Ground movements can occur for various reasons, regardless of the pressure that foundations exert on the soil. To avoid foundation failures, it is crucial to take appropriate measures against such movements. Mitigating ground movements may require either deepening foundations to reach soil that is not susceptible to such movements or constructing a foundation type that can withstand significant movements without causing structural damage. This article will cover different types of ground movements and recommend suitable foundation types for each.
Types of Soil Movements, Causes and Recommended Suitable Foundations
Soil movements can occur due to various factors, such as seepage of water and surface erosion, which can lead to ground movements. Vibrations, hillside creep, and mining subsidence can also cause ground movements that can affect structures built on top of the ground.
Foundation on filled ground can pose challenges as the soil may not have been properly compacted or settled, leading to uneven settling of the foundation and potential structural damage. Therefore, it is essential to ensure proper soil preparation and compaction before building on filled ground.
Machinery foundations are crucial to ensure the proper functioning of heavy machinery. The foundation must be designed to withstand the dynamic loads and vibrations generated by the machinery, which can cause ground movements and potentially damage the foundation or the machinery. It is essential to consider the soil conditions and potential ground movements when designing machinery foundations to ensure their stability and longevity.
Soil Movements
There are several factors that cause the movement of soils which are discussed in the following sections:
Wetting and Drying of Clay Soil due to Moisture Content Changes
Certain types of soil exhibit significant swelling and shrinking behavior as their moisture content changes. This phenomenon can lead to the formation of cracks in the soil during dry seasons, which then close up during wet seasons. These changes in soil volume can also cause the ground to rise and fall throughout the year. Interestingly, grassy soils are believed to experience more noticeable and pronounced movements than non-grassy soils.
The swelling and shrinking of clay soils can be particularly problematic for buildings constructed on shallow foundations, especially in areas where the underlying clay is dense. To mitigate the risk of damage, it is recommended to use pad or strip foundations that extend to a depth of 0.9 to 1.2 meters for low-rise structures. However, excavating to this depth for the entire building can be costly. As a result, it is often suggested to allow for some movement between the foundation walls and the ground floor slab.
Effect of Vegetation on Swelling and Shrinking of Soil
Trees and vegetation can have a significant impact on the soil in their surrounding area. Their presence often results in soil becoming dry to a depth ranging from 3m to 5m. Trees have been found to spread their roots at least as far as their height, causing damage to nearby structures. When trees grow in clay soil, their roots can cause horizontal and vertical shrinkage of the soil. This can result in soil settlement and foundation damage, which must be addressed by implementing necessary measures. Even after a tree is removed from an area, the effects of its movement can persist for up to 20 years. To address this issue, it is recommended that pile foundations be used in areas where trees are present.
Shrinkage of Clay due to High Temperature
Clay soil located beneath the foundation of furnaces, kilns, and boilers is prone to significant drying, leading to a serious issue of shrinkage. This problem is particularly relevant in situations where the heat generated by the kiln has penetrated up to 20 meters deep into the clay soil. To prevent this shrinkage from occurring, it is recommended that an insulating layer be placed between the foundation of these high-heat structures and the clay soil beneath them. This insulation will help to regulate the temperature and prevent the soil from drying out, thereby preventing any serious shrinkage from occurring.
Ground Movements due to Seepage of Water and Surface Erosion
Sandy soil can be problematic due to water seepage and surface erosion. Groundwater can seep into damaged culverts or sewer systems, carrying fine soil particles along with it and causing internal erosion of the soil. Additionally, water seepage can cause the degradation of soluble components of industrial solid waste material, leading to soil loss and potential failure of structures, particularly in mining subsidence areas.
Dry loose sand and loess soil are especially susceptible to water seepage, but compacting the soil with a heavy hammer can help prevent this issue in areas where the soil depth is less than 6 meters. In areas where the soil depth is greater than 6 meters, pile driving or blasting may be necessary.
Surface erosion can occur due to flowing water or strong winds. Fine sands, silts, and dry peats are particularly prone to surface erosion caused by strong winds. This can undermine the foundation of structures, but can be prevented by deepening the foundation to 0.3 meters and growing vegetation in the area, or by covering the surface with crushed stone, gravel, or clay soil.
Flowing water can also cause surface erosion, particularly in areas with heavy rainfall. Ordinary foundation depths of 0.9 to 1.2 meters may not be sufficient to prevent this issue, so providing necessary drainage and paving or other surface protection techniques is recommended. Pile foundations can also be employed to avoid foundation problems and potential failure of heavy structures.
Ground Movements due to Vibrations
Vibration equipment is often employed to compact freshly poured concrete and sandy soils. The use of vibration on concrete can lead to an increase in density as well as significant settling. Foundations constructed on sandy materials are also subject to this phenomenon when subjected to vibrations. Vibrations can come from a variety of sources, including rock blasting, unbalanced machinery, drop hammers, pile driving, and earthquakes.
Fig.: Soil Movement due to Vibration Caused by Earthquake
Laboratory tests, field experiments, and past observations of structural damage have demonstrated that significant and extreme settlements resulting from vibrations are caused by high frequency vibrations ranging from 500 to 2500 impulses per minute. To address the detrimental effects of vibrations on structural foundations, various measures have been proposed. One such measure involves vibrating sands and taking necessary precautions to reduce the impact of pile-driven foundations on surrounding structures built on sands. For foundation structures that support vibratory machines, it is recommended to use special vibrating damping devices or excavate the foundation until a soil layer that is not sensitive to vibration is reached. These measures are crucial for maintaining the integrity and stability of structures that are subject to vibrations.
