HERTFORDSHIRE COUNTY COUNCIL
Agenda Item No.
HIGHWAYS AND WASTE MANAGEMENT PANEL
TUESDAY 18 MARCH 2014
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SINK HOLES AND OTHER FEATURES: WINTER DAMAGE AND
POTENTIAL IMPACT ON FUTURE PROGRAMME
Including
 The formation of sinkholes and other features
 Other forms of damage to the network
 Potential mitigation options and impacts
Report of the Chief Executive and Director of Environment
Authors:
Mike Jarrett
Chris Allen-Smith, 01992 658167
Executive Member: Terry Douris (Highways & Waste Management)
1.
Purpose of Report
1.1
The purpose of this report is to:

Inform Panel of the damage done to the highway network by the
recent prolonged spell of very wet weather.

Inform Panel on work to assess the detail of the damage and on
work looking at options to address the damage

Inform Panel of progress on a bid to Department for Transport
(DfT) to secure funding to help address some of the damage.

Inform Panel of the potential impact on future planned
programmes of work
2.
Summary
2.1
The recent extremely wet weather has had a severe impact on parts of
the highway infrastructure with damage to road surfaces and bridges
as well as geotechnical problems such as sink holes and bank slips.
2.2
Work is ongoing to investigate, quantify, prioritise and address this
damage.
3.
Recommendation
3.1
The Panel is asked to note the report.
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4.
Background
4.1
In common with much of the UK, Hertfordshire experienced an
extended period of sustained heavy rain this winter which the Met
Office has provisionally called it the wettest winter on record.
4.2
This has caused damage to highway infrastructure throughout the
county and had a major impact on transport. Despite regular
maintenance of drains, the sheer volume of water has inevitably
resulted in flooding and some damage to highways.
4.3
Groundwater levels are still high, and are likely to remain so for weeks
to come, meaning even relatively small amounts of rain in the near
future are likely to cause more difficulties as there is nowhere for the
water to go.
4.4
A number of roads across the county have been closed or restricted at
times, either due directly to flooding or from other weather-related
damage such as fallen trees, bank slips, subsidence or sink holes.
4.5
When large quantities of water flow through or saturate the ground, as
we’ve seen over the last few months, the ground can soften and erode
both on the surface and below ground. This can lead to depressions
and holes forming in or under the road and earth banks slipping.
4.6
Immediate Response: Ringway and Fire and Rescue crews were
hard at work through the period to keep as many roads as possible and
to mitigate the immediate impacts of the floods and associated
damage. Ringway are continuing with the operation to clean up and
restore the network although it is likely to be several months before
normal service is fully restored.
4.7
As normal, the public can report any highways faults at
www.hertsdirect.org/highwayfaults and can also use the website to see
what faults other people have reported and subscribe to follow work
progress.
4.8
Longer-term Repairs: Repairing all the damage done will take time
and is likely to be expensive. The government has announced a fund to
help highway authorities with this task and we have applied to them for
additional funding. This is covered in more detail below. In the
meantime, we are prioritising repairs to ensure damage assessed as
requiring an emergency response is fixed straight away, and that other
repairs are done in the most efficient way possible.
4.9
Ringway are dealing with the most urgent damage under the Cat 1
service and we are assessing the sites with more extensive damage to
see if they need programmed works. This work is still underway at the
moment but it means that the 14/15 works programmes may need to
be reprioritised in order to accommodate some of the extra works.
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5.
Sinkholes and Other Ground Features
5.1
How Sinkholes Form: Sinkholes are depressions in the ground that
form over time. They can reveal themselves gradually or with sudden
collapses. Sinkholes are usually caused by underground water flows
which can dissolve and wash away the rock over time. The water
eventually flows down deeper into the underlying bedrock, gradually
whittling it away. In Hertfordshire this is especially true in areas such
as Watford, Hertford, Hemel Hempstead and Rickmansworth, where a
key layer is the Upper Chalk bedrock, which is soluble.
5.2
The flowing water dissolves the Chalk and creates underground
passages, through which more water will eventually flow. Eventually,
the deep bedrock will be so eroded that it can no longer support the
weight of the overlying ground and topsoil. When this happens, a
collapse will occur and the sinkhole will be visible, either as a
depression or as an actual hole.
