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Excavation support
4

Excavation support

Excavation support

(OP)
Hi all,
Please find the attached image.
The building has 2 basement floors+ 2 floors (used as shows)+ 4 typical floors (used as flats). The mat foundation is 800mm thick. The basement floors are 3.6m high floor-to-floor each. My first thought is using piles for excavation support. My worry is the piles will interfere with boundary columns. What is the best practice to avoid such a condition?

RE: Excavation support

I do not understand your question. Are you concerned that a soldier beam excavation support system between the existing and proposed buildings will interfere with the new building's columns? If so, the existing buildings should be underpinned with concrete piers beneath the existing walls and column footings. A section view could help explain your problem.

www.PeirceEngineering.com

RE: Excavation support

(OP)

Quote (PEinc)

Are you concerned that a soldier beam excavation support system between the existing and proposed buildings will interfere with the new building's columns?
Thanks. Can I use soldier beam excavation support system for such a height (3.6+3.6+0.8=8m)?

RE: Excavation support

Soldier beam excavation support systems are considered flexible systems (especially if cantilevered) and, as such, are not normally appropriate for supporting a closely adjacent structure. Flexible means wall movement. Wall movement means building movement and possibly damage. Now, can it work? Maybe, if made stiff enough and if installed carefully. I don't recommend supporting structures with soldier beam and lagging excavation support systems. With an excavation depth of 8m, you should need some type of lateral support, either tieback anchors or raker braces.
You also did not answer my question about having enough room to install the system without interfering with the new building wall and columns. If you can't underpin the building, your better options could be a diaphragm wall, a secant pile wall, or a tangent pile wall - all much more expensive than soldier beams and lagging and they still may need lateral support.
A section view could help explain your problem.

www.PeirceEngineering.com

RE: Excavation support

(OP)
Hi PEinc,
Thanks. I'm sorry I haven't been clear.

Quote (PEinc)

Are you concerned that a soldier beam excavation support system between the existing and proposed buildings will interfere with the new building's columns?
I meant the hatched area is the proposed construction area. My main concern is with interfering of the proposed piles (within this construction area to support the neighbor soil and foundations) with the column locations (within this construction area).

RE: Excavation support

As I said in my first response, if you are concerned that a soldier beam excavation support system between the existing and proposed buildings will interfere with the new buildings' columns or walls, then the existing buildings should be underpinned with concrete piers beneath the existing walls and column footings. The underpinning will prevent the existing buildings from settling and the underpinning, with proper lateral bracing or tieback anchors, will resist lateral earth pressure from under the existing buildings and resist any surcharges coming from inside the existing buildings.

www.PeirceEngineering.com

RE: Excavation support

(OP)

Quote (PEinc)

The underpinning will prevent the existing buildings from settling and the underpinning, with proper lateral bracing or tieback anchors, will resist lateral earth pressure from under the existing buildings and resist any surcharges coming from inside the existing buildings.
Thanks for your response. I don't think the tieback option is permitted. Will soldier beams (if not interfering with column locations) with struts be a choice? If so, can the walers be located between the flanges of the soldier beams so that the walers will not interfere with column locations?

RE: Excavation support

An 8m high wall generally requires lateral support - tieback anchors or internal bracing. If you try to use soldier beams, they will need to be closely spaced to provide stiffness. Installing a wale between the soldier beams would be difficult and expensive and still need a brace on each wale section or bay between soldier beams. This would be a nightmare for digging and for forming the new foundation walls. Also, the new foundation walls would need to be designed to support the lateral surcharge loads from the existing buildings.

You say you cannot use tieback anchors. Will the neighbors allow you to underpin their buildings? I guess probably not. Plus the underpinning would also need lateral support. I think that your only options may be a very stiff, cantilevered, secant pile wall that also acts as your new foundation wall or a very stiff, closely space, soldier beam wall with one or more levels of braced walers which get removed as the new foundation wall is constructed in shorter height sections which would need to be braced by the new floor slabs prior to removing the braces and walers. The second option would be expensive, tedious, and apply big concentrated loads to the new floor slabs.

If the neighbors are so disagreeable, maybe it is wise to move your building away from the existing buildings.

www.PeirceEngineering.com

RE: Excavation support

(OP)

Quote (PEinc)

I think that your only options may be a very stiff, cantilevered, secant pile wall that also acts as your new foundation wall
Thanks PEinc,
Please find the attached image.
Do you think a very stiff, cantilevered, secant pile wall that also acts as my new foundation wall will be applicable for this basement geometry (especially near the stair locations)?

