Bearing capacity of soil- Significance

The strength and stability of any structure depends on the ability of the underlying soil to safely carry and transfer loads to the ground. This load carrying capacity of the soil is referred to as the bearing capacity of soil.

Evaluating the bearing capacity of soil is a crucial step in foundation design to ensure the stability of structures and prevent excessive settlements or foundation failure. The ultimate bearing capacity is the maximum pressure that can be applied before shear failure occurs in the soil. However, an appropriate factor of safety is used to determine the allowable bearing capacity for safe foundation design.

The bearing capacity of soil depends on several key parameters like the shear strength, unit weight, and properties of the soil as well as the foundation dimensions. There are various field tests like plate load tests and standard penetration tests used to determine the bearing capacity of different soil types for a given site. Improving poor soils and maintaining adequate bearing capacity provides the robust foundation support structures require.

Types of Bearing Capacity of soil
Ultimate bearing capacity

The maximum pressure that can be applied to soil before shear failure and collapse occurs. This determines allowable foundation loads. It is highly dependent on the shear strength parameters of the soil, mainly cohesion and internal friction angle. It can be estimated using analytical equations, such as the Terzaghi or Vesic equations, or using chart solutions.

Depth of footing, footing shape and size, soil unit weight, and groundwater conditions affect the ultimate bearing capacity value. General shear failure, local shear failure, and punching shear failure are possible failure mechanisms when bearing capacity is exceeded. In design, an appropriate factor of safety is applied to the ultimate bearing capacity to determine the allowable bearing capacity for foundations. Typical factors range from 2 to 4.

Plate load tests can directly measure the ultimate bearing capacity of soils below a foundation. Remolding and sample disturbance negatively impact the soil’s bearing capacity compared to undisturbed conditions. Cyclic and impact loads may reduce the ultimate bearing capacity, requiring special analysis. Ultimate bearing capacity is one of the most important design parameters for developing economical, safe foundations.

Allowable bearing capacity

The maximum bearing pressure that can be safely applied to soil or rock to support foundation loads without causing shear failure. Allowable bearing capacity of soil is calculated by dividing the ultimate bearing capacity by a factor of safety (typically ranging from 2-4).
Provides collapse resistance – The factor of safety ensures the applied bearing pressure remains below the ultimate capacity to prevent foundation failure.
The safety factor accounts for uncertainties in soil parameters, analysis methods, and construction variability.

Allowable capacity may be reduced to limit total and differential settlements within structure tolerances. Building codes provide minimum factors of safety for determining allowable bearing capacity per foundation type. Allowable values are influenced by soil/rock type, strength, groundwater, slope stability, and loading conditions. A higher allowable capacity means smaller, less expensive foundations.
In layered soils, allowable capacity is determined separately for each distinct stratum. If allowable bearing greater than applied loads signifies the foundation will perform satisfactorily.
Geotechnical engineers provide soil data to calculate ultimate and allowable bearing capacity. Plate load tests can measure allowable bearing capacity in the field.

Methods to Improve Bearing Capacity of soil

Here are some common methods to improve the bearing capacity of soil:

• Compaction – Increasing the density of granular soils through compaction improves shear strength and bearing capacity.
• Reinforcement – Adding geotextiles, geogrids, or structural elements like stone columns or piles reinforces the soil and increases bearing capacity.
• Removing poor soils – Excavating weak, compressible soils and replacing with compacted fill creates a stronger bearing layer.
• Dynamic compaction – Dropping heavy weights on granular soils induces densification and higher bearing capacity.
• Grouting – Injecting cement or chemical grouts into the soil strengthens it by filling voids.
• Ground improvement – Methods like vibro-compaction, blasting, deep soil mixing, or injection can strengthen soils.
• Drainage – Installing drainage layers and sumps prevents build up of excess pore pressures that can reduce effective bearing capacity.
• Prestressing – Applying preloading to induce consolidation settlement prior to construction improves bearing capacity.
• Foundation width – Increasing the foundation width distributes loads over a broader area and engages stronger soil.
• Mats/rafts – Using a thick mat or raft foundation averages out soft spots and utilizes stronger depths.
• Deep foundations – Piles, piers and caissons transfer loads to deeper, stronger soil or rock layers.

The choice of method depends on soil conditions, improvement needed, cost, and project constraints. A geotechnical engineer can advise on the most suitable approach.

