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.
