Archaeological evaluation of waterlogged deposits
Where waterlogged deposits are known to exist on a site contractors need to have relevant experience of working on complex urban sites and the archives that they produce.
You'll be asked to undertake an archaeological evaluation of waterlogged deposits which:
- characterises the waterlogged deposits
- investigates the hydrological regime of the site
- references relevant previous archaeological fieldwork and any accessible borehole information
The evaluation must include:
- borehole survey
- specialist assessment for environmental potential
- specialist assessment for environmental condition
- monitoring of water levels for 6 months prior to planning application submission
Further stages of work or other mitigation measures could be required, depending upon the results of the evaluation.
The results of the evaluation are used to:
- make decisions based on the impact of a proposed scheme on the archaeology and continued waterlogged state of the site
- determine the nature of any mitigation measures that might be necessary
- understand the scale of archaeological work
The archaeological policy of the planning authority is to seek to preserve at least 95% of archaeological deposits underneath a new development within the city centre area; we'll advise how this preservation target can be achieved.
Where planning, listed building or scheduled monument consent is required for development; a final report on the evaluation results is required, as part of the information submitted to validate an application. No application will be determined until this information has been provided and mitigation measures agreed. This follows the archaeology policy we've adopted, and the contained in National Planning Policy Framework (NPPF).
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Methodology
During the evaluation, the following methodologies must be followed:
- window sample boreholes drilled using a compact tracked rig in locations where the proposed development will not prevent ongoing water monitoring post-construction
- boreholes should be located using Global Positioning System (GPS) survey equipment accurate to +/-25 millimeters - if this is not possible trench locations will be plotted using an electronic distance measurement (EDM) Total station, by measurement to local permanent features shown on published Ordnance Survey maps
- all measurements accurate to +/-10 centimeters, and locatable on a 1:2500 Ordnance Survey map
- the ground surface at each borehole location must also be recorded in meters above ordnance datum (mAOD)
- the sampling strategy must be agreed between the contractor and the City Archaeologist, and, where necessary, the Historic England Regional Science Advisor
- when organic deposits are reached environmental samples will be taken for General Biological Analysis from the cores a 100 millimeters diameter Shelby Tube will be inserted to recover 2 300 millimeter-long Class 1 undisturbed samples, per borehole, for further specialist assessment
Assessment will need to establish baseline conditions regarding preservation of organic remains and the quality and condition of the waterlogged organic deposits using the following techniques:
- triaxial permeability testing
- porosity/bulk density/moisture content testing
- particle size distribution analysis
- ph analysis
- chemical redox potential testing
Where the City Archaeologist considers it necessary, a programme of water monitoring work will take place. An initial 6 month programme of water monitoring work will be undertaken to understand the site hydrology and potential impact of the development. The monitoring and assessment will encompass both hydrology and water quality over the course of the stipulated time frame.
An interim report will be prepared after 3 months, using data collected to that point. Upon completion of the six-month monitoring, a final tier two hydrological report will be prepared, and the water monitoring equipment will be recovered for re-use.