5.3
There are two main types of sinkholes:

Cover-collapse sinkhole: This type of sinkhole typically occurs
where the upper layer is clay. Large voids can form in the chalk
bedrock beneath the clay. Gradually more and more sections of the
clay fall into the empty void below until such a time the clay is so
weak it fully caves into the void below. The sinkhole at Oatridge
Gardens is thought to be an example of this.

Cover-subsidence sinkhole: This type of sinkhole is usually
characterised by small voids in the ground, gradual collapse of the
surface and the abundant presence of water. These sinkholes are
often created by currently or previously leaking water pipes and
drains. The sinkhole at Park Lane, Broxbourne is an example.
5.4
Detection and Remedy of Sink Holes:
5.5
Once a significant sinkhole has been identified through collapse, or
suspected as a result in a depression in the surface, there are two
common techniques for analysing the ground conditions, and hence
establishing the size and extent of the underground hole:
 Dynamic probing
 Ground radar survey
5.6
Dynamic probing was undertaken in Oatridge Gardens, Hemel
Hempstead. This is an invasive technique that opens a 100mm square
hole in the surface, and the spacing of probes is often as close as
every 3m. The non-invasive technique of ground radar survey is
planned to be carried out in Cherry Tree Green, Hertford.
5.7
Once a hole is visible from the surface, aerated (foamed) concrete is
most commonly used material to backfill the hole. This is wet concrete
moulds to the shape, and therefore fills the entire hole. As an
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alternative, well graded granular material is used to fill small holes. It
requires compaction in layers.
5.8
If the hole is in the bedrock, mixed cement grout can be injected via
small surface openings. This treatment successfully stabilises the
ground, improve the strength of the bedrock and controls the flow of
underground water.
5.9
Proportionate Response: Not all dips in the road surface are as the
result of a sink hole and not all pose an immediate and urgent risk. The
cost of the kind of geotechnical investigations mentioned above can be
significant and is generally only warranted in the most severe cases. In
most cases the most appropriate response to a depression in the road
surface is to top up or patch the depression (if necessary) and visually
monitor it over a period of time. Only where there is clear evidence of
rapid and/or severe movement or collapse is it usually necessary to
carry out geotechnical investigations and/or deeper remedial works.
The vast majority of dips that appear in the road surface are dealt with
through such low-key techniques.
5.10
Some sink hole-type problems are man made. In some areas of the
county there are old chalk mines or clay pits that have been disused
and filled in for centuries. These are often poorly-filled and the ground
can subsequently move. Although fairly rare, such features can be
extensive and are potentially expensive to repair.
6.
Current Know Geotechnical Issues in Hertfordshire
6.1
Listed and described below are the specific locations that have been
subject, bank slips, subsidence or sink holes. Since the wet winter six
holes have appeared and one bank slip. In addition, there were two
locations where problems existed prior to the wet weather.
6.2
New Locations
6.2.1 St John's Road, Watford: Under the guidance of the geotechnical
engineer, Ringway carried out a local excavation by hand, to about
350mm deep. 2 distinct ‘holes’ were found, one about 600mm long and
extending about 300mm under the adjacent kerb and into the footway,
the other about 400mm long and extending some 350mm under the
road. On attempting to hand-dig deeper around these ‘holes’ the subsoil started to fall into the void and it was decided not to carry on
further, in case the road and footway became seriously undermined.
Further investigations, using mechanical diggers, revealed wet clay at
the bottom of the excavation. The findings we relayed to Affinity Water
and Thames Water. It is the intention to carryout CCTV of the sewers
in the road to identify if there are any breaches in the pipework.
6.2.2 Weall Green, Watford: Under the guidance of the geotechnical
engineer, Ringway carried out a local excavation of the sunken part of
the carriageway and footway. A ‘hole’ measuring some 300mm
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diameter and about 1.2m deep was encountered at one end of the
excavation close to a gully. The sub-soils under the adjacent kerb
appeared loose, probably causing the sunken footway paving slabs. A
road closure was implemented and the road was excavated, extending
from kerb to kerb and some 5m long, to about 1.3m deep. The gully
connecting pipe was exposed and no leaks were encountered. The
excavation was backfilled with new granular material to sub-grade
level. The section of road was resurfaced the following day.
6.2.3 Tudor Walk, Watford: Under the guidance of the geotechnical
engineer, Ringway excavated the footway to a depth of 1.2m. A ‘hole’
was evident in one corner of the excavation which extended to about
2m deep. It was first thought that the water stopcock in the excavation
was leaking, however the sub-soils appeared not to be wet. There are
two service pipes leading to the house, one of which is connected to
the water stopcock. It is possible that this pipe has leaked in the past.
