When the estimated settlement of a building exceeds the allowable settlement, the designer must consider different options to manage the excessive settlement. The designer has four possible methods to choose from, which are altering the structure, providing deep foundation, changing the foundation, or improving the characteristics of the soil. However, the designer must carefully evaluate the construction cost, long-term operation and maintenance costs, and construction time for each alternative.
In making the decision, the designer should prioritize choosing an option that is more reliable in its construction and performance, even if it has a higher initial cost. This approach can minimize the risks and consequences of potential settlement problems that may arise in the future. Therefore, the designer should not only consider the short-term financial implications but also take into account the long-term implications of their decision.
1. Alteration of the Structure
Possible reorientation or movement of a building can be considered to achieve better foundation settlement conditions based on site, location, and function. This option may be more feasible in rural areas where land values are low. Another solution is to redistribute the load of the structure to reduce settlement. This can be done by selecting different framing systems to make the structure more rigid or flexible, providing better load distribution.
To further reduce settlement, the load of the structure can be decreased. This can be achieved by reducing the number of stories or changing the operations within the building to remove the heaviest parts from the weaker parts of the soil under the foundation.
In addition, jacks can be placed under primary columns to adjust for differential settlement. Although this option may seem impractical and difficult to implement, it has been successfully done in Mexico City where high-rise structures are periodically adjusted for differential settlement. The provision of jacks under columns is necessary to make measurements and adjustments at the appropriate times.
2. Alteration of the Foundation
Deep foundations are commonly used to address excessive settlement of buildings. Depending on factors such as soil type, equipment availability, material availability, and skill level, different types of deep foundations can be chosen for a building under consideration. These options are selected based on their suitability to the specific project requirements.
Figure-1: Deep Foundation
A floating foundation can be a practical solution for mitigating excessive settlements. By using a compensating foundation, the net load on the soil is reduced, which in turn minimizes differential settlement. However, it’s important to note that if the area to be supported is large, opting for a large raft foundation may be more appropriate, as a floating foundation may not provide effective control over a larger area.
Figure-2: Floating Foundation
Excavating one basement level is equivalent to building ten stories in terms of weight and complexity, but it can pose challenges due to deep excavation. Therefore, the number of stories that can be constructed may be limited based on the geotechnical conditions of the site. An alternative option is to use combined footing, which can help reduce differential settlement of structures. Another approach is to vary the contact pressure beneath the foundation or adjust the initial elevation of the foundation to mitigate differential settlement. However, this option requires a comprehensive understanding of the soil variability at the construction site.
3. Alteration of Soil Properties
Engineers are faced with a variety of soil improvement techniques to choose from based on the nature of the structure and type of soil present at the construction site. The selection of an appropriate soil treatment method heavily relies on the expertise of the engineer. The available methods for soil treatment include grouting, soft soil consolidation with surcharge combined with vertical drains to accelerate compaction, dynamic compaction, replacement of soft soil with strong fill materials, lime and cement columns, electroosmosis, freezing technique, blasting, vibro-replacement and vibro-floating technique, geotextile and geogrid fascines, wager short sheet pile, root piles, and sand compaction.
When selecting a soil treatment technique, engineers or contractors need to consider several criteria. Operation criteria for the structure, such as permissible total and rate of settlement, stability requirements, and maintenance needs, is one of the crucial elements that should be accounted for during the selection of soil treatment. Other factors to consider include the type of soil and its initial properties, the depth, area, and total volume of soil that needs to be improved, the availability of materials, the availability of equipment and necessary skills, environmental factors such as erosion, waste disposal, water pollution, and influences on adjacent structures, cost, time available, and local experience and preference.
FAQs
What is allowable settlement?
The acceptable amount of settlement for a structure, taking into consideration a factor of safety, is referred to as the allowable settlement. This term is commonly used in the field of civil engineering, where engineers must ensure that a structure is safe and stable even under varying loads and soil conditions. The allowable settlement is an important factor to consider when designing a foundation, as excessive settlement can lead to structural damage or failure.
