Zak S

Most cost effective option for foundation -

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1 hour ago, Zak S said:

What is it I need in the report

A local SE will know the makeup of the ground, and if that material found, provides certainty of what is beneath.

eg If they have hit stone and stopped, is the stone there in bands or solid.

The survey company might have stopped in this kinowledge.

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2 hours ago, joe90 said:

Coming to this late but could you consider a raft and timber frame  build, less ground  loading and probably less than the existing build which I guess has not moved so far?

Would building control be willing to go against recommendation if SI report even if I change the build method?

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22 minutes ago, Zak S said:

Would building control be willing to go against recommendation if SI report even if I change the build method?

If the foundation slab was designed by a structural engineer such as William Hilliard (who has designed passive slabs for a few here) then why would the BC not accept it?

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1 hour ago, Zak S said:

Would building control be willing to go against recommendation

They will accept a design by a  Chartered Engineer. The individual BCO is likely to have it looked over by another SE, and doesn't have the qualifications (or any thought ) to dispute it with them.

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1 hour ago, joe90 said:

If the foundation slab was designed by a structural engineer such as William Hilliard (who has designed passive slabs for a few here) then why would the BC not accept it?

Thanks. I will speak with him to see if he has any tricks which dont involve piling in building the slab. Issue is Abby Pynford quoted me 90k for piled voided slab as due to poor ground conditions piling seems must. But I will chat with William. Thanks. Much appreciated. 

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With the passive slabs you may have to remove a lot of material and fill with compacted stone.

 

If you are piling, the augered method causes less disturbance to neighbours than driven.

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6 hours ago, Zak S said:

Bullivant is saying they need to go 5m below the depth of pile to provide the warranty. What is it I need in the report which is not there which would confirm that there is nothing under the sand stone which caused the Refusal. Is it drilling a new bore with different equipment. I really want having to pay for the GI report twice. I have not yet settled the invoice but the current GI covered by stating the Refusal in the quote and me being not technical did not understand the implication. Any thoughts?

 

Bullivant (and other piling contractors) will state they need this or need that, but there will be variation between one and another, so it may be worth running the logs you have past an alternative, like some of the companies I mentioned previously. All of them are serious contractors with the knowledge and gear to get past most work.

 

Piling is like a dark art and often it seems that different people and companies will suggest different methods to resolve given issues. it is a case of "there's more than one way to skin a cat", and so you do potentially have other options, which you'll need to look into. 

 

With respect to the first post on page 2 and to Saveasteading, local SE's don't always know what's below ground. No one does "exactly". That's borne out in paragraph 2, under limitations of investigation. The best you will get is an educated guess based on other / prior work within the proximity of the job in hand. The only way to get a real "feel" is boring test holes and inspecting what comes out, and what resistance is experienced at varying depths.

 

Have you asked questions of other piling companies? It may well be worth trying that to see what comes back in advice or suggested solution. If you hear the same as Bullivant have said, then fair enough, but from my experience, I'd not take one suggestion and feel like that was it. Bullivant are good, but they're not the only show in town.

 

As the guys here have said, it may be worth exploring alternative solutions to foundations. There is no harm in looking into it.   

 

 

Edited by Makeitstop

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10 minutes ago, Makeitstop said:

 

Bullivant (and other piling contractors) will state they need this or need that, but there will be variation between one and another, so it may be worth running the logs you have past an alternative, like some of the companies I mentioned previously. all of them are serious contractors with the knowledge and gear to get past most work.

 

Piling is like a dark art and often it seems that different people and companies will suggest different methods to resolve given issues. it is a case of "there's more than one way to skin a cat", and so you do potentialy have other options, which you'll need to look into.

With respect to the first post on page 2 and to Saveasteading, local SE's don't always know what's below ground. No one does "exactly". The best you will get is an educated guess based on other / prior work within the proximity of the job in hand. The only way to get a real "feel" is boring test holes and inspecting what comes out, and what resistance is experienced at varying depths.

