New Basement below an Existing House
New Basement below an Existing House
(OP)
I have to provide a method statement and outline design as part of a planning application showing how we intend to retrofit a basement under an existing building and demonstrate how we will maintain support to the existing structure and not distress it.
To make matters more complicated the house is part of a terrace and it is also a listed building. If you're not familiar with the term listed building, it means it's of signiicant historic interest.
The attached section shows typically what we're trying to create. The project is in it's infancy and therefore we do not have a contractor on board yet to benefit from their insight. Any thoughts?
To make matters more complicated the house is part of a terrace and it is also a listed building. If you're not familiar with the term listed building, it means it's of signiicant historic interest.
The attached section shows typically what we're trying to create. The project is in it's infancy and therefore we do not have a contractor on board yet to benefit from their insight. Any thoughts?






RE: New Basement below an Existing House
Most of those guys know what they are doing if they have been in business for like 20 years.
I guess you could shore the house in place and do the foundation - but you would be working with tight tolerances and limited working room.
Good luck and it won't be cheap.....
RE: New Basement below an Existing House
I was interested to know if anybody out there has done such a scheme and could suggest what works well in terms of both design and construction.
RE: New Basement below an Existing House
2. Insert stiff beams through the building, extend the beams beyond perimeter of the building on each side, and set the beams on temporary foundation.
3. Cut existing structures below the beams.
4. Insert vertical posts and horizontal shoring below the beams as excavation proceeds. Remove existing underground structural elements.
RE: New Basement below an Existing House
Cheers,
YS
B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...
RE: New Basement below an Existing House
I am doing a big one in London at the moment.
First of all, I suggest you do some reading such as:
CIRIA Report 139 - Water resistant basements
BRE CBG72 - Basement Construction and waterproofing
IStructE - Design and Construction of Deep Basements ....
The last one is for more serious basements but the principles are still generally the same.
There are two most commont ways in which this type of basement is done - contiguous/secant piles or by underpinning the party walls.
For the first method, you need to be able to get access for a mini pile rig - they do fit in houses if you have a big enough opening fo them to get in. The maximum pile size you will get drilled with one of these is about 350mm.
For the second method, it is simply a matter of extending the existing footings down on concrete to below the level of the new basement in sequential stages.
You may also need to provide props to these against earth pressures.
For any of these you will need to provide calculations for the temporary works as these will need to be approved by the neighbors engineers (under the party wall act).
I suggest you get hold of one of those references(at least) because there are a lot of things to know. Good luck.
RE: New Basement below an Existing House
Good luck, sounds like an exciting project. I understand that building planning/approval in London can be a detailed process.
RE: New Basement below an Existing House
Never, but never question engineer's judgement
RE: New Basement below an Existing House
That is a very American detail, it is much wetter here in the UK and the codes tend to be a bit more stringent because of it. Basements are typically reinforced with the equivalent of #5 bars at 8" each face to limit crack widths. Makes that sort of detail difficult.
RE: New Basement below an Existing House
Never, but never question engineer's judgement
RE: New Basement below an Existing House
I have done a couple. Lots of work to hand excavate.
Main Problems to look out for
Water how to stop it. and avoid changing underground flow patterns.You could damage neighbours foundations from this alone.
Underpinning and being very careful of disturbing soils under adjacent properties.
You will, have to ensure that you can maintain the original compaction under the adjacent properties .
This means grouting to stop soil slippage and wash out by water flow.
Summer is best time to take this on less water about.
You can pile or place the new foundations in 3 ft sections.
I always prefer not to pile as any vibration will cause law suits from neigbours over cracking etc. They will find cracks in walls they never even looked at before.
Survey existing foundation levels and nighbours properties to prove that they have not moved while you are working.
Just some things to think about.
Its an expensive process go cost plus or write a very
detailed contract detailing whats included and what is going to be extra carefully. You never know what you will find could be bodies, gas lines old sewers.Who is paying for
the work if its stopped or some other authority has to get involved. You may even get a permenant stop work order from the court if adjacent properties show damage.
Intrusion Prepakt /marineconcrete.com
RE: New Basement below an Existing House
RE: New Basement below an Existing House
Never, but never question engineer's judgement
RE: New Basement below an Existing House
I missed your section on the first read, but everything I suggested still stands.
What you have shown on the sketch will not work due to health and safety. The reason is that the underpinning needs to be done in 1.2m maximum sections and sticking rebar into the soil on either side is not acceptable as it inhibits the follow on construction of the next stage.
I would suggest that you do plain concrete underpinning and then follow up with a reinforced concrete liner on the inside.
Keep in mind that many of the comments on this have been from an American perspective, UK standard practice is actually considerably different in a number of ways even though the principles are the same.
RE: New Basement below an Existing House
Helicopter the house, dig a hole, drop a precast box, put house back in. Should be quite efficient - time wise.
RE: New Basement below an Existing House
thanks for the advice. will dowel bars inhibit construction?surely they can dig the soil out around the dowels or just dowel in bars after you've excavated for the next pin. like the idea of mass concrete underpinning but the width of this + the rc liner wouldn't this bring the face of the basement wall much further in than the upper floors.
RE: New Basement below an Existing House
Never, but never question engineer's judgement
RE: New Basement below an Existing House
Digging the dowels into the soil would diminish the support provided by that soil and make it harder to excavate.
Perhaps you could look at using rebar couplers between the pins.
I would think that csd72 has suggested the best solution, but you will lose some internal space. A drained cavity construction could also be an option. It's likely that insulation will also be required.
RE: New Basement below an Existing House
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RE: New Basement below an Existing House
First, the assumption is that the building in question is red brick as 95% of buildings in the UK are or at least it is some kind of brick. Is not this too heavy to shore up? Unlike wood frame structures that can tolerate minor movements while being shored up, will not a brick building crack and show it?
Since the building does not have cellar(basement) so it will have concrete floor or some solid arrangements, how will this be shored up during construction of a cellar under it?
Consider that the property in this thread is terraced(means it shares two walls with two adjacent properties so the shoring up will not be only for one property but for three properties.
This is too complicated to do, but I am sure people with deep pockets can do miracles. I wonder if a tear down is better option, but I remember it is a listed building !!!
RE: New Basement below an Existing House
apsix was correct in his explanation.
Another issue that I forgot to mention is that you cannot effectively tank (waterproof) the underpinning. You would need an internal wall anyway. Even if you could then the waterproofing for your house will be in the neighbors property - a party wall act nightmare waiting to happen.
It is generally accepted that basements like this will not achieve the full footprint.
I really recomend you read the references that I listed.
RE: New Basement below an Existing House
RE: New Basement below an Existing House
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Oxbridge, would it be possible to show the sort of location you will be working in and show the constraints?
RE: New Basement below an Existing House
BA
RE: New Basement below an Existing House
Even if you underpin the wall with concrete sections, minipiles, helical piles, can you guarantee that the soil on the opposite side of the party wall will not fall into your excavation? Your neighbors could come rolling in!
Soil stabilization is the real problem. What is your soil type? Have you inquired on the geotech forum? They may have some ideas. Remember what is your risk versus your reward--some projects may not be worth it.
RE: New Basement below an Existing House
The soils under the nieghbours would have top be stabilised with grout before excavating which is itself not an easy process.
The cost however would be very high and your construction insurance is questionable. If you have not got general coverage for this type work like a major company might have. Insurance companies are going to turn you down.
Intrusion Prepakt /marineconcrete.com
RE: New Basement below an Existing House
Prepakt1,
Exactly what I was trying to imply.
RE: New Basement below an Existing House
The one downside of this site. 787,000 engineers separated by a common language.