Clay and foundation engineering issues!

[tbirkey214]

I'm building a greenhouse for my mom out here in Texas just north of Dallas in the country. I just moved out here and am just starting to understand this soil *****.

We are trying to build a 11x15 foot greenhouse with a 3' pony wall that holds up a steel and polycarbonate upper like this picture below.

I hired an engineer and he told me to pour a footer to the specs shown below. I've dug down 3 feet and still am getting a 1.5 on my penetrometer from top to bottom of my trench. I don't want to dig down any further since I'll be laying cmu on this footer and I'm digging by hand.

I did say that we wanted to put DG on the inside or pavers or something and he drew up a footer, but now after talking to alot of people and dealing with this clay I'm really surprised he didn't try to talk me into doing a raft or waffle slab.

Its been a couplendays and I haven't heard anything back on fixing this design. Any thoughts?

 

[billconner]

I was thinking a hefty slab.

 

[pcmeiners]

The reality is you are not supporting a 4 story building, you should be down 3 feet around the entire foundation perimeter as per the specs you show, looks like on the one end you are not. For the end which is not 3 feet, just dig out that area to a 3 foot level, the foundation does not have to be at the same level all the way around, your poured crete or block does. At 3 feet your past organics and disturbed soil, unless you have tree roots which I do not see trees nearby. As to compaction even if it settle a bit it should settle evenly as long as you have removed the organics and have no tree roots (or buried bodies).

 

[tbirkey214]

The reality is you are not supporting a 4 story building, you should be down 3 feet around the entire foundation perimeter as per the specs you show, looks like on the one end you are not. For the end which is not 3 feet, just dig out that area to a 3 foot level, the foundation does not have to be at the same level all the way around, your poured crete or block does. At 3 feet your past organics and disturbed soil, unless you have tree roots which I do not see trees nearby. As to compaction even if it settle a bit it should settle evenly as long as you have removed the organics and have no tree roots (or buried bodies).

It says i have to get to a 2 (tonnes per square feet) reading on my penetrometer, which i have not in 3 feet on the one side so far

 

[PWC Repair]

11x15??........Good Lord, I would have just poured a slab on grade, added some rebar, then stacked some block for the wall.

 

[rancherbill]

That is the foundation spec for a much larger building. Your engineer pulled standard boilerplate. I don't believe Texas has much frost penetration in the winter. Three feet sounds excessive. The Texas guys can tell how deep for the frost line. I'd just check that at whatever level you stop digging is that the soil is undisturbed. It is extremely hard to get a uniform compaction if you need to refill.

 

[Rusted Nut]

It’s not a frost issue, it’s expansive clay soils. You either need to dig down to below the clay, use piers, or pour some sort of structural floating slab.

 

[PT Doc]

Good forms might helpful. Those foundation plans look very robust for your application.

 

[TRWham]

We use helical piers with abandon when we encounter bad soil, but that may be overkill, at least for your budget depending on how deep they need to go.

 

[ConCretin]

Hopefully your engineer will get back to you so you can have this discussion with him but in the meantime we can chew it over. In reading through his notes, it appears the concern is expansive soils, which swell and shrink depending on moisture level. Concrete doesn't like it when the ground underneath moves.

I suspect the footing depth is an attempt to find bearing at more stable moisture levels. The foundation design is a grade beam rather than a conventional spread footing, which will tolerate more movement. Presumably once the structure is up, the moisture level of the soils within would be more stable for whatever working surface is installed.

A properly designed 'floating' slab would be fine but would be subject to much more movement than his design. Based on your greenhouse design, you'd still need a few courses of block on top. Maybe he was concerned about twisting or racking, which would raise hell with the structure above.

The note on the penetrometer does appear to be boilerplate and possibly isn't applicable to your soil conditions. I'm just guessing here but I suspect that your engineer will tell you you're fine.

Let us know how the conversation with your engineer goes!

 

[cannuck]

Put me in the floating slab camp. I have built on exremely poor/unstable soils in far Northern bush and you had one of two things to choose from: pilings of floating slab. I built my house on pilings and the cost was high but tolerable. To build a greenhouse, garden shed or any other tiny building like that is just plain bonkers. I have seen really good floating slabs work all of the way up to aircraft hangar sizes, so it would be "back of napkin" engineering to do what OP needs.

 

[PopcornSutton]

Holy smokes. For a greenhouse? Those footings would support 10 times what you're doing. Two mats of #5 and banded at 16" OC? Sheeit. Plus bars and fully grouted block walls below grade? This A&E just copied a standard building design foundation without regard of what is really going on it, but he covered his ***.

 

[Firebrick43]

It says i have to get to a 2 (tonnes per square feet) reading on my penetrometer, which i have not in 3 feet on the one side so far

First off, tonnes or tons? We have freedom units here, not pink metric measurements and 2 tonnes = 2.2 tons.

Second, is that what your penetrometer actually measures in tons and not PSI? Are you using the proper tip on it? Or are you converting numbers?

I agree with @PopcornSutton. If you are getting 1.5 tons or 3000 psf on you soils you footers are so grossly oversize for even that.

