More trenches!

Just when we thought we'd seen the last of the trenches, more are starting to appear! However, these trenches are not for cables. Instead, this is part of the fence that will surround the two LOFAR fields. Because of the requirements, these fences cannot contain large lengths of metal; so the common wire fencing is not an option. Instead, they are constructed of timber and synthetic netting. The netting itself is buried into the ground to attempt to control the rabbits (evidence of whom has already been spotted on the LBA field).

Yet another trench!


Timber fence posts are dropped in along the trench ready for securing.

LBA field backfilling complete

There are no more open trenches! The last work on the LBA trenches has now been completed and the cable graves have been completely backfilled and dressed in. This has been a huge effort and a dangerous one too. In particular, the maze-like LBA has been difficult to negotiate with the heavy machinery. Still, the crews have managed to get the job completed and work has been a success. Thanks goes to everyone who has participated in this major phase of the project.

The sub-contractors from Coral Constructors Ltd start backfilling the last stretch of LBA trenches.


The LBA field completely filled in (with the rest of LOFAR Chilbolton in the background).

LBA cabling nearly done

Here are some photographs of the work being done on the LBA cable grave.

Martin Bell pulls back some of the final cables across the LBA grave. This is the last time they will look all neat and tidy.


Now that the cables have been all patched in, the cables in the grave are "shuffled about" to make them sit better and reduce stress points. This does make them look rather messy, but it does help ensure cable integrity.


The next step is the covering of the cables with the protective sand layer.


Then the crew move their way up towards the RF container, following the feeder trenches.


Almost there.


Finally, the sand covers all the cables. Warning tape is placed over the area (should anyone excavate in this area in future). Then all the graves are backfilled.

Backfilling the LBA

Now that the cables have been run out for the LBA, it is necessary to cover them with a layer of protective sand and then backfill with the soil and rock that was originally dug out. This is not easy, as the LBA trenching pattern is complicated and it is difficult for the diggers to manoeuvre between any partially filled trenches.

The first step is to apply the sand. A narrow bucket is attached to the digger and builders sand is scooped from the dumptruck into the trench. This is then carefully spread by hand to ensure that the cables are properly protected.


Once the sand is in place, the wide bucket is attached to the digger and the piles of chalk and soil are used to back fill the trench.

Final run of the LBA cables

The LBA cable work has now been complete. All the cables from the LBA have been run from the field into the RF container.

Looking down a trench, there are numerous cables stacked up, waiting to be routed through the LBA cable grave.



Harry Smith working a new line through the LBA cable grave. Like the HBA, the LBA also has a cable grave to take up cable excess before it goes into the final RF container.


Brian Finegan prepares to feed the final cable pair through the duct and into the RF container.

Sand People

Now that the HBA cables are in place and patched into the RF-container, the HBA cable grave can be filled in.

First a thick layer of protective sand is spread across the cables. The crew work the cables gently to ensure that the sand penetrates between all of them.


Here is the finished sand cover. This protects the cables against flint and other sharp rock as the spoil is back filled into the grave.


With all cables completely covered, the digger moves in to bulk fill with previously excavated material. And that is it! The HBA cables are fully covered and the crew can walk through the area without the risk of falling into the hole or collapsing a trench wall.


There have been many people involved in the laying of the HBA cables, and here are a just a few of them. From left to right: Dave King, Jon Eastment, Mike Willis, Alejo Martinez-Sansigre and Harry Smith. To these few, and everyone else involved in this stage of the project: thanks and well done!

Enter the Labyrinth

The LBA trenches have all been dug and the surveying of the centre pins is complete (the things which mark the cable exit points). The next task is to actually put the cables in, which means dealing with the maze of trenches. Unlike the HBA, the LBA is arranged in a scattered pattern.

(as shown below - click to enlarge the diagram).



This gives the resulting telescope great beam properties. However, it also makes it difficult to install.

Here, Griffin Foster (U.Oxford) and Mayaane Soumagnac (ENS-Lyon, France) start setting up the cables for one of the LBA aerial locations. There is a six-page document describing the procedure, as many of the steps done in this initial phase are critical to subsequent phases of the installation.


"You are in a maze of twisty little trenches, all alike". Here the LBA team are busy placing the LBA cables in place. From left to right: Harry Smith, Griffin Foster, Mayaane Soumagnac, Danny Price and Jack Hickish.