Quote (PEinc)

or a very stiff, closely space, soldier beam wall with one or more levels of braced walers which get removed as the new foundation wall is constructed in shorter height sections which would need to be braced by the new floor slabs prior to removing the braces and walers.
Can soldier beam wall be braced with struts without walers? If so, I'm planning on using the struts in the y direction.

RE: Excavation support

You will not know if a system will work until you determine the lateral and vertical loads and the soil and water conditions and then run some design numbers, including deflection calculations. If a cantilevered design can work, it will be a lot easier than working around walerless raker brace at each soldier beam, especially at the corners. I suggest that you talk to a contractor who is experienced in design-build secant pile walls. I don't expect stairs to be a problem if the subgrade is not significantly deeper than the rest of the perimeter walls. The common problem is architects placing deep elevator pits up against existing buildings.
Remember, a proper design is important; but, more so, an experienced contractor is critical.

www.PeirceEngineering.com

RE: Excavation support

(OP)
Hi PEinc,
The soil report evaluation on page 4 of the attached link on 26 Dec 23 18:42, mentioned that:
Due to the fact that no undisturbed samples could be obtained, no compressibility and strength tests performed. The minimum N-values obtained from SPT tests was 30 indicating hard consistency of the soil.
How one can calculate earth pressure coefficients Ka and Kp while no compressibility and strength tests performed?
Can phi be estimated from the SPT N-values?

RE: Excavation support

You have a poor soils report. It does not have much site specific soils information included. There are plenty of geotech/soils books and manuals with estimated soil properties based on soil type and N-values. Your boring shows predominantly clayey silt and clay. Look for estimated undrained and drained properties. I don't know your time frame for providing a design; but you may need to get a few more borings and some lab testing.

www.PeirceEngineering.com

RE: Excavation support

(OP)

Quote (PEinc)

There are plenty of geotech/soils books and manuals with estimated soil properties based on soil type and N-values.
Would you recommend Braja M. Das "foundation engineering" textbook for estimated soil properties based on soil type and N-values?

RE: Excavation support

Many people use Das' book. I personally don't have it. I use other books such as Bowles, Winterkorn & Fang, NAVFAC DM 7. Just remember, you may (should?) get greatly different designs for drained and undrained soil conditions. I prefer to use reasonable soil properties with the appropriate safety factor rather than using overly conservative soil properties with overly conservative safety factors.

www.PeirceEngineering.com

RE: Excavation support

(OP)

Quote (PEinc)

Many people use Das' book. I personally don't have it. I use other books such as Bowles, Winterkorn & Fang, NAVFAC DM 7.
Can you tell me what will be reasonable soil properties with the appropriate safety factor for my case when you use other books such as Bowles, Winterkorn & Fang, NAVFAC DM 7?

RE: Excavation support

No. That would not be prudent on my part. I know little about your project, your geographical area, or your building codes. It sounds to me like you need to speak to someone in your area who has more experience in designing and building non-gravity, anchored walls. Sorry.

www.PeirceEngineering.com

RE: Excavation support

(OP)

Quote (PEinc)

No. That would not be prudent on my part. I know little about your project, your geographical area, or your building codes.
But you recommended using books and manuals in your post on 27 Dec 23 22:58

Quote (PEinc)

There are plenty of geotech/soils books and manuals with estimated soil properties based on soil type and N-values.
Your help would be highly appreciated.

RE: Excavation support

(OP)
If I get enough geotechnical soil properties, I'm thinking of using soldier pile beams with walers connecting all perimeter soldier beams @ elevations -2.40 and -6.00 (so it's now above the splice length of the columns since the columns have 16mm reinforcing bars in it), and struts @ corners. Another level of walers may be required between the two other waler levels. Your opinions on this would highly appreciated.

RE: Excavation support

If you are going to brace the 8 meter deep excavation, the upper braced waler should be about the same elevation as the bottom of the existing building foundations. Preferably, each braced waler would be just below a proposed, basement, floor slab to minimize brace interference with new building framing. Bracing at the corners of the excavation can be tricky with raker braces. You probably need to use horizontal corner braces at the corners instead of raker braces. The locations of your soldier beams and your raker braces need to be coordinated with the proposed foundation mat and column locations. Walers with inclined raker braces will need roll chocks at each brace to prevent the waler from rolling upward from the vertical component of the raker brace load. Use as few levels of braced walers as possible. Make sure you check the intermediate design cases: Cantilevered soldier beams before installing the upper level of bracing, One level of bracing before installing the second level of bracing, etc.

