Plate load test

[Mr Blobby]

So a bloke came on our site today (from another building firm) and asked if we are doing a plate load test after the hardcore is compressed down for our KORE slab.

 

Before work started we did get an engineer on site to dig some holes and peer in to see the ground make up.  He was happy enough for the KORE slab to go in.  The site is not liable to flooding.  There was a house on the site previously although most of the new house is on virgin ground.

 

What is a plate load test and do I need one?  I don't want an extra day of a digger on site waiting to see if it sinks (if that's what a plate load test is). 

Did anyone else do this?  Seems a waste of time to me. Comments please.

Edited April 5, 2023 by Mr Blobby

[George]

Not usually done on domestic scale works. Check the KORE slab specifications, it may be a requirement of the warranty. 

 

A plate load test puts a known plate size with a known load and measures the deflection. This can give good, site specific information on the ground bearing capacity. 

[Gus Potter]

On 05/04/2023 at 13:10, Mr Blobby said:

What is a plate load test and do I need one?

Hello all.

 

A bit of technical stuff and a few tips on self building an insulated Kore (say) slab.

 

On 05/04/2023 at 15:45, George said:

Not usually done on domestic scale works.

Yes spot on. For a bit of fun here is an bit more of an explanation as to why and what is more important (in my view) for insulated raft design and construction on self builds.

 

Roughly the plate load test involves a stiff metal plate, usually 300, 450 or 600mm square, a jack and a strain gauge and a load gauge (ring). The plate goes on the ground, and you jack against something heavy. You can create a big cradle weighed with kentledge.. expensive for smaller stuff or you can use a 16 tonne excavator to provide the kentledge. Excavator is the easy way as you drive it about.. if you have the room.

 

You start to jack measure the amount the plate sinks by at what load, plot a graph etc and from that you derive what is called the CBR ratio (California Bearing Ratio) based as a percentage value. The CBR value is a number that is often desired for highway design. In a domestic context we may want to know this if say we are building a few houses and the shared access is to be adopted by the Highways agency or for some reason the fire brigade have a particular requirement for access during a fire.. they need to know the access will not sink under the weight of a turn table ladder.. you would have to have an exceptional site for this.. but just to mention anyway.

 

Another time you may need to know the CBR ratio is if you are doing piling, need a heavy rig and need to design a piling matt for the rig to safely get in and do the job. The piling Contractor will do due dilligance and often ask for this number especially if they are designing the mat.

 

If you find you get stuck on this there is the option of proof rolling. Here we may run a roller with a dead axle weight of 5 tonnes over the ground.. if it moves about like pastry then we know it is not promising.

 

Now a key thing about the CBR (plate) test is that it only gathers good information down to 300 to 600mm, much depends on the size of the plate and the ground you are testing. The pressure / stress the plate exerts on the ground is roughly the shape of a slightly extended (droopy) light bulb (incandescent). Lastly it is possible using some rules of thumb, charts to roughly convert the CBR value to a value that is roughly equivalent to how elastic a material is called the modulus of elasticity and this is good as we now have units that are compatible with our hard core, slab insulation and concrete slab above.. not quite but closer.

 

But on an insulated raft we are probaly using an insulation 300mm thick on 150 - 300 mm of well compacted hardcore. These spread the load a lot so the underlying formation ground is much more evenly loaded. The CBR value is now not really that useful any more to us on BH. What is useful is the value the SE puts on the soil they see in the hole as this gives them the allowable (presumed) bearing capacity of the ground. Often you see a value of 100 - 120 kN/m^2 in decent clay soils. That value equates to you being able to put about ten tonnes per square metre on the ground with the expectation you might get some 15 -25mm settlement in a clay soil. Houses are designed to move about a bit.

 

Now if we know the allowable bearing capacity of the underlying soil, know we are using good well compacted hardcore, know what kind of insulation thickness we need and roughly what kind of concrete slab is going on top we can then design all these layers for the loads. What 99% of the time drives the design (Kore etc) is the slab loadings (slab point loadings / spread out loads) and how these interact with the insulation directly below. This is the critical bit as it drives slab thickness, amount of rebar required and the complexity of the slab / rebar and that is one place where money can be lost. The SE then iterates the design to get the most economic slab by balancing rebar, concrete thickness, concrete strength, concrete shirinkage (movement joints, water bars if applicable) and the underlying layers of insulation etc.

 

OK. Things some you can look / think about when doing an insulated slab to set you on your way for a good job.

 

1/ When you excavate out the ground to the level that you are going to start from.. called the formation level look after the virgin ground surface, be quick to get a covering layer on it if say clay or friable chalk to stop it softening in the rain / frost or drying out too much. remember that value of 100 kN/m^2 .. if you soften the top 50 -150mm that bit it will move later. The last time that soil may have seen daylight could be some 100k years ago so treat it gently.