Our equivalent of Ariadne's Thread: countless cable trenching plans, diagrams, lists and tallies are used to keep track of the deployment within the LBA. However the ultimate is this dual-core backpack-computer, with headset, scanning gun, barcoded-trench plan, LBA antenna checking software, text-to-speech synthesiser and rocket launcher! (Okay, okay... maybe not the rocket launcher.)

Trench Hazards

During the digging of the trenches, the folks at Chilbolton have occasionally come up with some unexpected finds. One of these was some brickwork, close to a cable exit point on the High-Band Array.



From comparing the location to some of the historical maps, they suspect it may be part of a World War II searchlight facility. The Chilbolton Observatory site was once an RAF airfield (also see this Wikipedia article on RAF Chilbolton), used during the Second World War. According to old maps, there was a searchlight installation near this position and perhaps the brickwork is part of the surrounding structure.

It was certainly extremely hard and they had great difficulty in dealing with it, but it's all taken care of now.

Backfilling the HBA

The High-Band Array cables have now all been laid in the trenches. The folks at Chilbolton have now covered them over ready for the final surface preparation. However, this not a trivial task. First, the cables need to be covered with a protective layer of sand. This is to shield the cables from any sharp flint or other stones in the trench, which may cut or abrade them. Then the trenches themselves are filled in, leaving the cable exit points exposed. The cabling team then move in to remove the survey peg and holding ties, insert a marker, apply a guard at the actual exit point and then re-tie the cable to keep it clear of the ground. The cables are then subjected to another quality-assurance inspection. Once this is done, the final backfill can occur. So far, this process has been completed for about half of the HBA. When complete, the field can be levelled to high-accuracy ready for the tile antennas.

The first picture shows the sand which as been laid down, and the digger covering the main portion of the trench with pre-excavated material.



Then once the cables have been prepared the next backfill can occur. The equipment operators need to be very skilled to work this close to the exposed cables.



Finally, preparations complete! This pair of cables has now been secured ready for the application of the surface gravel.

LBA cable trenches open for business

Yesterday at Chilbolton they made the first incision for the LBA trenches. Unlike the HBA trenches which lie on a regular grid, the LBA aerials are scattered across the field in a pseudo-random pattern. This helps ensure good beam properties, but it also makes the cable trenches a bit of a nightmare to dig. However, the ground crew have demonstrated incredible finesse in negotiating the heavy machinery through the labyrinthine trench plan.

In this picutre you're looking down the line of the first LBA cable trench.




It looks like a giant Kanji character scribed into the ground. However these are the feeder trenches from the LBA aerials back to the RF-container.

HBA Cable Graves

In the foreground of this photograph you can see the so-called "cable grave" for the High Band Array (HBA). Because it is important to control the cable lengths for the LOFAR radio telescope, each cable is made to a specific length. And to ease the manufacturing process, these lengths are standardised to a small number of set lengths. Because of the positions of the HBA tiles, there can be several tens of metres of excess cable which needs to be carefully zigzagged back and forth before the remainder of the cable goes into the RF-container.

In the photograph, Harry Smith and Dave King are adding another cable to the cable grave. In the background, the ground works crew are adding a layer of sand to protect the recently laid cable.

Planting the cables

From each place where an antenna is located, a trench is dug into the ground. It leads into other trenches and gradually these make their way to the RF-container, where the signal is combined. The placing of the cables is critical; the exact lengths must be measured so that the exact timing of the signal arrival is known. This is because LOFAR uses software and timing between the signals from different antennas to "point" to different parts of the sky.

The cables must also be protected - ultimately, they will be safely buried underground. But for a short time they will be in open, exposed trenches, so this is a critical phase of the installation.

Below, Harry Smith is positioning the start of two HBA cables. These are measured to ensure they have sufficient ground clearance. They are then sealed and attached to the survey peg.


The cable laying is a carefully coordinated operation between the different groups involved. Here the digger-crew wait for the all-clear from the cable-team before proceeding to open the next section of the trench.

Trench Warfare

At the Chilbolton site, digging has now started for the cable trenches on the HBA field. Unlike the LBA, the HBA (High-Band Array) is arranged in a regular grid pattern (as shown below - click to enlarge the diagram).



There are a total of 11 "columns" which have varying numbers of cable exit points where the HBA tiles will be connected. The main trench is dug by a mechanical digger. Feeder trenches then are cut into the main trench to give access to the cable exit points. Cables are then fed down into the trench from the surveyed marker pegs and bundled along the bottom of the
trench channels.

In the picture, a digger carefully backs across the site as it digs in another
HBA cable trench.