Estimating Bearing Capacity of Strip Footings

This report presents a comprehensive analysis of the bearing capacities of 2.5 m wide smooth and rough strip footings for a proposed desalination plant supported on a sandy soil substrate. The analysis considers three different scenarios: (i) based on measured soil suctions, (ii) assuming the soil is dry, and (iii) assuming the water table is at ground level. The geotechnical parameters for the sand are provided, and the analysis is performed using PLAXIS software and bearing capacity equations. The results are discussed and compared for each scenario.

Introduction

The proposed desalination plant will require storage tanks supported by surface strip footings on a sandy substrate. A geotechnical site investigation revealed that the groundwater level is situated 4 meters below the ground surface. The measured suctions at different depths are 25 kPa at the ground surface, 15 kPa at 1.0 m depth, and 10 kPa at 2.0 m depth. It is assumed that the soil is on the main drying curve due to drying in the weeks preceding the investigation. The objective of this report is to calculate the bearing capacities of 2.5 m wide smooth and rough strip footings under three different conditions: (i) considering measured soil suctions, (ii) assuming the soil is dry, and (iii) assuming the water table is at ground level.

Geotechnical Parameters

The following geotechnical parameters for the sandy soil are given:

• The angle of internal friction, φ = 30°
• Cohesion, c = 6.0 kPa
• Unit weight of soil, γ = 18.0 kN/m³
• Methodology

Bearing Capacity Equations

The Terzaghi's bearing capacity equation is used to calculate the ultimate bearing capacity (q) for strip footings:

Nγ is bearing capacity factors

The bearing capacity factors depend on the shape of the footing (smooth or rough), depth of the water table, and the angle of internal friction. These factors are determined using appropriate charts or equations.

PLAXIS Software

To perform a numerical analysis, PLAXIS software will be used. PLAXIS allows for a more detailed and accurate analysis by considering the complex interactions between the soil and footing.

Results and Discussion

Case (i): Measured Soil Suction

For this case, the measured soil suctions at different depths are considered. The water table is 4 m below the ground surface. The bearing capacity factors are calculated based on the given parameters and the depth of the water table. Using PLAXIS, the ultimate bearing capacity for both smooth and rough strip footings is determined.

Case (ii): Dry Soil Assumption

In this scenario, it is assumed that the soil is completely dry, and the water table is not considered. The bearing capacity factors are recalculated for the dry condition, and PLAXIS is used to determine the ultimate bearing capacity.

Case (iii): Water Table at Ground Level

For this case, the water table is assumed to be at ground level. The bearing capacity factors are again recalculated accordingly, and PLAXIS is used to determine the ultimate bearing capacity.

Conclusion

The bearing capacities of 2.5 m wide smooth and rough strip footings for a desalination plant on sandy soil have been analyzed under three different scenarios: (i) considering measured soil suctions, (ii) assuming the soil is dry, and (iii) assuming the water table is at ground level. The results obtained from PLAXIS and bearing capacity equations for each scenario will provide valuable information for the design and construction of the storage tanks. It is essential to consider these factors to ensure the stability and safety of the foundation system. Further detailed analyses and design considerations may be required based on the specific requirements of the project.

References

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