Considering that we made the decision early on to have a concrete house we have to take into account the extra weight of the whole structure. The “soil” (sandy marine clay and rock fragment) we are building on is probably fill the developer threw on top of a marsh to create “dry” land and from the surrounding canals.
The method used on our site was to the drill the hole, fill with concrete and drop in a rebar cage to form the pier. This method was preferred by our current building designer.
The original method, which we did not use, was to drive precast concrete piers into the existing soil “driven to refusal”. That means a pile driver will hit the pier into the soil until there is enough compacted material to stop the pier from going farther.
It was an interesting method I did not see used before until recently.
Pier being aligned
A very large and powerful crane with a support cage will be used to drive a precast concrete pier into the existing soil “driven to refusal”. The soil at the site was poorer than at our lot. It is black marsh mud without any cohesion. The piers seemed to be about 12” x 18” or so and about 30 feet long. I’m sure they had test holes to determine how deep they would have to dig to get to base rock and find the cap rock support a structure would need.
The crane operator had the pier elevated by its rebar attachments. The helper would help maneuver the pier over the pre-drilled hole and keep it straight in line with the other piers. When the pier position was approved, it was dropped into the guide hole. The muck and water from the displacement was like a geyser and the helper had to jump away or be engulfed.
The crane operator added the support cage to the rig. With it, the rig has the drop hammer to drive the pier into the soil. As the pier is vertically aligned the drop hammer pounds the pier into the soil, inches at a time. As the concrete flaked off a section of tree stump was added to the top of the pier as a cushion against the blows of the concrete hammer. It took an hour to drive the pier about 30 feet into soil. Some of the drops from about 30 feet seemed like small earthquakes!
Pier part way in
Many times it seemed like the pier could not go any further. The operator just added more height to the drop hammer and slowly down the pier went. It took an hour to drive the pier the 30 feet into the soil.
Valley of Hearts Delight
P.S. The machine is a FMC crane. For those from the South Bay/San Jose, FMC was originally a company that made fruit packing machinery for the “Valley of Heart’s Delight.”
