How To Look At A House
McGarry and Madsen's home inspection blog for buyers of
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What is the difference between soil subsidence, heave, creep, and settlement?
Sunday, June 17, 2018
Although all four of these different types of soil movement are often lumped together as simply “settlement” by realtors and contractors, each one has a different cause and specific cure. Knowing which type of soil movement you are dealing with makes it easier to determine the correct fix.
Subsidence is the downward movement of soil, usually due to withdrawal of moisture. Soil shrinks when water is removed from it and, because trees pull a lot of water from the ground that they transpire through their leaves into the atmosphere, the roots can cause soil subsidence that corresponds roughly to the area under the drip line of the branches. Removing a tree will allow the moisture in the soil to rebound and raise the soil level. Soil subsidence can also be caused by a developing sinkhole, when water running through the soil is eroding the karst rock underlayment.
Heave is the upward movement of soil level, usually due to increase of the moisture content. Clay soils absorb and hold more water than granular soil, and have a corresponding increased heave when wet and subsidence as they dry out. They shrink during dry spells, then swell during seasons with heavy rains. The swell/shrink cycle causes the ground under a home built over a layer of clay soil to heave up and down in seasons that are extra wet or dry. A hilly band of clay soil that geologists have labeled the “Hawthorn Formation” runs through the middle of Alachua County, roughly north-south along I-75 and east-west along Newberry Road.
Creep is the gradual downhill erosion of the top layer of soil on sloped land. It can occur faster on the side of a steep hill and slower on the gradual incline of the soil towards a lake or creek on the surrounding land. The photo at the top of this page shows the corner of a house on a lot that backs up to a creek, and you can see where the ragged edge of the home’s concrete foundation has become exposed as the soil gradually migrated down towards the creek over time. The gutter drain termination next to the wall also helps to move the soil away from the foundation and down the slope.
Settlement is the downward movement of the supporting soil under a home due to loads imposed on it by the structure that exceed the soil’s bearing capacity. All new homes have a small amount of settlement in the first two years after they are built, as the soil responds to the new weight placed on it. Settlement can also occur slowly over many years. The addition of new loads is another way, such as by the enclosure of a patio slab to create an enclosed porch, and the additional weight bearing on a small thickened slab edge not designed for it.
Determining which type of soil movement is causing structural cracks in a home can get complicated. Engineering experts sometimes reach different conclusions about the cause of a structural problem when examining the same home.
But once you figure out the type of movement, the next step is to remove the cause or modify the structure to resist it. This can be done by actions like eliminating a tree, rerouting roof drainage away from the home, adding piers to the foundation that extend below the level of clay soil, removing the veins of clay soil, reinforcing inadequate footings, or adding stabilizing retaining walls at a sloped site.
Also, see our blog post How do I look for yard drainage problems when buying a house?
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