New Basement below an Existing House

[oxbridge]

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?

 

[MiketheEngineer]

The easy answer is get a house mover to lift the house. Then put in the basement and drop it back down.

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.....

 

[oxbridge]

The property is in London, UK and believe it or not these are becoming very common nowadays bacause of the the returns you get for habitable floor space. Typical basement will set you back £200k.

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.

 

[kslee1000]

1. Provide sheet piles a few feet around perimeter of the building.
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.

 

[youngstructural]

I am currently doing something similar to kslee's suggestion at an old courthouse to provide new cell space. Not cheap, but effective, and while I thought Mike's suggestion was a great idea, piles is probably your next best bet if you didn't like it.

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[csd72]

oxbridge,

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.

 

[BSE05]

It looks like you may be a good candidate for underpinning with either mini or helical piles. You will need soil boring and sample information. Talk to a good geothechnical engineer. You will have to calculate the spanning capacity of the existing foundation wall or if necessary you may have to reinforce it to span between piles. The piles will be of temporary use until your new concrete is in place. Helicals can be installed with offset plates. Both types of piles can be installed without undesireable vibration.

Good luck, sounds like an exciting project. I understand that building planning/approval in London can be a detailed process.

 

[COEngineeer]

How about something like this? I've done this before but I made the basement smaller than the existing walls. Just a rough idea. You need to do the excavation in sequence. Maybe 2 m at a time. I hope im making any sense.

Never, but never question engineer's judgement

 

[csd72]

COEngineer,

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.

 

[COEngineeer]

But what is wrong with I drew? I did not put any reinforcement. Since the wall is build in sequence, It is better to do the basement slab in one pour to have a better finish.

Never, but never question engineer's judgement

 

[Prepakt1]

Depends totally on the situation


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

 

[MaInspector]

interesting topic, I have seen a few projects where a full basement was add to a structure. Lifting the building and excavating the full basement should not be a hard project. My advised is to pour new full height walls to existing sill plates. Problems I have incountered are installing propper foundation ancors through existing plates. Also will you be required to install footing drainage? If so; where is it draining to? I sugjest doing some testing of the soil before any excavation begins, too much water in the ground can cause the excavated side walls to collapse. Good Luck!

 

[COEngineeer]

The problem with your structure is the foundation is not very strong. If you had a deeper foundation, it would have been easier because it can span.

Never, but never question engineer's judgement

 

[csd72]

oxbridge,

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.

 

[kslee1000]

Another American way:

Helicopter the house, dig a hole, drop a precast box, put house back in. Should be quite efficient - time wise.

 

[oxbridge]

csd72,

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.

 

[COEngineeer]

Ok.. underpin it 1.2 m at a time then. I am not sure what rebar into the soil you were talking about.

Never, but never question engineer's judgement

 

[apsix]

The 'rebar into the soil' is to provide dowels into the next pin (or lap length for the horizontal bars in the adjacent pin).

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.

 

[BAretired]

Is there room to drill piles adjacent to the existing wall? Is the soil suitable for friction piles? Are we concerned about undermining neighboring property? We do not have enough information to make an intelligent recommendation on this.


BA

 

[aayjaber]

In 1991 I fixed a house in England (Red Brick and also listed for historical reasons). I had a cellar (basement) under part of the house which was of decent size but was not water proofed so it was very damp and I did not use it. I did not know much about construction otherwise I would have trenched around the house and water proofed all the sides. The other part of the house had concrete floors.

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 !!!