To determine the allowable settlement, engineers must consider various factors such as the type of soil and its bearing capacity, the weight of the structure, and any potential loads it may experience during its lifespan. The factor of safety is also taken into account, which is a measure of how much stronger the structure is compared to the minimum strength required to support its loads. A higher factor of safety indicates a greater margin of safety against potential failure.
In summary, the allowable settlement is a crucial consideration in the design of any structure, as it helps to ensure its safety and stability over time. By accounting for various factors and incorporating a factor of safety, engineers can determine the maximum acceptable amount of settlement for a structure and design a foundation that can withstand a range of soil conditions and loads.
What are the alternatives used to cope with estimated settlement that exceeds tolerable limits?
There are various options available when faced with the challenge of addressing issues with a foundation. One alternative is to opt for a deep foundation, which involves excavating deep into the ground to create a sturdy base for the structure. Another possibility is to alter the structure itself in order to accommodate the existing foundation. This might involve making adjustments to the design or layout of the building in order to better distribute weight and reduce stress on the foundation.
Alternatively, it may be possible to change the foundation altogether, replacing the existing foundation with a more suitable option. This could involve using a different type of foundation or utilizing new materials and technologies to create a stronger and more durable foundation. Finally, improving the characteristics of the soil can also be an option. This may involve treating the soil to make it more stable or adding additional support structures to shore up the foundation and prevent further damage. Ultimately, the best option will depend on the specific circumstances of the situation, including the nature of the foundation, the characteristics of the soil, and the structural requirements of the building in question.
What are the criteria that should be considered during the selection of soil treatment technique
During the selection of soil treatment, several factors need to be considered. One of these elements is the operation criteria for the structure, which includes the permissible total and rate of settlement, stability requirements, and maintenance needs. These criteria play a crucial role in determining the appropriate soil treatment method to be used.
Soil type and its initial properties are also essential considerations when selecting a soil treatment method. This information helps to determine the appropriate approach to soil improvement and the expected outcomes. Additionally, the depth, area, and total volume of soil that needs to be improved must be taken into account.
The availability of materials is another crucial factor to consider during soil treatment selection. It is important to ensure that the required materials are available before starting the project. Availability of equipment and necessary skills is also necessary to ensure the successful implementation of the soil treatment method.
Environmental factors such as erosion, waste disposal, water pollution, and influences on adjacent structures must also be considered. Proper planning and preparation can help to minimize any negative impacts on the environment.
Cost and time available are other important factors to consider when selecting a soil treatment method. The cost of materials, equipment, and labor must be carefully considered, along with the time available to complete the project.
Finally, local experience and preference should also be taken into account. Knowledge of the local soil conditions and previous experience with soil treatment methods can help to determine the most suitable approach. Preferences and opinions of local stakeholders should also be considered to ensure that the project is well-received by the community.
What is a floating foundation?
A foundation that is known as a floating foundation, or balancing raft, is built by excavating soil in a manner that results in the weight of the structure being nearly equal to the weight of the soil excavated from the ground, along with the weight of any water present in the soil prior to the construction of the structure. This type of foundation ensures that there is zero settlement to the structure. For more information on floating foundations, click here.
What is raft or mat foundation?
A raft or mat foundation is a type of shallow foundation that consists of a large continuous concrete slab, either rectangular or circular in shape. Its main purpose is to distribute the weight of the superstructure evenly over the entire area beneath the building, thereby reducing the risk of differential settlement. This type of foundation is particularly useful in situations where the soil has low bearing capacity or where the building loads are very heavy. The raft or mat foundation is designed to provide a stable and level base for the building, which can help to prevent structural damage over time. By spreading the load of the building over a wide area, the foundation can also help to reduce the amount of excavation and soil removal required during construction. Overall, the raft or mat foundation is a reliable and effective option for many building projects, especially those that require a large and stable foundation.