 

Have you asked questions of other piling companies? It may well be worth trying that to see what comes back in advice or suggested solution. If you hear the same as Bullivant have said, then fair enough, but from my experience, I'd not take one suggestion and feel like that was it. Bullivant are good, but they're not the only show in town.

 

As the guys here have said, it may be worth exploring alternative solutions to foundations. There is no harm in looking into it.   

 

 

I am currently speaking with RB, Abby Pynford (way too expensive for piled slab option), Van Elle and some others for innovative piling options to see if I can find cost effective options. Current conversation seems to be pointing to that due to ground condition Auger Piling wity ring and beam might the way to go unless some trade marked solution can be found. Ground conditon is just very soft clayey with high water table and low significant sheer strenght does not help. Slab without piling on it's own for 330sqm would be c45k without insulation. Adding piling and insulation the cost would be around 85-100k at which point project becomes unfeasible.

Edited by Zak S

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Surely, if you have decent access and can get a serious CFA rig in there to auger out the ground, that wouldn't be a bad option. A simple ringbeam and B&B floor may be worth considering.

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Bullivant are not cheap when it comes to sorting the beam, and so it might be worth getting costings from another groundworks company.

 

Of course, having the one team on it to finish makes life easier, and you only have one company to deal with, but as with everything, that does come at a cost.

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24 minutes ago, Makeitstop said:

Bullivant are not cheap when it comes to sorting the beam, and so it might be worth getting costings from another groundworks company.

 

Of course, having the one team on it to finish makes life easier, and you only have one company to deal with, but as with everything, that does come at a cost.

So I had a quote from local guy he has done lots in the area and also did house opposite.  He said 60k (30k for 55 piles and 30k for ring beam) for foot print of 300sqm which included one out building. Bullivant said £38k for 55 piles so slightly expensive and including ring and beam it was coming to 65k. Abby Pynford said 75k uninsulated Raft invoiced and 90k for with void. All are VAT exclusive. Another local company West Midlands piling said £52k. With block and beam included cost would reach to 75k easily without insulation.

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Definitely worth looking at some of the smaller contractors.  We had bottom-driven steel case piles (26 x ~10m).  The piling rig arrived on a trailer behind a 4x4 pickup, so mobilisation cost was only about £350 from memory.  Also worth mentioning, perhaps, is that my understanding is that achieving a 'set' on a driven pile doesn't rely entirely on hitting something hard; friction on the sides of the pile also play a part.

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15 minutes ago, Roundtuit said:

Definitely worth looking at some of the smaller contractors.  We had bottom-driven steel case piles (26 x ~10m).  The piling rig arrived on a trailer behind a 4x4 pickup, so mobilisation cost was only about £350 from memory.  Also worth mentioning, perhaps, is that my understanding is that achieving a 'set' on a driven pile doesn't rely entirely on hitting something hard; friction on the sides of the pile also play a part.

Hi. Thanks. What was the cost per pile? Given the ground is very soft to 9/10m I suppose it would be easy to drive it down?

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30 minutes ago, Zak S said:

Hi. Thanks. What was the cost per pile? Given the ground is very soft to 9/10m I suppose it would be easy to drive it down?

Memory is fading, so just dug the quote out!  These are 2016 prices, but £200 for mobilisation, £9.5k for 24 × 12m piles (credit at £10 per linear metre if less depth reqd) plus £13,650 for 111 linear metres of ring beam.  No idea what % to add to get to current prices though...

 

Also just found the quote from one of the big boys for the same job...£52k.

Edited by Roundtuit
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15 minutes ago, Roundtuit said:

No idea what % to add to get to current prices though...

 

 I would hazard 25%

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10 hours ago, joe90 said:

Coming to this late but could you consider a raft and timber frame  build, less ground  loading and probably less than the existing build which I guess has not moved so far?

Me too.. to add some of my thoughts to yours.

 

Before you go down the piling route I would make sure that you rule out the other options first and can justify to yourself that you have given it your best shot.

 

@Zak S "To me rebuild could only be justified if I build a standard 2 storey plus a loft kind of house. Given the foot print it could easily reach 600sqm but I onky need 450-500 sqm. But want to have decent size ground floor.

