SINKHOLE INVESTIGATION BOROUGH OF MILTON KEYNES

NEED A SINKHOLE INVESTIGATION ?

Let Geoinvestigate quote for your next sinkhole drilling investigation, sinkhole desk study or sinkhole site investigation. At Geoinvestigate we have been providing expert sinkhole risk assessment reports, ground stability reports, land stability risk assessment reports, building subsidence surveys, chalk mining searches, swallow hole searches, limestone cave searches, chalk hole searches, dene-hole searches and gypsum dissolution subsidence geohazard searches for over 20 years. 

WHY DO SINKHOLES APPEAR IN THE UK?

Sinkholes occur wherever there is karst terrain – rock vulnerable to being dissolved by water, such as limestone, gypsum and chalk – and roughly 20 per cent of the country is underlain by it. Gypsum is particularly prone to the problem, with many large sinkholes appearing in areas around Ripon in North Yorkshire.

TYPES OF SINKHOLE

The term sinkholes is used to describe a whole range of natural and unnatural collapses of the ground including natural cavities, caves and pipes formed in chalk, limestone and gypsum by solution and dissolution as well as less natural collapses originating back to man-made features including mine shafts, mine adits, underground mine workings and mine galleries, chalk pits, tunnels, sewers, drains, basements, cellars and leaking pipes.

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collapse of old mine shaft                                             partial collapse of old chalk well

INVESTIGATING SINKHOLES

Geological engineers use several techniques to locate sinkholes called intrusive and non-intrusive techniques.

NON-INTRUSIVE TECHNIQUES

With non-intrusive techniques no physical penetration of the ground using excavations or boreholes or probing occurs.

Geologist try to either locate the tell-tale signs of sinkholes using desk top studies of historic maps, plans and aerial photographs, walkover inspections of the site otherwise known as reconnaissance surveys or they scan or image the ground using Geophysical instruments typically Ground Penetrating Radar (GPR) or Electromagnetic Conductivity (EM).

DESK STUDY

In a desk study the geologist is looking for circular features or holes, map descriptions including “chalk pits” especially along field boundaries, “coal pits”, “shafts”, “adits” “collieries”, clumps of trees, ponds or uncultivated ground in the middle of farmland map, terms such as “shake holes” or “dene” the latter indicating a valley which may have been formed by the collapse of caves or ancient cavern systems in limestone or chalk.

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aerial view showing circular features                                old map showing “old chalk pits”

In a desk study geological maps are a good starting point because they identify the type of soil and rock beneath a site and whether the ground conditions are likely to be susceptible to sinkhole formation.

RECONNAISSANCE SURVEY

During the site inspection (or recce) the geologist is looking for evidence of movement of the ground including subtle depressions or sagging of surfaces including lawns, driveways and floors, cracked or bowed walls, darker damper  patches/pools where water may collect and soft ground indicative of soil disturbance or leaking pipes.

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rainwater ponds above old mine shaft                              sagging paving above sinkhole

GROUND PENETRATING RADAR (GPR)

The most commonly used instrument for sinkhole detection is Ground Penetrating Radar (GPR). GPR creates an underground, cross-sectional picture or image of the subsurface. Soil layers and rock layers are clearly shown in the GPR image. Large areas of ground can be covered, detecting and mapping buried sinkholes with GPR. Site maps can be drawn showing the size and distribution of buried sinkholes.

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GPR Survey                                                                            x-sectional anomaly profile

ELECTROMAGNETIC CONDUCTIVITY (EM)

The Electromagnetic Conductivity (EM) method can map large areas fairly quickly to locate underground sinkholes. The EM method detects areas underground where there are relative differences in ground conductivity that may be indicative of sinkhole location. In some areas, the EM readings may increase due to an increase in soil moisture caused by water flow towards the sinkhole. In other areas EM may be lower due to air voids or disturbance in the subsurface near or in the sinkhole. Colour contour maps are made showing the location of the sinkholes on the property.

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EM field survey                                                                       surface anomaly map

USE OF GEOPHYSICAL INFORMATION

The information from the geophysical survey can be used to predict possible problem areas where buildings are proposed to be located. Areas on the property with sinkholes can be reserved for non-building areas due to their hazard potential.

Geophysical information can also be used to identify possible target areas for intrusive investigation using excavation, probing, penetration testing or boreholes. 

INTRUSIVE INVESTIGATION

Intrusive investigation can be carried out using simple, rapid hand surface probing, digging by machine excavation, mechanical probing and penetration testing using hand-portable 2 wheeled dynamic probes or mechanised small multi-purpose tracked penetration rigs both capable of reaching depths of 20m and more.

