The Warehouse


The squealing of machinery comes to a halt and the large red light on the operator’s panel turns green. A buzzer sounds and he steps forward, opening a salt-encrusted gate to reveal a small miner’s lift. On its floor more salt has formed into small drifts. It has walls on two sides, but apart from a safety bar the back is open, the rear wall of the shaft (and the gaps in it) clearly visible.

“If you’re not good with that kind of thing then best face forward.” Says Graeme McDonald, Head of Operations for Deepstore.

“Either that or we can just turn our helmet lamps off.” He says with a grin, gesturing up to the empty spot where, in a regular lift, the light fitting would be.


The miners’ lift from a distance and then close up.

The regular entrance to Deepstore’s facility is closed for maintenance. That this doesn’t stop us from entering is a rather effective demonstration of just what makes this facility so unique. For this is no run-of-the-mill secure storage facility, nor is it even in London. In fact we are at Winsford Rock Salt Mine in Cheshire, Britain’s oldest active rock salt mine. Deepstore inhabits some of its mined-out tunnels and thus with the main visitor lift out of action we are heading underground the same way the miners do.

The fact that we are here at all might seem to be a spectacular case of scope creep on the part of the inhabitants here at LR Towers. Deepstore are underground, certainly, but they are not Underground. Nor, quite obviously, are they within the M25.

150 metres below us, however, lie literally kilometres of London in the form of thousands of broken-down core samples – all carefully collected, compared and catalogued by Crossrail over a fifteen year period. Intended primarily to ensure that no surprises awaited its Tunnel Boring Machines (TBMs) as they carved a path through the ground beneath the Capital, in the process they also expanded our geological understanding of what exactly lies beneath our feet.


Down in the mine. Note the remains of an old narrow-gauge railway.

The big question

Before looking at the contents of Crossrail’s vault itself it is worth stepping back briefly and looking at the path which led to Cheshire. For the residents of LR Towers this journey began back in 2012 when we looked at the surveys Crossrail had to carry out to ensure the route was free of unexploded WW2 bombs. Whilst there was no need to retain the vast majority of the core samples taken as part of this particular process, it did prompt a question – core surveys to establish the geology of the route had been undertaken since the early nineties…

…so where were they?

We built this city on rock and loam

In the world of geological engineering there is an old adage:

You pay for a quality ground investigation whether you procure one or not.

Karl von Terzaghi, father of soil mechanics

The attribution in perhaps apocryphal, but the sentiment is certainly true. Any large scale construction project is at risk of failure if the ground on which it is built is not thoroughly surveyed. This is doubly true for tunnelling, where an unexpected change in the condition of the ground through which you are tunnelling can bring disaster. This was something Marc Brunel discovered to great cost during his efforts to build the Thames Tunnel, the world’s first tunnel beneath a tidal river. Although his efforts would eventually be successful, unexpected pockets of sand and silt contributed to the tunnel collapses and flooding that would bring the project to the edge of disaster and cost lives. More recently, a similar failure to understand an area’s geology would play a key part in the rampant delays and cost escalations that would plague Boston’s “Big Dig” .


Travelling through the mine to the Deepstore facility. It is difficult to grasp the scale.

More complex than you think

The geology beneath London is frequently over-simplified. The general narrative is often reduced to one where north of the Thames, London is largely clay (which is easy to tunnel) whilst the ground to the south is more problematic. The reality, however, is that over hundreds of millions of years the London Basin has seen vast changes in its makeup. High water levels, flooding, glaciation and a wide variety of other factors have periodically affected the area, and the Thames itself has changed course several times.


A conveyor belt stretches out into the distance, carrying salt towards the surface lifts.

All of this has had a profound effect on the geology of the Capital, to the point where a description of what lies beneath Greater London and its surroundings reads like some sort of alternate Shipping Forecast: Lower Greensand, The Upnor Formation, Thanet Sands, Harwich Clay, Shepperton Gravels, Bullhead Beds, Upper Shelley Clays, and more.