 

To follow up on Joe's point I would also ask myself.. can I design something that uses part of the existing foot print only and retain the existing founds? Then add some new founds similar to the existing to "fill in" the gaps. You have gathered some info by way of your GI report but you also have an existing building sitting there. I would want to have a look at the existing building, determine it's age and also when parts were added and altered. You see this with old steadings.. as say a farm grows they add bits, change it to accommodate larger farm machinery and so on. Different loads are added to the ground at different times.

 

Now this can be a mine of information as it lets you observe how the existing building has performed on the ground over many decades. You can see cracks, where it settles / heaves and so on. You can also calculate the load the existing building is putting on the soil under it's foundations. Already you have some hard evidence on what the ground can carry. This also gives you clues as to where you need to target any further investigation. Could be trial pits dug with a JCB.  It requires some experience to interpret what do see, if a lay person don't be put off using your common sense. It's your house / project so have fun with it and explore what you see. The ask say the Geotech / SE folk to explain your observations.

 

Also have a look at what is next door as the ground conditions can change a lot over a 100 metres, but right next door can provide the best clues as to what you may have just over the boundary and thus what you may have a few metres away.

 

I have had a quick look at the GI report (only in terms of the house though) and while underpinning the existing founds is always an option it's labour intensive and if you have high ground water.. a horrible job and difficult to get right. So this means cost £.

 

You have what looks like two water tables on the bore hole logs BH1 & 2. This is not uncommon. Very roughly the rain falls / your drains leak / you have old soak aways and want to put in new ones too. This water soaks into the ground and can get trapped on top of a less permeable layer, like CLAY. But below this layer of CLAY can be fractured rock or gravel overlying fractured rock, like sand stone and at this level lies the main aquifer. Often you find gravels / sands / silts before hitting more competant rock. This is due to weathering of the rock during ice ages and so on. This goes some way towards explaining the refusal term?

 

The lower water strike can be a primary aquifer that the water board use to extract drinking water from so watch out if you want to start messing with it! Always check with the water board early just in case. The upper water level could be attributed to what is sometimes called a perched water table. The water gets stuck by a layer of less permeable material and can't drain straight down to the lower water table.. thus it is "perched".

 

You see I have used capital letters above in places. You can also see this in the bore hole logs. Take borehole 1 log. At a depth of about 1.6m you find "..slightly silty sandy CLAY with fine roots" The capital letters indicate that the dominant portion of the sample is clay, hence in capital letters. Clay is less permeable than sand so it is preventing the water from draining down to the lower level.

 

Now above 1.6 you have ground with sand and sands can transmit a lot of water. Yes there is a bit of made ground but this may not be extensive. Farmers were / are not daft and don't tend to build steadings over made ground. But they do often have a midden outside (MADE GOUND) and this gets picked up on a GI!

 

Also if you pump like mad to keep your excavation dry you can suck out the fine portion of the sand, this weakens it and can cause a lot of trouble. Ideally you want to design a domestic found that is situated above the water level and avoid these issues if you can when dealing with sands.

 

There is much discussion on BH about clays swelling and shrinking but if you can be sure they are deep enough and always saturated this won't happen. I won't go into great detail but you can also find fine roots at depth. Yes they will rot but settlements are negligable and can be accounted for.

 

On the bore hole logs and in the "site work" section you can see they have carried out two types of penetration test. The SPT and dynamic probing.  The dynamic probing is complex so I'll leave that for now. But the STP number provides the Engineer with a rough (well very rough) feel for what might be worth a closer look. A rough rule of thumb is to take the STP number and multiply by ten to give you an indicative allowable bearing capacity. The allowable bearing capacity is a measure of what load you can put on the ground (with factors of safety) without the ground failing or you house settling too much.

 

Take BH 1 at 1.6 m SPT = 8 and BH 2 SPT = 9 at say 2.0m. Take the lower value 8 x 10 = 80 kN/m^2 ( 80 Kilo Newtons per square meter) which is about 8.0 tonnes per square metre.. quite a lot! But.. above this where want to put a found (could be a raft) there is water shown in you logs.

