Design and installation of the monitoring devices

In June 1995 an environmental monitoring programme was established by Hunting Technical Services (HTS) at the Marks & Spencer site in the City of York (PDF Report 1). The work was undertaken at the request of York City Council, who wished to investigate and monitor both the character of the archaeological deposits present and also the burial environment surrounding them. This second objective was of particular importance because the burial environment, both in its characteristics and stability, is thought to play a vital role in the preservation in situ of a site's archaeological deposits.

The project site is located within York city centre, primarily on the site of the former Curry's Electrical Store at 44-45 Parliament Street (NGR SE6044 5180). At the time of the project's inception the site was 'L' shaped, with access to both Pavement and Parliament Street, as shown in Figure 5. The site was selected because it formed part of the redevelopment and expansion proposals for Marks & Spencer who owned and operated the adjacent store. The proposals included demolition of the former Curry's Electrical Store and then expansion of the Marks & Spencer store into a new building to be constructed on the site.

Within the disused Curry's store, and prior to the demolition work, an archaeological evaluation was conducted in 1994/95 by the York Archaeological Trust (YAT). The work, involving hand excavation of a 3.0 by 3.0m trench to a depth of 2.0m, revealed archaeologically important deposits from the medieval, post-medieval, and modern period. The medieval deposits included various inorganic (e.g. iron, pottery and building materials) and organic finds (e.g. leather, wood, bone and horn), that were preserved in situ within a variable soil-like matrix. This soil deposit was described as a wet or very wet (possibly saturated) organic-rich silty clay, possessing inclusions of ash, charcoal and industrial waste. Much of the indeterminate organic material present was thought to be 'manure' or 'cess' and was probably faecal in origin.

The trench excavated by YAT not only provided an opportunity to investigate and therefore describe the archaeological deposits present, it also presented an opportunity to install a range of monitoring devices (Figure 5). The detailed information on the devices selected and their method of installation can be viewed in the detailed reports 1 & 2 supplied within this article.

In brief, the devices selected permit repeated monitoring, at the same location and in a relatively non-destructive manner, over a period of at least five years. The devices do not monitor directly the state of preservation within the archaeological deposits, but they do provide data on a number of parameters which are considered to be important in understanding the preserving burial environment surrounding the deposits. These parameters include possible changes in the deposit's moisture content, and changes in the chemistry of free water which is present within the deposits as a probable perched water table. A summary of the devices installed and the parameters they monitor is given in Table 1, and their location at the project site is given in Figures 5 and 6.

Type of monitoring device (and number installed) Location of device Parameters monitored by device
Dipwell (1) Within deposits below the pavement of Parliament Street Water level within dipwell (metres above Ordnance Datum), and recovery of sample for the determination of water chemistry parameters*
Neutron Probe Tube (2) Within deposits below the pavement of Parliament Street Moisture content of deposits at 100mm intervals down the archaeological profile
Moisture Cell (13) Within deposits below shop floor of new Marks & Spencer store Resistance (used to estimate moisture content) and temperature of deposits at the point of installation of each cell
Suction Sampler (5) Within deposits below shop floor of new Marks & Spencer store Recovery of water samples from deposit at the point of sampler's installation for the determination of water chemistry parameters*
Table 1: Monitoring devices installed at 44-45 Parliament Street

* The water chemistry parameters were determined using a range of portable dip probes that measured: temperature, electrical conductivity, pH, dissolved oxygen, and redox potential.

The installation of monitoring devices was originally conducted between 14th June and 19th June 1995, and the methods used plus the depths at which the devices were placed are detailed in HTS's original installation report) - PDF Report 1.

Unfortunately, on the 7th or 8th October 1995 damage was caused to several of the monitoring points by various construction operations being carried out as part of the site's redevelopment. The damage was largely confined to the suction samplers, and as a result two of the samplers (nos 6 and 7) were found to be damaged beyond repair, while the remaining three required minor repairs to their sampling tubes. To compensate for the loss of the two samplers, an additional sampler was installed by HTS on 18th October at a position close to the original sampler 7. The archaeological profile through which the replacement sampler was installed appeared to be very similar to that encountered when the original sampler was installed, and therefore it has been incorporated into the monitoring programme as the new sampler 7. The depth of installation for those monitoring devices which required installation down a borehole is given in Table 2, and the point of installation achieved with the moisture cells is summarised in Table 3.

Monitoring device, with identifying number Top of monitoring device (m AOD) Base of monitoring device (m AOD)
Neutron Probe Tube 1 13.40 (top of tube in manhole) 9.43
Neutron Probe Tube 3 13.22 (top of tube in manhole) 9.27
Dipwell 2 13.43 (top of dipwell in manhole) 7.37
Suction Sampler 4 14.30 (upper surface of former shop floor) 8.39
Suction Sampler 5 14.30 (upper surface of former shop floor) 9.10
Suction Sampler 6 14.30 (upper surface of former shop floor) 9.30
Suction Sampler 7 14.30 (upper surface of former shop floor) 8.46
Suction Sampler 8 14.30 (upper surface of former shop floor) 8.02
Table 2: Depth of monitoring devices installed down a pre-formed borehole

With the exception of the two neutron probe tubes, the above monitoring devices were installed within boreholes that went to the base of, or through, the highly organic archaeological deposits (largely medieval or earlier). Underlying the organic deposits a greyish-brown clay was generally encountered, which was identified as a reworked soil.

Cell numberFace within inspection chamber into which cell is installed Parameter monitored by cell Depth to cell from ground level (m AOD)* Distance cell inserted into deposit (mm)s
1 North facing Resistance & Temperature 13.18 50
2 North facing Resistance & Temperature 12.52 85
3 West facing Resistance 13.05 85
4 West facing Resistance & Temperature 12.49 85
5 East facing Resistance & Temperature 13.35 100
6 East facing Resistance 13.20 100
7 East facing Resistance & Temperature 12.95 114
8 East facing Resistance & Temperature 12.62 100
9 South facing Resistance & Temperature 13.31 160
10 South facing Resistance 13.08 115
11 South facing Resistance 12.84 110
12 South facing Resistance 12.66 110
13 South facing Resistance & Temperature 12.50 112
Table 3: Depth and position of moisture cell installation into archaeological deposits

*Ground level taken as the upper surface of the former shop floor, recorded as 14.30m OD.

All the monitoring devices were installed at a depth which would permit the recovery of monitoring data from the organic archaeological deposits only. Most of the devices recover data from a single point in the archaeological profile, i.e. for the moisture cells it is the deposit immediately surrounding them, and for the suction sampler, it is from water drawn through the deposits surrounding the device's porous ceramic tip. In contrast, the neutron probe tube permits the recovery of data from near the surface of the archaeological deposit to a total recording depth of 3.8m. Because the first reading is taken at a depth of 400mm (the first 300mm of tube is exposed within the manhole chamber created within the pavement) an effective deposit depth of 3.4m can be monitored by the neutron probe.


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