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Stress Cutoff Solution

Stress Cutoff Solution

Stress Cutoff Solution

30 ft deep excavation is proposed for mat foundation of 50-story highrise right next to 100 years old 4-story stone building with 18" wide stone foundation, 7 ft below grade. Rock is 20 ft below the excavation (50 ft below ground). To minimize settlement of the stone building, geotechnical engineer requires rigid excavation support system that extends to rock that provide stress cutoff and will not cause lateral or vertical settlement of the stone building foundations. I would think underpinning all the way to rock and provide temporary tiebacks with required dewatering and shoring which may be difficult given the high permeability of the material below water table. Secant shaft wall with tiebacks which may serve as wall for the highrise. would the wall be able to function properly and handle the high rise type vertical load and provide lateral support for the old building footings. The mat of the highrise is 15 ft thick and will be poured about 6-12" off the property line.

PS. caissons for the highrise was out of the window.

RE: Stress Cutoff Solution

I'd ask a firm such as Hayward-Baker for their thoughts. It's a mighty difficult problem you have.

RE: Stress Cutoff Solution

A stress cutoff would be unusual and expensive. This is a really challenging assignment. Tell your client to purchase the building and re-erect it far away.

RE: Stress Cutoff Solution


oldestguy - indeed it is difficult. I won't come to this forum with simple problems.

aeoliantexan - the 80 years old lady who owns the old house ain't going anywhere, oh and the building is marked historic.

RE: Stress Cutoff Solution

Interesting problem. We have an expert for temporary supporting systems in this forum (PEinc). Hope he can chime in soon.

RE: Stress Cutoff Solution

I think a secant piled wall with ground anchors (tiebacks) is an appropriate solution and could work. What are the ground conditions like above the rock?

Why are caissons for the highrise out of the window?

RE: Stress Cutoff Solution

You need an underpinning of the old house with jet grouting columns. Any retaining structure (diaphragm wall, secant pile wall) requiring soil excavation will produce horizontal movements and settlement already during the construction phase of this retaining structure. The real question is how much displacement the old building can take. If you want 0 movement then jet grouting prior to retaining structure construction is a good solution.

RE: Stress Cutoff Solution

Retrograde - above rock is few feet of highly weathered material and above that about 15 ft of high permeability gravel from river deposits. I believe cost is the reason behind choosing Mat over caissons.

BigHarvey - I don't believe that the old building is going to take a lot of movement before walls start to crack. I will dig more into the jet grouting solution and find some info about the expected horizontal deflection of that system. I think monitoring program is essential during construction.

RE: Stress Cutoff Solution

I have tried to make suggestions, but we do not have much information. For example, are there anything behind this stone building?

Anyway, I can tell one thing: Careful with jet grouting.

Jet grouting pressure may cause enormous movements. You cannot believe how much lateral movements it caused on a diaphragm wall, when we apply jet grouts to passive side of diaphragm walls, even without any excavation.

Also, you know that we use jet grouts to lift differential settled buildings.

So, any under-studied jet grout may effect the building even without any excavation. Literature does not contain extensive studies about that issue. If you will go with jet grouts, I can give more information.

RE: Stress Cutoff Solution

Jet grouting can be dangerous if not mastered. As long as you check that the spoil is arriving at the surface there is no danger of lifting the ground. I wouldn't use jet grouting to lift differential settled buildings because you don't control the amplitude of the movement you generate (just like with compaction grouting).
bdbd, can you share info about lateral movements generated by jet grouting on D walls ? It's an interesting subjecthttp://www.eng-tips.com/.

RE: Stress Cutoff Solution

Quote (EngMann)

I believe cost is the reason behind choosing Mat over caissons.

They may want to reconsider when they find out how much it is going to cost to build a retaining structure adjacent to the existing building.

RE: Stress Cutoff Solution

First, we have not seen anything about the location or dimensions of the adjacent building(s) and the property lines. There are several possible solutions - most are very expensive and not used for smaller projects. Slurry (diaphragm) walls, secant pile walls, tangent pile walls, and jet grouting are very expensive and not (very, very rarely) used on small jobs or jobs with small footprints. Therefore, you are left with cheaper, more common options: possibly underpinning or, if underpinning is not allowed, a very stiff drilled-in soldier pile wall, braced or tiedback as necessary.

Underpinning would not necessarily need to extend the extra 20 feet to bedrock. That would be very expensive. Design the underpinning to bear on suitable soil at or slightly below excavation subgrade. Dewatering will be very important and should be implemented prior to any underpinning or drilling for soldier beams. Soldier beam walls are considered flexible support systems. Therefore, they are often (usually?) not recommended for supporting closely adjacent buildings and underpinning is used. However, if you can't underpin, maybe because the adjacent owner will not let you on his property, you may have no reasonable option but to design an extra stiff soldier beam wall. The extra stiffness should help minimize (but probably not eliminate) movement and damage. Remember, even if you underpin the building, there may still typically be from 0 to possibly 1/2 inch (I edited "12 inches" typo) of settlement.

If the underpinning will bear on soil with much better bearing capacity than the soil under the original footings, you may not need as much bearing area and possibly spaced out underpinning piers can be used with lagging between. If the soil at the new excavation depth is similar to or worse than the original bearing soils, you will need equal or better bearing area for the underpinning. If the wall to be underpinned is a jointly owned party wall, you are allowed to underpin it but you still will need permission to install tieback anchors across the property line. For sheeting or underpinning, you may need to internally brace the support wall if the adjacent owner does not give a temporary underground easement for installing tiebacks across the property line.


RE: Stress Cutoff Solution

Quote (If the underpinning will bear on soil with much better bearing capacity than the soil under the original footings, you may not need as much bearing area and possibly spaced out underpinning piers can be used with lagging between.)

