Archaeological trial trenching and surveys on the Lower Thames Crossing
May 17, 2020
The Lower Thames Crossing is currently undertaking a series of ground investigations and surveys at locations throughout North Kent and Essex to help us to improve our proposals and to develop our application for the Development Consent Order.
The first phase of Archaeological Trial Trenching has now been completed with the second phase starting shortly in Kent.
- Mechanical excavators will be used to dig shallow trenches approximately 30 metres long and 2 metres wide.
- Before digging the trenches, a magnetometer (detector) attached to an all-terrain utility vehicle will be used to scan the area.
- We will be excavating the soil to look for archaeological features and we will replace the soil once the excavations are completed.
- Noise level will be comparable to that of farm machinery, such as a tractor.
- Before we dig the trenches,we will undertake non-intrusive electromagnetic surveys to detect any sub-surface anomalies such as utilities, waste and unexploded ordnance. This is a standard health and safety requirement.
- Our teams do not carry out any crop spraying or treatments on any vegetation
- The locations for the trial trenches will mainly be on private farm land on or near the proposed route.
- The work may be visible from nearby roads and properties.
- The second phase of these surveys will start in April 2020 and continue until October 2020.
- Most of the work will take place Monday to Friday 8am – 6pm with the possibility of some weekends.
What’s happening now
Trial trenching processes
Compacting trench machine
Backfilled arable field
Drawing section and Sampling
Hand excavated ditch
Excavation in the snow
Trial trenching processes explained
The percentage of each area tested by trenching varies: where cropmarks or geophysical survey indicate buried archaeology, trenches are targeted on buried features. Where soils are not suitable to reveal buried features through survey, the sample is often 5%.
Trenches used where it is unclear what may lie below the ground are commonly 30m long and 1.8-2m wide.
In areas where cropmarks or geophysical surveys have indicated the presence of below-ground features, trench lengths and widths may vary, depending upon the location and size of the features to be tested.
The depth of trial trenches depends upon the depth of overburden (topsoil and subsoils) over the natural geology, which varies depending on location. On hilltops and the upper part of slopes soils are often thin, but lower slopes and valleys can have a greater depth of soils due to erosion. Trenches are not dug to more than 1m deep without fencing and stepping.
After machining, archaeological features are usually visible as different bands of colour in the surrounding soil. In some circumstances, for example clayey soils, or where there are walls or cobbled surfaces, it is necessary to clean all or part of the trench by hand before planning and photographing the remains.
Archaeologists work all year round, and often in difficult weather conditions, provided the weather does not risk damaging archaeology, for instance by flooding.
Ditches appear as bands of soil within the surrounding soil, and a sample (usually 1m in length) is excavated to establish their depth and profile, the sequence of soils they contain, and to recover finds and environmental remains
Spoil from hand-excavation is added to the subsoil mound created by opening the trench by machine.
Once excavated, a vertical section through each feature is photographed and drawn, recording all of the fills in detail.
Finds can vary from tiny scraps to whole or nearly whole vessels, like this example from a Roman ditch. The positions of finds like this are planned and recorded before the find is lifted for excavation and further study in the laboratory.
Shovelling the soil, which is rich in charred plant remains, into sample buckets for flotation and sieving back at the archaeological offices.
And stretching out a tape as a horizontal scale for section drawings, and using a hand tape to measure vertical depths and draw them on waterproof permatrace.
If a layer, or a sequence of layer, representing buried soils, containing lenses suggesting how they were formed, or suspected of containing significant environmental remains are found, a column of the soil(s) is retrieved for detailed study and recording in the laboratory.
Metal tins are often used for these samples, but plastic drainpipe is also good at keeping the soils intact and undisturbed.
Sometimes, where the geology is varied, or where there may have been soils eroded from higher upslope, it is not certain that a layer represents the bottom of the archaeological sequence. In such cases a slot is dug by machine to check. Such slots are normally dug only to a maximum depth of 1m.
In areas that were formerly channels or lakes, and where successive soils have accumulated at the bottom of slopes, the archaeological sequence may be deep. In these areas trenches will be stepped or shored, and dug to greater depth.
This example shows the sediments that built up alongside a palaeochannel of the Thames above London, and includes two dark peaty deposits, as well as former land surfaces on which clusters of struck flints were found.
Backfilling will involve the machine starting at one end of the trench and clearing the spoil in front back into the trench, compacting with the bucket at intervals.
The machine then tracks along the area just cleared, so keeping crop damage to areas already affected. Topsoil can be reached from the other side, reducing further tracking.
The machine uses its arm and bucket to compact the spoil as it is put back into the trench.
If compaction is not sufficient, the machine may track up and down the backfilled trench before moving on to the next trench along the tracks already created.
In pasture fields, the small amount of spoil left on either side of the trench after backfilling is not normally a problem, as the grass will grow back through it.
The turf can be replaced after trenching, but this can mean gaps into which livestock can put their feet, so reseeding is often a better option.
Frequently asked questions
The majority of the work will take place on private property, such as farmland. The work will be carried out in multiple locations, north of the Thames in Essex and south of the Thames in Kent.
The first phase of these surveys will start in November 2019 and continue until March 2020.
The works will not cause any adverse noise disruption, the noise level will be comparable to that of a farmer’s tractor.
Road users will see staff wearing full Personal Protective Equipment (PPE) and machinery on private land.
Due to the location of the works and that they will be carried out on private land, there should not be any traffic disruption caused to road users.
We will replace the soil once the excavations are completed.
The surveys will enable us to identify and assess any archaeological features along the route. This information will be included in the Environmental Impact Assessment in our application for a Development Consent Order.
We have used previous records to obtain data, but we need to obtain more up-to date information to ensure that we can build the Lower Thames Crossing safely and efficiently with accurate information.
These first phase of these surveys will take 5 months.
Balfour Beatty will be carrying out these works on behalf of Highways England, alongside Oxford Archaeology who specialise in archaeological excavations. They will be wearing PPE which will be branded with Lower Thames Crossing, working on behalf of Highways England.
The surveys will enable us to identify any archaeological features along the route. This will help us to improve our proposals and to develop our application for the Development Consent Order.