Fellow Cheesehead,

I would offer the following:

1.  Underpin the existing footings before excavating the new basement.  This is the safest course and then you don't have to worry about how close to it you can dig.

2.  For the underpinning, layout an elevation view of the existing foundation wall and footing showing all existing grades and the bottom elevation required.  Divide the length of the wall to be underpinned in 3'-4' widths.  Number the sections in sequence 1-10 if using 4' widths and a 40' total length.

3.  You want to pick a sequence to work on, such that no new width is next to a underpinned width of less that 7 days old.  So, you might select widths 1, 5, 10 as your first sections to underpin.  Then select 3 and 7; then 9.  Now you will have to wait until 3 and 7 are 7 days old to complete 2 and 6. Lastly, work on 4 and 8.

4.  To justify the 7 days, I would use a 5,000 psi concrete mix, which should yeild 3,000 psi in 7 days.

I might add that this is not something for the do-it-yourself homeowner if they haven't done it before.  It is a lot of hand work, plus it can be a risky operation depending on soils, weather and building loadings.  

Good luck!

One word of caution: partial underpinning can cause long-term foundation problems.  The risk is dependent on a lot of factors - the major ones include subsurface conditions, weather patterns, the structural condition of the building and its foundation, the support of the new addition, and the owner's expectations.  I'd hire a local contractor to do the work - hire the one with the most experience and least number of lawsuits, not the cheapest or slickest.

cheesehead,

I agree with jheidt2543 on most but not all of his comments.

Draw out the elevation as suggested. Include the original ground elevation inside your existing building and the bottom of existing footing elevation.

Excavate to approximately 1 foot above the bottom of the existing footing to be underpinned.  Make sure the excavated area is gently sloped away from the existing footing to prevent ponding of surface water.

Make sure all roof drains and downspouts are detoured away from the work area to prevent accumulation of water in the work area.

The underpinning should be performed in sections of approximately 4 feet in width.  If the existing footing is rubble stone instead of concrete, it might be necessary to use less than 4 feet wide underpinning sections. Usually excavation for an individual underpinning pier is done by hand.  The excavation is usually shored with 2x10's if the soil is loose or the pit is deeper than about 4 feet.  Shoring also helps give a relatively uniform, vertical face to the poured concrete. Most importantly, it also protects the worker from collapse of the excavation.

Excavate an underpinning pit to about 6 inches or a foot below the proposed subgrade, shoring the hole as required.

Fill the excavated, shored pit to within 3 inches of the bottom of the existing footing.  Use about 3000 psi concrete.  Stronger concrete is rarely needed.  The next morning after pouring an underpinning pier, drypack the 3 inch space with a slightly moist (not wet) mixture of 2 parts sand to 1 part cement (non-shrink cement is not required).  This drypack mix should be rammed or packed into place between the bottom of footing and top of underpinning.  Reinforcing steel is rarely needed in underpinning and I do not expect that your underpinning will need any rebars either.

Do not excavate two pits simultaneously if they are within 8 feet of each other, edge to edge.

As soon as a pier is poured and drypacked, an adjacent pier may be excavated.  There is no need to wait 7 days as suggested by jheidt2543.  Overnight, the concrete is now stronger than the dirt it replaced and almost all of the concrete shrinkage will have occurred.

Continue excavating, pouring, and drypacking the individual underpinning piers until all 40 LF is underpinned.

The underpinning pits should be at least the same dimension as the width of the existing footing.  That is, if you have a 2 foot wide footing, the underpinning pier should be 4 feet wide (along the wall) by at least 2 feet under the footing.

If the existing building has any columns along the wall to be underpinned, then additional precautions must be taken to prevent undermining the column footing.  These precautions are beyond the scope of this lesson.

Underpinning will support the vertical load of the existing building. However, you must also consider the lateral load on the underpinning created by the height difference of the dirt under the existing building and the proposed excavation subgrade.  The underpinning acts as a gravity wall and must be checked for overturning and sliding.  Sometimes this will require larger, heavier underpinning piers to get the proper safety factors. What good is underpinning if it supports the building vertically but can't retain the earth and surcharge pressures laterally?

jheidt2543 is right about this not being for the do-it-yourselfers.  It is hard work and slow work.  I recommend you find a contractor who has significant experience in underpinning.  I've seen many excavators and concrete contractors who claim to know how to underpin but they rarely do it right.  They get by on luck rather than know-how and skill.

In any case, make sure you set up some survey monitoring controls on the existing building before the underpinning is started. That way, you can tell is you are causing any movements while underpinning.  Also, it is not uncommon for a building being underpinned to experience some small amount of settlement during the work.  Usually an 1/8 to 1/4 inch is not too noticeable.

