Holdfast Training Services Trial Blog

This week did not start well when South West Trains decided to terminate my train at Southampton at 9pm on Sunday evening due to an electrical failure. Having learnt from my last epic 10 hr train journey where a tree landed on the train roof a few weeks ago, I decided to promptly get on the next train back home to Bournemouth as there was no sign of anything going north.  The 5am start on Mon to get back set me up well to write a great complaint letter to the train company anyway!

After finally arriving on site the day could have gone one of two ways depending on the progress with the HV substation slab and the tapping of the old pipe.  I was happy to find that both tasks had been carried out but when I looked into them further I realised that the day had started at 5am as it meant to go on: a nightmare!  Firstly, I went to have a look at my concrete slab and found this:

A banana shaped trench that was meant to be straight!!  The as built survey showed that the trench was 47mm out about 3/4 of the way along the first picture.  Apparently the formwork had got wet before the pour and as a result it had warped or not been strong enough.  It is unlikely that the pour rate was too fast for the formwork design as some parts had deflected into the slab.   A more likely reason was crap carpentry from the two cowboy builders who won’t be working on site again!  The subcontractor may get away with it if the electrical company don’t have a problem with it when they visit next Mon.  Otherwise I will be producing a non-conformance report next week for some remedial action to be taken.

Then there was my old pipe that broke the utility guys tapping drill last week.  We excavated a straight part of the pipe and they were finally able to tap it and it was found to be an old gas pipe, not full of gas but full of water.  The drainage gang took the bung off and spent an afternoon letting it drain and pumping out the water.  A few hours later they gave up as it didn’t seem to be draining-back to the drawing board!  Plan B involved hiring in a water tanker pump vehicle to hopefully pump it out.  2000 litres and 2 tankers later they were finally in a position to break the pipe and carry on.  The south drainage area has now been nick named the aquatic area and they will have more problems to come when they encounter the run-off from the wheel wash that is also turning part of the site into a swimming pool.

The week ended in a slightly more positive note with a free trip to the Beyoncé concert at the O2 arena.  Our groundworks sub-contractor’s O’Keefe own a box there so they invited the construction team for a night out.  With free drinks and food and a great view it was a great night out.  However I have learnt that drinking on a school night isn’t much fun when you get given a list of RFIs to check if they are closed the next morning!

Today I genuinely enjoyed the construction of a spreadsheet, this is probably the day I’ve enjoyed more than any other since I’ve been here at Ramboll.

The Problem

We are part of a team tendering for a new large 1 km ish bridge over a river on my mudstone.  I have been asked to look at the West Tower which is a spread foundation on the mudstone that is currently sloping quite severely and has a minor road at the top of that slope.  The mudstone has discontinuities although orientations aren’t known.  I was asked to identify whether there are any orientations that are particularly dangerous to the foundation. 

Direction

Initially I was told to us Slope/W which is a simple program that can check large numbers of slip surfaces on a slope in a short time and come out with the least safe.  In order to model the rock with discontinuities I was advised to model the discontinuities as regions with weaker parameters to encourage failure along these lines.  It all made sense so I started learning Slope/W and built a model.

Issues

After running a few iterations of the model and speaking to the person who first tasked me it became obvious that things weren’t so clear cut.  The model didn’t seem sensitive to the properties of the discontinuities, I even gave them the friction angle of ice cream and it still didn’t fail along them.  Going back to original instruction revealed that she didn’t necessarily know how to get it to work, or even what the answer might look like (I can hear the Orator as I type if you don’t know the answer don’t use a program!  I would be fairly unemployable right now if I stuck to that advice).  She then promptly buggered off and after a day or two of wrestling with stupid answers spoke to the Director who had tasked the other engineer, he was equally confused but gave me a book on Foundation on Rock (along with a quip about when he was looking for it on Amazon all that kept coming up was music related, hilarious). 

Stability of a wedge

The book was quite useful and gave examples of calculating the stability of a wedge based on certain soil parameters and geometries of rock and loadings.  Because we didn’t really know the orientation of the cracks and that my brief was kind of a what if analysis I decided to stick it all into excel and see if I could make it work.  This is why I feel dirty, I had to design the spreadsheet on paper to make sure I got all the variables, lots of coordinate geometry and learning to use ‘data tables’ with 2 variables later I churn out a table that will autocalculate the lowest factor of safety depending on crack angle and the point at which is intercepts the front of the slope.  I can then fiddle with discontinuity parameters such as friction angle or rock unit weight or applied load and see how it changes the FOS.  Next week I will interrogate a true expert on Slope/W to see if they can get better answers on the problem.

What have I learnt?

• Software can be limited – There were issued modelling the surcharge in Slope/W because it was below the level of the ground so I had to model a rock with a unit weight of 2200 kN/m³
• Books are useful
• Clear briefing is a skill that isn’t necessarily easy to find in an office, this relates to other stuff I’ve put on here over the last month or so that designers don’t get the opportunity to really manage, I’ve just had things thrown at me a couple of times and been expected to get on with it with little to no brief

Other recent lessons

• People will just assume you can read your mind, or they don’t know what they really want when the task you.  An individual from another office working in structures wanted some pile group modelling done but didn’t explain how he wanted the output so the work took twice as long as necessary
• Geotechnics is about experience and experience, knowing where to look for the answers and best guesses are the currency.  We are looking at doing an extension to a quay wall in a dock basin that can’t be drained and need to know what angle the mudstone can’t be excavated to and remain stable, the answer is ‘who knows? But our best guess is ….’
• When you tell people what you’re doing and that you’re going to CPR in Jul they all reply sceptically ‘That’s a bit fast isn’t it?’ they are promptly silenced when politely informed that the Army allows you to cover all the DOs they can’t in a design role.  As an illustration a Resource Leader (read just below executive board level) gave a Business Update presentation the other day and I have genuinely seen Sappers do a better job.