What Things to Consider for Soil-bearing Capacity

Soil bearing capacity:

Soil bearing capacity refers to the maximum load a soil can support without experiencing shear failure or excessive settlement. It is a critical parameter in construction and engineering projects, as it determines the stability and safety of foundations. Poor soil bearing capacity can lead to structural failures, so understanding and assessing it thoroughly is essential.

Definition of Soil Bearing Capacity

Soil bearing capacity is measured in terms of the pressure the soil can withstand, typically expressed in kilopascals (kPa) or pounds per square foot (psf). The two key thresholds are:

Ultimate Bearing Capacity:

The maximum pressure before soil fails.

Safe Bearing Capacity (SBC):

A reduced value, incorporating safety factors, is used for design purposes.

Factors Affecting Soil Bearing Capacity

Soil Type and Composition

Granular Soils (sand, gravel):

High bearing capacity due to good drainage and low compressibility.

Cohesive Soils (clay, silt):

Moderate to low bearing capacity, depending on moisture content and compaction.

Organic or Peaty Soils:

Very poor bearing capacity, unsuitable for direct foundation support.

Moisture Content

• Water reduces soil strength by weakening inter-particle bonds, especially in clay soils.
• Saturated soils are prone to liquefaction or excessive settlement under load.

Density and Compaction

• Well-compacted soils have higher bearing capacities.
• Loose soils or voids reduce load-bearing ability.

Depth of Foundation

• Bearing capacity typically increases with depth due to the confining pressure of overlying soil layers.
• However, deeper layers may pose issues with water tables or weaker strata.

Soil Permeability and Drainage

• Soils with poor drainage can retain water, increasing the risk of settlement or shear failure.

Key Methods to Evaluate Soil Bearing Capacity

Accurate determination of soil bearing capacity requires site-specific assessments through testing and calculations.

Field Tests

Standard Penetration Test (SPT):

Measures soil resistance to a driving hammer. Provides N-values used to estimate SBC.

Plate Load Test:

This test involves loading a steel plate on the soil and measuring settlement under incremental loads. It is ideal for shallow foundations.

Cone Penetration Test (CPT):

This test uses a cone to measure resistance during penetration and offers continuous profiling of soil strength.

Laboratory Tests

Triaxial Shear Test:

Assesses soil’s shear strength under controlled pressure.

Unconfined Compression Test:

Suitable for cohesive soils, measuring maximum stress before failure.

Moisture Content and Density Tests:

Determines optimal compaction and moisture conditions for load-bearing.

Empirical Formulas

Engineers often use empirical formulas such as Terzaghi’s bearing capacity equation for preliminary estimates. This method considers factors like cohesion, soil weight, and foundation width.

Adjustments and Considerations for Site-Specific Conditions

Load Type and Distribution

Concentrated loads (e.g., columns) require more detailed analysis than distributed loads (e.g., slabs). Due to repeated stress, dynamic loads (machinery) demand higher safety margins.

Influence of Adjacent Structures

Nearby foundations or excavation work can affect soil stability. Lateral pressures and shared loads need consideration.

Seasonal Variations

Frost heave or soil expansion in wet seasons can impact SBC. Soil performance over time should be assessed.

Groundwater Table

High water tables reduce effective stress in soil, lowering bearing capacity. Dewatering measures or deeper foundations may be necessary.

Improving Soil Bearing Capacity

If the natural soil does not meet the required bearing capacity, various techniques can be used to enhance it.

Compaction

Mechanical compactors increase soil density and reduce void spaces. This is effective for sandy and granular soils.

Soil Stabilisation

Chemical Stabilisation:

Adding lime, cement, or fly ash improves cohesion and reduces moisture sensitivity.

Geosynthetics:

Geotextiles or grids reinforce soil structure.

Replacing Weak Soil

Excavating poor soil and replacing it with compacted granular material is a direct but costly solution.

Piling or Deep Foundations

Transferring loads to deeper, stronger strata through piles is ideal for weak surface soils.

Drainage Improvements

Installing drains or permeable layers can mitigate water retention, improving SBC over time.

Safety Factors in Design

Engineers use safety factors to account for uncertainties in soil properties, loading conditions, and long-term behaviour.

• Typical safety factors range from 2 to 3, depending on the soil type and project risk level.
• Conservative designs reduce failure risks but may increase costs.

Practical Considerations During Construction

• Avoid overloading temporary storage areas near excavation sites.
• Monitor settlements during construction using benchmarks.

Conclusion:

Assessing and ensuring adequate soil bearing capacity is fundamental to the success of any construction project. By considering soil type, moisture, depth, and environmental conditions and employing robust testing methods, engineers can design safe and efficient foundations. Where natural conditions fall short, soil improvement techniques and safety factors provide viable solutions. Proper attention to these factors ensures long-term stability and minimises the risks of structural failures. CJB Piling is the leading company that is always here to help you with each project within budget.