Immediate Settlement Calculation

Settlement of Foundations Analysis

The settlement of a foundation is an important consideration in foundation design. Many foundation design spreadsheets such as the CivilWeb Foundation Design Suite use a maximum allowable bearing capacity in order to simplify the analysis process. In the majority of cases this simplification is both reasonable and conservative.

Most methods of maximum allowable bearing capacity analysis include a maximum of 25mm of total settlement. As this requirement is included in the maximum allowable bearing capacity, a separate settlement analysis is not required so long as the maximum allowable bearing capacity is not exceeded.

However in many cases a separate settlement analysis is required. This can be in cases where consolidation settlement is expected which is not included in the maximum allowable bearing capacity calculations. In other cases structures may be very sensitive to settlement. In these cases the immediate settlement needs to be estimated in order to make sure that the structure is capable of accommodating the settlement.

Immediate Settlement Calculation

The total settlement of a foundation is made of several different components, each which must be calculated separately. The immediate settlement of a foundation is the almost elastic deformation of the soil when the foundation is first constructed and loaded.

Soil is not an elastic material when loaded close to its bearing capacity but can behave in an almost linear elastic manner when lightly loaded. In typical conditions foundations do not stress the soil to anything near its bearing capacity, and during working conditions the stress will generally be within the zone where soil behaves in an almost linear elastic way. For this reason it is often suitable to estimate the settlement of a soil under foundation loads by assuming a  linear elastic response.

The Bowles immediate settlement formula method is a variation on classical elastic theory analysis, with care taken to ensure that the elastic soil design parameters are determined in a representative way. By doing this the classical elastic theory can be used to complete immediate settlement calculations.

Elastic theory analysis such as those used in the Bowles method for immediate settlement calculations are extremely complex. The equations used are long and difficult to complete by hand. A number of graphs and tables have been developed over the years to try and simplify the hand calculation process but these can often lose a great deal of precision when values are being interpolated.

The process is further complicated when an optimum solution is need which requires the designer to complete the calculations several times. Indeed the immediate settlement calculations can sometimes need to be completed 10 or 12 times in an iterative approach in order to get any precision in the results. This can take days of difficult design work.

For this reason a simple design spreadsheet such as the CivilWeb Immediate Settlement Calculation Spreadsheet can save the designer many hours of work on every immediate settlement formula calculation. The spreadsheet completes the calculations instantly, allowing the designer to adjust and optimise the design quickly and easily without fear of making mistakes. The unique design graph also shows the designer exactly where the design can be improved at a glance, allowing a fully compliant, fully optimised design to be completed in minutes.

Immediate Settlement Calculation - Design Inputs

The CivilWeb Immediate Settlement Calculation Spreadsheet requires the designer to input the foundation dimensions and the maximum bearing pressure. Then the designer must enter the elastic soil design parameters. This includes the soils elastic modulus, the poisson's ratio and the depth of the soil layer. These parameters can be determined from site investigation testing.

The CivilWeb Immediate Settlement Calculation Spreadsheet then completes a stress analysis using elastic theory. The spreadsheet uses a method detailed by Bowles to compute the elastic settlement at the corners, sides and the maximum settlement at the centre of the foundation. This method has been found to be fairly accurate for estimating immediate settlements when used in appropriate conditions.

CivilWeb Immediate Settlement Calculation Spreadsheet

The CivilWeb Immediate Settlement Calculation Spreadsheet can be used to estimate the immediate settlement of any shallow foundation. The spreadsheet includes unique analysis tools which allow the designer to complete a full immediate settlement analysis in minutes, saving hours of work on every settlement calculation.

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