I would also like to suggest that you hire an experienced underpinning company.  You will also want a company that gaurantees their work from any future foundation movement and settling.  Be careful, so companies offer warranties with "acceptable levels of foundation movement".  That is acceptable for them, but should not be for you.

Rob

While I have done a lot of pit underpinning, it has been confined to heavy commercial buildings.  I underpinned a house converted to business with addition extending 8' below existing floor level last month with piles and shotcreted the excavated face with a few tiebacks.  For comparison sake, pit underpinning is over $1000/CY, while helical piles are less than $1000 in my area, and a small shotcrete wall is about $35/SF.  

g7mann [geotechnical]
Dear Chesehead:

You might also consider the use of driven-in-place small diameter [typically 2 or 3 inch] steel pin-piles to provide underpinning support.   These piles can be driven inside or outside the foundation, whichever is the most practical and least expensive.   The piles should be driven to "refusal" which is defined as "less than 1-inch of pile penetration after one full minute of contimnuous driving with a 90 pound jack hammer."  The piles shouold be driven as close to the perimeter of the foundation as feasible and, once driving and proof testing is completed, should be structurally connected to the exisitng foundation.   

The connection is typically in the form of a reinforced concrete pile cap or a continuous reinforced beam which is, where feasible, poured against and beneath the footing and is doweled into the existing foundation.  If it is infeasible [or expensive] to overexcavate to install the pile caps or grade beam they can be poured against the face of the foundation and/or stem wall and then structurally doweled in-place.

The pin-piles should be installed at approximately 4' - 0" center-to-center spacings and each should be capable of developing well in excess of four thousand (4,000) pounds in axial capacity provided the refusal criterion has been met.

Research we have performed indicates that if the refusal criterion is met axial load capacities in excess of twenty thousand pounds can be achieved.   If the piles are relatively short and/or are surrounded with reasonably firm soil [soil that may be compressible, but not junk]they should not be subject to any significant lateral loading or lateral deflection.   

This underpinning system has been used extensively in the Pacific Northwest for many years with virtually no detrimental impact.   One of the advantages is that you can limit the amount of overexcavation and backfilling, and all the equipment is man-portable a useful point in constrained access sites.

PEinc,

Your statement that overnight the concrete is much stronger than the soil is true but the typical specified waiting period is either 96 hours or until 50% of the concrete f'c is obtained,or by concrete cylinder testing. Also the typical spacing between pits dug at once is 12 foot not 8 foot as you indicate.Your statements are not in line with NJ and NY standards of good practice. Patience is required when underpinning. Excellent article you posted otherwise.

cap4000,

I have designed and built major underpinning projects in a number of states, including NJ and NY, for over 23 years.  Current projects include the University of Pennsylvania Archaeology Museum and Temple University Hospital.  Other projects included Princeton University, Harvard Medical School, Brigham and Women's Hospital, Thomas Jefferson University Hospital, Cornell University, the New Jersey State House Capitol Building, the Philadelphia Criminal Justice Center,  and others.  I respectfully disagree with you on several points.  Waiting 96 hours or until the concrete strength reaches 50% of f'c before starting another pit is not a standard specification.  In addition, rarely are concrete cylinders required.  Generally, the required strength of underpinning concrete is a fraction of the strength of the concrete ordered and placed.  If the concrete piers are 4 feet wide, constructing every third pier would mean skipping 2 piers or, as I said, 8 feet edge to edge or about 12 feet center to center.  This is common.

You are correct that underpinning requires patience.  But patience should not mean unnecessary delay.  By the time all of the underpinning piers are excavated and poured in an area, they are usually strong enough to support the building while the new structure is excavated.  A properly laid out sequence of pier construction will assure that the concrete has sufficient time to develop sufficient strength.

It is my experience that engineers who routinely specify high strength concrete, non-shrink drypack grout, treated timber shoring boards, and long waiting periods between piers or before drypacking [b]usually[b] have minimal real experience in underpinning.  They only know what the read - and there isn't enough practical reading available on the subject of underpinning.  Some of the above requirements may be valid at times, but those times are rare.

I'm glad you appreciate other portions of my post.  Thanks.

I recall one job where the inspector wanted to test the drypack strength so he scooped some into the grout cube molds.  We told him that's not the way it's installed and took a 2x2 and sledge hammer and pounded it into the molds. I don't recall the break but it was ridiculously high.