Additionally, despite having a temporary sheeting wall, the new building still needs to be designed for all lateral pressures and surcharge pressures from the existing buildings. As the building is being constructed, you cannot remove any temporary braces until the new building is backfilled, if needed, to the brace levels that need to be removed and until the foundation wall is properly and permanently braced by the new floor slabs.

Again, sheeting walls are generally flexible systems. Consider underpinning the existing buildings.

www.PeirceEngineering.com

RE: Excavation support

(OP)

Quote (PEinc)

Preferably, each braced waler would be just below a proposed, basement, floor slab to minimize brace interference with new building framing.
The building framing is RC columns, beams, slabs, and walls. If each braced waler would be just below a proposed, basement, floor slab, then it would interfere with new building framing since bar splicing for columns from the basement floor would interfere with braced waler. My thought is each braced waler should be above bar splicing for columns. So my proposed elevations in the previous post. Am I wrong?

RE: Excavation support

hoshang, I am sorry to say that you are way out of your depth. If you do not know how to estimate phi from an SPT then you shouldnt be designing an 8m high retaining wall, or any for that matter.

Its great that you are coming hear and trying to get advice on what to do but the consequence's of an 8m high wall failing right beside a building are very serious. Someone could die.

If you are coming hear just to learn and ask questions then that is fine. But if you are coming hear to get advice on how to design the wall with you being principal designer then this thread should not go any further IMO.

RE: Excavation support

I agree with EireChch. Wall-line soldier beam sheeting with walers and braces will cause many construction problems. You could brace every soldier beam without walers but your concrete contractor and waterproofer will not be happy.




www.PeirceEngineering.com

RE: Excavation support

(OP)
Hi all,
I made a preliminary soldier pile layout as in the attached file (not designed yet) at the elevations in my earlier post. Some struts are longer than 13m. The right and left boundary would be soldier piles braced by rakers. Your thought would be highly appreciated.

RE: Excavation support

It would be nice to know the soldier beam spacings. My first impression is that, for holding up buildings, the W8X31 soldier beams are very small and the 8-inch struts are very small and very long. The struts need to be designed for axial load and bending from self weight and any surcharges. In addition, where will your lagging be installed - behind the front flanges or behind the back flanges? Are your struts inclined raker braces or horizontal cross braces? How are you providing lateral support for the sheeting walls along Col. Lines A and E? Is there sheeting at the bottom, angled side of the site from Col. Line A to E? How will the site be excavated with all of those long, closely spaced struts? You need help from someone with extensive, successful, sheeting and underpinning design experience.

www.PeirceEngineering.com

RE: Excavation support

(OP)

Quote (PEinc)

In addition, where will your lagging be installed - behind the front flanges or behind the back flanges?
behind the back flanges, at the property line.

Quote (PEinc)

Are your struts inclined raker braces or horizontal cross braces?
I'm thinking of using this configuration for struts longer than 13m.

and horizontal cross struts for struts less than 13m long.

RE: Excavation support

You need help from someone with extensive, successful, sheeting and underpinning design experience.

www.PeirceEngineering.com

RE: Excavation support

(OP)

Quote (PEinc)

You need help from someone with extensive, successful, sheeting and underpinning design experience.
Thanks. I'm trying to understand whether my thought is correct. My thought from my post on 5 Jan 24 08:55 came to my mind as I read other threads here recommending struts not longer than 13m. Isn't my framing model for the struts bracing the soldier piles applicable? If not, I wonder why?

RE: Excavation support

I do not understand your strut sketch. Nothing is labelled. Don't you need soldier beams on all 4 sides of the excavation? Are you trying to brace the soldier beams with a truss? Will you have more than one level of that type of bracing?

www.PeirceEngineering.com

RE: Excavation support

(OP)

Quote (PEinc)

I do not understand your strut sketch. Nothing is labelled.
Sorry for that. Please find the attached image.

Quote (PEinc)

Are you trying to brace the soldier beams with a truss?
Yes, I'm trying to brace the soldier beams with a bent frame (rather than a truss) if the struts (in the attachment in my post on 4 Jan 24 18:38) are longer than 13m.

Quote (PEinc)

Will you have more than one level of that type of bracing?
If necessary, then it would be a multistory of this type of bracing.

RE: Excavation support

(OP)
Hi PEinc,
my proposal on the last post is same as would be for a steel structure frame resisting wind pressure (rather than resisting soil pressure). Is this simulation reasonable?

RE: Excavation support

Final response: You are in way over your head. Get help quickly (not internet help!).

www.PeirceEngineering.com

RE: Excavation support

(OP)
Hi PEinc,
Thanks for your Final response.
My Final query: is my proposal on 6 Jan 24 09:50 doable?

