My problem is this.

In Column A I have a list of depths, (i.e. 1 - 10m in increments of 0.5m)

Column B has different levels corresponding to ground strata (i.e. Sand to
1.5m, clay below that to 4m, gravel below that to 6m etc..)

Column C has a designated shear strength value according to the type of soil.

What I want to do is sum the values in column C according to the value in
column A. For example for a depth of 7m, I want to sum all the values in
column C for the given levels in Column B between 0 and 7m

Hope this is explained well enough