The excavation was backfilled with foamed concrete, and the footway
reinstated.
6.2.4 Park Lane, Broxbourne: Following a report, Ringway attended site
and made-safe. Ringway excavated to a depth of about 1m to
determine the cause of the ‘hole’. This investigation revealed a broken
Thames Water pipe. The pipe was reported to Thames Water. The
company is responsible for the repair of the pipe and back-filling the
hole.
6.2.5 Cherry Tree Green, Hertford: Originally reported anonymously, and
then again by East Herts District Council. The hole reported is on
private property managed by Riversmeed Housing Association.
Following probe investigations, the hole has been filled. There is
however there is further evidence of ground subsidence to a parking
bay in the vicinity of the initial hole, possibly indicating the presence of
a sink hole. The maintenance of this parking bay is the responsibility of
East Herts District Council. Whilst the carriageway appears not to be
affected, there are signs of cracks in the footway. It has been
recommended that ground radar investigations are carried out in the
footway, carriageway, parking bay and verge. Joint working with East
Herts District Council will be pursued.
6.2.6 Oatridge Gardens / Wood Lane End, Hemel Hempstead: This was
reported by the police out of hours. The Police advised that Wood
Lane End needed closing. Ringway attend site and closed the road.
Ringway also constructed a temporary access, as instructed by HCC.
Over the weekend the police had a presence on site, until private
security had been arranged
6.2.7 Following the initial work, it was established that a sinkhole (or
crownhole), resulting from an historical chalk mine, appeared in private
property, adjacent to public highway in Wood Lane End. Within a week
the initial hole was filled foamed concrete under the instruction of
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consultant engineer Dr Clive Edmonds, Peter Brett Associates.
Subsequent voids have also been filled with foamed concrete.
6.2.8 Dynamic probe investigations are to start on Wood Lane End, week
commencing 10 March and continue for about 1 week. Peter Brett
Associates has been engaged to supervise the investigations and
analyse the results, in conjunction with the probe results from the
private property. Dependant upon the advice received from Peter Brett
Associates, the footway maybe re-opened within a fortnight. It may
also be possible to re-open half the road, and introduce traffic signals.
6.2.9 Warren Lane Braughing: This is a bank slip. It measures about 20m
long by approximately 4m high. Whilst the bank supports the farmer’s
field it appears to be part of the highway. The bank slip occurred
following the collapse of a fairly mature tree and saturated ground
conditions. The bank conditions have been made worse as the farmer
has attempted to push-back the slipped ground into the bank. This was
reported by the police. Ringway closed the road and considered that
the road clearance could be delayed. Geotechnical and structural
engineers have been engaged to design a solution. Whilst no decision
has been made, gabion baskets would offer one solution although soil
stabilisation may be a more appropriate solution. As these are
specialist operations it will take time to mobilise and construct. In the
meantime the road remains closed.
6.3
Existing Locations
6.3.1 Orbital Crescent, Watford: Under the guidance of the geotechnical
engineer, an excavation in the footway measuring roughly 2.2m x 2.8m
across was opened up to a depth of about 200mm. Once excavated
the sub-grade was found to be sound, with no evidence of holes or
water. The cause of this footway subsidence was probably as a result
of previous poor reinstatement, as opposed to the result of the wet
weather. The excavation was backfilled with granular material followed
by two layers of asphalt.
6.3.2 Highbarns, Hemel Hempstead: In May 2007 there was a partial
ground collapse in a garden (now established to have been caused by
a shaft collapse). At that time, Hertfordshire County Council, in
consultation with Dacorum Borough Council, commissioned a specialist
consultant to assess the condition of the land within a 50 metre radius
of the affected site. This assessment involved a geological survey and
an investigation of historical activity on the site in order to establish the
cause of the incident and establish if any further remedial work was
needed.
6.3.3 A nearby mine which was discovered beneath the road and pathway
was found to need urgent filling. This urgent work was completed in
2008, and the cost of remediation was met by Dacorum Borough
Council. The geological survey indicated that the mine works spread
over a defined area and a Derelict land Clearance Order (DLCO) was
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issued for that area by the Homes and Communities Agency. This
enabled Dacorum borough Council to apply for central government
funding for treatment works. Between 2008 and 2011 funding was
sought from central government to fully investigate and repair the
mines from the Land Stabilisation Fund. Funding was finally approved
by the Homes and Communities Agency in November 2011.
