Exterior residential tank access?

[Redacted]

Hi there, I am looking into putting constructing an access path to an existing concrete water tank underneath a house. The access chamber will be 4.5’ deep 3.3’ wide and is around 3’ away from the existing water tank wall. I plan on excavating down, doing bottom up construction (foundation, retaining wall and suspended slab on top.). However, I am trying to figure out the best way of getting underneath the existing house wall safely.

I have considered 3 options.

Option 1 : Is the safest option, just prop the roof and remove 3.3’ of the exterior wall to the ceiling. However, this is the most costly.

Option 2: cut out a 6”x3.3’ opening into the wall and cast a rc beam inside to support the upper wall when carrying out the works below.

Option 3: This would be the cheapest and I am aware when putting a new foundation such as a bench footing or mass concrete foundation under an existing wall, they do this by excavating 1m of earth every other meter under the exterior wall. They then fill the excavated portions with mass concrete and then do the same process again until the entire bearing surface is mass concrete. I note in my case that the opening that I need to cut out for this access is only 1m. As this is a relatively small opening, whilst the house has full bearing on either side, can I rely on this alone as a temporary case to hold up the house without putting in a lintel?

The CAD drawing is a plan of the tank area. The red dashed line is the new tank construction and the green is the existing. Hand sketches are elevations.

 

[SlideRuleEra]

I am trying to figure out the best way of getting underneath the existing house wall safely.

Option 1 : Is the safest option...

IMHO, you have answered your own question, and I agree.

Options 2 and 3 rely on optimistic assumptions about existing design / construction. When working with any existing structure unintended consequences happen everyday... and (even is their are no injuries or significant property damage) cost of recovery can easily exceed the price of performing the work right the first time.

http://www.SlideRuleEra.net

 

[r13]

What type is the existing foundation? Will under pinning helps?

 

[Redacted]

Thanks for the responses :

@SlideRuleEra that is true, just wanted to make sure I wasn't being overly conservative and spending money where it wasn't needed.

@retired13 the house is old (over 100 years) it wasn't actually built on a proper foundation. CMU or stone block (exterior wall) was placed directly on the ground.

 

[LittleInch]

You have a large concrete water tank UNDER your house??

Really?

Your hand drawn option 2 has no bearing support for the RC beam which looks like it will just fall down when you remove the bricks under it....

I had to underpin a section of a 100 yr old house where the "foundations" were about 12" below ground level and consisted of two bricks laid side on and then a double brick wall built over them.

They did the 1m thing, with props inseeted as they removed the 1m section which they later concreted in the blocks.

Maybe an alternative is to insert the two 1m blocks either side of your 1m cut out first, let then go off and then do the work. Even option 2 will increase loading on the rest of the "foundation" and may lead to movement and cracking.

So option 2a - Underpin the foundation and then insert beam with at least 150mm support onto the blockwork and then excavate to your mysterious tank.

Or fill the tank in and build a tank somewhere else or connect to the mains / well direct.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Redacted]

Hi Littleinch

Yes, there is a concrete water tank under the house. Quite a long time ago, an addition to the house was built over the tank, which was originally outside. I believe it has been that way for over 25 years. The floor is a suspended slab.

The sketch is just indicative, if I went with option 2 I would set it to bear a minimum of 6" on adjacent blockwork.

Just to note, I am not dealing with large loads here. When I took a look at the required steel for Option 2 it only required around 50mm2 of reinforcement to take the factored loads. Although I would've bumped this up to meet the minimum steel requirements based on the area of the beam itself.

Although I would prefer to not have the tank under the house, filling it and moving it would be cost-prohibitive.

 

[r13]

filling it and moving it would be cost-prohibitive.

If the tank is not affecting your living space, once filled, it becomes part of the subgrade, why the think of remove?