciate some feedback onmy approach or pointers to resources that deal with this sort of scenario.

 

[BigHarvey]

A far better solution would have been to carry out some pre drilling. You still face the problem of having some loose material behind your wall. You can grout them to restaure the initial mechanical characteristics.
Be careful when driving your sheet piles 1 foot below trench depth : the top of the sheet pile might go down as its foot stays at the same level ( it's a matter of energy and sheet pile inertia )
It sounds like sheet pile might not have been the best choice for your project

 

[gwynn]

Having some choice in methods and materials would have been nice, and may have resulted in a different solution. It's a case of this is what's been done and I'm left with designing the waler frame(s).

 

[DRC1]

Typically, a trench is hogged out, backfilled with good material (free draining) and then the sheets are driven. This makes it much easier to backfill and stick the sheets. The driving does densify the material somwhat. I would use the friction angle for the fill, as it may be hard to control excavation limits, espcially if the hard driving is due to bolders or the trench is below watertable. Also, you may want to consider a lower frame rather than trying to get sufficent toe.

 

[gwynn]

Thanks to both of you for the advice and information. We would like to see concrete or grout poured into the base of the trench, but are restricted by the amount of time they can leave a trench open (this is in the middle of a road).

My original post may be a bit off, as I was missing some information and haven't dealt with this before. The trenching is proceeding as DRC1 described.

We're now planning on filling the lower sections of the trench with road base, so it will be free draining. The main issues now are how far down they can actually trench (which may mean the bottom of the trench is above the bottom of the required excavation depth) and how low we can put the bottom waler frame. This is for a pipeline running under a creek, so they will be drilling out the side of the cofferdam. In the end we can get it to work, but it probably means more cash than anyone was planning on spending.

 

[DRC1]

Anytime there is a change involving cofferdaming, it means more money than anyone was planning. Are you planning to proceed halfway across the creek and allow flow on the other half or are you planning to dam the whole creek and pump around?
If you can trench through the obstructions, you should be able to stick and drive sheets. Otherwise you will have to hold the sheets with frames, dig out below the sheets and adavvance them one at a time. To set the lower frame you may need a temporary intermeadite frame, depending on how much toe you have below the lower frame.

 

[gwynn]

The cofferdam does not impede the flow of the creek, it is outside the riparian zone completely. The pipe runs well below the bottom of the creek bed.

Digging out and driving the sheets a pair at a time is essentially plan B. Hopefully this can be avoided.

 

[Zambo]

gwynn,

what is the depth of excavation required within the cofferdam?

Is the cofferdam a jacking pit for pipe jacking or a drilling pit for a directional drill across the creek?

Are the sheet piles being driven with a vibro or a hammer?

I don't really understand about the road. It is often hard to explain the facts in these short posts. In one post this sounds like a jacking pit, in another a road crossing trench.

Unfortunately the best option seems to come from BigHarvey - predrilling.

 

[gwynn]

It is a bit of a moot point for this project now as the piles are driven, but input for future circumstances is always good.

what is the depth of excavation required within the cofferdam?

The depth below original ground is 28'. The trench was dug to a maximum of 30' (the reach of the bucket). The piles were driven 32-34'.

Is the cofferdam a jacking pit for pipe jacking or a drilling pit for a directional drill across the creek?

It's for directional drilling.

Are the sheet piles being driven with a vibro or a hammer?

Vibro.

I don't really understand about the road. It is often hard to explain the facts in these short posts. In one post this sounds like a jacking pit, in another a road crossing trench.

The actual pit is being dug in the middle of an existing road. This is followed by directional drilling under an adjacent creek bed.

By the time we were involved in coming up with a solution, a nearly full depth (~25-28') trench had already been dug and filled around much of the perimeter of the required excavation.

 

[Zambo]

Sorry gwynn didn't notice the date of your post.

I was going to suggest you drive with an air or hydraulic hammer instead of the vibro.

Sounds like you succeeded - thanks for the feedback