Fig. Providing Dampers to Absorb Machine Vibrations
Fig.: Anti-Vibration Mounts
Ground Movements due to Hillside Creep
Hillside slopes with shallow soil are prone to slipping and sliding over an extended period, which can be indicated by the presence of leaning trees. To assess the likelihood of future debris movements, it is recommended to dig trial pits on these slopes. It is important to note that the weight of structures does not significantly affect the stability of the slipping ground, but construction processes can influence slope stability. Vegetation can improve stability, so its removal during construction should be avoided. The measures to be taken to address instability vary according to the type of soil present on the hillside slope. For rocky slopes, grouting or rock bolting can be effective, but construction should be avoided in areas with clay soil as remediation is nearly impossible. Alternatively, the foundation of the structure can be designed and constructed to move as a single object to mitigate the effects of soil movement.
Fig.: Bolting Slopes to Obtaining Stability
Fig: Leaning of Tress due to Hillside Creep
Ground Movements due to Mining Subsidence
Many old mining locations that are buried underground can cause serious problems for the foundation of buildings and other structures. While various methods, such as geophysical techniques, have been used to identify these locations, trenching across the construction site remains the most effective technique. The magnitude and lateral range of mining subsidence are determined by the technique used for material extraction, such as dredging, pumping, or mining. Different approaches have been employed over time, leading to different impacts on the surrounding soil and structures.
The United Kingdom has experienced many instances of foundation issues caused by concealed mining. The thickness of the soil above the area from which materials were extracted is a determining factor in the extent of the damage to the foundation. If the soil is deep enough, it may not significantly affect the structure. However, if the soil is not sufficiently deep, the foundation could suffer considerable deterioration. The figures provided above and below depict the movement of soil due to inactive coal mining and its effects on the surrounding area and structures.
Fig.: Subsidence Above Inactive And Unidentified Coal Mining
Fig: Effect of Subsidence on the Building at the Site
To ensure the protection of structures in mining subsidence areas, it is important to consider several recommendations. Firstly, it is advised to use fully rigid or flexible structures with simple spans, and a flexible superstructure should be used where possible. Additionally, shallow foundations like raft foundations are recommended as they are able to better protect against compression and tensile fracture in the ground.
There are also other methods available for protecting structures in subsidence areas. These include using superstructure or substructure articulation, providing hydraulic jacking under the walls or columns, and digging trenches around the structure to release compression strain. These methods can be effective in mitigating damage to structures caused by subsidence.
Foundation on Filled Ground
Ground fill settlements can pose a risk to foundations built on filled ground. There are three main causes of ground fill settlements, namely consolidation of compressible fill under foundation weight, degradation of ground fill due to its own weight, and consolidation of the soil layer beneath the fill under its own weight plus the weight of the fill. When the foundation is not heavy, soil movement due to consolidation of compressible ground fill is negligible. However, if the foundation is heavy, the movement needs to be calculated and properly addressed.
Soil movement due to ground fill weight is influenced by various factors, such as fill layer composition, thickness, compaction, placement conditions, and exposure to the environment. The presence of chemical materials in the fill can cause significant settlement due to chemical reactions. The extent of movement due to consolidation of natural material beneath the ground fill depends on factors like soil layer composition and thickness. Soft clay soils are susceptible to considerable settlement, while dense soils like sand experience slight settlement.
Several solutions are available to tackle ground fill movements, such as adding reinforcement to strip foundation to prevent stepped cracks, proper compaction of fill material before considering any foundation type, removing the upper loose material if the fill thickness is substantial, opting for raft foundation for poorly compacted fill, and considering piles for heavy structures like industrial buildings.
Machinery Foundations
When it comes to placing machinery, it is important to design the foundations in a way that distributes the load evenly onto the ground. This is done to prevent excessive settlement or tilting of the foundation in relation to the floor or other fixed objects. If the machinery is non-vibrating, it is relatively easy to determine the appropriate size of the foundation base, as long as the soil compressibility and shear strength are known. By estimating the permitted bearing pressure, excessive settlement can be avoided, and by placing the center of gravity of the machinery on that of the foundation block, tilting can be eliminated.
However, when it comes to foundation vibration, there are other factors to consider. Vibration can come from outside sources such as earthquakes, or from objects that are supported by the foundations such as machinery. To prevent excessive movement due to resonance, it is advised to avoid coincidence between the operating frequency of the machine and the natural frequency of the foundation. When a machine operates over a wide range of frequencies, it is recommended to test the influence of resonance on foundation movement to ensure that it remains within acceptable limits.
To reduce the impact of vibrations, various measures are implemented. One approach is to increase the mass of the foundation to minimize the reflection of vibrations within the structure. However, this solution may not always be practical, and in such cases, mountings are used to dampen the vibrations.
Several types of mountings are available for this purpose, including cork slab and rubber pads, rubber carpet mountings, bonded rubber mountings, and leaf springs. Each type of mounting is designed to absorb vibrations in a specific way. The choice of the most suitable type of mounting will depend on various factors such as the level of vibration, the type of structure, and the available space.
Using cork slab and rubber pads is one option for damping vibrations. Rubber carpet mountings are another type of mounting that can be employed for this purpose. Bonded rubber mountings are a more advanced type of mounting that can handle higher levels of vibration. Finally, leaf springs are another type of mounting that can be used to reduce the impact of vibrations. These mountings are designed to provide a degree of flexibility that can absorb and dissipate vibrations.