Rakers for 40 ft Deep Excavation 2

[EngMan40]

Neighbors across street don't like tiebacks below their property so I am thinking 3 levels of rakers with reaction blocks, any precautions/issues to look out for given the depth of excavation? top 20 ft is medium dense fill/natural, bottom 20 ft is good dense to very dense natural sand and gravel. If you have some details to share, photos of this being done in the past that will be great.

 

[PEinc]

If the temporary sheeting wall is about 3' or more off the proposed foundation wall, you can use horizontal walers and spread out the raker braces to one per every other soldier beam. If the sheeting wall is to be located at the face of the new foundation wall, you can't use walers and will need to install rakers at every soldier beam. Raker braces increase the cost of excavation, wall forming and hole patching, floor slab box-outs and patching, waterproofing. Raker brace removal can be costly since the braces have to be cut up into smaller pieces to lug them out of the building. Raker braces need to be installed so that they do not interfere with structural framing, columns, and building corners. Be careful how you brace a corner soldier beam, especially with wall-line sheeting systems. Also, the braces and heel blocks for the raker braces need to be located clear of or under proposed footings, drilled shafts, interior walls & columns, and floor slabs. Therefore, the raker braces may need to be angled horizontally in addition to vertically. Raker braces that support horizontal walers require roll chocks at the soldier beams to prevent upward rolling of the walers. If you want to uses walers with less raker braces for wall-line sheeting, the waler will need to be located inside the building, in front of the foundation wall and its wall forming system. See last photo.

Try to reduce your design to 2 levels of raker braces.

The first 3 photos are of a raker-braced, wall-line sheeting wall (no tiebacks allowed) that is 41.3' high with 2 levels of bracing. The fourth photo is on a wall-line sheeting wall, braced with waler inside the proposed building. Note the roll chocks and the outlooker beams needed to space the waler off the sheeting and inside the proposed foundation wall.

Show this to the concrete contractor!

http://www.PeirceEngineering.com

 

[EngMan40]

Thanks PEINC for the information.

My SOE is one-sided shoring but if I can relocate an electric line, the SOE can be 3 ft away from the proposed wall which seems to be more cost-effective than placing a raker at each pile. your 4th photo is an interesting concept, would you mind sharing a detail?

I am planning on rotating one of the corner piles that sits at the street intersection at 45 deg so it will be drilled into the intersection which is what I think you did in the photo. But for the other one, and if the piles are all on-line, how would you brace it without interfering with the proposed building wall? I have a short line of one-sided shoring in the perpendicular direction to hold back a building that sits off the property line and therefore I can't underpin it. In the first photo, you conveniently transitioned back to tiebacks before reaching the second corner but in my case, all neighbors across the street are opposing the tiebacks (maybe the whole development!).

 

[PEinc]

The last photo pretty much shows the details. There is a horizontal beam welded between the soldier beam and the waler. Its length needs to be enough to give room for forming and stripping the forms. The angled roll chock is longer than usual because the waler is inside the new building. The roll chock needs to resist the axial load in the raker brace. It has a relatively short length so it can resist a high load, but it needs a lot of weld.
Near the corner, it may be hard to install lower levels of tieback on the sheeting wall that is perpendicular to the braced wall because of raker brace interference. Good luck.
In my first photo, not all of the neighbors objected to tieback anchors. Sheeting is flexible and not supposed to support buildings. However, if you can't underpin the building, you have few economical ways to support the building. If you sheet in front of the building, you need to design a very stiff sheeting system where there is a level of tiebacks or braces at or very slightly below the bottom of footings of the adjacent building. Also, remind your client that you should not be responsible for minor damages that probably will occur without underpinning. Someone will be saving a lot of money by not underpinning the building. They can use the savings to handle any required repairs.

http://www.PeirceEngineering.com

 

[hotmailbox]

PEinc,

Do you have any pictures showing the support (heel blocks) of those groups of rakers (3 each group)? Those have to support massive compressive forces from rakers and typically soils engineer does not give bearing capacity more than 2000psf.

Another question, assuming you can't use tiebacks for the wall on the other side, are horizontal struts the better only option than rakers on both sides? Is there any reason for not using diagonal corner braces at corners?

Thanks,

 

[PEinc]

hotmailbox, I don't have good photos of the raker brace heel blocks. Here is a plan detail. For the triple raker heel block, the dimensions were 6' wide by 6' deep x 6' long (parallel to the sheeting wall) minimum. The allowable bearing pressure per the geotech report was 12 ksf. In order to avoid interference of the triple rake braces with the proposed columns and foundations and to provide some space between the bottom ends of the 3 rakers, the actual heel block length was 10'. Ground water was encountered while digging the raker heel blocks and the contractor installed so sheet piling in order to control the water and minimize the excavation. If the raker braces need to be installed flatter than about 30 degrees, the heel blocks design should consider passive resistance rather than just allowable bearing.

Cross braces may be a good way to support opposite facing walls if the distance between them is less than about 40 feet. If the spacing is greater, you my need to install some vertical and lateral support for the cross braces. Long braces weigh a lot and cause bending in addition to the axial compression. As braces get longer than about 40 feet, it might be better to use pipe braces instead of beam braces. Don't forget to add a surcharge load per linear foot of brace to account for build-up of excavation spoils during excavation.

Corner braces can be used but you have to consider the longitudinal force the corner braces impart to the braced, perpendicular, sheeting walls. You need to be able to distribute that force to the two perpendicular walls. This is easier to do with continuous sheet piling than with individual soldier beams. Also, it is hard to accurately determine how far along each wall the load needs to be distributed. For example, if you have a perpendicular brace load of 100 kips, at a 45 degree angle, the brace load is 141.4 kips. The 100 kip load needs to be supported by or be distributed to both perpendicular sheeting walls. The 100 kip load will be applied in the plane of each sheeting wall, which means the soldier beams or SSP need to support the 100 kip load in the members' weak direction.

http://www.PeirceEngineering.com

 

[hotmailbox]

Thanks,

12ksf is only my dream. I typically got 2ksf (2.5ksf if lucky) from soils engineer. Most of the times I have continuous heel blocks (7'x7' or deeper) and if I can't do 7ft or more vertical cut for them, I use modified block with bearing surface perpendicular to the rakers to avoid vertical cuts.

 

[PEinc]

Don't be afraid to challenge/question the geotech on his or her recommended(?) allowable bearing pressure. If you know how, try to determine the ultimate and allowable bearing pressure yourself. What is the sheeting wall supporting? Perhaps a lower safety factor is reasonable for a short-tern condition?

http://www.PeirceEngineering.com