Raft foundation advice

[Saints1]

Hi all, 

I feel like recently I am constantly on here asking questions and nagging everyone but I value everyone’s advice. 
 

I am just getting my plans back from the structural engineer to submit to the BCO and they have gone from piling to a raft foundation which is fine. My architect has now stated that on top of the raft I will need a block and beam floor. 
I just wanted to query is this normal practice? 
I understand that I will need to insulate above the raft but I was hoping that I could insulate and then 75mm screed on top with UFH? 
I am no way a builder or in the building trade so could be way off with my theory but any help or assistance is greatly received. 

[Nick Laslett]

9 hours ago, Saints1 said:

 

I am just getting my plans back from the structural engineer to submit to the BCO and they have gone from piling to a raft foundation which is fine. My architect has now stated that on top of the raft I will need a block and beam floor. 
I just wanted to query is this normal practice? 
 

This is my best guess, but remember I’m just an amateur like you. 

What did the Structural engineer’s drawings specify? Their raft foundation design will be based on the structure that is being built and the ground conditions. They would have to incorporate the extra weight of the block and beam floor in the raft design, if that is what is specified. You can discuss this with your Structural Engineer, or talk directly with BCO. You are allowed to challenge your architect. 

Based on my very limited knowledge, I would be very surprised if you need a raft and block and beam, but I could be wrong. Others with more experience are sure to come along and comment. But not every post gets attention. 

 

Sounds like crossed wires, or lost in translation, or missing info.

 

These two gentlemen @Gus Potter, @saveasteading are very knowledgeable, and much better qualified than me, and might happen on your post. Good luck. 

[saveasteading]

27 minutes ago, Nick Laslett said:

might happen on your post

@Nick Laslett good thinking to do the @ thing. I am set up to see what about 8 members are discussing so can miss new posts.

 

Yes I think you are right.

The purpose of beam and block is to jump over poor ground to reach foundations on poorish ground. Or it can be to overcome a slope.

 

To build a structural raft then build a completely extra floor seems wierd.

@Saints1 I'd really like to know your Architect's logic for this.

Firstly on the 1% chance that there is a good reason.

Secondly to understand what he doesn't understand. 

 

I've never used a structural raft because of cost and have built on bogs, fill sites and even refuse pits.

On marginally soft ground I can see that a raft may be optimum. 

 

A raft is expensive in itself. That should suffice. 

 

One more thing. I have many times changed a specification from driven piles to vibro improvement. In itself that showed that the original SE wasn't up to speed on the possibilities, perhaps thinking it was expensive. It would be worth asking them if it is a possibility.

 

The ground is compacted by inserting gravel, and then the ground can take standard construction.  Not always suitable though.

 

What is the issue with your ground being weak?

 

[Saints1]

57 minutes ago, saveasteading said:

@Nick Laslett good thinking to do the @ thing. I am set up to see what about 8 members are discussing so can miss new posts.

 

Yes I think you are right.

The purpose of beam and block is to jump over poor ground to reach foundations on poorish ground. Or it can be to overcome a slope.

 

To build a structural raft then build a completely extra floor seems wierd.

@Saints1 I'd really like to know your Architect's logic for this.

Firstly on the 1% chance that there is a good reason.

Secondly to understand what he doesn't understand. 

 

I've never used a structural raft because of cost and have built on bogs, fill sites and even refuse pits.

On marginally soft ground I can see that a raft may be optimum. 

 

A raft is expensive in itself. That should suffice. 

 

One more thing. I have many times changed a specification from driven piles to vibro improvement. In itself that showed that the original SE wasn't up to speed on the possibilities, perhaps thinking it was expensive. It would be worth asking them if it is a possibility.

 

The ground is compacted by inserting gravel, and then the ground can take standard construction.  Not always suitable though.

 

What is the issue with your ground being weak?

 

Hello and thank you for your quick response. 
 

so there is a mine shaft on the site situated about 9 meters from the property however this has been capped at rockhead. 
The survey completed on the ground came back with good ground albeit made ground over the shaft entrance which is to be expected but the build is over 9 meters away from that. 
this was the originally reason for piling however structural engineer has stated rafters now which is fine. 
there has been no subsidence in the area ever recorded either. 
I just felt it was a little strange to put a raft then a block and beam flooring in.

[saveasteading]

1 hour ago, Saints1 said:

structural engineer has stated raft

That sounds right to me. Piling 9m from the shaft isn't that far  and might disturb the ground there. A raft will spread the load wherever it needs to go.

But do ask why beam and block. It will cost thousands without need.

You can't sensibly just ignore the Architect's instruction/guidance so you need them to justify or remove that design. Or get the SE to over-rule it and somehow persuade the Architect to do as instructed.

Good luck, and keep us informed please.

[Saints1]

1 hour ago, saveasteading said:

That sounds right to me. Piling 9m from the shaft isn't that far  and might disturb the ground there. A raft will spread the load wherever it needs to go.

But do ask why beam and block. It will cost thousands without need.

You can't sensibly just ignore the Architect's instruction/guidance so you need them to justify or remove that design. Or get the SE to over-rule it and somehow persuade the Architect to do as instructed.

Good luck, and keep us informed please.

Thank you all for your advice, I spoke to the architect today and he has agreed that block and beam isn’t required. To be fair to him, he hadn’t finalised all the drawings as there have been some too’ing and throwing between him and SE and he did state it required a re-design. 
If I hadn’t of come on here and sort advice I’d of probably let it go. Maybe the architect may have changed it regardless but at least I was forearmed. 
thank you all again.

 

i am sure i will have many more questions to come considering i am very early in to the project 🙂 

[BotusBuild]

26 minutes ago, Saints1 said:

more questions

Keep them coming. Thats exactly what the sites for

[Gus Potter]

8 hours ago, Saints1 said:

I spoke to the architect today and he has agreed that block and beam isn’t required. To be fair to him, he hadn’t finalised all the drawings as there have been some too’ing and throwing between him and SE and he did state it required a re-design. 

Good point, design is an iterative process.