Holdfast Training Services Trial Blog

So in my last post I said I would cover a little about working with rock.  I am currently excavating in low to medium strength phyllite. Phyllite is a metamorphic mudstone somewhere between slate and schist.  It is highly foliated and has fine clay like material in between the layers. The foliation is at 70 degrees to the horizontal across the site.  Despite being fairly weathered it is still relatively intact with only a few faults across the site.    RQD was between 50-75 % across site.  The problem is it weathers so incredibly quickly when wet.  I am about 15 m below the water table and water coming through the anchors at about 5 litres per minute.  So what I hear you cry – well its rock but it is a galactic pain in the back side to work with.

I am now down at footing level and I am faced with an even worse problem than the bulk excavation.  The design has called for individual pad footings which are all at different levels and a core that is 3 m below any of the other pad footings.  There are zone of influence clashes everywhere, the architect/structural engineers have very helpfully not included any footing RL so I have had to work them out, as well as how we are going to get the batters and benches in while still being able to move.  The geotechnical report states that with low strength phyillite I can achieve a batter of 0.7 H and meduim strength 0.5 H.  With H being the height of the batter (not exceeding 3m).  So rock strength can be pretty important.Footings Handover Schedule

What strength is that rock?  You cannot just take the results of the lab test for Ultimate compressive strength and point load tests.  There is a very good website that helps with estimating rock strength via the Rock Mass Rating (RMR) process (RMR rating)that can be then used to calculate the allowable/safe bearing pressure for foundation design.  Ultimately though this counts for very little as to get anything signed off you end up with a geotech going mental with a geo hammer on site before telling you its half of the strength you have calculated.

A conversation with the local geotech went along the lines of “how do you come up with the safe bearing pressure do you use RMR.”

“RMR what’s that? Nah, I just use experience, umm what does the drawings say this should be?”

Incidentally I got an RMR of around 45 -50 across the site.  That gives between 2880- 1510 KPa am I right?

I made the mistake of being over confident with the bench and ended up having get the geotech back in.  In my defence I excavated a pad footing expecting it to be open for the maximum of a week and the PM didn’t make the call on the steel contractors in time so it was open for nearly 3 weeks.  The result was water got in and the sides blew out, I had to get the geotech and subbies back in to batter back a ridiculous amount.  This all meant time, concrete and money.

There is worse to come – the other Site engineer (little e deliberate)- to be now known as ‘the lone ranger’ took responsibility for the crane base and has massively ‘dropped his baseball’.  He wrongly measured the zone of influence of the crane base to the core and failed to gain approval for the design of the pad footing from the geotechs.  We are now faced with delaying the tower crane erection or digging the pad footing deeper.  It hopefully won’t end up like the crane around the corner though, which has a lovely lean to it. 

Prima furniture may be getting some renovation work done in the future.