Category: Foundation

At about a week forms for the grade beams were stripped and the excavator came to fill around and over them. We ended up putting down about 40 tons of stone over the grade beams to provide a base for a cement slab for the garage/storage area.

The plans called for 19 12″x12″ concrete piers to support the main floor of the home. The concrete crew tied reinforcement to the dowels coming up from the grade beams, erected forms, and then we poured concrete again.

Drawings showed 4 each 16″ long 3/4″ bolts at the top of each pier, at the corners of a square with 5″ sides. It was not clear to me how they were going to place these bolts, seemed like the plan was to just stick them in by hand after the concrete had been poured to the required elevation. That did not need like a great plan to me, so I did some poking around and found some adjustable anchor bolt jigs called AJ Speedsets. Concrete crew probably thought I was nuts, but I ordered these jigs and got the bolts (and nuts and washers) from Portland Bolt. I assembled the jigs and the crew tied them in to the pier reinforcement. Unfortunately they did not really keep an eye on centering as they poured the piers, but turned out that alignment of the bolts between piers was not super critical. Nuts were threaded on to the bolts and used to adjust 1/2″x11″x11″ steel plates on which the steel beams would rest (with an additional bearing plate that fit between the bolts). Then the space below the plates was filled with non-shrink grout. So after everything was together the bolts were no longer supporting anything, the steel beams were on plates that were on grout that was on the top of concrete piers.

I am sure they are strong, but the piers came out a little rough. The sub offered to let me pay for rental and deposit of new forms, in hindsight perhaps should have done that. At some point will clean the piers up a bit, maybe grind them a bit and/or apply some stucco.

Once the piles were in the excavator came out to dig trenches for the foundation’s grade beams, which were drawn as 2’6″ wide by 2′ high, with the bottom of the beams at an elevation of 3′, which was 4’8″ below finished grade. The structural engineer indicated that the bottom of the grade beams should be at least 3’6″ below grade to avoid any frost problems.

Next the concrete subcontractor’s team assembled the forms for the grade beams and cut the piles off such that the pile tops would be embedded into the grade beams by 6″. I had asked the pile driving contractor about treatment of piles where cut but they either did not know what to do, or didn’t want to have to do anything. The plans called for cut end treatment per AWPA M4-84, I figured out that meant a product called tenino copper naphthenate 2%, so I ordered a gallon of that from poles.com and treated the cut ends of the piles myself. No cap was required, the grade beams just get poured on top of/around the pile tops.

I had gone back and forth with the structural engineer on #6 (3/4″) vs #5 (5/8″) rebar for the grade beams, the concrete sub said that #6 was unusual for residential construction, but if we dropped from 6 to 5 the engineer wanted to add bars. So we decided to use the #6, but have the rebar supplier, Harris Rebar, pre-cut and pre-bend for us, including a bunch of #3 stirrups. The structural engineer also called for galvanized, after discussing we backed off on that and went with plain black bar, as this stuff was going to be encased in concrete and below grade, as well as probably over engineered. The engineer also wanted placement/shop drawings for the rebar, when I figure out what that meant I did the drawings myself.

We were in to December now, which is not an ideal time to pour concrete. The structural plans called for 5000 psi cement, I managed to get that dialed back to 3500 psi to save a little money, and I think we ended up using 4000 psi. When we were ready to pour daytime temps were in the 40s and overnight temps in 20s and one night in the teens. So O&G used hot water, a non-chloride accelerator, and a mid range plasticizer. It took 7 truckloads of cement, I think about 63 cubic yards.

I pushed the cement sub to put blankets on the grade beams for some thermal protection, and also pushed back against pulling the forms off too quickly.

For this first pour I had a lab come out to catch and test samples of the concrete. Two cylinders tested at 3980 and 4120 psi at 7 days, and six cylinders from two samplings averaged about 4500 and 4900 psi at 28 days. So despite the cold the concrete strength was good.

Based on two test borings our Geotechnical Engineering Report concluded that piles, either timber or helical, were required to support the home. This seemed consistent with foundations for a few recently completed homes in the neighborhood, one was on timber piles, and a couple were on helical piles with grade beams. Our test borings reached refusal at 39 and 41.5 feet below the surface.

Our structural engineer thought that timber piles were superior from both engineering and cost points of view. The downside was concern about damage to an adjacent home from pile driving vibrations. Helicals were not a lot more expensive, but the chance of overages with them was probably greater, they would have to go down about 22′, and at that depth there would be multiple sections with joints, so if you fetched up against a rock or something before you got to depth you cut the pile off and try again in a different spot, and the different spot means you are redesigning your grade beams. So we decided to give the timber piles a go. I suppose we could have run them right up to the main deck, but our foundation design called for grade beams and concrete piers, a much cleaner looking approach, and better suited to the prefabricated construction approach as the precision of concrete piers coming off grade beams would be higher than timber piles sticking out of the ground (and probably requiring a bunch of bracing).

First step was to get the surveyor to come out and mark the pile locations. He used giant nails with pile numbers written on a piece of plastic. I was not sure whether excavation for grade beams would be before or after piles were in, answer was after, so a bit more work for the excavator to dig around the piles.

Our lot only has one home on an adjacent lot, the other side is a small park. The owner of the adjacent home was quite concerned about damage from the installation of the piles, so in addition to engaging a geotechnical engineer to monitor and log the pile driving operation we had them do a survey of the adjacent home to document the condition before the pile driving operation, and during the pile driving they monitored vibration along the lot line. A couple of piles were 15-20′ from the adjacent home. Thankfully the soil conditions contributing to the need for a deep foundation system were also conditions that were not conducive to transmission of vibrations, which remained below acceptable levels even for the piles closest to the adjacent home, and way below acceptable levels for the piles that were further away.

The timber piles were 45 foot long CCA treated Class B southern pine with 8-inch tip and 12-inch butt minimum diameters.

The piles were installed with a Vulcan #2 single-acting air impact hammer with a McDermid base. The ram was 3,000 lbs and dropped 2.4 feet. Required pile working load was 25 tons, based on that they figured out that piles needed to be driven until it took at least 8 blows per inch. As the movement of the pile slowed down they marked 1 inch intervals on it and someone stood there watching the marks and counting blows. I don’t totally follow, but they did some sort of wave equation analysis to confirm we were getting a working load capacity of at least 25 tons.

The hammer and rails for same were suspended by a crane, and the crew jockeyed the thing around to get it vertical and then started driving. I forget the size of the air compressor that they used, but it was a big one.

Piles were installed to depths of 18 to 41 feet below the existing ground level. There was an intermediate layer that most of the piles pounded through (I think the crew was always hoping they would fetch up in that layer, would have been quicker that way). Occasionally they would also hit a soft spot and the pile would drop several feet with a single blow. Piles were cut off just above ground level and then we did an as-built survey, which showed that all piles were within inches of where they were supposed to be (plans indicated a tolerance of 3″, which I think we were within). We were very happy about that because it meant we did not need to make any adjustments to the plans for the foundation.