Which is worse, heave or subsidence?

[epsilonGreedy]

When designing foundations which dynamic force worries a structural engineer more:

 

1. An upwards force, or.
2. The ground beneath the foundations dropping away.

 

In the context of this question I am not referring to diagnosed heave or subsidence that is already damaging a building, my interest is which presents more concern when designing trench foundations to cope with anticipated heave or subsidence.

[Tennentslager]

It's all a matter of degree and likelihood and then engineering an acceptable solution.

Professional competence coupled with experience and knowledge of the published standards will provide an independent answer for which I guess someone will want you to pay for.

Or in other words, I've no idea whatsoever

[Ian]

@epsilonGreedy

the real answer would be that it depends on the type of ground eg:

clay - the engineer would typically worry about heave 

peat - the engineer would worry about settlement, especially differential settlement between the building and the drain connections as they enter the building.

In the case of larger steel frame buildings they can also worry about wind uplift forces. I once designed a 100,000 sq. ft. factory with a shallow pitch metal roof where the engineer sized the concrete pad foundations by weight to resist wind uplift loads acting on the roof

[ToughButterCup]

I'm no expert in heaving or subsiding, but I am good at searching for sources of relevant information

[epsilonGreedy]

8 hours ago, Ian said:

clay - the engineer would typically worry about heave 

 

 

I had not considered the issue of drain disconnection.

 

The Cranford Soilscape map describes the soil for my plot as "Slowly permeable seasonally wet slightly acid but base-rich loamy and clayey soils".

 

http://www.landis.org.uk/soilscapes/index.cfm

 

The prompt for my question was a thought about the best time of year to pour foundations. If a clay soil is prone to some seasonal expansion/contraction I wonder if the best time to create foundations is half way through this cycle i.e stress the foundations with 10mm of heave and 10mm of subsidence as opposed to pouring foundations in late August with subsequent 20mm of heave.

[Ian]

7 minutes ago, epsilonGreedy said:

 

The prompt for my question was a thought about the best time of year to pour foundations. 

The best time to do ground works is always when it’s dry weather (summer would be good) especially if your subsoil is clay based. It can turn into a sticky quagmire when you put machines onto wet clay. The engineer will design the foundations in a way which prevents heave - eg to a certain minimum depth and lining the trenches with a slip membrane/board to stop the clay grabbing the concrete. They will also need to consider proximity of any trees to your foundations.

Edited February 20, 2018 by Ian

[nod]

11 hours ago, epsilonGreedy said:

When designing foundations which dynamic force worries a structural engineer more:

 

1. An upwards force, or.
2. The ground beneath the foundations dropping away.

 

In the context of this question I am not referring to diagnosed heave or subsidence that is already damaging a building, my interest is which presents more concern when designing trench foundations to cope with anticipated heave or subsidence.

I think that you can prevent subsidence 

But ground and trees may make Heave enivitable 

Weve put clay board around all foundation to prevent the likely heave

[ToughButterCup]

Have you worked out yet whether you are likely to be covering your clay with (say) MOT1?

 

Our site is on a slight slope; we needed a piling mat: that is built up on MOT1. So despite lots of clay (alluvial till), in a real sense, the problem disappeared under 400 tonnes of MOT1.

 

As soon as you step off the foundation build-up you can see the effects of the clay: bogged down Land Rovers and Great Crested Newts, waltzing to their hearts content.

 

[StructuralEngineer]

On 2/20/2018 at 11:25, recoveringacademic said:

 Land Rovers and Great Crested Newts, waltzing to their hearts content.

 

What a sight that must be!

 

On 2/19/2018 at 22:41, epsilonGreedy said:

When designing foundations which dynamic force worries a structural engineer more:

 

1. An upwards force, or.
2. The ground beneath the foundations dropping away.

@epsilonGreedy Ground heave is more difficult to remediate once it's happened, for sure. Ground dropping away can be dealt with by underpinning (if a foundation) or resin injection (if a slab). If a slab is lifting, more invasive interventions could be needed such as breaking out the slab and reinstating -- not cheap, not easy to design, and very time consuming (while the home owners have to move out). This is one more reason why a suspended ground floor is a good choice at design stage.

[Dreadnaught]

3 minutes ago, StructuralEngineer said:

This is one more reason why a suspended ground floor is a good choice at design stage.

 

Is the downside  of a suspended floor that it compromises the insulation by necessitating an air gap beneath? 

[Ferdinand]

19 minutes ago, Dreadnaught said:

 

Is the downside  of a suspended floor that it compromises the insulation by necessitating an air gap beneath? 

 

Not convinced by that .

 

I don't see the air gap on the outside, ie the atmosphere, as compromising the insulation of a wall.

 

F

 

Edited April 4, 2018 by Ferdinand

[StructuralEngineer]

You'd usually have insulation over the slab if it's concrete or beam and block. For Timber you can have it between the joists. A gap allows for air circulation which generally is a good thing (especially if radon or methane is present. For a cast in-situ concrete slab you would have void formers, so no air gap as such.

[gravelld]

There are interesting discussions about filling voids under floor boards, see Sophie Pelsmaker's work and also discussions in GBF.