RE: Excavation support

I am sorry, but my opinion is that your multi-level, segmental, truss brace scheme is both unbuildable and uneconomical.

www.PeirceEngineering.com

RE: Excavation support

(OP)

Quote (PEinc)

your multi-level, segmental, truss brace scheme is both unbuildable and uneconomical.
Why my truss brace scheme is unbuildable? Can you elaborate more on this, please?

RE: Excavation support

Why don't you discuss your scheme with a local contractor who might bid the project's SOE/Underpinning?

www.PeirceEngineering.com

RE: Excavation support

(OP)

Quote (PEinc)

Why don't you discuss your scheme with a local contractor who might bid the project's SOE/Underpinning?
I seek the difficulties in building my truss brace scheme you posted here:

Quote (PEinc)

your multi-level, segmental, truss brace scheme is both unbuildable and uneconomical.
I have limited options here since we don't have the underpinning & tieback option. The cantilever soldier pile with this height (8m) must be braced (using either raker or horizontal cross struts). The raker option is omitted since it obstructs the excavation & construction for this limited area. So I have the horizontal cross strut option for bracing the cantilever soldier piles. Since the horizontal cross struts are longer than 13m, I proposed my truss brace scheme. I wonder if one can explain the difficulties in designing & building my truss brace scheme so it can be solved or proposing other options available.

RE: Excavation support

Your truss brace scheme: too big, too heavy, too many pieces, too many connections, needs lateral bracing between trusses, takes up too much vertical space between floor levels, bracing is difficult to remove as floors are being installed.
Braced walls are not cantilevered walls. The sheeting walls will be low, cantilevered walls only until the upper level of bracing is installed.
For 13m long braces, try walers with horizontal pipe braces between opposite sheeting walls. Sometimes, long cross braces need some vertical and/or lateral support to reduce their unbraced length. Install horizontal, corner braces at sheeting corners. This will be easier to build if the new building walls are about 1m off the sheeting walls (the 1m space will need to be backfilled before removing bracing). If you cannot underpin the building, make the sheeting system very stiff and use drilled-in soldier beams, walers, and braces at every other soldier beam. Do a pre-design and a pre-construction survey of the existing buildings and monitor the buildings and sheeting walls during construction.
Again, I ask, why don't you discuss this project with a local contractor who might bid the project's SOE/Underpinning? It does not matter what you design if you cannot find someone, with great experience, to build it.
There is nothing more that I can add.

www.PeirceEngineering.com

RE: Excavation support

(OP)

Quote (PEinc)

Sometimes, long cross braces need some vertical and/or lateral support to reduce their unbraced length.
I think you want to say:
Sometimes, long cross braces need some vertical with/without lateral support to reduce their unbraced length.
In my case I would not prefer some vertical support to reduce long cross braces length since it would interfere with excavation operations. So how one can deal with these long cross braces?

Quote (PEinc)

Install horizontal, corner braces at sheeting corners. This will be easier to build if the new building walls are about 1m off the sheeting walls (the 1m space will need to be backfilled before removing bracing).
The new building walls will be at the sheeting walls (the sheeting wall will be the exterior form for the new building wall).

RE: Excavation support

(OP)
Hi PEinc,
I followed your valuable posts on my and other threads. I appreciate your help. You made it clear the possible solutions. Now, my thought is that the load path of the soil pressure is from the soldier piles to walers, then from walers to cross struts (a straight cross strut is much better than a broken one since it will be subject to compression only from earth pressure with little bending from self-weight). I appreciate your recommendation on corner braces. This may solve a lot. For the perpendicular sides the corner braces are easy to visualize. But the angled (skewed) side is what worries me. Your opinion would be highly appreciated.

RE: Excavation support

If you cannot get permission to installed tiedback underpinning, consider a very stiff sheeting layout something like shown here. Remember, the soldier beams take up room and will cause your building to have a smaller footprint. Talk to the project's structural engineer, architect, and a contractor about how close to the existing buildings the soldier beams can be.
https://res.cloudinary.com/engineering-com/image/upload/v1705342464/tips/Sheeting_Scheme_hvasyr.pdf

www.PeirceEngineering.com

RE: Excavation support

(OP)

Quote (PEinc)

Pre-load the braces to minimize soldier and brace deflections.
Thanks, PEinc. Can you elaborate more on Pre-loading of the braces? A detailed sketch, or a reference would be helpful. Is it pre-loaded using jacks? How it comes to be pre, rather than post? I mean is it done after installation of the braces or before the installation?

Quote (PEinc)

Possibly four horizontal corner braces
Do mean you've drawn two in this corner. It may need four braces?