 

Sands get their strength (bearing capacity) from partly inter granular friction in laymans terms. For this to work each grain bears on the ones around it and each particle has a density of say 20 kN/m^3. But when the water rises the density is halved (see Archimedes or the nautical members of BH) so you have half the friction. Thus when doing conceptual stuff we divide the 80 kN/m^2 by two when in SAND type soils to give an allowable bearing capacity (ABC) of 40 kN/m^2.

 

Now you can often on a low rise building you can get a simple ground bearing raft with insulation under to work (with a fair wind) using an ABC of 40 kN/m^2. Make the insulation thicker and you are in passive house terratory.

 

You may ask.. what about the roots rotting? Well we just design the raft to account for the ground settling a bit and the odd soft spot. You keep the raft as high as you can and away from the ground water.

 

I would kick the tyres on this first before committing to piling, even if to rule the idea out and free up brain space.

 

I hope this gives you a bit of further insight and is of use.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Edited by Gus Potter
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@Zak S

 

Za.. an after thought.

 

When you are comparing piling quotes check what they require in terms of you preparing the site, access you may see mention of a piling matt. Much will depend on the size of the rig and pile type.

 

Often they will say you need to have the site preped up to X,Y,Z talk about making it hard to compare quotes.

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51 minutes ago, Gus Potter said:

To follow up on Joe's point I would also ask myself.. can I design something that uses part of the existing foot print only and retain the existing founds? Then add some new founds similar to the existing to "fill in" the gaps.

Thanks for such a thorough helpful advice. Greatly appreciated. Can I please check the followings:

 

Does this mean given its currenlty a banglow I will be limited to banglow (no build upward) option unless I go down the underpinning option which is a not an option at the moment.

 

55 minutes ago, Gus Potter said:

Could be trial pits dug with a JCB

So I had two full bore holes and third extended via with dynamic probe with two trial pits as marked on the plan. If I need to do more trial holes I would need to get ot sorted via a professional but what is it I am looking for which could not be done previously. Sorry just trying to clarify my understanding. 

 

58 minutes ago, Gus Potter said:

allowable bearing capacity (ABC) of 40 kN/m^2.

So that is 4tonne per sqm. Would this be enough for a banglow only or traditional block and brick two and half (loft) storey house?

 

1 hour ago, Gus Potter said:

I would kick the tyres on this first before committing to piling, even if to rule the idea out and free up brain space.

Definitely, I would consider that if it's possible and a cost effective solution. I had a chat with Hilliard Tanner (Ireland) as @joe90 suggested and he was to going to look at it but he mention thin steel cases concrete beam with raft ok top among one option so I am keeping fingers crossed. 

 

I had a chat with a guy from Helical screw piles and he said the raft on it's own (without insulation) costs 150 per sqm so for 330 sqm it would be c50k plus insulation and screed on top to sub floor level. Does that seem right?

 

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Hello Zak.

 

The thing I enjoy about BH is that you can chew the fat without getting your head bitten off. Have copied and pasted you text in italic with my comments/ fat!

 

"Does this mean given its currenlty a banglow I will be limited to banglow (no build upward) option unless I go down the underpinning option which is a not an option at the moment."

 

No. what we look to do is to see how we can redistribute the loads to "trick the ground" into thinking that nothing has changed too much and then we design for the little extra anticipated settlement.. yes my name is Potter but not Harry Potter, but do wear glasses.

 

So I had two full bore holes and third extended via with dynamic probe with two trial pits as marked on the plan. If I need to do more trial holes I would need to get ot sorted via a professional but what is it I am looking for which could not be done previously. Sorry just trying to clarify my understanding. 

 

You need to take a few steps back and look at what you have on site, you just missed the earlier steps? maybe call these earlier steps a walk over site study, preceeded by a desktop study. But unless you do this all the time then you probably won't know this. Don't worry though, your GI is of much value. I don't think you have done anything wrong, maybe just got the order / sequence mixed up?