 

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shallow hand probing      excavation           portable dynamic probing     

Alternatively cavities and voids may be located using more sophisticated “state of the art” small volume air or water flush injection type rotary drilling equipment similar to Geoinvestigate’s New Microdrill capable of depths of 30m in <1 hour.

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small penetration testing rig                                            rotary microdrill     

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Microdrills tilting mast allows declined drilling to be safely undertaken several meters from the anticipated surface position of a sinkhole. Small diameter microlaser borehole scanning and CCTV instruments can be inserted down through the borehole casing into the void to image and map its configuration and work out the volume of aerated/foamed concrete needed to stabilise the cavity.

SINKHOLE CASE STUDY

Geoinvestrigate were requested by a Hertfordshire Council to carry out an investigation of a 1m deep 1.4m dia circular  “sinkhole” which suddenly appeared overnight in a garden 3m from a house in Hemel Hempstead in February 2016.  As it was an emergency Geoinvestigate’s roving geologist Jack Harper attended site that afternoon.

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Following his site inspection the hole was cordoned-off by the Council creating a 6m safety exclusion zone. Jacks advice was that the feature was temporarily stable and as such there was currently no need to evacuate the elderly residents of the nearby sheltered housing scheme. Subsequently as advised by Geoinvestigate the hole was monitored for further signs of movement by the Council but remained stable.

A desk study carried out on the same day at Geoinvestigate’s Reading Office. This suggested that  because the feature was located in an earlier building it might be a Chalk Well which had partially collapsed. This importatnt information was immediately relayed to the Council’s housing manager

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Though the initial evidence was encouraging it was important to keep an cautiously open mind about the origin of the hole.

Because there was no brick or timber well lining the Council were advised of several possibilities that it was either a chalk well, a dene-hole (ancient chalk mine working) typical of the area or a dene-hole which had subsequently been re-used as a well

The most worrying outcome with regard to ground instability was a dene-hole (typically a circular shaft 1-3m dia up to 30m deep descending to a large mine chamber and/or several offshoot galleries/tunnels running in any direction and potentially under the nearby housing..

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To cover for all possibilities Geoinvestigate carried out a clustered borehole investigation commencing with a borehole down thee centre of the sinkhole/shaft to establish its full depth and therefore the zone over which a chamber and horizontal offshoot workings might lie.

Drilling was carried out from a scaffold safety platform using Geoinvestigates New Microdrill. This quiet, clean  lightweight powerful fast drill enabled boreholes to be sunk quickly and safely with reduced risk of de-stabilising the ground while causing minimal disturbance to this sensitive residential area.

Microdrills angled drilling capability allowed holes to be drilled from the platform reaching both under a drain and nearby buildings.

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randallsangledrill

 

 

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OUTCOME

Drilling confirmed the sinkhole feature on this occasion to be an infilled chalk well extending to a depth of 36m.

The well was probably previously located in a pump room or a well house at the rear of a large Georgian or Victorian Manor House next to a yard and stables.

Boreholes clustered around the feature confirmed that there was no chamber or mine galleries at depth and the sinkhole was purely a deep infilled circular well shaft of 1.4m diameter.

As the well infill hadn’t settled appreciably over several weeks – not even after disturbance by drilling and the injection of drill water into it, it was concluded that the feature was semi-stable and the appropriate remediation currently was to infill it with sand and to monitor at 2 monthly intervals for 1 year or so to determine its longer term stability.

It was considered that the situation was safe and did not warrant expensive stabilisation works by injection groutingat this time though this may be the solution ultimately.

On this job Geoinvestigate implemented a safe, measured, flexible,  efficient and cost effective sinkhole investigation strategy.

The Council were routinely kept up-to-date with the progress of the investigation with regard to the latest findings and their implication on site safety and cost.

PLAN B

Had drilling confirmed that the feature was not a chalk well but a dene-hole with underground chamber and lateral offshoot workings, Geoinvestigate was, subject to Council approval, ready to implement an appropriate alternative grid drilling strategy based on the following conceptual target model.

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FEEDBACK

“Thanks very much for all of your help and support on this Geoinvestigate, I am pleased it was a fairly inexpensive result for us, although I am sure you would have preferred to find something more interesting” 

“Just to echo Adrian’s thanks – I know the Council is very relieved and reassured with the findings and outcome”

NEED SINKHOLE ASSISTANCE ?

If you require a quote for a sinkhole investigation or a sinkhole risk assessment report or you just need some advice on ground stability problems  please do not hesitate to contact us at Geoinvestigate on Free Call 0800 171 2011 or Reading 0118 4027662 or Head Office 01642 713779 or email us.