Gravel vs. Cement Walkway to Prevent Erosion

Cement and Gravel

• Cement walks contain Portland cement mixed with sand and water to produce slurry that dries to a solid block. It is poured over several inches of gravel, depending on local building codes. The slurry requires wood or metal forms to contain it until it dries. Gravel walks contain two layers; a base of rough gravel and a top of pea gravel or other fine aggregate. Wood, concrete paver or stone edging contains the walk. Each level is compressed with a plate compactor until the finished walk is solid enough for even a bicycle or wheelchair to traverse.

Percolation

• The formation of clouds and precipitation form only part of the water cycle. Not as familiar but just as important is the role of aquifers, wide underground areas bounded by rock layers where water collects. Unless you live near a freshwater lake or river, your drinking water probably comes from a well drilled down into one of these aquifers. The quality and abundance of these underground water sources depends on how much water is allowed to infiltrate through the soil and rock above them. This percolation effect keeps the topsoil in place and the subsoil moist. Water percolates easily through the spaces between bits of gravel, but solid cement presents an obstacle.

Runoff

• When built objects are placed on the surface or the grade is changed by adding fill, for example, the pattern of infiltration changes. Some of the water that falls as precipitation drains cannot penetrate the soil and must run off to other areas until it can begin to infiltrate or join a surface body such as a lake or river. The river beds were originally formed by the erosion caused by runoff, but open land still allowed plenty of space to feed the aquifers, too. Cement pavement changes the patterns of movement, encouraging erosion of the new path. Although the gravel and sand under cement walks allow movement of water, insufficient underlayment and the earth beneath it may also erode because of the walk’s runoff.

Water Bodies

• Conservation departments in states with extensive shorelines mandate the use of pervious materials such as gravel for walkways near waterways, lakes and the ocean. In addition to erosion caused by runoff of precipitation, these walkways also face water movement from seasonal flows, waves and tides. Cement walks present a solid barrier for runoff, contributing to erosion alongside and, eventually, under them. In addition to appearing more natural, gravel walkways allow easy passage of water, much as would a layer of sand or rocks.