Foundation heave is the movement of expansive soil due to changes in its moisture content. The foundation heave occurs in various patterns like doming, edge, cyclic, and lateral movement.
Differential movement of expansive soil is more damaging than a uniform heave. It mostly affects lightly loaded structures like one or two-storey structures, highways, retaining walls, canal linings, and reservoir linings.
Building damage due to heave occurs within a few months after construction and may develop over about five years, or it may not be visible till some changes occur in the soil moisture.
What are Patterns of Foundation Heave?
Doming or Edge Down Heave
Foundation heave is generally erratic but can occur with an upward, long-term, dome-shaped movement that develops over several years. The greatest heave occurs toward the center of the structure.
A doming heave causes the external walls in the superstructure to lean outward, initiating horizontal, vertical, and diagonal fractures with larger cracks near the top.
The roof of a building restrains the rotation due to differential vertical movement, causing additional horizontal fractures near the roofline at the top of the wall.
A dry, hot, semi-arid climate and deep water table can be more conducive to severe and progressive foundation soil heaves if water becomes available.
Cyclic Heave
A cyclic expansion-contraction associated with drainage, frequency, and amount of rainfall and evapotranspiration may be superimposed on a long-term heave close to the perimeter of the structure.
Localized heave may occur around the ponded area or water leaks. Evapotranspiration is the moisture evaporation from the ground surface and the transpiration of moisture from heavy vegetation into the atmosphere.
Downwarping from soil shrinkage can develop underneath the perimeter of the building during dry and hot periods or from the desiccating influence of trees and vegetation close to the structure. These edge effects may extend inward around 2.4 to 3 m and become less effective in a well-drained land.
Heavy rain periods may cause ponding adjacent to the structure with edge lift and reversal of down warping.
Edge Heave
Edge or dish-shaped heaving of portions of the perimeter of a building may become evident shortly after construction, particularly in semi-arid climates where preconstruction vegetation has been removed and there is minimal topographic relief. The removal of vegetation can result in increased moisture in the soil, while the absence of topographic relief can cause ponding. Edge heave may also occur beneath foundations due to factors such as consolidation, drying out of surface soil from heat sources, or lowering of the water table. Lowering the water table in uniformly loaded soil beneath structures may not contribute to differential heave. However, buildings with deep foundations, such as drilled shafts with slab-on-grade, may be severely impacted by changes in the water table or soil moisture if the slab is not isolated from perimeter grade beams, and if the internal walls and equipment are not designed to accommodate slab movement.
Figure-2: Cracks in Building Because of Edge Heave
The building in Clinton, Mississippi, constructed on expansive soil with a deep water table in a wet humid climate, experienced significant damages as depicted in Figure-3. One of the notable damages was the heave of the floor slab, which exceeded 6 inches. This uneven upward movement of the floor slab was a result of the expansive soil swelling due to increased moisture content, leading to significant structural deformation. Additionally, the lack of construction joints in the structure exacerbated the cracking in the building. The differential movement of the soil underneath the structure caused differential settlement, resulting in cracks in the walls, ceilings, and other structural elements. These damages were aggravated by the combination of expansive soil conditions, high moisture levels, and the absence of construction joints, highlighting the importance of proper design and construction techniques when building on expansive soils with deep water tables in wet humid climates like Clinton, Mississippi.
Lateral Movement
Lateral movement has the potential to compromise the integrity of a structure. When expansive soil exerts horizontal force on a basement or supporting walls, up to the passive earth pressure, it can result in bulging and cracking. Unlike cantilever retaining walls, basement and supporting walls are less tolerant of lateral movement and require a higher level of stability in their design.
Additionally, foundations and walls constructed on slopes steeper than 5 degrees may be susceptible to damage caused by downhill soil creep of cohesive expansive soils. This downhill soil creep can create shear force on shaft foundations and lead to deformation. The depth of soil creep can vary from a few inches to several feet, posing a risk to the stability of the structure. Proper consideration and design measures must be taken into account to mitigate these potential issues and ensure the durability and safety of the building.
FAQs
What is a foundation have?
Foundation heave refers to the shifting or movement of expansive soil caused by fluctuations in its moisture content. Expansive soil, also known as swelling soil, is a type of soil that has the ability to expand when it absorbs water and contract when it dries out. This property of expansive soil can lead to significant changes in its volume, resulting in movement of the surrounding foundation. When the moisture content of expansive soil increases, it swells, causing the soil to push against the foundation, which can result in heaving or lifting of the foundation. Conversely, when the moisture content decreases, the soil contracts, which can cause the foundation to settle or sink. These changes in moisture content and subsequent movements of expansive soil can have detrimental effects on the stability and integrity of structures built on such soil, requiring appropriate measures to mitigate the risks associated with foundation heave.
What are the patterns of foundation heave?
Foundation heave can manifest in different patterns, each with its own distinct characteristics. One such pattern is doming, where the foundation rises in a rounded shape, resembling a dome. Another pattern is edge movement, where the foundation shifts along its edges, resulting in a tilted or angled appearance. Cyclic movement is another type of foundation heave pattern, which occurs periodically over time, with the foundation alternately rising and settling. Lastly, lateral movement is a pattern of foundation heave where the movement occurs horizontally, causing the foundation to shift laterally. These various patterns of foundation heave can have different implications for the stability and integrity of the structure built upon the foundation, and require specific techniques and measures for mitigation and repair.
What types of buildings are mostly affected by the foundation heave?
Foundation heave is a common issue that primarily affects lightly loaded structures, such as one or two-storey buildings, highways, retaining walls, canal linings, and reservoir linings. These types of structures are more susceptible to the effects of expansive soils because their relatively low loads do not exert enough pressure to counteract the upward movement of the expansive soil. As a result, foundation heave can occur, causing uneven settling and potential damage to the structures. It is crucial to consider the potential for foundation heave in the design and construction of these types of structures to mitigate the risks associated with expansive soils.
What are the effects of doming heave on a structure?
A Doming heave is a phenomenon that results in outward leaning of the external walls in a superstructure, leading to the formation of horizontal, vertical, and diagonal fractures. These fractures tend to be larger near the top of the walls. The roof of the building acts as a restraint, preventing excessive rotation caused by differential vertical movement. As a result, additional horizontal fractures often occur near the roofline, particularly at the top of the walls.