RE: Excavation support

Pre-loading a brace entails jacking a load, usually 50% to 75% of the brace design load, into the brace after it is installed but before digging deeper than that brace's waler installation elevation. The brace has one or two jacking brackets welded to the brace near the waler so that one or two hydraulic jacks can be set between the bracket(s) and the waler. The resulting gap between the brace and its waler then gets packed with welded plate(s) and the jack is then released and removed.

I was referring to possibly 4 total corner braces, not necessarily 4 in one corner. You need to run the design and see how many braces you need and how big a waler you need to span between all braces.

You need to talk to someone in your area who has done this type of work, both design and construction! You are into this way over your level of experience.

www.PeirceEngineering.com

RE: Excavation support

(OP)
Hi PEinc,
What should be done on the lower right corner? Isn't necessary to provide a brace there?

RE: Excavation support

The waler along the street side braces the end of the right side waler. The walers need to be designed for combined axial and bending loads. The 4 walers would be connected at the 4 corners.

www.PeirceEngineering.com

RE: Excavation support

(OP)
Hi PEinc,
Thanks for your contribution.
On the right side there exists a three floor & basement building. Does the length of the soldier piles on this side be same as those on the other sides?
Another thought came into my mind.

Quote (PEinc)

Pre-loading a brace entails jacking a load, usually 50% to 75% of the brace design load, into the brace after it is installed but before digging deeper than that brace's waler installation elevation.
After pre-loading (pre-compression) a brace, doesn't digging deeper than that brace's waler installation elevation (followed by lagging installation behind the back flange) relieve some pre-loading amount? Should the soldier piles be higher or lower than the bottom of the existing building foundations? If the former, the pre-loading won't put the soil beneath the existing foundation in passive condition. My thought is that the latter will put the soil beneath the existing foundation in passive condition (hence the correct condition for soldier pile installation). Is my approach correct?

RE: Excavation support

Usually, you can brace a taller wall against a lower wall. Remember that the lower wall needs to develop enough passive resistance to resist the load from the taller wall. The lower wall's walers and braces would receive a greater load than what is needed to support the lower wall. Generally, the soldier beams for the lower wall need only the length required for the lower wall design - as long as the lower wall can provide sufficient passive support for the taller wall.

When you pre-load a brace or tieback anchor, the pre-load amount should be close to the maximum expected load for the worst case. Therefore, if you dig down about 2m or 2.5m to install the first level of bracing or tieback anchors for a 10m excavation, you would pre-load that level for its required load for the 10m wall.

www.PeirceEngineering.com

RE: Excavation support

(OP)
Thanks, PEinc, for your responses.

Quote (PEinc)

Generally, the soldier beams for the lower wall need only the length required for the lower wall design - as long as the lower wall can provide sufficient passive support for the taller wall.
Do you mean for multilevel bracing, the upper bracing will be 3-sided brace? angled (skewed) side, angled (skewed) opposite side, and left side?

Quote (PEinc)

When you pre-load a brace or tieback anchor, the pre-load amount should be close to the maximum expected load for the worst case. Therefore, if you dig down about 2m or 2.5m to install the first level of bracing or tieback anchors for a 10m excavation, you would pre-load that level for its required load for the 10m wall.
But after pre-loading (pre-compression) a brace, doesn't lagging installation behind the back flange relieve some pre-loading amount?
I interpreted your post as a response to a thread as if you were calculating earth pressures rather than net pressures. Do you mean you calculate it as in the attached image?

RE: Excavation support

I don't know what you mean in your first question.
When lagging is installed, there usually is no excavation behind the back flange of the soldier beam. Lagging is usually installed behind the front flange or is attached to the front of the front flange.
Multi-level braced or tiedback walls usually are designed for an empirical earth pressure diagram, such as a trapezoidal or rectangular loading diagram. I would not use theoretical, triangular, earth pressure unless the soil was very soft or the wall had a single level of support, positioned very high on the wall. Throw away that earth pressure diagram that you posted.
I am sorry but, the more questions you ask, the more certain I am that you need in-person help.

www.PeirceEngineering.com

RE: Excavation support

(OP)

Quote (PEinc)

I don't know what you mean in your first question.
I'm sorry if the question isn't clear. At the right boundary we have this condition. Please find the attached image.

Do you mean for multilevel bracing, at level 0.00 the waler will be 3-sided waler? angled (skewed) side, angled (skewed) opposite side, and left boundary side (3.9m to be retained)?
At level -4.50 the waler will be 4-sided waler: angled (skewed) side, angled (skewed) opposite side, left side, and right boundary side (8m to be retained).