Factors Affecting Bearing Capacity of soil

some of the key factors that affect the bearing capacity of soil:

• Soil type – Cohesive soils like clays have higher bearing capacity than granular soils like sands. Bearing capacity increases with cohesion.
• Density/compaction – Denser soils have higher bearing capacity. Compaction improves dense granular soil capacity.
• Moisture content – Very dry or saturated soils have lower capacity. Optimal moisture provides maximum density and bearing capacity.
• Shear strength – Higher cohesion and internal friction angle give soils greater bearing resistance before failure.
• Soil layering – Capacity depends on the weakest layer. Thick strong layers over weaker layers help distribute loads.
• Depth of soil – Capacity increases with depth due to overburden pressure. But too deep reduces stability.
• Groundwater level – Higher water tables reduce effective stress and capacity. Proper drainage improves bearing soils.
• Footing width – Wider footings distribute loads over larger areas and engage stronger soil.
• Load inclination – Inclined loads reduce bearing capacity compared to vertical loads.
• Rate of loading – Rapid loading gives soils less time to adjust, decreasing bearing capacity.
• Cyclic loading – Repeated loads degrade soils through pulsation and fatigue.
• Slope of surface – Bearing capacity is reduced on slopes due to soil weight acting laterally.

Proper accounting of these factors through soil testing and analysis provides the most accurate evaluation of bearing capacity.

Methods to measure and evaluate bearing capacity of soil in field

• Plate load test – A loaded plate or footing applies increasing pressure to the soil while settlement is measured. The load causing shear failure indicates ultimate bearing capacity.
• Standard penetration test (SPT) – A thick-walled sample tube is driven into the soil using a hammer. The blows needed provides an empirical correlation to bearing capacity.
• Cone penetration test (CPT) – A cone tip is pushed into the soil while measuring penetration resistance. This indicates bearing strata.
• Vane shear test – A vane is inserted in soft clays and torque applied until shearing occurs. The torque correlates to shear strength and bearing capacity.
• Pressure-meter test – A cylindrical probe inflates in the borehole, measuring stress and displacement. This provides soil deformation properties related to bearing.
• Dilatometer test – A flat bladed probe expands in the soil, measuring pressure and displacement to evaluate bearing capacity.
• Dynamic load test – Impact loads are applied by dropping weights or other methods to assess dynamic bearing behavior.
• Plate load test with settlement monitoring – Settlement instrumentation helps determine both ultimate and allowable bearing capacity.
• Test pilings – Instrumented piles are loaded to failure to directly measure axial and lateral pile capacities.
• Observation of existing foundations or pilings – The load-settlement behavior of existing foundations can indicate bearing strata capacities.

Field data provides supporting evidence for bearing capacity assumptions and replaces or augments laboratory testing. This leads to more accurate foundation designs.

Factors that influence the selection of an appropriate bearing capacity test methods

• Site conditions – The ability to conduct certain tests may be limited by space constraints, subsurface obstructions, groundwater depth, or weak surficial soils.
• Soil type – Cohesive vs. granular soils may dictate suitable tests. Penetration tests work better in cohesive soils.
• Depth of interest – Shallow tests like plate load tests evaluate shallow foundation capacity. Deep tests like pilings assess deeper strata.
• Cost and schedule – Simple tests like SPT are lower cost. Advanced tests like pressuremeter and pile load tests are more expensive and time-consuming.
• Ultimate vs. allowable capacity – Plate load and pile tests can determine both ultimate and service limit capacities. Other methods estimate ultimate only.
• Test standards – Building codes may require certain standardized test methods and procedures.
• Equipment availability – Availability of specialty testing equipment like dilatometer or pressuremeter probes may dictate viable options.
• Accuracy needs – Some projects demand high accuracy bearing data justified by costly tests. Other situations may only require basic estimates.
• Past experience – Local experience with certain tests provides correlations to bearing capacity for similar sites.
• Disruption – Tests like pile load tests greatly disturb sites. Minimally invasive methods may be preferred.
• Field verification – Even when lab testing is conducted, some projects benefit from field verification of bearing assumptions.