- Dip-wells will be protected during the re-development and re-cored if necessary to facilitate a 5 year post-construction water monitoring programme, if required
- Dip-wells will be 50 millimeters in diameter with lockable caps
- Remote sensors will be installed for water level monitoring (for example TROLL unit and BARO TROLL unit to record water levels and barometric pressure)
- Water levels will be automatically logged using in-situ data-loggers; the data will be assessed with reference to the levels measured by the Viking Recorder on the River Ouse (the closest Environment Agency monitoring station, along with weekly rainfall levels recorded at the University of York Heslington Campus and hosted by the Electronics Department. Alternative data sources will be used if these are not available)
- REDOX sensors will be installed - the location of these sensors will depend on the sequence recorded, focussing on potentially water-logged organic deposits and will be agreed with City Archaeologist
- REDOX sensors will measure 4 variables: Conductivity, Redox, PH level, and Dissolved Oxygen level - these measures provide an accurate assessment of what the current organic conditions are, how they change and why they vary over time to allow an impact assessment to be made, in accordance with our policy as informed by Historic England guidelines; the sensors can potentially be re-used if monitoring is required elsewhere on the site at a later date
All appropriate records must be made and kept:
- all boreholes will be recorded using standardised pro forma record sheets and related to Ordnance Datum - borehole cores will be examined in the field by an archaeologist suitably experienced in the deep stratigraphic nature of York’s archaeological deposits
- all records must be indexed, ordered, quantified, and checked for consistency; photographs of work in progress and recovered cores will be taken - the photographic record will comprise of digital photographs of not less than 12 mega-pixels
- all site photography will adhere to accepted photographic record guidelines
- all artefacts and ecofacts recovered and retained from the evaluation must be packed and stored in the appropriate materials and conditions to ensure that minimal deterioration takes place and that all their associated records are complete; the rest of the material archive must be assessed for its potential to contribute to artefactual research
- all archaeological contexts must be sampled in accordance with a sampling strategy which must be agreed in advance with the Science Advisor, Historic England (York Office), and approved in writing by York City Council
- all sampling must be in accordance with the recommendations contained in Historic England guidance, in addition, the advice of the Regional Science Advisor must be sought with regard to all other aspects of archaeological science, including dating, that might arise on this site - their recommendations must be followed and confirmation of the adoption of their recommendations supplied in writing to Assistant Director (City Development and Sustainability)
- all finders of gold and silver objects, and groups of coins from the same finds, over 300 years old, have a legal obligation to report such items under the Treasure Act 1996. Prehistoric base-metal assemblages found after 1 January 2003 also qualify as Treasure.
- if anything is found which could be Treasure, under the Treasure Act 1996, it is a legal requirement to report it to the local coroner within 14 days of discovery
- the Archaeological Contractor must comply with the procedures set out in The Treasure Act 1996; any treasure must be reported to the coroner and to the Portable Antiquities Scheme Finds Liaison Officer, who can provide guidance on the Treasure Act procedures
Report
The evaluation report should include:
- a plan of site with scale showing position of boreholes, site levels (AOD) and 8 figure National Grid Reference
- portfolio of drawings and where appropriate, drawings of artefacts
- an interpretation of the waterlogged sequence, setting the site into the context of the known archaeology of the area
- a tier 3 analysis of the hydrological regime on the site based on 6 months of monitoring and how that may impact upon archaeological deposits
- suggestions for areas of further monitoring post-construction
- a deposit model of the archaeological potential of the remainder of the site
- an assessment of the artefacts and ecofacts and where produced reports on any further analyses
- list of sources consulted
- recommendations for further work
- an index to, and details of, the location of the physical and digital archive*
* The long term care of the evaluation archive must be provided for; all the original material and paper archive must be prepared for deposition with an approved archaeological depository such as the Yorkshire Museum. These Institutions will charge to cover the long-term curation of the archaeological archive. The requirements of the receiving Institution must be identified at the time of producing an estimate for the scheme of investigation. The digital archive must be deposited with the Archaeology Data Service (ADS) and costed at the outset.
Our Historic Environment Record (HER) supports the 'Online Access to Index of Archaeological Investigations' (OASIS) project; the archaeological contractor must complete the online OASIS form. We will only accept reports as digital files. The report must be submitted as portable document format (PDF) files.
Once a report has become a public document by forming part of a planning application, we will place the information on our website.
- The contractor must give at least 7 days notice in writing of the start of works on site to City Archaeologist
- The contractor must produce a written synopsis of the results of the work and submit this to us no later than 2 months after the completion of work on site
- The contractor may be subject to regular monitoring visits by us
- Reasonable access to the site must be given at all times to the City Archaeologist
- All work must be undertaken in a professional manner paying attention to CIFA Standards and Guidance and English Heritage, 2008, Geophysical Survey in Archaeological Field Evaluation
Reinstatement
Contractors must ensure that the question of backfilling and surface re-instatement is discussed with the client/landowner prior to any works commencing on-site.
Following re-instatement, where necessary, 60 millimeter diameter standpipes will be inserted into each borehole, surrounded by gravel and Bentonite surrounds and capped with a lockable cover to allow dip-well monitoring.