Ground Investigations and surveys on the Lower Thames Crossing

Ground Investigations and surveys on the Lower Thames Crossing

Why we’re carrying out ground investigations and surveys

We’re carrying out a huge programme of ground investigations and surveys under the Thames and across Essex, Thurrock, and Kent, to help us design what we’re building and how we will build it.

These preliminary investigations are vital to the design of our scheme. The data we get from them will help us build the Lower Thames Crossing safely, in a way that protects the environment, while keeping costs down.

The geology of the 14.5 mile (23 km) route is very varied and includes Chalk, London Clay, Gravel, and in some areas man-made ground.

We are looking for things ranging from ground cavities, pockets of water, and geological faults, to unexploded ordnance and archaeology.

What we’re doing and where

Our investigations will take place at over 700 individual locations along the route, and beside it at the sites of proposed depots or highways or utility diversions.

The vast majority of work will take place on private property with no public access, such as farm land and landfill sites. We have agreed access to work with each of the landowners.

In towns or cities you may see barriers around a small section of a pavement or road, with a diversion around it for traffic or pedestrians.

We will be using a variety of different techniques for the work. Here are the most common:

Cable percussion boreholes

Cable percussion boreholes are used to drill through soils up to depths of 60m, to identify the geology, obtain samples, and carry out tests. The cable percussion drilling rig is a mobile A-frame which is towed as a trailer by a 4×4 into position, before it is erected to 7m high to carry out drilling, which generally lasts for a few days. Boreholes are progressed using drilling tools and equipment which are raised and lowered within the A-frame using a cable.
Upon completion the borehole is either completely backfilled or installed with instrumentation, such as a monitoring well. If instrumentation is installed then a metal well cover will be left in place to protect it.

Rotary core boreholes use a rotating drill barrel to core through hard soils and rocks up to depths of up to 100m. Recovered core samples are then used to identify the geology, whilst tests can be carried out during the drilling. The rotary core drilling rig and can either be a lorry type rig or come on caterpillar tracks, generally with a drilling mast upto 8m high. Each borehole can generally be completed within a couple of weeks.
Upon completion the borehole is either completely backfilled or installed with instrumentation, such as a monitoring well. If instrumentation is installed then a metal well cover will be left in place to protect it.

Concrete cores use a small hand portable drill with a small rotating drill barrel. This technique is used to investigate the thickness and composition of existing structures, such as bridges, retaining walls, buildings and pavements. A small piece of the structure is cored for examination and laboratory testing, and the hole is filled in. This method can usually be completed in a matter of hours.

Cone penetration testing involves pushing a small cone into the ground on the end of a series of rods. The cone is mounted inside a truck or smaller vehicle, which powers a motor that pushes the cone downwards.

The cone is generally used to investigate softer soils, and special sensors on the cone can measure a variety of properties to produce a profile of the geology. Each test is generally short (30-60 minutes) so we can carry out multiple tests each day.

Dynamic sampling uses a small tracked drilling rig. The body of the rig is about the same size as a motorbike, and the drilling mast is upto 3m high. This technique is used in softer soils to drill boreholes to around 10m deep, which can generally be completed within a day. Boreholes are progressed by driving steel tubes into the ground to recover soil cores which are then used to identify the geology and obtain samples for lab tests. Upon completion the borehole is either completely backfilled or installed with instrumentation, such as a monitoring well. If instrumentation is installed then a metal well cover will be left in place to protect it.

Surface water sampling. This technique is simply the collection of a water sample at a particular location. Some simple tests can be carried out on site, but generally the sample is sent to a laboratory.

Trial pits are used to identify the geology of soils and obtain samples upto 4.5m deep. These are excavated using a mechanical digger and a typical trial pit is 1m wide by 4m long. Upon completion trial pits are backfilled with excavated material, which is left slightly mounded to allow for some settlement.

Geophysical surveying

Geophysical surveying techniques are a quick and easy way to collect data about the ground with minimal disruption. The following is a summary of the techniques we will be using on the Lower Thames Crossing project.

Electromagnetic conductivity

Electromagnetic conductivity tests the ability of the ground to conduct an electric current. These variations in conductivity can be used to identify the presence of different soils and rock, groundwater and buried objects.

Ground-penetrating radar (GPR) uses radar pulses to create a profile of what the ground looks like beneath the surface.

GPR is commonly used to detect objects, changes in material, voids and cracks. The radar may reach upto 15m underground or only a few centimetres, depending on the type of soil and terrain.

Magnetometer measures the strength and sometimes the direction of a magnetic field, which can indicate the presence of buried objects.

Microgravity surveys are typically used to detect underground cavities (both natural and man-made) or changes in ground density.

Resistivity is used to detect and map underground features and patterns. The technique uses steel pegs inserted into the ground at regular points. Resistance is measured by introducing an electric current into the ground via the pegs and recording the resistance between other sets of pegs.

