Geotechnical and archaeological data from Bristol and Dover

 

 

Bristol

 

A comparison of geotechnical and archaeological recording was made at Redcliffe Street, Bristol.  The area lies within the medieval suburb of Redcliffe.  Development since the 14^th Century has been concentrated in and around this area.  The site was chosen for study because of the completeness of the archaeological and geotechnical records, but also because of its position.  Redcliffe is situated on the outside of a meander bend of the River Avon on deep alluvium.  This provides a favourable environment for the preservation of archaeological artefacts.

 

The study highlighted a number of important points. 

 

1. A lack of detailed recording during geotechnical investigations meant that important archaeological boundaries were missed.  For example, the boundary between the modern cellar fill and medieval fill below the cellar flagstones was missed.
2. Drillers routinely logged additional important information such as depth to ‘natural’ on site. 
3. Interfaces within made ground are often distinguished through the types of inclusions present.
4. Borehole heights were rarely accurately constrained in three dimensional space. 

 

Dover

 

Dover was selected as an example are due to the familiarity of one of the investigators, Dr. Martin Bates, with both the field geoarchaeological program as well as the archaeological investigations.  Much of the archaeological information was collected as part of a mitigation strategy during the construction of the new A20 Road and Sewer Scheme in Dover during the early 1990’s. 

 

 

The archaeological investigation involved the use of historical archaeological borehole and section data, geotechnical data, geoarchaeological borehole data and excavation data carried out by Canterbury Archaeological Trust.  The study highlighted a number of important findings that build on those summarised above.

                                                        

1. Geotechnical drill crews often log much more detail than is included in the interpretative geotechnical report, which is written according to client specifications.  The study highlighted that geotechnical drillers could record enough detail to permit basic archaeological site interpretation.

 

2. The depth to ‘made ground’ was often over-estimated by the drillers.  Many of the layers containing archaeological artefacts or occupation surfaces were classed simply as made ground.  This is because no criteria exist for recording made ground.  Simply classifying the ground as modern or old made ground would have provided the archaeologist with more detail. 

 

3. Methods of drilling can destroy complex stratigraphic sequences, such as where the vertical distance between layers is small.  Where significant changes in archaeology occur, important information may be destroyed by over-reliance on borehole logs.

 

4. The general geoarchaeological and geotechnical interpretations of the stratigraphy of the site were similar, but that the geoarchaeological logs provided more detailed local information.  This illustrates that geotechnical logs can be useful in directing future archaeological investigations. 

 

 

  

Summary

 

The Dover and Bristol studies showed that geotechnical drilling crew could record enough detail to provide archaeologists with useful data for assessing the archaeology of the site.  When briefed with a set of simple recording criteria for made ground, geotechnical workers were quickly able to produce detailed borehole logs.  Geotechnical drill crews routinely provide relatively detailed borehole logs, suggesting that more detailed archaeological recording is achievable. 

 

However, the study has highlighted some limitations of the geotechnical records:

 

1. Interfaces within the made ground were not recorded as a matter of routine.  Recording the depths of stratigraphic changes in the nature of made ground would be very useful archaeologically.

 

2. Although inclusions were noted in many geotechnical logs, their depths were not recorded.  By recording the depth of inclusions, useful data for archaeological site assessment may be recorded.

 

3. Complex archaeological stratigraphies are better recorded from geotechnical test pits than boreholes because drilling can destroy fine stratigraphy.

 

 

The full text of these reports can be obtained by contacting us at the address above