Good point, PSlem.  Even relatively weak drypack or concrete should be much stronger than the dirt that originally supported the footing. A footing with a bearing pressure of 5000 psf is still only 35 psi bearing!  Why even sample the drypack or concrete? There could be some cases where the concrete strength is more critical (for example, deep tiedback, plain concrete underpinning piers or post-tensioned interpier beams) and may need testing, but not usually.

PEinc.

While I was intially reading your post I knew you were a real rare expert in underpinning. However,leading books written by Tomlinson,Bradbury,and Prentis and White,and Jordan don't exactly advocate your aggressive expertise. While practical underpinning experience is a real must and clearly you have, I wonder if your your Elite Projects Required "Temporary Shoring" of the wall loads prior to your underpinning??. I also have worked on Historic Projects whereby crack monitoring was very closely watched. Typically 12 foot was clear pit edge to pit edge dug simultaneously. Finally, have you ever considered writing an article to ASCE on your works? It certainly seems like you easily could..As Usual Nice Job.

PEinc

Your soil bearing pressure of 5000psf=35psi is an interesting correlation. ACI concrete bearing strength is 0.7 x 0.85 x f'c at 28 days. For 3000psi concrete that would = 1785psi.Thus,2 percent of that would be your 35psi required for the soil. Your aggresive preachings I dare say are clearly and easily confirmed.

cap4000,

Thanks for the kind words.  But a few more comments.

Usually, the structures being underpinned have not needed any temporary shoring.  Several times, while underpinning small column footings, I've used temporary shoring but wall footings and large column footings usually don't need this extra support.  If you undermine no more than approximately 25% of a column footing at one time, you should not have problems unless there are other non-average conditions to deal with.  Walls that do frequently need shoring are walls that were originally common party walls where one of the buildings was removed and there is now nothing to keep the wall from moving laterally, away from the remaining building, due to vibrations, wind, etc.  These walls need lateral bracing.

Monitoring is important for all projects.  Some projects require additional (excessive?) monitoring.  These projects include historic structures, highly ornate structures, and structures whose owners are looking for a lawsuit.  If you didn't survey the structure before and during the work, you won't know if you caused the problem or how much of the problem should be yours.  Also, pre-construction surveys can discourage frivolous lawsuits.

If you can work pits no closer than 12 feet edge to edge, great. Do it.  If the soils are weak or the building has already undergone unacceptable settlements, then work pits farther apart so as not to aggravate the situation.  But on most projects, time is of the essence and soils are not weak.  The biggest killer to a GC's or CM's schedule is watching a subcontractor underpin a building.  As far as they are concerned, all underpinning takes too long.  They seem to forget that the purpose of underpinning is to protect people and property.

Writing articles would be very interesting. However, while I am doing that my clients would be waiting for their designs.  Most of my work is for contractors who need everything yesterday.

Most engineers never realize that underpinning concrete and drypack are so lightly loaded.  Underpinning piers are usually large in size because the worker needs to be inside the pit while digging and a certain bearing area is needed on the dirt beneath the concrete pier.  Sometimes, with unreinforced concrete underpinning piers, there is some bending and therefore tension.  Adding reinforcing steel into a pier should be a last resort.  Usually the pier has a large enough section modulus that the tension stress in the pier is below the allowable.

With respect to having a waiting time for concrete pier shrinkage to occur, remember that most of the concrete shrinkage occurs in the first 12 hours.  So, if you pour a pier this afternoon, by tomorrow morning, most of the shrinkage has already occured, the concrete should be stronger than the dirt that was previously there, and the pier should be ready for drypacking.  The sooner you get the building resupported (drypacked), the better off you are.  Because drypack is relatively dry and placed in a thin layer (about 3 inches), it too will have insignificant shrinkage.  Therefore, non-shrink grout is usually a waste of money.

I know you think I am being aggressive, and I know what the few reference books say about underpinning but that's how underpinning is done by companies who do real underpinning for a living, on an everyday basis.  Authors generally try not to be too aggressive.  They are writing for people who are not experienced.  And they don't want readers to get into trouble.  Excavators and concrete subcontractors who dabble in shallow underpinning are not real underpinning contractors.  A recipe for a problem is to hire an excavator to dig the holes and hire the concrete sub to pour the concrete.  When this is done, no one is totally responsible and neither really understands underpinning or its requirements for pit spacing, drypacking, shoring, etc.  They know excavating; they know concrete.  They just don't know underpinning and don't know that they don't know.  Underpinning is a specialty.

Finally, I'm not trying to be aggressive.  I want to design it right, build it right, have it perform as intended, be economical, and get the job done ASAP.  I can't afford any problems.

Sorry to be so long.

