What is a Michigan Basement?
You might not know it by name, but you have probably seen a Michigan basement. A Michigan basement is a term used in and around Michigan for a crawlspace that was later excavated to the depth of a basement. Transforming a crawlspace into a Michigan basement seems to have been popular in the roaring twenties, when credit was easy and the American family needed extra room for storage of the junk they began to accumulate. They are also known by other names, such as, a Long Island basement.
A Michigan basement is usually shallower than the normal basement and requires the occupant to duck to avoid banging his/her head on the floor framing or furnace ducts above. The floor and the side walls of the excavation sometimes remain as exposed dirt, but are usually covered with a mud slab (1 to 2 inch coating of concrete or cement). Or, the side walls of the excavation may be finished with brick or block.
The finish on the side walls of the excavation keeps the soil from the basement, but provides little strength for restraining the lateral (horizontal) pressures of the soil and ground water. The shelf between the top of the excavation back to the original crawlspace wall is often covered with concrete, cement, or brick. This shelf often provides a handy place for storage. See Figure MB-1.
The excavation is usually set inward a few feet from the original shallow depth foundation walls of the crawlspace to avoid undercutting the original footings. The set back also reduces the lateral pressures on the soil and the finish coating of the side walls.
Lateral soil pressures are generated by the weight applied by the original footings. That is, as the original crawlspace footings push downward on the soil, the soil is squeezed inward, toward the basement. If the soil is too soft, it will squeeze out like soft ice cream between two cookies of an ice cream sandwich.
The most common failure we have seen with these types of basements is the collapse of the concrete, cement, or brick finish that covers the side walls of the excavation. A common cause of failure is poor handling of roof or surface runoff on the exterior (or occasionally a water line break) that saturates the soil, causing it to become somewhat soft or even fluid, and thus, too “heavy” to be restrained by the finish and too soft (weakened) to stand on its own without sliding into the basement (the excavation).