6.3.4 The Land Stabilisation Fund has now closed and Dacorum has
committed additional funding from its capital programme to ensure that
the work could be completed. Following the surveys, works have now
been completed on the existing void and extents have been agreed for
the repair works to the highway. It is intended that this work will repair
sections, as opposed to just patch repairs. This will include the removal
of the physical road closure allowing traffic to use Highbarns. A
contribution to the resurfacing works has been agreed with the
contractors BAM Ritchies to cover their reinstatement commitments.
6.3.5 At the end of October 2013 a collapse occurred in the garden of 1
Meadow Road. This was outside of the DLCO boundary and thus
required a new investigation, which was part funded by HCC and
Dacorum Borough Council. As a result of this HCC implemented an
emergency road closure at the junction with Meadow Road/Highbarns.
Probe investigations have now been completed and we await the full
design from the consultants so as options can be explored into funding
any stabilisation works required. The completion of this work will allow
the road to be re-opened.
6.3.6 To date, around 8,300 cubic metres of filling and grouting material (the
equivalent of 83 double decker buses) has been used to fill the mine
within the DLCO boundary.
7.
Damage to Road Surfaces and Bridges
7.1
Although the sink holes and similar features have been some of the
most high-profile damage to the network, there has also been
extensive damage to road surfaces throughout the county and to a
number of highways structures.
7.2
Bridge Damage: Two main types of bridge damage can occur during
flood events:
7.2.1 Scour damage: Fast flowing water when forced to pass a restriction in
a watercourse, such as a bridge, erodes and scours foundations.
Bridges without scour protection can be critically damaged and in
extreme cases can collapse.
7.2.2 Impact damage: Flooded rivers have enough power to carry fallen
trees, abandoned cars and other bulky items downstream at speed.
Such items can be forced into colliding with, and becoming stuck
against, bridges leading to bridge damage.
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7.3
Carriageway Damage:
7.3.1 In addition to the damage that flowing water can do to the underlying
layers of roads, standing water on the surface of the road for an
extended period can also cause problems. Standing water is forced
into cracks and joints in the surface under considerable pressure by the
action of vehicle tyres. This hydraulic pressure can widen the cracks
and loosen surface material, especially if it occurs over an extended
period. In time the water can get into the horizontal joints between
layers of the surface and can cause the top most layer(s) to ‘blow off’
the road. This problem has been quite common this winter in areas
covered in high friction surfacing (HFS or anti-skid); we have seen a
considerable amount of damage to the road surface where HFS has
blown off the underlying surface, doing damage to that surface as well.
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Assessing and Addressing the Longer Term Damage
8.1
Geotechnical issues: The various sink holes and other geotechnical
issues have been assessed on an individual basis. Investigation and, in
some cases, remediation works are already in hand. In the more
severe cases, the design and cost of necessary remedial works will
depend on the results from ongoing investigations. Table 8.1 below
summarises the known costs although, at this stage, most are to be
finalised.
Table 8.1 Summary of Geotechnical Issues
Location
Value (if known)
St John’s Road, Watford
(Ringway to forward)
Weall Green, Watford
(Ringway to forward)
Tudor Walk, Watford
(Ringway to forward)
Park Lane, Broxbourne
Not applicable
Cherry Tree Green, Hertford
To be established
Wood Lane End, Hemel Hempstead
To be established
Warren Lane, Braughing
£100,000
Orbital Crescent, Watford
(Ringway to forward)
Highbarns, Hemel Hempstead
£600,000
8.2
Bridge Damage: The Highways Structures team have inspected a
number of bridges around the county since the flooding, both in
response to reported damage and on the basis of those structures
known to be potentially at risk. Table 8.2 below summarises the
damage found. As with other issues, some of this damage is minor but
some will require more extensive investigation and assessment and
could ultimately require costly and significant remedial work.
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Table 8.2 – Observed Damage to Structures
Bridge
Bridge Name
Identified Damage
Number
0011
GOLDINGS NORTH
Scour to downstream wing walls
0021
POPULARS
Damage to abutments and wing walls,
currently over flooded.
0023
ROXFORD
Damage to abutments and wing walls,
currently over flooded.
