Ground is all sand, foundation questions?

glenn kangiser · January 22, 2007, 02:28:37 AM

For a bit more hold down you could drill, lag and washer the 2x12's to the bottom of the post before dropping it in the hole, but other than that I didn't see any detail on fastening.

If you have questionable soil I would still go with the concrete mix with rebar. That option is near the bottom of the page. Note that the PT wood said for well drained soils and to field treat all cut ends. See the option for the 8" cardboard tube with a rebar in it. If using the crawlspace option note the bigger center footings with extra rebar.. The first thing you may want to do is study over all the footing and foundation details an the sheets that show them.

Note that John may have more detailed information - I just volunteer here and can look the plans over. I'm really a steel guy and sometimes wood butcher.

I'll move you to plans support and he may catch it sooner.

John Raabe · January 22, 2007, 05:31:36 PM

The detail I show for the PT post footings will rely on good bearing soil under the Dbl 2x12 plate (grain running each direction) spike in into the post with a few hot dipped nails (if you don't need to worry about uplift) or lag screws. If you are in questionable soils then dig a larger hole and line the bottom with crushed rock to spread out the bearing. This is a two story house and these will be serious point loads that need to replace the bearing capacity of a full perimeter foundation. Not a problem if the soil supports 6,000 lbs/SF but a real issue if it's only good for 1,500.

Also see this post and pier foundation piece: http://countryplans.com/foundation/index.html

For a house of this size and expense you should not be shooting from the hip on the foundation. I would review whatever you decide to do with a local foundation contractor and perhaps a soils engineer. Especially for post and pier, local conditions can make the difference between success and expensive problems.

January 22, 2007, 08:12:14 PM

phalynx:
I live just east of the DFW airport. We have some of the worst soil conditions in Texas.
The roads in LasColinas and North Irving are just like a roller coaster.
I was doing a repair for a customer that had his home built on concrete pier and beam.
Even the pool and the decks were elevated. The homeowner showed us were the clay can
contract and expand up to 16".
In the same development, another home was built with concrete pier and beam. The
drilling contractor thought he had hit bedrock. It was a rock shelf 2-3 feet above bedrock.
The weight of the home punched through the rock, causing foundation problems. A
$40,000 repair.
Another home was built on a hill using soil stabilization. The home seemed fine until a
large pool was installed in the back yard. There is a 6' retaining wall in the alley. The
pool has moved 6-8". The home has cracks 3-4" wide at the foundation. The retaining
wall is about to fall over. There is a lawsuit and possible demolition of this million dollar
home. Needless to say, the builder has not been invited back to the development. All new
homes require concrete pier and beam.
We will be building a retirement home at Lake Whitney, Texas. We have the opposite
problem. A couple of inches of soil, about 16" of coarse gravel and then bedrock. It took
me a full day to trench a 80' electric line.
Sorry to be so long winded. But, it could cost more in the long run if the foundation
isn't done right.
Here a couple of web sites explaining Texas soil stabilization, and Perma-Jack
foundation repair.

http://www.landstabilizers.com/gpage1.html
http://www.slabfix.com/advantage.html

John Raabe · January 23, 2007, 02:06:48 AM

Interesting report BayviewPS.

Did I understand correctly that new homes in this clay soil area are having better luck with concrete pier and beam foundations rather than a perimeter stemwall foundation? If so, I'm guessing that the pier can be drilled or driven to bedrock whereas the stemwall foundation (generally the gold standard in most places) is still floating on unulating clay.

bayview · January 23, 2007, 11:04:22 AM

John:

Stabilizing the soil in this development seemed to work best in flatter, lower lying areas. Floating slabs were being used following the stabilization. This procedure was utilized to save money. $40,000 / $60,000 per home. Homes are 3500-5000 sq. ft. ($200.00 / $300.00 per sq. ft.)

A dozer with a series of injection rods forces lime into the soil at a depth of 10-12 feet. The lime is injected under pressure as the rods are extracted. The grid pattern in this particular area was every 3-4 feet.

The home I previously mentioned was built on a hill with the soil stabilization method. Possibly the stabilization didn't reach far enough into the soil.

Due to this experience, the final phase of this development requires builders to use the concrete pier and beam.

This area is very hilly. To reach bedrock for the concrete pier and beam sometimes exceeds 40-50 feet for the drilled pier.

First, piers are drilled and concrete is poured in forms. Then concrete beams are formed and poured across the piers. A heavy corrugated metal is placed across the concrete beams. A mesh of steel is tied. Then a concrete floor is poured. Everything is elevated, even the decks and garages. The crawl space is filled with crushed rock. A drain field exits to a sump pump that is directed to the city sewer. Everything is covered with a vapor barrier. The crawl space is then fan vented with a humidity controller. Afterwards the soil is pushed against the concrete beam.

Then I've seen this foundation hybrid. A home being built in Coppell, Tx had piers drilled to bedrock around the perimeter and load bearing walls. A "waffle iron" grid was prepared in the soil. Cushion sand was added. Then a post tension foundation was poured over the prepared soil on top of the piers. This property raises 8-10 feet for a depth of about 200 feet. A series of drainage pipes were laid and then vented through the roof of this home. A vapor barrier was then placed over the ground before the pour. This was for venting radon gas.

Again I'm long winded.

Here is a site helping explain post tension foundations.

http://www.agtmengr.com/foundation.htm

January 23, 2007, 11:23:03 AM

Thanks for taking your time to post this, Bayview. Take as much room as you like. Great info.

phalynx · January 23, 2007, 11:28:47 AM

Bayviewps,

This is all good info. Thanks. I am going to build this house myself with my wife and I doing all of the work. I would pour a concrete floating slab if I could get a truck back there. I have built a large shop before so I am capable of doing it all. My problem is cost. I have to balance the cost/performance options. I really want t asolid house but I can't afford to over engineer. It's a real delima and not a terrific one to be in. I know there are several houses around here with monolithic slabs. There are several mobile homes around here with just piers on pads. Granted, I am talking about a 2 story house. My brother's house is just a little ways down the street and he has class 4 noncompressable clay. I hope that's what I have. His neighbor has sand and had to have an engineered tension slab.

bayview · January 23, 2007, 02:22:03 PM

phalynx:

You might want to consider renting a boom truck to pump the concrete if you are concerned about getting trucks stuck.

Years ago . . . (Back in my day) We were pouring a deck behind a house and around a pool that had limited access (extensive landscaping). The boom truck was parked in front on the street. The boom was long enough to reach over the house and into the backyard. Back then, it cost about $150.00 an hour. We saved that and more if we had to hire extra help to wheelbarrow.

On another job we had access to the side yard. We would fill up the bucket on a Bobcat (skid steer). We would drive along side the house, dump, then return to the redi-mix truck. Bobcat rental for the day was $175.00. We generally worked a 5-6 man crew.

Jonesy seemed to have a great system with his adjustable piers. Have you checked out his site? He used a lot of steel. Maybe a similar system can be used with lumber.

Make sure you have plenty of help during a concrete pour. Nothing worse than being short-handed when a form busts out.

http://users.tpg.com.au/jonsey/progress.htm

phalynx · January 23, 2007, 02:36:00 PM

It's a good suggestion and I wish it were possible, but I am in a rural part of TX on a piece of property 12 acres in size. The foudnation is in the middle of the property. It's 700' back there in dense oaks. I wonder if they make a 4wd mini concrete transfer truck or something like that that could run between the 2.

January 23, 2007, 03:00:41 PM

The Bobcat (skid steer) might be the solution. Use the 1/3 cu. yard bucket - modify the bucket or maybe you can rent the Bobcat with the Optional dump hopper.