 

2/ Lay your hardcore in thin layers 100 -150mm thick and get a good whacker. Take extra care at corners.. try and get the compaction even.. no soft spots but also no really hard spots. Hard spots can break the back of a foundation.. strip foundations can suffer in particular if you have say a big boulder that does not move. What you are trying to go is to create a platform that moves up and down in an even way. Rafts are less succeptible but never the less.

 

3/ Often you need to run drains under and that involves excavating a trench and back filling. The objective here is to try and backfill so the filling is about as squashy as the ground next to it. If you were to say backfill with a lean mix 10N concrete you create a really hard spot on a line that invites a floor slab to crack, if you chuck disturbed clay back in that has got wet you'll now have a nice soft line under the slab.

 

Hope this helps someone.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

[Mr Blobby]

Thank you @Gus Potter for your detailed explanation.  I sat down with a fresh cup of coffee and read every word. 

As I had suspected the builder (not my builder) who visited the site and told us we needed the PLT once the hardcore was installed was talking out his backside.   He'd seen it once before so had jumped to the wrong conclusion that it was compulsory for a raft-type foundation.

 

 

12 hours ago, Gus Potter said:

What is useful is the value the SE puts on the soil they see in the hole as this gives them the allowable (presumed) bearing capacity of the ground. Often you see a value of 100 - 120 kN/m^2 in decent clay soils. That value equates to you being able to put about ten tonnes per square metre on the ground with the expectation you might get some 15 -25mm settlement in a clay soil. Houses are designed to move about a bit.

 

This is good to know.  I had thought the SE trial-pit investigation was perhaps a bit of a waste of time, but now I am reassured that it was a very worthwhile exercise.  I went back to read the SE report again in light of what you have written above.  Our SE report says that we have sandy boulder clay with bearing capacity of 150kN/m2.  Which sounds pretty good to me given the desired ranges you list above. 

 

It looks like we are blessed with good ground conditions, though I did suspect it would be pretty rocky after trying, and failing, to install an earth rod some time ago 🙄

 

 

[Gus Potter]

On 15/04/2023 at 10:29, Mr Blobby said:

Our SE report says that we have sandy boulder clay with bearing capacity of 150kN/m2.

Yes looks like you have some good soil to work with plenty spare capacity. You can get rafts (strip founds to a lesser extent) to work well below these values.

 

Here is a thing for all. If you get a ground investigation done you may see the soil descriptions. Maybe written like this CLAY with sand or.... SAND with clay, silt etc.

The word in capital letters denotes the dominant component of the soil.

 

Now if we are working in soil that has a dominant CLAY content we are interested in the swell shrinkage potential of the CLAY as that is driven by proximity of say trees and where you live in the country. Up north it rains more so excepting trees the founds don't need to be so deep, but down south you need deeper founds as the drying effect penetrates deeper.

 

Where you have a dominant SAND content we are much less interested in the swell shrinkage potential but very interested in where the water table is and where it might be in the future. The soil investigation may give us a bearing capacity of the SAND. Sand gets it's bearing strength from intergranular friction between the particles and this is driven by the density of the sand and what is above it adding load. But when the water table rises the water reduces the effective density of each sand particle so there is less friction.

 

In practice if we start with a bearing capacity of a SAND type soil of 100 kN/m^2 but find the water table may rise to within the pressure bulb under the found we then often say we are losing half the friction between the particles so we will half the bearing capacity to 50 kN/m^2.

 

 

 

[saveasteading]

1 hour ago, Gus Potter said:

Up north it rains more

Not that simple!

 

North West yes, North East no.

I noticed that Tunbridge Wells has much the same rain as Nairn (600 miles ish north). 750 to 770mm

TW has had very serious flooding in recent years, hence my interest.

 

Nairn and surrounds has more rainy days, but fewer torrential days. Ideal for the barley and the resulting whisky.

 

Back to trees. Your point is good though that the TW trees have more chance of drought, and so cause more problems.

 

[ProDave]

4 minutes ago, saveasteading said:

Nairn and surrounds has more rainy days, but fewer torrential days. Ideal for the barley and the resulting whisky.

One of my memories of living in Oxfordshire, were the periods when it rained non stop for days on end, and it was quite a relief when the rain finally stopped.  We never get that up here.

[Gus Potter]

1 hour ago, saveasteading said:

Not that simple!

Yes your are right but in the round and to give folk a flavour of the things that occupy our minds..

[Kelvin]

Really interesting stuff. My groundswork guy explained some of this to me when doing our foundation. Especially the bit about digging out the drainage under the slab and backfilling to the same consistency as the surrounding ground. He also wondered about the parallel two thickening layers in our slab that support the stairs. The SE design had deeper parallel Vs that then were filled with more concrete than the surrounding slab. He was more used to seeing blockwork doing the same job.