0036
BIGGIN BRIDGE
Scour to upstream and downstream.
0044
LIME KILN HILL
Lower half of A10 bridge wing wall
missing through scour
0046
SANDON LANE
Apron Scoured
0053
DARNICLE HILL
Scour to upstream causing brick loss to
abutments
0266
FOUR ACRE WOOD
Scour to downstream and exposed tree
roots, damage to retaining walls.
0399
SACOMBE POUND
Scoured invert to south span.
0576
BRICKENDEN LANE
Flooded at this point in time, damage to
CULVERT
culvert due to debris.
0594
STAPLEFORD POST
Scour hole behind east abutment
OFFICE
0771
GASCOYNE WAY
Damage due to over flooding.
1541
OUDLE LANE FB
Damage due to floating cars striking
bridge.
1656
MONTAYNE ROAD
Massive scour to upstream and
downstream
1866
BRICKENDEN
Known issues, that have been made
worse
1759
CARAVAN PARK
Foundation failure caused by scour. .
CULVERT
8.3
Damage to Carriageways:
8.3.1 Throughout the winter period, Ringway have continued to carry out
safety inspections on the network (where possible) and to receive
public fault reports. These have fed into the Cat 1 responses and,
where appropriate, the Cat 2 planned programme. This intelligence has
been captured in the Confirm database and used to create a
countywide picture of those roads that have suffered extensive
damage.
8.3.2 At the same time officers have been preparing and refining the first
draft of the 15/16 IWP for circulation early in the new financial year.
This has given the opportunity to visit and reassess some of the
potentially vulnerable sites on the network.
8.3.3 The results of these two processes have been combined to produce
the information needed to make a bid to DfT for additional funding (see
below) and are part of an ongoing process to identify and prioritise in
more detail sites that need additional urgent works as a result of the
damage over the winter.
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8.3.4 The list of damaged roads produced by Ringway from Confirm is
extensive although most of these have only minor or isolated defects.
8.3.5 However, for the purposes of the bid to Department for Transport and
using strict criteria set down by DfT on the type of damage which would
be eligible for funding, we have identified 42 miles of road and the 16
bridges listed above as being badly affected by the severe winter
weather. The cost to repair the identified carriageway damage is
estimated to be around £7m. As mentioned previously, estimates for
the other areas of damage are still under preparation.
8.4
Bid to DfT for Additional Funding
8.4.1 In response to the severe weather, the government announced a Flood
Recovery scheme, making money available for roads damage as well
as other issues.
8.4.2 DfT have made available a total fund of £173.5m to help local
authorities cope with damage from the recent severe weather and
resultant flooding.
8.4.3 £103.5m will be allocated pro rata to highway authorities in England
outside London, based on road length. We do not need to take any
action to receive our share of this funding.
8.4.4 The remaining £70m needs to be bid for and will be awarded only to
authorities affected by recent flood events. To qualify for a portion of
this funding, we need to identify how many bridges and how many
miles of roads were damaged directly in the recent flooding.
8.4.5 Using the strict criteria set down by Department for Transport on the
type of damage which would be eligible for funding, we have identified
42 miles of road and 16 bridges badly affected by the severe winter
weather. The cost to repair the damage is estimated to be around £7m.
8.4.6 We are expecting to hear within the next few weeks whether we have
been successful in this bid and, if so, how much we are to receive.
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Financial Implications
9.1
Work is currently ongoing to estimate the likely costs of some of the
geotechnical issues and damage to bridges and other structures.
9.2
The estimated cost of repairing the identified 42 miles of damaged
roads is around £7m; under the DfT rules this only includes lengths of
road that suffered damage as a result of the flooding etc between
December 2013 and February 2014.
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9.3
Although we hope to receive some additional assistance from central
government, it is unlikely that this would fully fund the damage
identified in this report.
9.4
The damage will therefore need to be prioritised and existing planned
programmes of work may need to be revised in order to accommodate
the highest priority works within existing budgets.
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Next Steps
10.1 Ringway and WCS officers are working together to prioritise and plan a
winter recovery programme to deal with the highest priority and most
pressing areas of damage to the network.
10.2 Members will be kept informed of this work although, given the urgent
nature of some sites, lead times for information may be less than
normally desirable.
10.3 Members will also be kept informed of any impacts on existing
programmes from any reprioritisation necessary to support the winter
recovery work.
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