PEinc,

Once again you have made an interesting statement where you say Authors- "are writing books for people who are not experienced in underpinning". I can only say in closing that should be the crystal clear understanding when you give your responses to people in this Forum as they are only looking for general conservative advise and not to go into a business based on ENG-TIPS particularly underpinning.
As for those impatient CM and GC's just show them the many failures that have occurred during the underpinning specialty process. As you said they don't that they don't know and that's their fault not your's.

Agree with Focht3 on the partial underpinning comment.  In smaller buildings 1 and 2 storey and residential seasonal loads such as snow in my neck of the woods can be significant and not occur as design loads for several years in which case in you may find differential settlement and resultant cracking due to the different characteristics of the bearing stratum.  If i understood you correctly Focht3?

Currently underpinning a 5 unit residential foundation left exposed to freezing over the winter and destroyed the clay soils for 12 in below the footing. heaving and cracking was extensive and wide spread throughout the foundation and know we have 12 inches of muck below the footing.  Solution was sequential underpinning with new footings founded on undistrubed sub-base.  New footing poured to within 2 inches of underside of existing footing and then drypack ... simple enough.... not so from the Contractors perspective.

He's fought the dry-pack issue from the get go feeling that he can vibrate enough concrete into the form and under the footing etc., etc.,... Comments?

Additionally our responsibility is actually the structural side of the foundation we have two different Geotechnical Engineers looking after the 'Dirt' side, CYA so to speak, our premise being that we'll start the foundation repair once we know the building isn't moving, but we havent specified continuous underpinning and new footing, instead looking at the load on the new footing from a bearing capacity and settlement perspective and then the shear capacity of a deep unreinforced concrete beam (ie. wall) to space the new footings... Comments?

To make it more challenginging, when i showed up on the site today, the Contractor had drywalled all the units and painted two, but only one unit was partially underpinned... comments?

Yes we wrote a letter saying do not perform anymore work until the underpinning work was completed.



 

You are correct, connect2.

This is a very good thread.  PEinc did a great job in talking about the 'real world' of underpinning.  I agree with his comments.  I have yet to find an underpinning project where high early strength concrete was truly needed; it has usually been specified to satisfy the owner and/or architect.  Depending on the site and soil conditions, I usually have piers installed in an alternating sequence - every other pier, not every third one - and only require a 24 hour "wait" time before the next one is excavated.  Consequently, twelve feet between excavations sounds excessive to me.

But sequencing is often the key to successful underpinning.  I was on a job site at the Galleria Mall in Houston in the mid 1980's when a GC (with no real underpinning experience) tried to underpin a footing supporting a fire escape walkway.  The column between the footing and walkway already made me nervous - it looked like a toothpick at a distance when the soil was excavated from around it.  (The column's L/r was increased by at least 50% as a result of the foundation excavation.)  Then the GC dug the four underpinning piers beneath the footing - at the same time.  I raised nine kinds of hell with the superintendent - he insisted that I was being overly conservative in raising the alarm with the owner and structural engineer.  

The footing, column - and walkway - failed while the structural engineer and I were talking about my heated conversation with the superintendent.  The walkway 'unzipped' from the building - beautiful and terrifying at the same time.

No one was hurt.  But it was a needless accident caused by a GC's superintendent who "knew better" than the engineers and specialty subcontractors.  If he had only left that work to a qualified subcontractor...

Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.  See FAQ158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

Gentlemen,

My apologies up front for invading your forum (I'm not an engineer, though I do have a degree in Chemistry - but am in Finance by way of career), but I came across it and you all look like a group that can definitely give me some advice. If it's inappropriate for me to write here, please forgive.

I'm a homeowner in NJ - pretty handy - but am planning a project that's clearly too big to do myself, involving excavation below grade to build an attached, partially below-grade garage onto my 100 year old house.  The roof of the garage would be formed by a concrete (brick or stone or tile facade) patio, about 4 ft. above grade.  I won't waste your time further with the details as to why I want to do this, but I have pretty good reasons.  My problem is that I'm having a hard time finding an architect and/or structural engineer to do the necessary planning. I just don't know where to look, besides the yellow pages which I hesitate to do, and never having hired an architect or structural engineer before, I don't know what kinds of qualifications to look for, what kinds of questions to ask, etc.

If anyone can give me some tips on where to begin, I would really appreciate it.  If it would be more appropriate, you can reply directly to my e-mail on drobezisi@yahoo.com.  I won't disturb your forum any more after this -- it just seemed too good an opportunity to pass up.  Again, sorry for the intrusion.

Kind regards,
Drobezisi

Try the yellow pages, 'Consulting Engineers', there will be lots of good experienced firms in your area to specifically deal with your situation.