Thankyou for the responses and apologies for not responding sooner. Some of the assumptions that people have queried, ie fixed at base, soil pressure uniform function of depth - are those stated in the software (for clarity this is the RCC spreadsheet for the design of Basement walls (RCC61) provided by the concrete centre in the UK). I was not attempting to question the engineering assumptions and am fully aware that some of them (eg in relation to soil properties) are simplifications.

I have previously found a number of mathematical errors in these spreadsheets (eg one reported in previous post – which apparently went unnoticed for 9 years). From the diagrams and other aspects I suspect that this sheet was written with the assumption that the retained soil level was below the level of the prop (described at being at the top of the wall). However it is possible to enter a soil height that is higher than the prop height (see enclosed screen dump) I was concerned that in this case the calculations might not be valid.

My concerns arose because the spreadsheet has no representation for the wall above the prop (e.g. it is not shown on 'Wall Geometry') – it only calculates bending moments etc. for the wall between the base and prop and the calculated weight of the wall (e.g. when it calculates bearing pressure) is only that up to the height of the prop. My statement about ignoring loads above the prop and load transfer was an attempt to articulate the fact that if you had no knowledge of what was above the prop one could not assume anything about its properties.

My first thought to verify the calculations was to compare them with those calculated from an equivalent situation (my original query) – and here I was thrown by the representation used by the spreadsheet, as I compared it with the software's calculation for soil height equal to the prop height with a surcharge equivalent to the weight of the soil above. I now realise that if the soil height is higher than the prop height as far as calculating bending moments is concerned the software's calculations implicitly model the upper (cantilevered) part of the wall above the prop and my 'equivalent' is not equivalent since as stated in responses the rotation of upper cantilever reduces the bending moment on lower part of the wall. Though other elements of the software's calculations are incorrect if the soil height is above the prop, as far as bending moments are concerned the calculation is correct. I make no comment on whether the engineering assumptions behind this calculation are appropriate (triangular soil load etc.) - they were not my concern.

I sorry if anyone thinks that it is presumptuous of a computer scientist to check engineering calculations ('way over my head') but the issue (software failing to check whether entry values are outside the valid range assumed during design) is a software issue. My approach (comparing the results from two alternative paths through the software) is a standard one - if you get two different answers then query which is right. My error (which was an engineering one – in choice of 'equivalent') was caused by taking the representation given (the diagram of 'Wall Geometry') at face value and not thinking carefully enough about the engineering reality.