Bearing Capacity of soil Calculations

1. Determine soil parameters:
   

• Unit weight (γ)
• Cohesion (c)
• Friction angle (φ)
• Footing width (B)
• Factor of safety (FS)

2. Estimate depth of embedment (Df) based on frost depth, groundwater, and loading.
3. Calculate bearing capacity factors (Nc, Nq, Nγ):

• Use bearing capacity equations/charts based on φ

4. Calculate ultimate bearing capacity (qu):

• qu = cNc + γDfNq + 0.5γBNγ

5. Apply safety factor:

• Allowable bearing capacity (qa) = qu/FS

6. Compare qa to applied bearing pressure:

• If qa > applied pressure -> Okay
• If qa < applied pressure -> Increase footing width or depth

7. Key considerations:

• Use appropriate analysis method (Terzaghi, Meyerhof, Vesic, etc)
• Account for groundwater impacts
• Check for eccentric or inclined loads
• Check potential settlement
• Consider shaley or layered soils

Bearing Capacity of Different Soil Types

• Rock – Very high bearing capacity.
• Sands – Low cohesion leads to medium capacity.
• Clays – High cohesion gives higher bearing capacity.
• Peats – Very low capacity due to high organic content.

Importance of Bearing Capacity of soil for Foundations

some key points on the importance of bearing capacity in foundation engineering:

• Prevents foundation failure – Verifying adequate bearing capacity prevents shear and plunging failures leading to structural collapse.
• Controls settlement – Bearing capacity limits settlement to acceptable levels, maintaining structural integrity.
• Drives foundation design – Bearing capacity determines the type, depth, and dimensions of foundations required to support structural loads.
• Impacts economics – Higher allowable bearing pressures permit smaller, less expensive foundations. Maximizing capacity provides cost savings.
• Influences construction – The bearing strata controls excavation needs, dewatering, and foundation construction methods.
• Requires geotechnical input – Geotechnical engineers provide critical subsurface investigation and analysis for bearing evaluations.
• Safety factor – Appropriate factors of safety must be applied to the calculated ultimate capacity.
• Code compliance – Building codes provide minimum bearing capacity and safety factors for foundations.
• Capacity can be improved – Ground improvement techniques can increase bearing capacity if inadequate.
• Needs verification – Field plate load tests help validate bearing capacity assumptions.
• Dynamic loads – Special analysis is needed for earthquake, wind, wave and vehicular dynamic foundation loads.

An accurate assessment of bearing capacity soil is crucial for safe, cost-effective design and construction of foundations in civil engineering projects. It is a limiting design parameter that must be carefully evaluated.

Effect of Groundwater on Bearing Capacity of soil

Higher water table decreases effective stress in soil reducing friction and cohesion. This lowers bearing capacity of soil significantly. Proper dewatering is essential.

some of the ways to account for the effects of groundwater on bearing capacity of soil :

• Use effective stress parameters – The cohesion and friction angle used in bearing capacity equations should be effective stress parameters, which account for pore water pressure.
• Adjust depth terms – The depth used should be measured from the groundwater table, not the ground surface. This represents the effective overburden providing confining stress.
• Check seepage effects – A sloping groundwater table or vertical seepage forces can reduce bearing capacity and require special analysis.
• Consider uplift forces – If the water table rises above the foundation depth, uplift forces will reduce bearing capacity.
• Compute buoyant unit weights – Below the water table, buoyant unit weights of soil should substitute dry unit weights in bearing capacity equations.
• Check for potential liquefaction – Loose saturated sands may liquefy under seismic loading, causing bearing failure.
• Evaluate drainage – Well-drained soils maintain effective stress and higher bearing capacity. Proper drainage may be required.
• Monitor water fluctuations – Water levels should be monitored over time to enable bearing capacity adjustments for seasonal highs.
• Increase factor of safety – A higher safety factor may be warranted to account for water table variability and drainage uncertainties.

Proper accounting of groundwater effects through both analysis and construction practices (drainage, seepage control) is needed to evaluate and maintain adequate bearing capacity.

Conclusion

The bearing capacity of soil is a critical design parameter that determines the load carrying ability of foundations. It must be evaluated to prevent shear failure and excessive settlement. Bearing capacity depends on several factors including soil cohesion, friction angle, unit weight, and depth.

These are used to calculate the ultimate bearing capacity, which is then reduced by a safety factor to determine the allowable bearing capacity for design. Settlement potential should also be assessed to ensure applied loads remain within tolerable limits.

The geotechnical engineer plays a vital role in investigating subsurface conditions, determining soil parameters, calculating bearing capacities, and advising on appropriate safety factors and field testing needs.

Safe and cost-effective foundation design relies heavily on a thorough evaluation of bearing capacity of soil using both engineering analysis and practical site-specific data.