Seismic refraction and reflection uses seismic waves to measure the soil and rock layers in order to understand the ground conditions and structure.

Seismic waves are generated by striking the ground with a hammer. Monitoring instruments called geophones, laid out in a line along the ground, record when these waves reach the surface again.

By studying the timing of the waves returning to the surface, it is possible to build up a profile of the ground and its variation in stiffness.

Laboratory testing

Laboratory testing is carried out on samples collected during the ground investigation. It allows us to understand the properties and behaviour of the ground beneath a specific site. In this way, it supplements tests carried out on site and any relevant published information.

Laboratory testing is divided into two broad categories: physical and chemical.

Physical tests

Physical tests provide information on the strength, stiffness, compressibility and permeability of different soil and rock types. We are also interested in how these qualities vary naturally within each material, either with depth or across a particular site.

The test results are interpreted by geotechnical engineers and used in calculations for the safe and cost-effective design of foundations, retaining structures, earthworks, tunnels and excavations.

Chemical tests provide information on any negative effects that soils, and rocks may have on construction materials – in particular, any concrete in the ground. Testing also confirms the type and extent of any contamination that could affect construction workers, nearby residents and workers, or the environment.

The knowledge gained from testing also allows us to identify contamination hazards and deal with them appropriately and safely, during and after construction.

Frequently asked questions

Where will the work take place?

The work will take place at over 700 separate locations along and next to the 24km route in Kent, Thurrock and Essex.

If work is going to take place next to residential properties, we will write to those properties in advance to let them know what is happening and when.

We’ve been undertaking these surveys throughout North Kent and Essex since late 2019. They will continue until Autumn 2021.

Some of the techniques we will use will inevitably be noisy and are likely to be heard if you are located within 50 metres of the machine.

For example, a percussion bore hole rig involves a long, steel pipe that is hammered into hard ground to keep it open, in order to take a sample of the deep soil.

However, there are many different types of techniques that are non-intrusive and virtually silent, such as the Geophysical Surveys.

Regardless of how noisy it is the majority of our work will be carried out during working hours, between Monday to Friday. Where we think there will be an impact we will take measures.

The GI works are not much different from the type of works that we see on our roads and pavements every day – for example, in towns or cities you may see barriers around a small section of a pavement or road, with a diversion around it for traffic or pedestrians.

Many of the site locations will not be visible from the road; some may be on private property with no public access such as a farmer’s field or a landfill site. You may be able to see parts of our rigs from over hedge rows in farmland but most of the works will not be visible.

Our contractors will be required to leave every site as they found it – whether that means laying new turf on a patch of grass or resurfacing a small section of road. Landowners will be compensated for any damage arising from works, although the timing of investigations and access routes will be negotiated to minimise the effects of the disturbance. Sites will always be left safe and secure.

The information we get from the ground investigation will give our engineers data about the existing ground conditions and any contamination in the ground, which will help us build the new road safely and economically. The information we get from studying the ground conditions will confirm the best methods for excavating the tunnels and the most appropriate foundations for bridges and viaducts.

Initially our design relied on records of previous ground investigations and other geological information held by the British Geological Survey and others. This information is extremely valuable but unfortunately it is not typical of the exact Lower Thames Crossing route, especially in rural areas; moreover, it may not provide relevant information or reach a depth sufficient for our proposed tunnels.

We have 24 kilometres of the route to investigate. The investigations will be split into a number of packages, each relating to a specific length of the route. Each package will typically take a few months to complete, including work on site, laboratory testing and reporting of the results. Investigations on site can take a matter of hours, or days, depending on the location. Additional investigations may have to take place if something unexpected is found and further information is required.

We will be working with five specialist contractors to carry out the work in different areas. If you have any questions or concerns, you still have a single point of contact at Highways England who you can contact here.

Our contractors are:

  • Perfect Circle
  • Structural Soils
  • Soil Engineering
  •  Geotechnical Engineering Limited

Safety is always our highest priority and nothing will happen until we are certain it can be done safely. We will ensure that the site risks associated with the ground investigations are identified, assessed and managed. We will assess the contractors’ health and safety performance, and security will be managed at all stages, so sites are safe and secure during and outside working hours.

We have been undertaking a range of different surveys along and near to the proposed route of the Lower Thames Crossing since late 2019. This has been to improve our designs which in turn also helps us to better plan how we would build the project safely.  

The information we gather from the surveys not only helps us to refine our plans for our Development Consent Order (DCO) application, but they will also feed into the DCO process, between acceptance and examination.

Survey works continuing after DCO submission is normal practice for major projects. They are often undertaken both before DCO submission and during the subsequent DCO process, this is usually in relation to the design concerning both environmental factors, ground conditions and existing utilities.

Watch this video for information on the DCO process and or read more here.