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.
- Why we are doing it
- What is involved?
- Where will the investigations take place?
- When will work take place?
- We are digging shallow trenches to assess the archaeological features of a site.
The surveys will help us to improve our proposals and our application for a Development Consent Order.
- 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
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.
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
Once excavated, a vertical section through each feature is photographed and drawn, recording all of the fills in detail.
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.
Metal tins are often used for these samples, but plastic drainpipe is also good at keeping the soils intact and undisturbed.
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.
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.
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.
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.