Quote (PEinc)

When lagging is installed, there usually is no excavation behind the back flange of the soldier beam. Lagging is usually installed behind the front flange or is attached to the front of the front flange.
Please find the attached image:

This is from Handbook of Temporary Structures in Construction by Robert T. Ratay.

RE: Excavation support

If you look at the newest, 2012 edition of Ratay's book, I am the co-author of Chapter 7, Earth Retaining Structures. There is a lot more info in the 2012 Chapter 7 than in Chapter 8 of the edition you have.
How deep is the existing building on the left side? Is there a basement? If the left side of your Section B-B is that much higher than your right side, you probably cannot use cross braces, unless they can line up with the floor levels of the existing right side building. Even then, the neighbor probably will not allow you to brace to their building. You may need to use inclined raker braces for both sides.
If you cannot underpin, cannot use tiebacks, and cannot cross brace to another building; the project has a major problem - unless you can design very stiff, cantilevered, secant pile foundation walls that will not need temporary tiebacks or braces. This would involve significant redesign of the proposed building.

www.PeirceEngineering.com

RE: Excavation support

Although lagging can be installed behind the back flanges of the soldier beams, it is not done very often, except maybe for the upper few feet of the beam where the lagging may need to be removed and the top of the beam may need to be cut off. Soldier beams are usually designed as being fully supported in their weak direction y-axis because there is usually soil or fill concrete and lagging on either side of the web. If you lag behind the rear flange, you lose this lateral support and you will need to use lower allowable stress when designing the soldier beam.

www.PeirceEngineering.com

RE: Excavation support

(OP)

Quote (PEinc)

If you lag behind the rear flange, you lose this lateral support and you will need to use lower allowable stress when designing the soldier beam.
I think we need lagging behind the rear flange not to lose lot area. Does that mean pre-loading will be non-beneficial? Can corner struts and horizontal cross struts be used without pre-loading? My thought is that pre-loading can be applied by calibrating the pre-loading jacks after each lagging phase to engage the passive condition. Am I right?

RE: Excavation support

@ hoshang
we seamed to be in working at the same region, send your info to my email
---> lazem02(at) gamil and will help you out as much as possible ...
check following picture of similar work i did before

RE: Excavation support

hoshang, how do you think you will be abke to install horizontal reinforcing steel through the webs of very closely spaced soldier beams? The more you explain and question, the more concerned I am about what you are trying to do. Get help.

www.PeirceEngineering.com

RE: Excavation support

(OP)

Quote (PEinc)

hoshang, how do you think you will be abke to install horizontal reinforcing steel through the webs of very closely spaced soldier beams?
My mistake. I didn't consider horizontal reinforcing steel of the basement wall. It seems the underpinning approach of earth retention would be the best choice for my case. Till now the neighbors didn't agree intrusion into their ground. If we make them agree explaining the risks of collapse of the soil under their foundation in case bracing didn't work perfectly, didn't underpinning 8m deep need bracing or anchorage?

Quote (ALK2415)

we seamed to be in working at the same region, send your info to my email
Please check your email box.

RE: Excavation support

Generally, any underpinning supporting about 2.5m to 3m of unbalanced earth pressure will need lateral support. I expect any underpinning for your project would need tiebacks or bracing.

www.PeirceEngineering.com

RE: Excavation support

(OP)

Quote (PEinc)

Generally, any underpinning supporting about 2.5m to 3m of unbalanced earth pressure will need lateral support. I expect any underpinning for your project would need tiebacks or bracing.
Thanks for your response. Can underpinning bracing be done by corner struts and horizontal cross struts? or rakers?
Are there any ways rather than underpinning not to lose lot area?

RE: Excavation support

Underpinning can be laterally supported by tiebacks, soil nails, cross braces, inclined raker braces, or corner braces.

Temporary ground freezing (not recommended due to other resulting problems) and permanent jet grouting are other methods of building support. Underpinning can include concrete piers, helical piles, micropiles, jacked piles, jet grouting, chemical grouting (risky voodoo?), bracket piles (which take up room), and slant-augered piles. See any edition of Robert Ratay's Temporary Structures in Construction, preferably his 2012 3rd edition. Concrete piers, jacked piles, jet grouting, chemical grouting, and slant-augered piles are installed under the existing building and encroach very little or not at all into the proposed building. Jet grouting, chemical grouting, and ground freezing will require being trimmed back in order to not encroach into the new building. More underpinning information can be found in Winterkorn & Fang's 1975 Foundation Engineering Handbook and Alan Macnab's 2002 Earth Retention Systems Handbook.

www.PeirceEngineering.com

RE: Excavation support

(OP)

Quote (PEinc)

Underpinning can be laterally supported by tiebacks, soil nails, cross braces, inclined raker braces, or corner braces.
I'm not sure, but if tiebacks aren't available here, what's the simplest form of tieback one can use? I mean DIY (Do It Yourself).

