Consolidation Settlement Calculation Spreadsheet

Primary Consolidation Settlement Calculations

Primary consolidation settlement of soil occurs when clay soils are loaded under certain conditions. Any soil under sudden load such as when foundations are built and loaded experiences an increase in pore water pressure. In most soils this pore water pressure quickly dissipates as the water percolates through the soil away from the zone of influence of the foundation. This movement of water away from the foundation causes the soil to lose volume resulting in settlement beneath the foundation.

In many soils such as gravels or sands this happens within hours because the soil is very permeable. However clay soils are very fine grained and have very poor permeability. This means that the increased pore water pressures dissipate only very slowly and the associated settlement continues for months or even years after the foundation is built.

The magnitude of primary consolidation settlement of the soil is dependent on the soil conditions and can continue to occur even when the foundation bearing pressure is well below the soils maximum allowable bearing capacity. The bearing capacity of the soil includes a requirement that the soil will not settle by more than 25mm. However consolidation settlement is not included in this figure. This means that in some conditions with clay soils a separate settlement analysis is required along with the bearing capacity analysis.

Primary Consolidation Settlement Calculation - Classical Analysis

The CivilWeb Consolidation Settlement Calculation Spreadsheet includes two different methods of estimating the ultimate consolidation settlement of a shallow foundation. The first method is a classical method. This uses a Boussinesq style analysis of a multi-layered soil system.

First the designer must input the foundation shape, dimensions and loading conditions. The spreadsheet uses a total axial load which should include both the imposed loads and the self-weight of the foundation.

The designer can input details for up to 10 different soil types, more than any similar analysis spreadsheet. The designer must input details for the soil layers depth, consolidation index, recompression index, preconsolidation margin and the unit weight of the soil. These parameters can be obtained from laboratory testing of the soils. The designer must also input the depth of the groundwater which can be taken as an average depth.

The spreadsheet then calculates the bearing pressure acting on each layer of the soil using Boussinesq principles. This average bearing pressure is then used along with the soil design parameters to estimate the amount of ultimate consolidation settlement which can be expected from each layer of the soil. To improve the precision of the calculation, the spreadsheet automatically splits the soil into 100 separate layers. This would take many hours of calculations to complete by hand but the CivilWeb Consolidation Settlement Calculation Spreadsheet completes all the calculations instantly. The spreadsheet then combines the results for all 100 soil layers to estimate the total consolidation settlement  which can be expected.

The CivilWeb Consolidation Settlement Spreadsheet also includes a second method of estimating consolidation settlement formula. This method is much simpler as it applies only one soil design parameter, the coefficient of compressibility.  The coefficient of compressibility of the soil can be determined from laboratory testing or for preliminary design the parameter can be estimated or taken from typical values. In this way this method is often used for quick preliminary design.

Both spreadsheets include unique analysis graphs which show the designer at a glance exactly what  level of settlement they can expect for different sized foundations. This can be used to optimise the design of the foundation in moments. The optimisation of foundations in this way would usually take many hours of iterative design work but the CivilWeb Consolidation Settlement Spreadsheet makes this process quick and easy.

The total amount of consolidation settlement can be added to the estimates for other types of settlement such as elastic or immediate settlement to determine an estimate for the total settlement of the structure. Often this is limited to a total value of less than 25mm for normal buildings though this will depend on the building type and use. Some buildings can accommodate much more settlement so long as it is constant along the whole building. Some structures cannot accept any significant settlement.

CivilWeb Consolidation Settlement Calculation Spreadsheet

The CivilWeb Consolidation Settlement Calculation Spreadsheet can be used to estimate the amount of consolidation settlement which can be expected for any shallow foundation in any soil. This calculation is not practical to complete by hand as it requires a large number of calculations, particularly in deep stratified soils. The CivilWeb Consolidation Settlement Calculation Spreadsheet completes all these calculations instantly allowing the designer to complete a detailed analysis in minutes.

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