There was either a huge communication gap between him and you. Or your engineer is the R%$#@ laziest engineer there is.

2 tons or 4000 psf soils are decent soils, not clay typically, so why is he requiring those numbers? Look at this chart. A 3 story solid masonry wall structure on 4000 psi soils doesn't require a footer that large. 24" is absolutely stupid. And you would be at the opposite corner of the chart as its a very light single story structure.

 

[tbirkey214]

First off, tonnes or tons? We have freedom units here, not pink metric measurements and 2 tonnes = 2.2 tons.

Second, is that what your penetrometer actually measures in tons and not PSI? Are you using the proper tip on it? Or are you converting numbers?

I agree with @PopcornSutton. If you are getting 1.5 tons or 3000 psf on you soils you footers are so grossly oversize for even that.

There was either a huge communication gap between him and you. Or your engineer is the R%$#@ laziest engineer there is.

2 tons or 4000 psf soils are decent soils, not clay typically, so why is he requiring those numbers? Look at this chart. A 3 story solid masonry wall structure on 4000 psi soils doesn't require a footer that large. 24" is absolutely stupid. And you would be at the opposite corner of the chart as its a very light single story structure.

Maybe he is a retarded. Clay soil usually read at 1.5. And y3s i did mean tons, I'm not using metric or converting anything. I don't know what this guy is thinking nor qhats normal. I paid $ 500 for this

 

[tbirkey214]

Hopefully your engineer will get back to you so you can have this discussion with him but in the meantime we can chew it over. In reading through his notes, it appears the concern is expansive soils, which swell and shrink depending on moisture level. Concrete doesn't like it when the ground underneath moves.

I suspect the footing depth is an attempt to find bearing at more stable moisture levels. The foundation design is a grade beam rather than a conventional spread footing, which will tolerate more movement. Presumably once the structure is up, the moisture level of the soils within would be more stable for whatever working surface is installed.

A properly designed 'floating' slab would be fine but would be subject to much more movement than his design. Based on your greenhouse design, you'd still need a few courses of block on top. Maybe he was concerned about twisting or racking, which would raise hell with the structure above.

The note on the penetrometer does appear to be boilerplate and possibly isn't applicable to your soil conditions. I'm just guessing here but I suspect that your engineer will tell you you're fine.

Let us know how the conversation with your engineer goes!

Tha ksnfor the detailed reply. I'm going to call him today. Hopefully it's boilerplate because it's probably clay all the way to bedrock, or atheist that's whatbi hear and this guy is in the same county as me and I assume he knows what he's doing and not whipping me up something quick and sloppy for 500 bucks.

I am very new to these soils and the movement is scaring me, but I will definately be installing a drip line around the foundation

 

[tbirkey214]

11x15??........Good Lord, I would have just poured a slab on grade, added some rebar, then stacked some block for the wall.

I'm not a concrete guy and new to this part of the country so we just asked an engineer and this is what we got

 

[tbirkey214]

The reality is you are not supporting a 4 story building, you should be down 3 feet around the entire foundation perimeter as per the specs you show, looks like on the one end you are not. For the end which is not 3 feet, just dig out that area to a 3 foot level, the foundation does not have to be at the same level all the way around, your poured crete or block does. At 3 feet your past organics and disturbed soil, unless you have tree roots which I do not see trees nearby. As to compaction even if it settle a bit it should settle evenly as long as you have removed the organics and have no tree roots (or buried bodies).

Yes , right now it's 1' down in the low and 2.5' on the high side(level all around). I was hoping to only another 1'

 

[tbirkey214]

It’s not a frost issue, it’s expansive clay soils. You either need to dig down to below the clay, use piers, or pour some sort of structural floating slab.

That's right!
Now after all my research I'm puzzled why our engineer didn't try to get us to do a raft slab. Fonyou think footers in this soil is a bad idea?

 

[billconner]

I think a slab on grade with footers - thickened edges - would have been fine for this light weight building with no concentrated loads but the perimeter. Raft or mat slabs are really for supporting concentrated loads like a column or load bearing wall anywhere. If you did do separate footers for a stem wall hard to believe a minimum 12" wide by 8" deep wouldn't be more than sufficient. I did that for a 28 x 32 garage with tall block stem walls and stick frame on it based on knowing the bearing ground was at least 2000 psi - just from looking at it.

 

[dcg9381]

I'm not a concrete guy and new to this part of the country so we just asked an engineer and this is what we got

Did you provide the engineer with a soils sample? Because without that, I don't think an engineer can "stamp" any type of foundation.

It "sounds" like you're getting generic building advice for the area?

And it might be that this engineer has 3 different solutions to the conditions, but just provided you one.

Here (I'm south of you) you dig perc holes, submit soil samples, give those to an foundation engineer who comes up with a stamped "engineered foundation". IMHO, total overkill for 11x15 and non-residential. But if you want peace of mind, you do an engineered slab.

Down here we just use perimeter concrete beams and rebar. But I'm not on clay. I have had to drill piers on left over soil spoils...

