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If the estimated settlement of a building exceeds the allowable settlement, then the designer can choose the following methods to cope with the excessive settlement:
i. Alter the structure
ii. Provide deep foundation
iii. Change the foundation
iv. Improve the characteristics of the soil
During the selection of an alternative, the designer must investigate both construction cost, and long-term operation and maintenance costs. Additionally, the time required for the construction of an alternative should be taken into consideration. It is better to choose an option that is more certain in its construction and performance even if its initial cost is higher.
What to do if the Estimated Settlement of a Building Exceeds Allowable Limits?
1. Alteration of the Structure
It is possible to reorient or move the structure based on the site, location, and function to achieve a better foundation settlement condition. The movement of the building is feasible in rural areas where the value of land is low.
Another option is to redistribute the load of the structure to decrease settlement. The selection of different framing systems to make the structure more rigid or flexible might provide better load distribution.
The load of the structure can be reduced to decrease settlement. For example, reduce the number of stories. Moreover, change operations within a structure to remove the heaviest part of the building to the strongest part of soil under the foundation.
Finally, provide jacks under primary columns to adjust for differential settlement. This option may seem impractical and undoable, but it has been implemented in Mexico City, where the number of high-rise structures are adjusted periodically for differential settlement. The provision of the jacks under column is needed to make measurements to make adjustments at the right time.
2. Alteration of the Foundation
The deep foundation is the most commonly adopted option used to tackle excessive settlement of a building under consideration. Various types of deep foundations are available that can be selected based on the type of soil and material, equipment, and required skill availability.
A floating foundation is an excellent option to choose to overcome excessive settlements. The compensating foundation reduces the net load on the soil and consequently reduces differential settlement. If the area is large, then select a large raft foundation instead of the floating foundation because it cannot be controlled over a large area.
For a given soil condition and building weight, excavation of one basement level is nearly equal to ten stories of average building height. Nonetheless, one should know problems arise with deep excavations.
Therefore, there are limits as to how many stories’ compensation can be achieved based on the geotechnical conditions at the construction site. Combined footing is another option that can reduce the differential settlement of structures.
Variation of contact pressure beneath the foundation or changing the initial elevation of the foundation is another feasible option for the reduction of differential settlement. One may not be able to adopt this option unless a complete picture of the soil variability on the construction site is available.
3. Alteration of Soil Properties
Different soil improvement methods are available from which engineers can select a suitable option based on the nature of the structure and type of soil on the construction site. The expertise plays a crucial role in the selection of soil treatment techniques.
Soil treatment methods include:
- 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,
- Freezing technique,
- Vibro-replacement and Vibro-floating technique,
- Geotextile; and geogrid fascines,
- Wager short sheet pile, root piles,
- Sand compaction.
Engineers or contractors should consider the following criteria during the selection of soil treatment technique:
- Operation criteria for the structure, such as permissible total and rate of settlement, stability requirements, and maintenance needs, is one of the elements that need to be accounted for during the selection of soil treatment.
- Soil type and its initial properties
- Depth, area, and total volume of soil needed to be improved.
- Availability of materials
- Availability of equipment and necessary skills
- Environmental factors viz erosion, waste disposal, water pollution, and influences on adjacent structures
- Time available
- Local experience and preference
The allowable settlement is defined as the acceptable amount of settlement of the structure and it usually includes a factor of safety. Read more about it here.
The alternatives may be a deep foundation, alter the structure, change the foundation, or improve the characteristics of the soil.
1. Operation criteria for the structure, such as permissible total and rate of settlement, stability requirements, and maintenance needs, is one of the elements that need to be accounted for during the selection of soil treatment.
2. Soil type and its initial properties
3. Depth, area, and total volume of soil needed to be improved.
4. Availability of materials
5. Availability of equipment and necessary skills
6. Environmental factors for example erosion, waste disposal, water pollution, and influences on adjacent structures
8. Time available
9. Local experience and preference
A floating foundation is a type of foundation constructed by excavating the soil in such a way that the weight of structure built on the soil is nearly equal to the total weight of the soil excavated from the ground including the weight water in the soil before the construction of the structure. A floating foundation is also called balancing raft and causes zero settlement to the structure. Read more about it here.
A raft or mat foundation is a large continuous rectangular or circular concrete slab that carries the entire load of the superstructure and spreads it over the whole area beneath the building. It is considered as one type of shallow foundation and is useful in controlling the differential settlement. Read more about it here.