Holdfast Training Services Trial Blog

Issue

Keltbray have commenced their bulk dig to B2 level in Zone A of The Stage, Shoreditch, experiencing silty gravel down to clay.  Although not scenes from The Tempest, they are experiencing groundwater seepage with the following photo showing what was experienced over the course of one night (having left the excavation dry at the close of works yesterday, and yes I have raised the issue that if someone is working that close to deep water it should be barriered off and have rescue means in place).  The Keltbray RAMS had a plan to deal with groundwater by managing with sump pumps before installing a full dewatering system, discharging to Thames Water drainage via sediment tank on the suspended slab above.  The issue here is that Keltbray have got a discharge permit in place but not a connection to the manhole for the water.  This poses the issue of what to do with the water?

Source

The excavation sits on top of an approx. 15m thick layer of London clay and is surrounded by a secant pile wall installed by another piling contractor before Multiplex took over the site.  The wall has a number of out of tolerance piles and gaps in it (as previously blogged by Fred Kiddie).  There is also an adjacent Zone B currently at B1 level which has a contiguous wall separating the zones, therefore gaps between the piles.  This means there is about a 5m head difference in the zones.  My assessment is that this groundwater is flowing from Zone B to Zone A due to the newly established head difference.  There is also the likelihood of groundwater flowing from outside the site, passing through some of the gaps in the secant wall into the excavation ie finding the shortest flow path.  The site is in Hackney and is located in a former mainly industrial area with mixed uses for the land around.  This means to me that there is a fairly high chance of some contamination in the groundwater.  The final option for where this water is from is that the pilers have been experiencing more polymer being used in Zone B piles than has previously been experienced.  This could mean that the new head difference from the excavation could be causing more flow from the polymer fluid out of the pile and into Zone A.

Options

Option one was to sump pump the water to a hole in the excavated material before mixing it with the spoil and removing using muck-away trucks.

Issues with this option:

• It is a slow process.
• Some of the water flows back into the excavation.
• The muck-away trucks leave a trail of wet dirt on the road with very wet spoil being carried in the trucks – likely to cause complaints locally from the dirty roads even with one road sweeper employed full time.
• Each muck-away truck is tested for contaminants which would pick up any issues with contaminants in the groundwater.

Option two was to use the polymer pump and pump the groundwater into some of the polymer tanks and use it with the polymer for the ongoing piling.  The polymer tanks can act as sedimentation tanks and ultimately conserves using mains water.

Issues with this option:

• Much faster at pumping the water from the excavation compared to a sump pump therefore making the excavation safer, quicker.
• The large amount of fines in the water cause the tanks to block up over time.  However, these fines are contamination if not filtered out.
• The water needs treating to increase the pH before being mixed with the polymer.
• The water needs testing for contaminants as you could potentially be putting contaminated water down a pile and into a separate groundwater course much deeper down, causing wider contamination issues.
• There is a limited volume of the tanks for removing the groundwater as the piling ultimately reuses the water.
• The tanks don’t fully act as sedimentation tanks as their outlet valve is at the bottom therefore fines will pass through.

Summary

I have been discussing with the sustainability team here on the project who have cautiously approved the polymer tank method as it sort of allows for sedimentation of the water.  The water isn’t currently being tested for contamination and the volume of the tanks has nearly been used up.  I will be inspecting the water for any traces of polymer this afternoon to see if this is the source.  If it is, then recycling the water through the polymer tanks isn’t such a bad option.

This issue only exists because a connection to the manhole has not been established yet.  This is an item which is sitting with the local council and is an ongoing task to get approval.