RE: Excavation support

If tiebacks are not allowed to cross over the property, you cannot use tiebacks. The same with any support members that could be placed under the existing footings. In my area, if the foundation wall is jointly owned, you would have permission to underpin but not necessarily install tiebacks.

www.PeirceEngineering.com

RE: Excavation support

(OP)
Thanks.
Please review my post on 21 Jan 24 20:38.
So, if tiebacks aren't available in my area, what's the simplest form of tieback one can use? I mean DIY (Do It Yourself).

RE: Excavation support

This project is not one for someone to be learning how to install DIY tieback anchors.
If I remember correctly, you have mostly very stiff to very hard, cohesive soils. Am I correct? If you are allowed to use tiebacks, I would look at using auger-drilled (preferably using a single-stroke, continuous auger), re-groutable, solid bar or strand tieback anchors. I would use an auger with a 150mm to 200mm diameter. Much of what you chose to do will depend on the tieback anchor design loads that you calculate.
Installing helical tieback anchors can be relatively simple BUT your very stiff to very hard cohesive soils will make installing multi-helix anchors difficult. Without multiple helices on an anchor, your anchor capacity will probably be less than you need.

www.PeirceEngineering.com

RE: Excavation support

(OP)
Hi PEinc,
can you elaborate more on auger-drilled (preferably using a single-stroke, continuous auger), re-groutable, solid bar tieback anchors? I searched YouTube for this, is it this:
https://www.youtube.com/watch?v=fAHW0jDfwdo

RE: Excavation support

(OP)
Also, for 8m pier underpinning, should concreting be a 1_pour process? Is it analyzed as a sheet pile wall?

RE: Excavation support

Your YouTube video shows good information on tieback anchors but not on single-pass anchors or hollow stem auger tieback anchors. You need to know more about tieback anchors than I can type on Eng-Tips. See if you can find online related design manuals on tiebacks and permanent anchored wall. Search for downloadable manuals on the US FHWA Publications web site. Search web sites for tieback drill manufacturers.

Underpinning piers should be excavated full height and then be concreted full depth in one pour. It is poor practice to perform deep underpinning using multiple lifts of shorter piers. Do not underpin the underpinning. Read Ratay's book.

www.PeirceEngineering.com

RE: Excavation support

(OP)
Thanks, PEinc,
what about my query:
Is it analyzed as a sheet pile wall?
I mean is pier underpinning analyzed as a sheet pile wall?
I used Figure 3-4 in USACE EM 1110-2-2504 and found phi to be 30 degrees. Is it reasonable?

RE: Excavation support

If you have continuous concrete underpinning, it can be analyzed as a sheet pile wall to get the bracing loads, bending moments, and shears. Non-continuous (intermittent) underpinning piers with lagging between them can be analyzed as soldier beams. I expect that a 30 degree phi angle could be reasonable for a very dense, drained, cohesive soil. You need to review the project Geotechnical Report, analyze the borings, and choose soil properties you believe to be appropriate. I can't/won't do that for you. Check to see if you have any testing results, such as Atterberg Limits and plasticity indices.
Get some help from an experienced, local, design engineer and sheeting/underpinning/tieback contractor.

www.PeirceEngineering.com

RE: Excavation support

(OP)
Hi, PEinc,
You have been of great help. I uploaded soil reports on 26 Dec 23 18:38 & 26 Dec 23 18:47.

RE: Excavation support

hoshang - man just quit while you are massively ahead

RE: Excavation support

(OP)
Hi, PEinc,
As you can see in the soil test report, PI is 22 to 25. So using Figure 3-4 in USACE EM 1110-2-2504, phi will be 30 degrees. Is it reasonable?
Another point came into my mind:

Quote (PEinc)

Non-continuous (intermittent) underpinning piers with lagging between them can be analyzed as soldier beams.
How the lagging is installed into underpinning piers?

RE: Excavation support

Charts of PI vs. phi shows phi to be about 30 degrees for PI of about 22-25. Use properties that you believe to be correct.
Read the chapter on Underpinning in one of Ratay's books. I have his 1984 and 2012 editions which both show a detail for lagging between underpinning piers.
Again, get some help from an experienced, local, design engineer and sheeting/underpinning/tieback contractor.

www.PeirceEngineering.com

RE: Excavation support

(OP)
Hi PEinc,
does drypack on top of underpinning piers engage at-rest soil pressure on the piers? Or shall they be considered as if they were subject to active soil pressure (without considering drypack engaging at-rest soil pressure)?

