Foundations are essential structural elements that support the loads acting on a building’s superstructure. They serve as the link between the structure and the underlying soil or rock that provides eventual support. The load transfer can occur directly through the soil or rock or with the use of intermediary elements such as piles or caissons. Foundation failures typically refer to the failure of both the structural elements of the foundation, such as footings or piles, and the failure of the soil itself.
One type of foundation failure occurs when the structural elements of the foundation, such as footings or piles, become overloaded. This type of failure can happen when the weight of the building or structure is too great for the foundation to support, resulting in structural damage or collapse. Another type of failure occurs when the foundation soil loses bearing strength due to poor location selection or adjacent construction work. These failures can occur due to negligence and inadequate planning.
Although it is rare, foundation failures can result from the collapse of the footing. The ten most common reasons that lead to foundation failures include poor soil conditions, inadequate foundation design, poor construction quality, insufficient site investigation, poor maintenance, poor drainage, environmental factors, expansive soil, inadequate compaction of soil, and nearby excavation or construction work. Understanding these factors and taking preventative measures can help avoid foundation failures and ensure the longevity and stability of structures.
1. Load Transfer Failure
A rigid-frame structure that is well-designed and properly constructed can withstand significant foundation movements. This is because when the walls, floors, frames, and partitions of a structure are interconnected and rigidly connected, the system can adjust itself to differential foundation movements. Load transfer is achieved through the support provided by the foundation and the frame action. If this interconnected rigidity is absent, the load transfer will be through a single support, causing the load to act directly on the soil. This can lead to structure failure if the single support is absent in the soil.
If adjacent rigidity is present but lacks sufficient strength, the adjacent structure may fail. For instance, if four people are carrying a log with its weight uniformly distributed over everyone and one person steps into a ditch (an inadequate foundation), their portion of the load is suddenly transferred to another person who may not be able to support the additional load. This scenario is illustrated in Figure-1. Thus, it is crucial to ensure that all parts of a structure are interconnected and rigidly connected to facilitate load transfer and prevent structure failure.
2. Undermining Safe Support
Prior to commencing a construction project, it is crucial to conduct a thorough soil investigation. This investigation should not only focus on the soil strata directly underneath the proposed structure, but also on any existing adjacent structures that may be impacted. It is essential to review these adjacent structures with care to ensure that they will not be negatively affected by the construction project.
One critical aspect of ensuring the stability of the construction site is the provision of a sound bracing and shoring system. This system should be designed to prevent any lateral shift in the soil that may compromise the stability of the foundation. By providing proper bracing and shoring, the construction site can be stabilized, and the risk of structural damage can be significantly reduced.
In cases where the new construction will undermine an existing support system, it is necessary to install a permanent support structure such as underpinning. Underpinning provides additional support to the foundation and helps to distribute the load more evenly, reducing the risk of structural failure. Failing to implement these measures can result in cracks in the existing structure, and in some cases, tragic collapse of the foundation may occur.
In summary, a thorough soil investigation should be conducted prior to starting any construction project. The investigation should not only focus on the soil strata beneath the proposed structure, but also on any existing adjacent structures that may be affected. Proper bracing and shoring systems should be provided to prevent lateral shifts in the soil, and underpinning should be installed to support the foundation where necessary. By taking these measures, the risk of structural damage can be minimized, and the construction project can be completed safely and successfully.
3. Lateral Movement
Foundations are vulnerable to lateral movement, which can cause more damage than vertical settlement. This lateral movement can result from the removal of existing lateral resistances or from the addition of active lateral pressures. Soil saturation often exacerbates the problem by increasing active pressures and reducing passive resistances. Buildings can collapse due to lateral flow of soil underneath them, which can occur when heavy storms wash out soil by breaking drains alongside the footings.
Unreinforced concrete basement walls are especially prone to failure due to changes in pressure intensity against the walls. These walls are typically not investigated for high soil pressures. Surcharging, or adding weight to, the soil adjacent to structures can cause large lateral pressures. The practice of piling debris from demolitions next to basement walls can also contribute to their collapse, since such walls are usually not designed to withstand these loads.
Overall, it is important to be mindful of lateral movement and the various factors that contribute to it in order to prevent foundation failure and collapse of structures.
4. Unequal Support
Foundation mechanics adheres to a fundamental principle that load transfer does not occur without deformation. When loads are transferred to the soil through a foundation, the soil undergoes deformation. Consequently, all footings experience settling when subjected to loading. The degree of settlement is equivalent for various footings that have similar soil resistances and load distributions. However, if soil resistances are not the same, the footings experience differential settlements, which can cause structural tilting. The weaker soil causes the part of the structure founded on it to tilt away. Additionally, if the building’s framework is not continuous, the brittle masonry enclosure can crack during shear transfer.
Fortunately, all of these deficiencies in soil support can be remedied, but the cost of doing so is usually high. Rectifications often involve underpinning of the weaker soil. The most effective and commonly used methods for this are the installation of additional piles or jack piles.
5. Heave
The support provided by the soil beneath a footing in response to a load is due to its ability to yield and compress. Granular soils compress quickly, while clays compress much more slowly. Once the compression occurs, the foundation remains stable, and settlement ceases. The stability of the structure depends on the soil area directly below the footing or near the pile tip in the case of piles. If the soil below the footing is disturbed or removed, it can lead to settlement or lateral movement of the foundation.
