Spatial variation of soil properties

Transcription

1 1 Introduction Spatial variation of soil properties The undrained strength of a soft soil deposit is frequently a function of the effective overburden; that is, the strength varies with depth. Sometimes, the undrained strength can also vary laterally perhaps due to some past loading of a confined area. A typical case is illustrated in Figure 1. The foundation contours show the spatial variation of the undrained strength (cohesion). The undrained strength of the peat varies laterally under the dam. The cohesion in the embankment is constant. The total unit weight may also vary in a similar fashion. Figure 1 An example of spatial variation of undrained strength (copied from the text book, Soil Strength and Slope Stability, by J.M. Duncan and S. G. Wright, page 147, published by John Wiley) This type of situation can be defined in SLOPE/W with what are known as Spatial and Linear functions. 2 Foundation soil 2.1 Undrained strength - cohesion The foundation undrained strength is defined with a spatial function. The undrained strength is specified at a series of data points. It is necessary to specify the x-y coordinates of the Points and the Cohesion at that point. The edit dialog box for specifying the data is presented in Figure 2. The data can be specified by entering the numeric values for the x- and y-coordinates together with the corresponding cohesion. Alternatively, data can be specified by entering a cohesion value and then Drawing the points by clicking at appropriate locations. SLOPE/W Example File: Spatial variation of soil properties (pdf)(gsz) Page 1 of 6

2 Figure 2 Illustration of x-y data points with a specified cohesion Once the data points have been defined, SLOPE/W will contour the data within the region that has the property of that particular spatial function, as illustrated in Figure 3. Figure 3 Specified data points and resulting contours of the cohesion SLOPE/W Example File: Spatial variation of soil properties (pdf)(gsz) Page 2 of 6

3 2.2 Foundation unit weight The same can be done for the total unit weight of the foundation soil, as illustrated in Figure Figure 4 Variation of unit weight in the foundation 3 Peat layer The lateral variation of the cohesion and unit weight in the peat can be defined with Linear functions, as in Figure 5 and Figure Peat strength Cohesion (psf) X (ft) Figure 5 Variation of the undrained strength in the peat layer SLOPE/W Example File: Spatial variation of soil properties (pdf)(gsz) Page 3 of 6

4 Peat weight Unit Weight (pcf) X (ft) Figure 6 Variation of the unit weight in the peat layer 4 Embankment material The properties of the embankment material are constant with respect to space or distance. 5 Material property contours The final contours of cohesion and unit, as defined for the whole problem, are shown in Figure 7 and Figure 8. Mohicanville Dike Figure 7 Contours of cohesion SLOPE/W Example File: Spatial variation of soil properties (pdf)(gsz) Page 4 of 6

5 Mohicanville Dike Figure 8 Contours of unit weight Note that the contours are not continuous across the geometric regions. Each region or group of regions with the same function is contoured separately. 6 Cohesion along the slip surface Figure 9 shows a typical potential slip surface. Figure 10 shows the variation in cohesion along the slip surface. Figure 9 A typical slip surface position SLOPE/W Example File: Spatial variation of soil properties (pdf)(gsz) Page 5 of 6

6 Cohesion Cohesive (psf) X (ft) Figure 10 Variation of cohesion along the slip surface Figure 10 is created with the Graph command in CONTOUR. It is always advisable to spot check that the definition has been correctly used in the factor of safety calculations by plotting parameters like cohesion along the slip surface, particularly when using material property functions. 7 Concluding comments It is important to note that these spatial material property functions are unique to a particular problem and a specific set of x-y coordinates. They cannot be used in other cases, unless the coordinate space and problem definition are very similar. This example vividly presents an illustration of how material properties that have a spatial variation can be defined and used in SLOPE/W. SLOPE/W Example File: Spatial variation of soil properties (pdf)(gsz) Page 6 of 6