Though none of that geology is particularly unusual (or indeed unfriendly to tunnelling) it still needed to be charted. And just as similar surveys for the Jubilee Line extension had significantly expanded our geological understanding of the London Basin, so too did Crossrail. Most importantly, from an engineering perspective, it revealed the existence of at least eight more geological faults beneath the city than were previously known – something that, alongside the discovery of a number of geological scars, necessitated extra work at Farringdon station and some subtle changes to tunnel depth and cross passage location beneath the Thames.


A salt bridge over lower works, lit only by the headlights of our minibus, on route to Deepstore

Keeping the cores

Given the geological tale they tell, keeping core data has long been an established part of tunnelling, especially with regards to the Underground. That information can be useful long after a core was drawn if ground conditions change. This may seem like an infrequent occurrence, but within the last few years London Underground have had reason to look at old ground data again not once but twice – when issues related to London’s rising water table (in part the consequence of changes in the level of heavy industry in the capital) affected both the Jubilee and Northern lines in different ways.

In recent history this philosophy has expanded to include the retention, wherever possible, of the original cores as well as the data produced, and such was (and remains) the case for Crossrail.

Which brings us back to our earlier question – where are the cores stored?


Emerging into the light at the Deepstore facility. The vehicles gives a sudden sense of scale.

In the short term, the location of those cores was relatively easy to trace – a large rented warehouse in Canning Town, East London. This would never serve as a permanent home, however, because the cores would need to be kept in a climate-controlled facility to prevent deterioration, a potentially costly exercise for an organisation critically aware of its need to be seen as constantly seeking value for money. At the time Crossrail were unable to confirm to us precisely where they intended to store them in the long term.


Work underway to expand Deepstore’s storage areas.

Getting by with a little help from a friend

That this question continued to occasionally prey on our minds may seem strange, even for the residents of LR Towers. That it did was largely down to some basic back-of-the-envelope mathematics – Crossrail had dug some 300 boreholes. Generally these ranged from 40m – 60m in depth, but occasionally they reached over 100m. Broken down into sections small enough to be shipped and stored this translated to an awful lot of containers, and thus by definition a particularly impressive place to keep them.

As is often the case with such things, in the end the answer ultimately emerged almost by accident. Whilst researching TfL’s move from 55 Broadway, it became clear that they would require additional space in which to store their own archive and other materials.


Storage space waiting to be populated at Deepstore.


Document storage in Deepstore.

TfL’s archive (and its management) is a worthy topic in its own right and one to which we will return in a future article, but what mattered at the time was that TfL publish all expenditure over £5000 in their public accounts and thus their partner in this exercise was easy enough to determine – Deepstore.

Crossrail have always enjoyed a close relationship with TfL, not least because the latter is one of the former’s project sponsors. This even extended in early years to secondment of staff and resources from TfL to Crossrail. It thus seemed logical that when looking for a place to store cores Crossrail’s first call would have been once again to their organisational friend and their supply chain.

If TfL used Deepstore, we thought, then perhaps Crossrail did too.


Floor to ceiling document storage. A vault like this can store approximately 150,000 boxes in this way.

Discretion over secrecy

It should be emphasised that Crossrail themselves were happy to confirm this was the case when asked, just as TfL were happy to confirm their own relationship with their out-of-London archive. Indeed both organisations are justly proud of their current setup. It was discretion, rather than secrecy on their part (as well as Deepstore’s respect for the privacy of their clients) and our own low level of interest in finally solving the mystery that had resulted in it lying unsolved for so long.

Nonetheless, solution brought a sense of satisfaction. In Deepstore both TfL and Crossrail had found a perfect storage partner.


