Holdfast Training Services Trial Blog

A series of events has unfolded over the past few weeks which has reinforced my utter contempt for the reactionary nature of site works; a common theme on Phase 2 WhatsApp chat. BPS Phase 3 basement is a 13m excavation, 16m in some locations.  Along the southern boundary (adjacent to the Northern Line Extension site) the embedded retaining walls used are sheet piles.  The contractor is unable to use vibratory hammer techniques to drive the sheet piles due to the proximity of the largest brick clad structure in Europe (Battersea Power Station).  The 23m long sheet piles are thus driven using a silent press.  The geological succession is typical London stratum.  Made ground (crap), Terrace Gravels (medium dense sandy gravels – very porous), London Clay (very stiff – not very permeable) and Thanet Sands (very dense silty sand – porous).  Due to the required toe depth, in the London Clay, pre-augering and water jetting techniques must be used.  For the civils students the problem bears remarkable similarities to Ex COFFER.

The following photos below provide an indication of the works.

 Pre-augering Works

Sheet Piling Works using the silent press

The aftermath of Water Jetting

The Inevitable Collapse

The previous photos show the steep batter adjacent to NLE site and there is a risk of undermining the adjacent site. However, sheet piling works continued whilst the contractual wrangling from the piling contractor and ground worker simmered in the background.  Both parties claimed the other was responsible for fixing the problem.  The inevitable happened and the batter gave way, undermining the NLE site, next to Polymer tanks.  This incident has added further friction and incurred additional expense as BPS Phase 3 must repair the damage to NLE site.  All in all a foreseeable incident that could have been avoided if stakeholders had looked beyond their blinkered “it’s not my responsibility” approach.

Batter Collapse

My original comment: “utter contempt for the reactionary nature of site works” may seem a little unfair given the circumstances detailed above. Never-the-less I think my comment is well founded given the final photo taken AFTER the batter collapse which was the piling contractor’s response to dealing with one of the contributing factors (disposing of the excess water)…

The contractor’s solution to the excess water – discharging the water behind the sheet piles further washing away any batter!  However in their defence apparently contractually: “It’s still not their responsibility“