During sand and gravel extraction, washing and scrubbing are part of the processes which removes fine particles. Water which is used in this process with suspended silt is pumped into lagoons. Over time silt can accumulate in these lagoons. Currently the use for such silt has not been fully explored when it is excavated periodically, being seen as a by-product. Mineral operators such as Tarmac typically have agreements for an after care period of 5 years to return the land back as close as possible to its original state after aggregate extractions. The challenge being faced by Tarmac and other mineral operators is the lack of restoration medium especially for subsoil during the aftercare period. This project, developed in close collaboration with Tarmac, proposes to conduct a study in Yorkshire, Nottinghamshire and Hampshire to evaluate the quality of silt coming from these lagoons as a replacement media for subsoil.

Aim : To evaluate physical, chemical and biological parameters of silt that will influence its successful re-use as subsoil media in restoration of mining sites to arable land

Objective 1 : Assessment of characteristics of silt from lagoons as suitable subsoil medium

This objective will entail design and implementation of a strategic sampling program to facilitate visits to Marfield (Yorkshire), Lockington (Nottinghamshire) and Blashford (Hampshire) Quarries. Representative silt lagoon samples will be collected to assess their quality in terms of bulk density, shear strength, porosity, pH and organic matter as indices of silts suitability for providing nutrients for crop establishment. Information on the potential quantity of silt that can be excavated from lagoons to determine the supply of material that can meet the demand for subsoil in restoration sites will be collated. There will be laboratory analyses to determine a range of suitable physical, chemical and biological characteristics that will form the basis of an assessment of biotic and abiotic barriers to crop growth.  

Objective 2 : Assessment of biotic and abiotic barriers to effective agricultural restoration using silt from lagoons

The characteristics mentioned in Objective 1 will determine the suitability of this medium as a potential subsoil that is susceptible to minimal compaction and also facilitate drainage in restoration sites. A pot study and field trial will be carried out to verify this. Cranfield University’s repository on the Land Information System (LandIS) (www.landis.org.uk) will be used to match the properties of silt from the lagoons to silty sub-soils (and associated crop suitability, drainage, compaction risk assessments) in order to enable the suitability of this material in restoration sites to be assessed.


Figure 1: Sand and gravel washing plant (Left) and silt lagoon (Right) at quarry site of Tarmac in Blashford (Hampshire)


Target sites have been identified in discussion with Tarmac to build on previous collaboration with Cranfield University. Silt samples will be collected from these sites and brought to Cranfield University to carry out pot experiments. Simultaneously there will be field trials set out at target locations. The project will apply cutting-edge analytical instruments including laser mastersizer (characterising silt), colorimetric flow injection analyser (for nutrient and microbial biomass analysis), phospholipid fatty analysis, organic elemental analyser (carbon analysis) and ICP-MS/ICP-OES (determining metal concentrations). The pot trials will be established in greenhouse facility at Cranfield University. Analysis at University of Birmingham’s Facility for Environmental Nanoscience Analysis and Characterization (FENAC) laboratories would involve use of Scanning Electron Microscope (SEM) for detailed structural analysis of silt and characterization of the surface area of silt through the Brunuer, Emmett and Teller (BET) method as these properties will provide information about the pore network in soil cores that can be used to infer potential for root penetration.

Training and Skills

The student will attend two MSc modules at Cranfield University, namely Soil Systems and Soil Carbon, Nutrient and Contamination to provide the intermediate and advance information on soil and crop science. There will also be a 2 day course on Land Resource Planning (as part of a module) to provide background information and site survey. Tarmac will provide a 5 day training course spread out throughout the study (see attached letter on training)

The student will also benefit from training available to PhD students under Dr S Ullah (co-supervisor at Birmingham University) in the use of colorimetry, spectroscopy, SEM, BET and chromatography to equip the student with the necessary analytical skills for undertaking this project successfully.


Year 1: Site selection and detailed characterisation of silt and establishing pot experiments, poster presentation at a local conference + training

Year 2: Establishing field trials in target sites, oral presentation at an international conference + training

Year 3: Data collection, analysis and paper writing + training

Partners and collaboration (including CASE)

This is a CASE project in collaboration with Tarmac Industries whom Cranfield University has been working together for over four years where they have funded Group and Masters Thesis projects. This project idea as proposed represents a natural extension to confirmatory MSc Group and individual thesis studies previously conducted in 2017.

As a CASE partner, Tarmac has agreed to contribute cash £5k per year over three years, with an in-kind contribution in excess of £30k (please see attached letter of support). A paper has been submitted to Geoderma (Maskova et al. (2018)) from the earlier MSc project with Tarmac that demonstrates the scientific viability for, and framework for the feasibility of this project.

Further Details



The student will be based at the Cranfield campus at Cranfield in Bedfordshire - https://www.cranfield.ac.uk/About/How-to-find-Cranfield