Earth Pressure for Temporary Shoring 1

[dsharp]

I am involved with the design of a temporary shoring project that will allow for the construction of a new building with a basement that is to be located adjacent to an existing building without a basement. The excavation will range from 10 to 14 feet below grade (7-11 feet below the existing building's foundation). The existing foundation is a mat foundation (likely with thickened sections at the edges for frost protection) for a single story building. The current design provides for a H-pile wall (14' piles) with wooden lagging to be located approximately 3 feet away from the existing building. There is some speculation to some of the designers involved as to whether active earth pressure or at rest pressures should be used on the wall. I personally don't like the arrangment of using a Ka and a cantilever wall system as I fear any movement of the wall would promote settlement of the existing building's foundation. I would prefer to use Ko and plan on using at least one row of tie-backs. Does anyone have experience with similiar applications?

 

[Ginger]

Using a cantilever system is not the best idea for this application as you have correctly identified. Using the king-post type wall with timber laggings will attract additional deflection through deflection of the timber laggings between the posts.

Using tie-backs will be a problem as the tie will have to extend beneathe the foundation of the property you are looking to retain. The property owners may have a thing or two to say about that! Looks like you need to use internal props in your excavation and then construct your works around the props, finally removing them when your basement is structurally capable of withstanding the imposed ground load. I would use Ka with the adjacent structure as a surcharge load. Retaining wall could be either king post type or driven sheet piling (using a Still Worker) or other silent piling method.

The adjacent structure being on a raft is a bonus as differential settlement will be unlikely (depending on the design of the raft foundation).

 

[GeoPaveTraffic]

Good post Ginger.

dsharp, I would add that you can model, approximately, the deflection of the proposed wall. When you show how much deflection could occur a the top of the cantilevered wall that should convice people that you need a braced excavation. I would add that it may be possible to install the ties under the existing building, I have seen it done many times.

 

[PEinc]

Two very good responses. Usually, when excavating that close to and that far below an existing foundation, the foundation needs to be underpinned. With an existing mat foundation, it might be reasonable to use a stiff sheeting wall rather than underpinning. Usually installing tieback anchors beneath an adjacent building is not a problem but, if the property is owned by someone else, you will need their permission and a temporary underground easement. Usually, you can convince the owner that tiebacks are the best way of protecting their property.

If you are digging 12 to 14 feet below grade, 14' long soldier beams are probably too short. In general you should have at least 5 feet of soldier beam embedment below proposed subgrade for braced or tiedback soldier beams. Cantilevered soldier beams usually have embedment at least equal to the height of the wall.

I would not cantilever next to the building under the conditions you have described. I would try to use tieback anchors installed just below the bottom of the existing mat foundation. I would design for an area surcharge equal to the design bearing pressure under the mat. I would not be against using active earth pressure. A 10' to 14' high wall does not need to deflect much to develop active pressures. A sheeting wall this low with tiebacks will usually have minimal deflections, with most of the deflection occurring before the tiebackeds are installed. That's why you want to install the tiebacks just under the mat. However, for a wall this low, if tiedback, using at-rest pressures should not have a significant effect on the design.

I would install the soldier beams at a conservatively close spacing, maybe 5 to 6 feet on center. This way the lagging boards are short, and not too heavy even if they are 3 or 4 inch nominal thickness. You may want to drill in the soldier beams rather than drive them in order to reduce vibrations to the existing structure. If you drill in the soldier beams, after placing the soldier beam, you should completely fill the drill hole with lean concrete or flowable fill. You do not want any voids around the soldier beams or behind the lagging. Although I feel that treated lumber is used too often when not needed, you may want to use it along the existing building.

Make sure that someone does a preconstruction survey of the existing building and that the building is monitored for vertical and horizontal movements during construction.

 

[BigH]

We used secant piled walls in a similar situation. While these are cantilever, they are stiffer. Ginger's suggestion of use of rakers is a good one - see Figs 9.23 and 9.24 of Tomlinson's book on Foundation Design and Construction (6th Edition).

 

[dsharp]

Thanks for the responses. Good information. I plan to share info with other designers to validate my concern of the cantilever wall in this application. I do have another question, however. I would like to stipulate in the technical specificaitons that the Contractor pay special attention to not allowing any voids behind the lagging or piles. What is the best method to excavate in front of the piles, install the lagging, and then immediately fill the void behind the lagging? Lets assume that the contractor only excavates to a depth of 6-8 feet on a Friday and will not be back at the site until Monday. Would it be reasonable to require them to grout behind the lagging and resume excavation on Monday. I guess they will be able to grout the lagging installed after Monday's excavation by injecting grout part way down the wall since the void from the surface to the 6-8 foot level would have already been grouted.

 

[DRC1]

A very interesting project. If you are trying to retain a building, definately design for Ko. Some tie back support should be used, with at least one brace at the depth equal to the building set back, although more levels will probably be required. I would use prestress strand anchors. Most local codes require the builder to get an easment fot the tie backs, but most local laws also require the building owner to give you reasonable access to support his builing if you are building on an adjacent lot, or he must shore the building himself. The trick with tie backs is to lock off at 100% design load. At the brace level, the pile will actually move slightly back into the retained soil and will reduce observed movements during excavation. Not every one accepts this view.I would not use rakers unless the contractor has had a lot of experience with them.They are not as tight as anchors and will allow considerable movement of a wall as compared to an anchor. If a contractor is not experienced, significant potetial exists for large movements.
Having said that I would like to point out that shoring is not the most effective method for foundation protection. I would recomend pit underpinning. For your structure it may be possible to do soldier pits 6 to 8 ft on center and lag between. I have never heard of gouting behind lagging and would be leary for 2 reasons 1.) Grout usually needs pressure to be injected which is resisted by overburden pressure. Grout would just come out between the boards. 2.) should any water devlop in the retained soil (leaking pipe, rainwater, etc.), the lagging allows the water to drain. Grouting would make the soil behind the boards impermiable. However, since a building is being retained, the wall should not be left until the lift is 100% complete.

Good Luck

 

[PEinc]

dsharp,

Lagging is not usually grouted. You install lagging by excavating in maximum 5' high lifts and installing that lift's lagging from the bottom up., backfilling each board as it is placed. Be careful not to excavate behind the rear face of the lagging boards. After the first 5' lift is lagged, the next lower 5' lift is excavated and its boards are placed and backfilled from the bottom up. If the dirt is loose, you may need to reduce the height of the lift as required to maintain a vertical cut while the boards are being installed. The keys to lagging are to not overexcavate, not lose dirt from behind the soldier beams, and to backfill the lagging properly so that the dirt behind a lift or lifts will not run out when you dig the next lower lift. Also, whatever is excavated, should be lagged the same day. Dig & expose only what can be lagged the same day. Lagging should be installed one board at a time, not in preassembled panels of multiple boards. Lagging is not supposed to be slid down as lower lifts are being excavated.

 

[geo00]

This is probably a loaded question. In general, when is it appropriate to underpin a structure?

 

[PEinc]

When to underpin? Generally, if the adjacent structure falls within a 1V:1H slope from the edge of the excavation, underpinning should be considered. If the adjacent structure falls within a 2V:1H slope, the building should be underpinned. There may be other reasons that could influence this guideline. One big factor to consider is easement onto someone else's property to underpin their structure. They may not let you on their property.

 

[SlideRuleEra]

geo00 - As PEinc says, there can be other considerations, here is one

http://www.structuremag.org/archives/2005/December 2005/From-Experience.pdf

http://www.SlideRuleEra.net