---
Created: 2024/12/20 12:15
---
# Definition of Foundation

Foundation is the structural element of a building that transfers its load to the underlying soil or rock, ensuring stability and support. It distributes the weight of the structure evenly to prevent excessive settlement, tilting, or collapse.

# Difference between shallow and raft foundation

## Shallow Foundation

A shallow foundation is a type of foundation that transfers building loads to the near-surface soil layers — at shallow depth from GL. It is typically used when the load-bearing capacity of the upper soil layers is sufficient to support the structure.

- **Depth:** Generally placed at a depth of less than 3 meters.
- **Cost:** More economical compared to deeper foundations.
- **Usage:** Suitable for light structures such as residential buildings and small commercial buildings.
- **Examples:** Spread footing, strip footing, mat foundation.

## Raft Foundation

A raft foundation (A.K.A mat foundation) is a large, continuous slab that spreads the load of the building over a large area — typically the entire area of the bottom structure (floor). It is used when the soil's bearing capacity is low, and loads need to be distributed more evenly. It keeps all the columns under a single foundation and thus distributing load evenly in the soil.

- **Depth:** Similar to or deeper than shallow foundation.
- **Cost:** Higher cost compared to regular shallow foundations due to the amount of concrete and reinforcement required.
- **Usage:** Suitable for heavy structures, such as multi-story buildings, or when the soil has poor load-bearing capacity.
- **Examples:** Mat foundations under high-rise buildings or industrial facilities.

![](Figures.pdf)

# Sample bore log


![](Bore%20logs/Borehole%20locations%20-%20Sample%20bore%20log,%20Rajshahi.pdf)

![](Bore%20logs/Sample%20bore%20log%20-%20Rajshahi.pdf)

# Understanding about the bore log

The bore log provides critical insights into the site subsurface conditions. They can be interpreted as:

- There are three distinct soil layers — clayey silt (0 – 9m), silty sand (9 – 15m), and medium to dense silty sand (15 - 21m).
- The SPT-N values show consistency across borehole locations indicating uniform subsurface conditions. This reduces the risk of differential settlement.
- The SPT-N value is 2 – 4 at 1.5m, 4 – 6 at 3m, 7 – 10 at 4.5m, and 11 – 13 at 6m below GL. It increases gradually to 41 – 47 at 21m below GL. This indicates that the upper layer has lower bearing capacity while the lower layers offer better support for structural loads.
- There is a presence of groundwater at 4.5m depth. Foundation must be designed to mitigate potential issues such as buoyancy effect.

# Incorporation of choice of foundation

Based on the bore log data, between shallow and raft foundation, raft foundation is to be selected for this site.

- The upper soil layer (0 – 9m) is clayey, and has low SPT-N values, indicating a relatively weak soil with low bearing capacity (see the table from figure 3 on earlier page).
- The weak upper layer makes the building susceptible to subsidence due to its low bearing capacity as it may compress unevenly under building's weight leading to differential settlement issues.

A raft foundation will mitigate these issues by distributing the building load evenly over a large area, reducing per square meter load (stress).

On the other hand, a shallow foundation, though generally cheaper, is unsuitable because the required bearing capacity is only found at depths greater than 21m, where SPT-N values approach 50. Excavating to such depths would be impractical and likely be more expensive given the large footing area.

# Procedure for completing foundation

The step by step process for completing the foundation from scratch is detailed below.

Construction of the foundation is be to done starting with site exploration and soil testing. Considering they are done and their data is indicated by the attached bore logs, the next step would be to,

1. **Determine foundation depth**: The bore log indicates that the bearing capacity of upper soil layer is low which can lead to considerable settlements. One approach for improving this capacity is by increasing the depth of foundation since soil is denser at deeper depth. For this case, foundation depth of 9m is to be considered. At this depth, the soil can be compacted to further improve bearing capacity.
2. **Shore pile construction**: When foundation area is excavated, nearby soil may subject to lateral tilting due to nearby structural loads. Shore piles to be installed all around the excavation area to prevent such collapse.
3. **Excavation:** Once shore piles are constructed, the soil is to be excavated up to the required depth of the foundation, which is in this case 9m below GL.
4. **Formwork:** Install formwork for defining the edges and ensuring that the foundation dimensions are maintained.
5. **Levelling and dressing:** The excavated base is to be compacted to make the base flat and uniform as well as to make it stable and improve bearing capacity.
6. **C.C. layer construction:** Bricks (for Brick Flat Soiling, B.S.F) and cement concrete (C.C.) layer is to be laid over the base to provide a smooth and strong platform for the foundation.
7. **Column positioning:** Raft foundation has all columns under single a foundation, the columns are to be marked in such way. It is important that the loads from these columns are distributed uniformly. So, columns are to be positioned with proper design measures to minimize eccentricity which can lead to differential settlement.
8. **Reinforcement:** Steel reinforcements are to be placed in designated positions and above ground using clever clovers to avoid rust and reinforcement corrosion.
9. **Concrete casting:** Cement concrete to be poured into the formwork from a low drop height and proper compaction to be ensured to avoid segregation. 
10. **Curing:** The concrete needs to be kept moist for the required duration to achieve necessary strength and prevent micro cracks.

By following the above steps, a raft foundation for the target site would be completed ensuring long-term structural durability and mitigating risks such as settlement.