Holdfast Training Services Trial Blog

Has nine months really passed, time on attachment has gone very quickly. As I leave Liverpool Street here is a quick update on my areas of responsibility.

Site in General. At the time of my departure we had just completed the concrete pour for the penultimate slab at 78SSL and works had commenced on the excavation to the final level 73 SSL. All in all some 2,400m3 of concrete poured and nearly 2000 tons of steel fixed in the preceding slabs. The final excavation would prove the most complex as a the break out of the pile wall to create the access tunnels to the platform would need completed. During the breakout of the pile wall a temporary section of slab would be required carry loads across the sections of pile wall. I have intentions to return to the site to witness the pile wall breakout as the plan is to use a steel and diamond rope in a winch to cut the piles like a cheese wire……I would like to see that as the piles are 1200diameter with heavy steel reinforcement, but I have been told it is possible.

Installing steel at the penuletimate slab level & view of the running tunnels adjacent to site

Depressurisations wells. My part completed wells installed, working and a steady decline of the pore pressure had allowed the cessation of works to be lifted and the excavation to continue. Having now entered the Lambeth Groups of soils and with the wells firmly in the course grain gravelly layers the amount of water being pumped had dramatically increased from 200-300litres per day to20,000 per day. All was going well up until my last day when we arrived at work to discover the site flooded from an over flowing settlement tank, the cause turned out to be that the contract excavating the tunnels adjacent to our site had turned their dewatering system off on the Monday and it had taken a week for the recharge of the ground water to reach our site. Although the wells we had installed were capable of pumping the increased amount of water we did not have sufficient storage capacity. It also turned out that the young apprentice responsible for monitoring the flow rate had neglected to do this and so the increase in water flow had not been picked up.

Installing the depressurisation wells & the well head of the vacuum edjector depresssurissation system

Precast Concrete. Leading the charge for Laing O’Rourke I had at the time of leaving managed to secure my greatest legacy to the Liverpool Street site, the use of precast concrete sections to complete the Northern Wall of Blomfield box. Through the use of negotiation skills, professional engineering judgment, coercion and by being a deviousness and sneaky bustard I coordinated efforts and produced the Value Engineering proposal that convinced Crossrail Engineers that the use of Precast Concrete was in fact the best option in terms of engineering risk, H&S and commercial risk.

The complicating factor in the design of the precast concrete sections was Crossrails insistence that as they would be required to take structural design responsibility then they would design the wall sections and inter wall connections. As the wall is to be next to live LU track and undercover the wall must have little to no maintenance burden and cannot use any materials that release toxic fumes when burnt. As a result Crossrail had designed the walls connections as an insitu concrete stitch creating two problems for us as the contractor to overcome; Firstly this insitu stich requires the reinforcement within the wall to be tied to the structural frame of the building and then concrete poured between the steel to join the wall and structural frame together to form what engineers have described as a monolithic structure. Secondly how do we turn and ten support the wall sections during construction. The walls have a long slender overhanging nose section that cannot be used to support the wall during fixing and pouring. With the temporary works team we had developed a rough scheme of manoeuvre using Perri Strong backs to create legs to support the wall. These legs then had to be offset from the wall sections to allow the site operatives access to fix steel and pour concrete. As a result at my time of departure I was working with the temporary works team to model the loads on the connections. At the time of departure we were on the 6^th iteration of concept designs from the design consultants Motts….

I have now moved to Arup, London office without a speck of mud insight. I am now surrounded by a mix intellectual geeks and glamorous Europeans. I have discovered the free lunches that accompany the lunch time CPD sessions and have attended two this week already. The festive season has already started with a wear your s***test shirt to work day and festive if not geeky pub quizzes…John I am sorry to disappoint but my team came last at the geotechnical Christmas quiz…….

My final walk in the tunnels and on site