 

I would put together a little "portfolio" on your project. A summary of how you came by the property, what you know about it, what you have done say planning wise, your research and so on. Include a title plan, the sort of things that show you have done your best to get as far as you can. Then a short bit on what you want your project to deliver, make it personal. Take that to a few local SE's and ask them if they can help. Don't mention fee cost at this stage. SE's know this is coming down the pipe anyway. You may well get a pleasant surprise and an SE will drop into your lap, pick up the ball and you can work away together finding the right solution for you.

 

You have an interesting project here that is technically challenging and there are plenty SE's that love this kind of stuff.. provided they get renumerated at a fair rate. In return they will often seek to mitigate their fee by saving you money.

 

So that is 4tonne per sqm. Would this be enough for a banglow only or traditional block and brick two and half (loft) storey house?

 

At 40kN/m^2 ball park you could achieve two storeys plus the loft. Just don't be gready with huge spans, beam and block for serious acoustics. You can still create a great family working home future proofed. The trick here is to get an SE in early and they can work with you to support the Architectural design, which will help avoid you going down blind alleys and racking up design cost.

 

Definitely, I would consider that if it's possible and a cost effective solution. I had a chat with Hilliard Tanner (Ireland) as @joe90 suggested and he was to going to look at it but he mention thin steel cases concrete beam with raft ok top among one option so I am keeping fingers crossed. 

 

By all means talk to Tanners but you may want to consider a local SE that also has a black book of local contacts, brickies, sparks etc and reliable local folk.. and hypothetically a really black book of who to avoid locally contractor wise, not that they do keep such a book as this may be illegal.

 

In terms of insulated rafts you may see insulation load bearing values of 130 - 150kPa (150 kN/m^2) at 10% compression but put this in at 250 -300 mm thick and it will squash a lot 25 -30mm.. too much. Now what SE's look at is what can the building tolerate.. say 10mm as this has to be added to any ground movement. We look at the load bearing value at say 1 -2% compression and now were are down to a bearing capacity much closer if not less than the 40kN/m^2 I have been mentioning.

 

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Thanks for the great pieces of advice 🙂. It seems that a structural engineer is a must at this stage to decide most cost effective design option (remodel/extend v rebuild) which should determine the foundation options, I could go down to. This seems very sensible.

 

PS: reading my comments in italics; a bit shocked at my spelling/typing errors. Apologies :)

Edited by Zak S

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1 hour ago, Gus Potter said:

You need to take a few steps back and look at what you have on site, you just missed the earlier steps? maybe call these earlier steps a walk over site study, preceeded by a desktop study. But unless you do this all the time then you probably won't know this. Don't worry though, your GI is of much value. I don't think you have done anything wrong, maybe just got the order / sequence mixed up?

This was sent by one of the company as history/background of the site along with their quote which was the highest c7k for SI and hence I did not choose them but they provided some history if the site. I suppose a desktop study is lot more detailed. Is it worth asking the GI company who did the SI for desktop study or should this be independently sourced?

 

 

 

Screenshot_20220115-004526_Acrobat for Samsung.jpg

Edited by Zak S

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54 minutes ago, Zak S said:

Thanks for the great pieces of advice 🙂. It seems that a structural engineer is a must at this stage to decide most cost effective design option (remodel/extend v rebuild) which should determine the foundation options, I could go down to. This seems very sensible.

 

PS: reading my comments in italics; a bit shocked at my spelling/typing errors. Apologies :)

Take a bit of time to digest and go back and read the other comments again from BH members. Play about on BH and read around, you'll pick up some great nuggets for later anyway. Then take a view on whether getting an SE feels right for you.

 

I'm not strong on spelling either and get carried away as it's social media!

 

Folk don't pay enough for a desk top study in general.. they see it as a box ticking excercise so they get what they pay for from the professionals. Nowadays professionals just provide a churn that looks good often with little value and try not to accept liability. It's a mess. No wonder there are reports floating about exploring why UK build costs are apparently high.

