Schmertmann Settlement Spreadsheet

Settlement of Foundations

The settlement of foundations is a critical calculation in any foundation design. All foundations will settle to some degree. The magnitude of the settlement depends on the size of the load, the size  of the foundation and the characteristics of the underlying soils.

The importance of the settlement depends on the type of structure and its purpose. In many cases large  settlements of 25mm or more are acceptable so long as they are consistent across the whole structure. This is easier to achieve when only one foundation is involved for example mat foundations or raft foundations. If the foundations are designed so that the bearing capacity of the soil is not exceeded, it is unlikely that the settlement will exceed 25mm.

Some structures however need to control total settlement to much smaller levels. This is usually due to the purpose or construction of the structure. In these cases a detailed analysis of the settlement of the foundation is required as well as a traditional foundation design based on the bearing capacity of the soil.

Schmertmann Method

There are many different methods of estimating the settlement of a  foundation. Many of these methods are specific to a particular type of settlement such as  elastic settlement or consolidation settlement.

The Schmertmann method estimates the total settlement for any foundation subject to axial  loads. The calculation method uses an analysis of the stresses caused by the foundation on the soil at different levels. It then uses a simple soil modulus value to model the soil response.

CivilWeb Schmertmann Settlement - Inputs

The CivilWeb Schmertmann Settlement Spreadsheet requires the designer to input the foundation dimensions and the total load. The spreadsheet uses this to determine the stress in the soil using the classical Boussinesq analysis method. The spreadsheet includes the total axial load and the density of the overlying soil layers to determine the stress in the soil. Note that the designer should adjust the total axial load input to include the self weight of the foundation. If the designer alters the foundation size the total axial force should be altered to account for the change in foundation self weight.

The CivilWeb Schmertmann Settlement Spreadsheet requires the designer to input the groundwater level and the soil characteristics. The Schmertmann method requires the soil modulus of each layer of underlying soils. The spreadsheet allows the designer to include up to ten different soil layers of any thickness. The soil modulus for use with the CivilWeb Schmertmann Settlement Spreadsheet can be determined through site investigation testing of the soils.

The spreadsheet then divides the input soil into more than 100 different slices for analysis. The spreadsheet calculates the average stress acting on each soil layer then uses the soil modulus to estimate the settlement of each layer. Finally the designer inputs the time period for the settlement analysis and the spreadsheet  returns the result.

CivilWeb Schmertmann Settlement Spreadsheet Analysis Tools

The CivilWeb Schmertmann Settlement Spreadsheet includes three unique design and analysis tools to assist the designer. The spreadsheet presents the designer with a graph showing the level of settlement expected at different soil depths. This can be used to assess whether particular soils at depth are affecting the settlement of the foundation.

The spreadsheet also presents the designer with a graph showing the total settlement and bearing pressure for different sized foundations. This allows the designer to see at a glance exactly how large the foundation needs to be to achieve settlement below a particular critical level. This unique feature saves the designer a great deal of time with iterative calculations to optimise the design.

Finally the spreadsheet creates a graph showing the designer the level of settlement expected over time. The Schmertmann method assumes that the settlement of the foundation will continue indefinitely, though at a slowly reducing weight over the years. The designer specifies a time period for the design which will usually be the design life of the structure. However the designer may be interested in the total settlement of the structure over the first year,  or ten years for example as well as over the total design life of the structure. This graph shows the designer at a glance  exactly how much settlement can be expected over the full life of the structure, and the total settlement expected after 1,000 years.

CivilWeb Schmertmann Settlement Spreadsheet

The CivilWeb Schmertmann Settlement Spreadsheet calculates the settlement expected from any foundation using the Schmertmann method. This calculation cannot be completed by hand and the spreadsheet gives a result instantly. The spreadsheet also includes unique analysis tools which the designer can use to complete the optimisation of a  foundation design in minutes.

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