A few weeks back we made some progress on our structural engineering designs which I neglected to post about. As I covered in my previous post, our extension will require a foundation more substantial than the one under our current shack of an extension. When building an extension of our size the DOB requires a number of tests to inform how the extension is designed.
Test number 1 is called a Pit Test. This test is required because we need to know the depth of the foundation under our neighbor’s extension which is directly next to our planned extension. When 2 foundations are next to each other they must be the same depth otherwise one foundation puts unequal pressure on the other and one of them will eventually crack.
Lucky for us, the neighbor’s foundation is only a concrete slab on grade of about 1 foot thick. One foot isn’t deep enough to cause these pressure differences so we don’t have to be concerned with her foundation depth or do any underpinning. Underpinning is the task of digging the dirt out from under a foundation and filling in the gap with concrete.
Test number 2 is a known in New York City as a TR4 Soil Inspection. The purpose of this test is to learn the weight bearing properties of the soil under our future extension. I used to think of soil as a pretty solid substance but in truth it’s not. It’s really a fluid and the foundation of a building kinda floats in the soil. If the soil is significantly soft (like sand) we may need to drive deep pilings into the ground to support the load of our building. If the soil is hard enough we can simply do a shallow foundation also called footings. The shape of these footings will also depend on the soil quality.
To perform the soil inspection an engineering team sets up a boring machine which drills 20 feet into the ground and collects samples in hollow tubes which are then reviewed in a lab. It pretty much looks like those ice core samples you see sometimes on NOVA. Except of course it’s filled with earth.
A few weeks back we completed the boring test and the good news is that our backyard is mostly made up of clay, silt, and “sandy silt” which doesn’t require anything special for our foundation design.