RE: Excavation support

Drypack will be subjected to the same lateral pressures and compressive stress that the underpinning pier will see. No special analysis is required for drypck. It is usually much stronger than the soil upon which the footing was originally bearing. The compressive stress or bearing on drypack is usually insignificant (often about 10 to 50 psi or 70 to 350 kPa) for a properly prepared and installed sand-cement mixture. Instead, worry more about the shear and bending stresses in the underpinning concrete and the bearing at the bottom of the underpinning.

www.PeirceEngineering.com

RE: Excavation support

(OP)
Hi PEinc,
Sorry for not wording it correctly.

Quote (hoshang)

Or shall they be considered as if they were subject to active soil pressure
Consider the concrete pier without the compressive stress from the building above the concrete pier. The concrete pier will be subject to lateral active soil pressure, correct?
Now, consider the compressive stress from the building above the concrete pier. Doesn't the existence of compressive stress from building above concrete pier change the active soil pressure to at-rest soil pressure?

RE: Excavation support

I have never designed underpinning for at-rest pressure and have never had a problem.

www.PeirceEngineering.com

RE: Excavation support

(OP)
Appreciate your valuable posts.
About drypack, should it be dry? Why? Can one use wedges?

RE: Excavation support

Drypack is described in Ratay's books. Drypack is a damp mixture of sand and cement, about 2 parts sand maximum to 1 part cement. It has just enough water added that it can be formed into a stiff ball. It gets rammed into place using a small sledge hammer and a piece of 2x4 lumber. You hit on one end of the 2x4 with the hammer. The other end of the 2x4 compacts the drypack. High strength, non-metallic, non-shrink, flowable grouts are unnecessary. Although steel wedges are often described, I have never used wedges and believe that they may be capable of damaging the existing foundation, especially rubble stone foundations. Even if you use wedges, you still need to drypak between the bottom of foundation and top of underpinning.

www.PeirceEngineering.com

RE: Excavation support

(OP)
Hi PEinc,
appreciate your valuable reply.
It may be a newbie query:
Is it acceptable to use a plastic sleeve in the position of tieback while concreting the underpinning piers so it makes a hole through the concrete pier for the tieback? Is the tieback anchor through the underpinning interpier lagging or through the underpinning piers? If through the latter, what about the piers at the corners of the lot (the drilling rig can't reach this location)? In this case my thought is that the sequence of work should be such that the interpier lagging should be at the corners. If so, what detail should be used at the corner?

RE: Excavation support

The tiebacks are installed through the underpinning piers. Usually an angled boxout is installed in the piers at each tieback location before the pier is concreted. The boxout provides the proper angle for the tieback and provides a big enough flat area for the tieback bearing plate and the hydraulic jack. The hole through the pier can be drilled or be a sleeve set in the pier before it gets concreted.
If the pier is 3 to 4 feet wide (about 1m to 1.25m), it is usually possible to install a tieback in a corner pier at the end of the building. Some drills are more articulating than others. Talk to the tieback contractor about how close to the corner a tieback can be drilled.

www.PeirceEngineering.com

RE: Excavation support

(OP)
Thanks PEinc,
can drypack take loads from the building above and transfer it to the underpinning piers in the case of excavation support?

RE: Excavation support

The drypack is usually, probably always, much stronger than the soil upon which the existing foundations are bearing, unless bearing is on very hard bedrock. The drypack almost always has the same bearing area as the existing footing.
You still need to check that the building does not slide off or overturn from on top of the underpinning. This depends on how much soil is pushing on the back side of the existing foundation wall and footing.

www.PeirceEngineering.com

RE: Excavation support

(OP)
Hi PEinc,
Does ground anchor need casing? If so, how it's installed?

RE: Excavation support

(OP)
Hi PEinc,
Does ground anchor need casing? If so, how it's installed through underpinning piers?

RE: Excavation support

It there is any chance that a drilled tieback hole might collapse, the drill hole needs to be temporarily cased. Even fractured rock can require temporary drill casing.
My 18 Feb 24 21:12 post talked about installing tiebacks through underpinning piers.
You need to give this design to someone else who knows what they are doing. You are heading into big trouble!
This is the last response I am giving to this thread. I am done.

www.PeirceEngineering.com

RE: Excavation support

(OP)
Hi PEinc,
Your help is highly appreciated, thanks.

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