 

[ConCretin]

For those saying the foundations are over designed and/or recommending a slab on grade, the issue at hand is not frost or loads, it's expansive clay soils. Maybe it is overkill but unless you are also and engineer and familiar with the OP's specific soil conditions, off the cuff advice isn't very helpful to an inexperienced builder.

 

[tbirkey214]

Did you provide the engineer with a soils sample? Because without that, I don't think an engineer can "stamp" any type of foundation.

It "sounds" like you're getting generic building advice for the area?

And it might be that this engineer has 3 different solutions to the conditions, but just provided you one.

Here (I'm south of you) you dig perc holes, submit soil samples, give those to an foundation engineer who comes up with a stamped "engineered foundation". IMHO, total overkill for 11x15 and non-residential. But if you want peace of mind, you do an engineered slab.

Down here we just use perimeter concrete beams and rebar. But I'm not on clay. I have had to drill piers on left over soil spoils...

I didn't know that is what protocallnis, but he said he was familiar with the small town I'm in and I did tell him prior that we have very heavy clay soil that expands and contracts. I think it is very very common put here but don't know by how much variation.

 

[tbirkey214]

Were just about done digging the trench and will by laying rebar this week. One thing I have been thinking about is having 1.5' of one side (this is on a sloped hill) below grade. Should i do something extra to damp proof the cells so that they don't leak?? I'm not sure ifnit would but am curious if I'll have probl3ms when it rains. I. Also curious if i sould have some sort of damp proof course, or if since my structure is only steel and polycarbonate that I shouldn't worry about

 

[tbirkey214]

Were planning on painting the inside and I want to avoid what happened to this wall i found on the internet

 

[PWC Repair]

YES, you will want to french drain and watertight any portion below grade. The whole backside of my shop (48ft long) is about 4ft below grade.........I've NEVER had any water come in.

 

[ConCretin]

If there are areas where the outside grade is higher than the slab, I would apply damp-proofing to the outside face at a minimum. in fact, I might apply damp-proofing to all the exterior walls from top of footing to finish grade.

Btw, did you ever talk to your engineer? What is the current plan for excavation?

 

[tbirkey214]

If there are areas where the outside grade is higher than the slab, I would apply damp-proofing to the outside face at a minimum. in fact, I might apply damp-proofing to all the exterior walls from top of footing to finish grade.

Btw, did you ever talk to your engineer? What is the current plan for excavation?

Yes, I called him and said I was hitting the same soil 3 feet down, although I'd did start to get slightly differant after I had to dug another couple inches to even stuff out and he said " it will be fine". I think he game some boilerplate design and charged me 500 bucks for a quick payout. He says it's OK and will handle the load and I got ripped pretty hard on here and another forum for the overkill so I think I'm good, although it is hard to find a contractor or engineer or whatever that really cares, but we will go with it.

 

[dcg9381]

I got ripped pretty hard on here and another forum for the overkill so I think I'm good, although it is hard to find a contractor or engineer or whatever that really cares, but we will go with it.

You got something out of an engineer for $500. That's astounding.

Engineers don't have to care. They have to do the math (more likely use the software). Around here a STAMPED engineered foundation would cost $1200+ after the perc holes were dug and you do the soil analysis.

The GJ math seems to work here. Build the darn thing!

Engineer won't help you avoid water... French drain the **** out of it and if you're "really" worried about it (and building below grade) there are various products that deal with water... But isn't this above grade?

 

[tbirkey214]

You got something out of an engineer for $500. That's astounding.

Engineers don't have to care. They have to do the math (more likely use the software). Around here a STAMPED engineered foundation would cost $1200+ after the perc holes were dug and you do the soil analysis.

The GJ math seems to work here. Build the darn thing!

Engineer won't help you avoid water... French drain the **** out of it and if you're "really" worried about it (and building below grade) there are various products that deal with water... But isn't this above grade?

The other bid i got was 1500 and soil samples were needed so maybe it was a good price.




Really the interior grade will be maybe a foot down or a course of block, but I figure i ask anyway. Seems there's always something you learn along the way or wouldn't expect.

 

[dcg9381]

Really the interior grade will be maybe a foot down or a course of block, but I figure i ask anyway. Seems there's always something you learn along the way or wouldn't expect.

I got it. So you're building into a hill... Definitely french drain the hell out of it. See my above photo for how they did my foundation water proofing (my lower level is 10' underground). Probably a much easier / cheaper way to seal a small accessory building than using that stuff, but you get the idea.

 

[1/4atatime]

11x15??........Good Lord, I would have just poured a slab on grade, added some rebar, then stacked some block for the wall.

I'm glad I'm not the only one. They will be able to build a small house on the slab once he's done with all this engineering.

 

[tbirkey214]

I'm glad I'm not the only one. They will be able to build a small house on the slab once he's done with all this engineering.

Live and learn?... better go overboard on the part i don't know.

But also, with cmu walls, and several thousand pounds of 14g steel framing isn't that basically a house?

 

[1/4atatime]

Live and learn?... better go overboard on the part i don't know.

But also, with cmu walls, and several thousand pounds of 14g steel framing isn't that basically a house?

Fair enough with that much engineering in the winter you can rent it out as an Airbnb tiny home