How to pile through a railway station
As we aren’t able to add pictures to the replies, this is a quick post to answer Richard’s questions about the pile heads.
First things first – the time lapse camera looks west, from the ‘back’ of the site into the narrowest end. The view in my first blog An engineer without a site… for now. shows the remnants of a considerable amount of steel. This steel was formerly a piling grillage which can be made out below.
August 2018 – piling from the grillage at 21 Moorfields
The retained deck (RD),a composite steel beam and RC deck which is both the ground floor of this site and the roof of Moorgate station, is only rated for 10kPa UDLs so could not bear the weight of piling rigs. To get these rigs on site a grillage scheme was installed (3000T of steel – more impressive temporary works!) on plinths tied into hard-points on the RD.
Piling model showing grillage, RD, rail platform, pile casing and soil strata
From this platform a steel casing was driven through the grillage, through the Moorgate platforms, through the made earth, through the gravel and into the London clay. The piles were then bored, from the grillage into the Lambeth beds, to depths of up to 57m.
The upper part of the pile acts a column through the station platform level and the casing remains in place. The piles are heavily reinforced in the upper sections in order to transfer moments imparted at the head by the base plate connections. The steel columns are fixed to the pile heads and tied into the reinforcing cage at the head.
Pile function
Moments are ultimately transferred by the piles to the ground beneath platform level as shown in the sketch above.
A smaller 1.8m diameter pile (with engineer for scale). Total 12 No 2.4m dia piles and 4 No 1.8m dia
The piles have also been fitted with fibre optic strain gauges and grout tubes. The 5×4 anchor grid (seen below) was used to connect 4 No piles as reaction piles to the test rig for preliminary testing of a 1.2m pile. Axial design load on a 2.4m pile is 58MN.
Pile reinforcement (28 No H50 longitudinal bars with H16 links at 150mm centres), fibre optics and grout tubes.
Holdfast Training Services Trial Blog 
A few points; you say a casing was driven . I doubt it I am not sure how they got through the platforms but in London you’d be lucky to get approval even for vibrating the casing in to place
The pile moments are due to the horizontal base shear applied at an eccentricity above the level of pile embedment
Interested in the pile instrumentation:
On the fibre – I would guess there would be higher than normal instrument redundancy ; I am sure that the longitudinal strain is being assessed by light frequency change but interested to know where the strain is measured
Not sure about the grout tubes and these to grout optical fibre into place or are this inclinometer tubes to measure pile deformed shape?
Cheese interesting post.
I think the grout tubes around the edge are for cross hole sonic logging. This essentially looks for changing in the cross section, i.e. Lumps of soil in the concrete and voids. This, thermal integrity profiling and the standard stuff like tremi logs and bentonite testing give you some assurance on the pile from a structural point of view. There is usually a large number of durability and pile load NCRs from the piling phase.
The stain gauges might give a better guess at the shaft resistance with depth in the different soil layers. This combined with the osterberg cell can be used calibrate the Fleming pile settlement cal and more accurately confirm presetting etc.
Do the piles act together through a raft? If not settlement of individual piles could be very important.
They used fibre optic strain gauges on 22 bishop gate to measure axial shortening as they where building. Adjusting the preset as they went. At 17 floors I wouldn’t have though it an issue on your site.