PEinc - when you say lagging do you refer to, for example, a steel beam to be placed directly beneath the existing footing and then propped with piers beneath? One issue we had with small job underpinning job was that the contractor was ready to prop piers directly beneath an existing old un-reinforced concrete footing with checking that the footing could span between the micro piles. Subsequently we installed a beam support the concrete footing.

RE: Stress Cutoff Solution

Frequently, underpinning piers can be installed with a space between them. This space need to have horizontal, wood, lagging boards placed between and attached to the piers at each end of the boards. What you are referring to is a steel, interpier beam that supports the existing footing between spaced underpinning piers. First you install the piers with seats formed where the interpier beams will sit. Then, you install the interpier beams, encase them in concrete, and drypack the top of the concrete to the bottom of the existing footing. The interpier beams cannot be very long or else you would be undermining too much of the existing footing. If the interpier beam needs to be significantly longer than about 4 feet, you would need to use a post-tensioned, segmental, cast-in-place beam between the underpinning piers, or use more closely spaced piers. Often, concrete wall footings can span between piers without needing interpier beams, especially if there is a good concrete wall sitting on the wall footing.


RE: Stress Cutoff Solution

Thanks PEinc for the clear explanation. Great pictures too.

I would imagine that a post tension, cast in place beam would be the least desirable option. How would the fibers be tensioned considering there is an existing footing on top of the proposed beam?

PEinc - On a side note, and a risk of hijacking the existing thread (which i apologize for), is there an aspects of your work that you would consider needing further investigation? I am doing a masters at the minute and finding it difficult to nail down a topic. A previous idea i had got kicked to the kerb.

RE: Stress Cutoff Solution

The individual segments of the post tensioned beam are installed before the underpinning piers. The segments have PVC sleeves installed near the bottom of the concrete segments which are drypacked as they are installed. Then, the underpinning piers are installed up to the bottom elevation of the segments. Then, the segments are post-tensioned with coupled threated bars which provide the reinforcement. Then the remaining upper portion of the underpinning piers are concreted and drypacked. As excavation progresses to subgrade (or tieback grade), the interpier lagging is installed.

Post-tensioned, interpier beams are expensive to install; BUT, a lot of time and money can be saved if you can eliminate some underpinning piers. You will need very good bearing (rock?) at the bottom of widely spaced underpinning piers.

Topic? How about investigating the amount and rate of shrinkage for concrete and drypack in underpinning piers? Compressive strength of properly mixed and installed drypack in its first 12 and 24 hours after placement?


RE: Stress Cutoff Solution

PEinc - I draw a sketch for reference. This is a high rise project and not a small project so secant shaft might be a justified alternative. Would you agree that if underpinning doesn't extend to top of rock, it will settle with the mat and cause damage to the existing building?

EireChch - you are welcomed and also encouraged to use any of my posts to bring in interesting topics the enriches the discussion.

RE: Stress Cutoff Solution

Can you not just use pile a contig/secant wall and use it as a permanent foundation for the building? Two birds one stone. If it can key in to the rock it should achieve very high capacities I would have thought? Then specify a temporary shoring system during excavation to limit the piled wall movements.

PEinc - this is definitely a dumb question, but how do they actually get the piers directly underneath the existing footing? I understand in conventional underpinning for small depths the concrete is installed in a hit and miss manner so that only small parts are undermined at a time. But those piers look very long (and sqaure?). How does the excavation for the pier remain stable, what rig is used to excavate and how does it get under the footing? Also what is the point in post tensioning, would conventional RC drypacked not do the same thing? Similarly, if micropiles were used to underpin, how can they be installed under the footing?

Sorry for the long list of silly questions, but interesting stuff!

RE: Stress Cutoff Solution

EngMann, how much mat settlement do you expect? What is the bearing pressure under the mat? You are removing 30' of soil which could be a load reduction of about -30' x 120 pcf = -3600 psf.

BowlingDanish, checkout my web site for information on underpinning. Underpinning piers are hand excavated as vertical, rectangular shafts under the existing foundations. Check out my web site presentation on Causes of Building Collapses in Philadelphia. The segmental post-tensioned beam is installed in short sections so that you do not undermine too much of the existing wall footing at one time. Then, the short segments need to be post-tensioned into a continuous lintel beam that will bear on underpinning piers. Micropiles are not installed under the existing footings. This would required too large a hole beneath the footing to fit the drill rig. Micropiles are usually installed through the existing footing or foundation wall or adjacent to the foundation and then are attached. Search for info on "jacked pile underpinning." While not the same as micropiles, this will give you an idea of the trouble you will need to go through to get piles under an existing footing.


RE: Stress Cutoff Solution

PEinc - The mat is estimated to settle about 2-3 in. the preliminary loads are in the range of 15 ksf below the mat.
I don't think it would be 3,600 psf because you remove 30 ft of soil but pour 15 ft concrete mat so most likely to be 1.8 ksf and ok say 13.2 ksf! I am sure final loads will get higher. thinking about the micropile idea you have discussed above, it seems reasonable to extend the continuous underpinning down to some level and pick up the load with micropile but oh boy you need to make sure the micropiles don't pick up any load from the mat or they will snap like a toothpick.

RE: Stress Cutoff Solution

Maybe the new building needs 20 feet of ground modification below subgrade to reduce the 2 - 3 inches of predicted settlement? Or, more typically, a building that heavy would be on caissons (drilled shafts) instead of a very thick mat. Replacing the mat with caissons could mean less excavation, less dewatering, less concrete, less underpinning, no micropiles. I don't think I ever saw a new building with a mat that thick, except for a building's deeper core area that was not close to adjacent structures.


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