Locating a Scottish Mineshaft

A coal mining report recorded the possible existence of a 19th Century mineshaft in the general proximity of a proposed telecommunications tower site in Scotland. No detailed information was available indicating an exact location for the mineshaft. To determine an accurate location for the shaft STS undertook a geophysical survey utilising inductive ground conductivity and magnetic field strength mapping techniques to provide information to a resolution of 1 metre.

Significant geophysical anomalies were recorded from use of both techniques and, as can be seen from the conductivity results diagram left (reduced from A3 size), these correlated well with the approximate mineshaft location detailed in the coal mining report.

Radar Survey at Greenwich River Wall

STS recently completed a surface penetrating radar survey of the river wall frontage at the Royal Naval College, Greenwich, under the direction of DTZ Debenham Thorpe, International Property Advisors. Information on the wall construction/backfill materials was required as part of a general assessment into the structural condition of the wall. No documented information on the structural detail of the wall was available.

A GSSI SIR2 digital radar system was used to carry out the survey over successive periods of low tide to allow the appropriate wall elevation access. Information concerning the general wall construction was obtained, together with locations of construction variations and regions of possible structural deterioration. From this survey data, a limited series of intrusive investigations were targeted to determine the structural adequacy of these anomalous regions.

Winchester Cathedral Column Check

During the 19th century it was found that one of the main piers within the nave at Winchester Cathedral was under structural distress and four cast iron columns were built into a section of new stonework to help reinforce it.

A more recent concern of the Cathedral Architect was that corrosion of the cast iron reinforcement

would result in deterioration and spalling of the stonework. Whilst copies of the original 19th century drawings existed, some verification was equired as to their accuracy.

Consulting engineers, Mann Williams, commissioned STS to investigate the apparent continuity of the cast iron reinforcement and the possible presence of any additional undocumented iron work.

Accessible pier sections were surveyed using a GSSI SIR 2 digital radar system and electromagnetic metal detection equipment and plan and section diagrams were derived from the historical drawings. The results indicated the cast iron columns to be apparently continuous along the entire length investigated. Evidence of a number of features which could be horizontally aligned fixings/dowels was also obtained from the radar survey results.

Air Raid Shelter Collapse

Recent localised subsidence within an allotment site in North London was suggested to be due to the partial collapse of a buried air raid shelter. Historical and anecdotal information indicated that this could be the case, although there was no precise information recording the size of this air raid shelter.

The local District Council required an investigation to assess the extent of the buried air raid shelter and commissioned STS to carry out a geophysical mapping survey. An inductive ground conductivity meter and cesium vapour magnetometer were selected as appropriate investigation instruments for these specific site conditions.

The results of the survey provided evidence of a likely buried

The Cobb, Lyme Regis

The Cobb at Lyme Regis is a Grade I listed structure, believed to have originally been constructed in the 13th Century. It is the oldest working breakwater in the country.

As part of the planned refurbishment works to The Cobb, West Dorset District Council required a preliminary investigation into the possible existence and extent of subsurface voiding that may

have been caused by washout of the fill materials. STS proposed carrying out a reconnaissance ground penetrating radar survey to one section of the structure and to investigate variations in fill quality of a further section, a trial survey using the Sigmax seismic transmission method was proposed.

It was planned that the survey should be completed during late

Autumn when there would be fewer visitors to The Cobb, and the site works were timed to be completed during low tide and fair weather conditions. A number of locations provided evidence of anomalous radar reflections of the type that could be caused by either shallow underslab voids or loose infill materials which were recommended for further investigation. The Sigmax seismic transmission survey allowed variations in bulk material seismic velocity properties to be calculated and indicated considerable differences in the quality of infill of the structure under investigation.

Power Station Foundation Investigation

A former power station site in South London was planned for use as a housing development. However, structural remains of the power station construction included a number of buried concrete ground slabs beneath rubble fill materials.

One option open to Fairview Estates (Housing) Ltd, developers of the site, was to incorporate the

ground slabs within the foundation design for the new buildings. However, to assess the feasibility of this, representative information concerning the nature of construction of the slabs was required.

STS utilised ground penetrating radar to investigate the excavated ground slabs to identify the location of buried pipes/ducts and general slab thickness variations which could affect the load bearing capacity of the slabs.

The survey works were carried out using a GSSI SIR3 subsurface interface radar system over a series of visits, as each section of the slab was exposed.

Analysis of the radar data allowed representative slab construction information to be obtained and a series of exploratory trial pits to be targeted to investigate specific structural features.