When a new structure is built close to an existing building, it causes additional compression in the soil volume, leading to unexpected settlement of the previously stable structure. If the new building is not separated from the existing construction, the settlement caused by the new load will overload the previously stable footing. In plastic soils, new settlements are often accompanied by upward movements and heaves. However, if the entire structure settles at a slow rate over a long period, these settlements may be acceptable as long as they are uniform. Large differential settlements, on the other hand, can cause damage to the structure.
6. Drag Down
The drag-down phenomenon is a result of soil shrinkage caused by water table recession or tree growth, leading to differential settlements. When piles are embedded in soil layers, they will consolidate due to dewatering or load surcharge acting on the ground. This surcharge load increases the soil density, thereby increasing surface friction, and causing new soil loads to hang on the piles. This phenomenon is referred to as negative friction. However, the added load may cause a significant increase in settlement and could even pull the pile out of the pile cap.
7. Design Error
Inadequate prior subsurface investigation during the design phase of foundations can lead to insufficient support for the superstructure, resulting in costly corrective work later on. Designers often make the mistake of prioritizing initial construction costs over long-term stability. For example, they may use pile support for walls and roofs, while placing the main floor on compacted sand overfills. Similarly, in industrial plants, roofs are supported on piles while the floors supporting expensive equipment are placed over soil fill to save money. However, it makes more sense to support expensive equipment on piles and allow the roof to settle on soil fill.
Placing floor slabs on inadequate soil fills may seem like a cost-effective solution in the short term, but it can end up being a false economy. True cost efficiency should be measured by considering both the initial and in-life service costs. Heavy machinery in factories requires precisely leveled floors, and even small differential settlements can cause problems. Therefore, it is essential to prioritize stability and proper support during the design phase to avoid costly corrective work in the future.
8. Construction Errors
Construction errors are commonly categorized into two types. The first type involves temporary protection measures that are put in place during the construction phase. This includes shoring and bracing, cofferdams for lateral protection, and pumping operations. Unfortunately, these temporary structures are often built with minimal safety measures due to economic reasons, which can lead to failures.
The majority of foundation failures fall under the first type of construction error mentioned above. This is because they are caused by issues related to temporary shoring, bracings, and cofferdams. These structures are only in place for a limited time and are therefore more likely to be built with reduced safety measures. As a result, foundation failures often occur due to errors made during the construction of these temporary structures.
The second type of construction error is related to improper concrete work in the foundation. This includes the use of poor-quality concrete and haphazard placement of rebar. One common issue is the presence of cavities within cast-in-place piles, which can occur when quality control is lacking. The failure to implement verification methods early in the construction process is largely to blame for the significant losses caused by these errors. It is important to address both types of construction errors to ensure the safety and stability of any structure being built.
9. Change in Water Level
The extraction of groundwater by water companies can lead to changes in the water content of the soil, which can affect its dimensions and structure. This often results in receding groundwater levels that cause settlements and severe damage to the surrounding area. The stability of existing footings can also be impacted by pumping from adjacent construction excavations. Additionally, the construction of new dams has been found to lower river levels, which can cause severe damage and cracking of nearby structures.
In clayey soil, oversaturation can lead to a heaving problem. Therefore, structures built on such soil must be designed to withstand upward displacement or the supporting soil must be protected against flooding. This is important because failure to address these issues can result in significant damage to the structures and surrounding areas. It is crucial to carefully consider the impact of water content changes when building structures in areas with clayey soil to ensure their stability and longevity.
10. Vibration Effects
Soil can experience changes in volume when subjected to vibration impulses from various sources such as construction equipment, pile drivers, mechanical equipment in a completed building, traffic on the pavement, and blasting shock. When the soil is not fully consolidated, these vibrations can cause foundation failure, leading to heavy damage. The use of both impact hammers and vibratory hammers during pile driving can result in severe damage, causing entire rows of buildings to be deemed unsafe and demolished.
Blasting operations are another source of vibrations that can cause damage to adjacent structures. To avoid serious damage, maximum particle velocity should be established as a criterion for measuring the intensity of vibration. Seismographs should be used to take constant readings to monitor the vibrations. Due to the potential for severe damage, the size of explosive charges used in blasting operations may have to be limited to ensure that the vibrations remain at tolerable levels.
FAQs
How does excavation for new sewer trenches affect the foundation of existing buildings?
During the excavation process for new sewer trenches that are located near existing buildings, there is a risk of undermining the footings. This can lead to serious structural damage, including cracking and collapse of the building. It is important to be aware of this potential danger and take measures to prevent it from occurring. The consequences of an undermined footing can be severe, so it is crucial to approach any excavation work in close proximity to existing buildings with caution and careful planning.
What are the types of foundation failures due to loads?
The types of foundation failures that can occur due to loads can be categorized into three main types: punching shear failure, flexure failure, and one-way shear failure. Punching shear failure occurs when a concentrated load, such as that from a column, is applied to a slab or foundation that is too thin, causing the slab to fail by punching through the foundation. Flexure failure occurs when a foundation is unable to support a bending moment caused by an applied load, leading to the foundation cracking or breaking. One-way shear failure occurs when an applied load causes a foundation to fail in shear along a single plane, typically resulting in a diagonal crack. It is important to identify and address these types of foundation failures as they can have serious consequences for the stability and safety of a structure.