Part of Compass Minerals, Deepstore was set up in 1998 to take advantage of some unique attributes identified in its Winsford Mine. Located only 150m below the surface, the mine almost naturally maintained a temperature of 14°C. As a salt mine, it was also dry. Almost since it opened, salt has been excavated from here in a grid pattern, leaving large, regular, pillars to maintain the integrity of the mine.

By 1998 this meant that Winsford was a near-natural climate-controlled environment blessed with wide open passages and easily configurable chambers. In other words, it was as close to a perfect place to store archival documents and other items as one was likely to find, and Deepstore was born.

Engineering entry

The same discretion that, until now, has kept both organisation’s presence in Cheshire generally out of the public eye also meant that engineering a visit was also something that required some effort – much of it on the part of the press offices at both organisations rather than ourselves. Finally, however, this week saw ourselves and fellow rail and London obsessive IanVisits make the trip up north.


Moving through Deepstore.


Outside Crossrail’s vault at Deepstore.

This article ultimately represents the result of that exercise, with the photos hopefully providing as good an impression as one might get on screen of what we found. As a facility, Deepstore itself is impressive. The sight which greeted us on entering Crossrail’s vault, however, confirmed our mathematical suspicions: Crossrail had certainly required a very large area for their stored cores.


First sight of the Crossrail vault.

The scale of the vault is difficult to fully describe – at least not without resorting to comparisons with films such as Indiana Jones, or shows such as Warehouse 13. The vault is full of carefully stored and labelled pallets, each of which contains a selection of cores from a particular section of the Crossrail route.


Rows upon rows of core sample boxes can be found in the vault.

Having been lucky enough to secure the company of Crossrail Geotechnical Specialist John Davis for our expedition, we are able to open a number of crates to see the cores therein.


Boxes open, wrapped core samples visible.

Fascinatingly, in many cases once unwrapped the differences between the geology of differing cores is very obvious. Generally speaking they are also both intact and in good condition, thanks to the climate controlled environment. Some of the more brittle cores have cracked and split, but this is expected and not considered a problem.


Something that’s hard to capture on film is the width of the vault as much as the depth. In part this is perhaps because it is an “L” shaped space, stretching well round the corner from the entrance.


Crossrail assure us they have top men working on these cores. Top. Men.

A certain sense of atmosphere is also added to the scene by the fact that many of the crates have off-white labels that have acquired a layer of dust. It gives an impression of age that isn’t entirely accurate, at least not yet.


It’s a curious effect, which seems to add a sense of gravitas to the space. Visual dramatics aside, however, it does also repeatedly demonstrate how well Crossrail’s cores have been categorised and labelled. Something that will prove an enormous advantage to those engaging in future study.


A closer look at the crate labelling, which is highly precise.

That said, there’s no escaping the fact that despite the space, it is at least in some places closely packed with pallets. Something that really highlights the overall length of core samples taken.


Crates of cores packed tightly together.

Finally, tucked away in the corner, is evidence that TfL’s active cooperation with Crossrail is far from over. Clearly lacking a similarly configured space at Deepstore (TfL’s vault space is largely setup for documents), they have borrowed some space in which to store their own cores.

These cores, produced for the Northern line extension, help highlight that construction work – and planning for it – never stops on the network.


Less neatly packed but equally useful – TfL’s cores.

Amongst these samples is a large box marked as being from “Battersea Dog & Cat House” containing a large number of closely packed cores.


A small mystery solved

Ultimately the resting place of the cores was merely a minor mystery, one without real effect beyond establishing that the cores themselves were preserved for current and future generations to study.

Nonetheless it is somewhat gratifying to have an answer as to their location. And if nothing else that locations serves to highlight once again just how large a project Crossrail is, and how carefully it is being managed to ensure success.


Our thanks to Deepstore, Tfl and in particular to Crossrail for successfully navigating the paperwork to enable a trip to vault.

Written by John Bull
John Bull is the Editor of London Reconnections. A transport journalist and historian, his writing often focuses on the political or strategic challenges facing London's transport network and beyond.