Building a Basement Under Two Garages

Possibly simpler to knock down the garages?...depends on the thickness of the slab they're standing on?
Use the rubble as hardcore.

Oh BTW...remove the cars first.
Del

I would venture a guess that it would be less expensive to move the garages out of the way, dig the hole, build the basement walls and access, and then move the garages back to a new foundation. Either that or leave the garages where they are and build the basement elsewhere and then move the garages to the new location.

Thanks for the comments but moving the garages is not an option.

Both of them are blockwork buildings. They would have to be completely levelled and re-built. Too much cost implications.

i fear this would also contribute to why you shouldn't try undermining...?

Undermine is a great idea,shoring has been done since the sun startrd shining

Even in London at the height of the property boom (a distant 6 months back just before the crash), it was not worth excavating beneath a single storey garage for a basement.

The cost for the underpinning and the interfaces between old and new created too much cost compared to demolition and re-build with new basement.

So if you cannot afford to demolish (or deconstruct) then you cannot afford to create a basement with the original building in place.

You could perhaps create a small basement. If both garages are 18m x 6m then this is 18m x 12m overall? Assuming a maximum of 3m height from ground to basement foundations and that a 45 degree slope would protect the current foundations from undermining, then you could construct a smaller basement through the middle about 12m x 6m.

The central wall would need a transfer beam from one end to the other perhaps with posts along the way.

If this were an option, then a soil report is the first thing you need based on a simple scheme design.

GA. Digging out a foundation isn't impossible 3 of my neighbors have done it as you've suggested.

Or another neighbor borrowed a bunch of kids from boys home and they dug a trench under the center and few others very narrow for supports. After the supporting poured concrete sections were complete then they finished the digging.

Hell, just get some sky hooks and appropriate hoisting apparatus and pick them up until you get your basement built.

Seriously, with a structure(s) the size you noted, the number and sizes of the needle beams and main beams that would be needed to permit raising it without major cracking and damage seems to me to be an unlikely prospect unless you have an experienced house mover nearby who has such beams in his possession and can have them tied up for the length of time it would take to excavate for and build the basement.

It seems to me that the cost of preparing the existing structure(s) for jacking and the jacking process, along with the higher level of difficulty in building the basement, would more than offset the cost of razing them and building anew on the new basement. You might be able to salvage the roof structure, minus the roofing material, to save on that aspect of the re-building. If the existing structure(s) are very old, I can almost assure you that you or the owner would be better off with an entirely new building.

As the old saying goes, "If you can't afford to do it right the first time, where do you expect to get the funds to re-do it?"

kcoleman --

First and foremost you need to get an idea of the soil conditions before you start this job. For all you know there may be a local water table there or the garage may be built on fill. That should be the first contract. Are there soils engineers with portable core drilling rigs in your part of the world?

When our California house needed a lot of work after the 1989 earthquake a couple of guys brought in a portable core driller they unloaded from a truck and wheeled into the backyard for getting the cores in exactly the places the civil engineer doing the study told them to drill. This could even be done inside the garage using a diamond core drill to cut clearance holes for the soil drill if itself can't cut the concrete with possible rebar in it.

My guess is that if everything looks OK the construction approach will involve diamond sawing the floor slab into sections so you'll have nice straight lines along the walls to line up the new foundation with and then working section by section.

Have you figured out how you will build the ceiling to carry the live and dead loads from the structure above and live load from whatever will sit on the floor (the basement ceiling)?

My house is conventional USA single story wood frame construction approx 110square meters. Earthquake repairs consisted of lifting and leveling the structure an average of 12cm with almost 50 separate jacks. Then the perimeter foundation was built up to the hanging mudsill.

A room size area 5m x 10m was excavated to a distance of 2.45m after removing the support posts previously used and spaced in a 1.2m x 1.2m grid. This room became a basement workshop.

The support posts were replaced by a US wide flange steel beam 8" high, 6.5" wide and 32 feet long with support columns 1/4" wall square 4" x 4" steel tube 86" long 10 feet on center. This beam down the 10m length of the room was designed to carry approx. 50 pounds per square foot live load along with the 4 walls of the room. Note here that 2 foot x 2 foot x 6 inch thick concrete pads were poured with rebar extending from the perimeter of the pad under each column. This was done before removing temporary support jacks (screw type house jacks in the ends of 2" (50mm) schedule 40 steel pipe lengths and pouring the rest of the floor. We used Simpson adjustable post caps on US 4x4 lumber lengths when we ran out of house jacks.

OK, done rambling. Hope at least some of this helps.

Ed Weldon

Put a basement under a existing home one time would never do it again.

Check compaction of the ground under your new footers and basement floor.

If footer sinks oops would not cover it.

you have not said if there is a slab but I assume there is. The design of a slab for a garage relies on the support of the foundation soils below. The slab transfers the weight of the vehicles and other material kept in the garage to the soil below and primarily exist as a hard surface, not as the load bearing member. consider howmany times you have seen a driveway or sidewalk break under the load of a vehicle when the soil has been distrubed under it. The cost to support the structure and replace the slab (with one designed as load-bearing ceiling to your proposed basement) as a part of your construction cost, along with new supports for the structure will surely end up more expensive than following the suggestions you have already had made: demo the garages and replace them or move them out of the way. I have dealt with supporting existing small buildings to do work under them in my 20 years as engineer with a public utility. you end up, ALWAYS, with some damage to the building, it willl be due to uneven compaction or support or in the process of installing or removing the temporary system. You will end up paying more in the end.

An old friend tried to build one of those under a house.

He had to take out some rock and uncovered a cavern under the building.

He dropped a rock tied to a 250 ft line and the rock never hit bottom.

I am uneducated in the field of civil engineering. My opinion is that asking for this kind of help here means you're not qualified to do this estimate. If you're cornered and must come up with a plan of action, get your soil appraisal done. Hope that the ground is not suitable for a basement. Pay the outside soil analyzing team, take the small handling fee for getting the analysis done by an outside group and leave. Don't tell the owners this can be done without demolition.

kcoleman,

First if you do not know EXACTLY the way the concrete was made for the garage floors, DO NOT ATTEMPT THIS without demolition. The probability of the concrete being correct to support the cars without earth under the entire slab is extremely slim. The liability of parking a car on it is not one that you or the owners would want to take on. Just Walk away from the job. It is not worth the risk of that kind of an accident.

Second if you do know the way the concrete was built, is it sufficient to support the vehicle weight as a bridge. If not, demolition is required. If it is, then under pin it and start the excavation.

I can not see any code that would allow that much wieght to be undermined for a basement.

Espically when unemploymenr is so high the extra jobs of destruction and then building new should get a low interest rate loan from the SBA.

Close in the ground floors and build a third floor for parking.

Now you have an above ground basement, and two floors of parking above.

I believe I would price surrounding realestate as a buy option to use for what ever storage your company might need.

Surely buying a small lot and building a storage building would be cheaper than what than building a basement under those garages. You could build a large enough building for growth instead of a fixed basement limited space.

Something tell me this is a test of your ability to say no or speak truth to power.