 

I've done a few desktop studies and a good one takes time.. some several days / weeks to gather the info and then a day /s or so and a night/s to sleep on it. This approach has saved some Client a fortune that has well out weighed the cost of the study and mitigated the cost of the on site ground investigation.. The really hard part is to convince a Client that what you are doing is worth while!

 

You can do an lot to help yourself here. I would put your back into it and do you own research, it's only your own time you are spending.

 

I clicked on the link for the desk top study but for some reason it is not downloading? can anyone help?

 

 

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Thanks.

 

7 hours ago, Gus Potter said:

Then take a view on whether getting an SE feels right for you.

Members here have said quite correctly to pay greater thinking time if existing building can be salvaged. With 1960 build being extended at the rear, one side and bit on the front, I will only be left with one wall and some internal walls. In order to get the UFH I will have to do bit of work on flooring as well.

 

If costings only allows one and half storey then I will be certainly keen to extend/remodel but costing allows traditional jouse building then I need to seriously consider the cost differential between rebuild and extend back and up  and remodel. Speaking with SE just the discussion of the plans/option would help declutter the thinking so I need to seriously consider that.

7 hours ago, Gus Potter said:

Nowadays professionals just provide a churn that looks good often with little value and try not to accept liability. It's a mess. No wonder there are reports floating about exploring why UK build costs are apparently high.

Very valid points. I will speak with my existing GI company to see given they have done the site visit if they can provide the desktop study. They are based on Yorkshire and I am in west midlands so someone local might be better.

 

7 hours ago, Gus Potter said:

You can do an lot to help yourself here.

Yes definitely keen to always  learn something new. I will explore various options. May be give Bham central library a visit and see what resource I can find on the area in the form of maps and previous studies. Issue is I have found some desktop studies but the ground condition seem much better there.

 

7 hours ago, Gus Potter said:

I clicked on the link for the desk top study but for some reason it is not downloading? can anyone help?

 

This was just an image from a quote provided by Soilution and confirmed what you had said. It not a link to a document.

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Hi @Gus Potter I had written to my GI survey company as to the possibility of softer ground below hard ground asking what safeguards were in place to reduce the risk of aforementioned issue. They came back with the following. what are the view in respect of accepting their comments/going for desktop study/reinviting them for few other boreholes after demolition.

 

Quote from GI as follows:

The risk of softer ground below the borehole termination depth is
considered to be extremely low, with the cementation of the sands
indicating the beginning of the Sidmouth Mudstone Formation, part of the
Mercia Mudstone parent bedrock group from c.7.5m, with the bedrock
formation for this area, based on British Geological data for the site,
being indicated to be in excess of 150m thickness.

As to safeguards of no softer ground below the final borehole depths,
with any investigation there is a cut-off point where further progress
cannot be practicably achieved with the equipment on site without risk
to the drilling rig/samplers, etc.  Given an SPT value of 45 is
generally sufficient to undertake the pile design, a 'refusal' N value
of 50 was adopted in the quote, as strata with values above this level
when using conventional bottom driven piles are generally sufficient to
prevent pile penetrate much beyond this point/level.

The direct standard penetration tests confirm increasingly competent
ground from c.7.5m depth with very dense/strong strata at termination
levels.  The probe test in BH3 also confirms the continual increases in
strength of the apparent weathered sandstone with depth. Final blow
counts of 73 and 111 were recorded indicating very competent ground with
a bearing capacity of >750Kn/m2, this value incorporating a minimum
factor of safety of 2.5, or, the ground at refusal depth in the
boreholes can take a load in excess of 15T/m2, with anticipate loads
from piles being considerably lower than 500kN/m2.

If the above still does not give you the confidence regarding the ground
and you want/need to prove the competent strata continues even deeper
than the termination depth of the existing boreholes this will need to
be done either; after demolition is undertaken to allow access to the
rear garden area or; using the drilling rig we are in the process of
purchasing and which was only delivered to the UK last week and which we
will be collecting this week, the rig being specifically designed for
limited access site having an access width requirement of 800mm, as well
as the ability to dynamically drill and rotary core/open hole into
bedrock which has until now not been possible as all available rotary
hire rigs need >1.5m width access.


 

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