First, a retaining wall is a structure that holds back, or “retains,” lateral faces of dirt, water, or other materials. Retaining walls are used to prevent erosion or collapse of higher level ground onto a certain building, structure, or general Seattle retaining walls
. The earth that is retained is often referred to as backfill. The word backfill is also used as a verb to describe placing more dirt behind a retaining wall to make the ground more level.
Retaining walls can be the answer if you are blessed (or cursed) to a contoured lot. They can turn a steep slope into a terraced yard, creating a stair step effect instead of a slope.
Retaining walls can be constructed from many different materials using many different techniques. There are several factors that influence which choice will be the best choice for each situation, including cost, height of the wall, soil characteristics, and ground water conditions.
Generally, most municipalities require that any retaining wall over four feet in height be engineered or approved by a licensed engineer. It is important to check local building codes before starting any retaining wall projects, even if it is below four feet tall. After all, retaining walls should be structurally sound before there is any thought of aesthetic appeal. The retaining wall must be able to stand up to the pressure exerted by the backfill, also called lateral earth pressure. This lateral earth pressure is a majority of the force that the retaining wall will need to bear. Lateral earth pressure is determined by studying the weight of the soil unit, slope of the soil, soil type (for plasticity and adhesion purposes with clay soil types), and ground water levels. Engineers do many calculations with these factors to determine what design to implement with the wall. To homeowners and contractors that do not want to go through engineering steps, it is generally a good rule to overdo it. Many structural methods to build retaining walls with withhold earth, at least temporarily, with a height up to four feet, are adequate, which is why most local building codes require engineering if the wall will be over four feet tall.
However, even methods that may seem adequate for retaining earth can fail if they are not built properly. Many times, it is elements below grade that can make (or break) a retaining wall. Problems including bowing, slanting, or cracking are evidence of retaining wall failure because it was not successful in retaining the dirt. Cutting costs with a bad contractor, insufficient materials, or neglecting the below grade elements can turn out to be more expensive over time if replacement or repair is necessary often or if collapse of earth causes damage to nearby structures. For example, erosion can cause foundation issues if dirt erodes from under the foundation. This is just one example how a retaining wall failure can cause expensive problems aside from the cost to repair or replace the retaining wall.
Another aspect that is often neglected or underestimated that can cause failure of a retaining wall is drainage. Inadequate drainage of retaining walls can mean that water is trapped behind the wall, adding to the weight that the wall must hold back. Usually weep holes and/or drainage pipes are used to flush rain water and ground water from behind the wall. It is important that the drainage system, whatever it be, have a filter barrier to keep dirt from clogging the drain or weep hole.