Highway Structures General Advice – Ridley Road Emergency Works
Final Report on Site Investigations
London Borough of Bromley Environmental Services Department
5 December 2006
Table of Contents
1 Introduction ..................................................................................................................... 4
2 Microgravity Survey........................................................................................................ 6
3 Borehole Site Investigation............................................................................................ 7
4 Historical Data ............................................................................................................... 10
5 Monitoring of Levels and Crack Widths ..................................................................... 11
6 Interpretation of Results............................................................................................... 13
7 Treatment of void found in Borehole No. 3 ................................................................ 16
8 Reinstatement of the “Sinkhole” at No 122 Ridley Road.......................................... 17
9 Reinstatement of Foul Sewer and Surface Water Drainage ..................................... 18
10 Consultation with Network Rail ................................................................................... 19
11 Recommendations for further work and remediation of site ................................... 20
12 Conclusions................................................................................................................... 21
Location Plan Ridley Road Location Plan
Figure 1 Results of Microgravity Survey and Location of Boreholes
Figure 2 Idealized Geological Section Through No. 122 Ridley Road
Figure 3 Location of Monitoring Points
Figure 4 Schematic Features in Chalk
Figure 5 Geological Section Parallel to Ridley Road
Figure 6 Section Through Reinstated Sinkhole
Figure 7 Extent of Drainage Reinstatement
Figure A1 Ridley Road North Carriageway – Profile A-A
Figure A2 Ridley Road South Carriageway – Profile B-B
Figure A3 Marina Close Centre Carriageway – Profile I-I
Figure A4 Change in Crack Width at Point A No. 16 Marina Close
Figure A5 Change in Crack Width at Point B No. 16 Marina Close
Figure A6 Change in Crack Width at Front of No. 128 Ridley Road
Faber Maunsell Highway Structures General Advice - Ridley Road Emergency Works 4
1.1 Early on the morning of Tuesday 11 April 2006, the residents of No 122 Ridley Road are
understood to have called the fire brigade at 04:30 when they found their front door
jammed. Collapse of north east corner and front of property occurred within the next two
hours. Properties in the area were evacuated.
1.2 Later that morning Bromley Council mobilized Faber Maunsell Limited to carry out
surveys and provide emergency structural advice with respect to the public highway.
Three probe holes were drilled to a depth of approximately 4.0m in the centre of the
carriageway outside No 122. No voids were found beneath the carriageway but the sub-
soil at a depth below 1.0m was very soft. Later that day cracking of the highway was
noted at several locations at some distance from the partially collapsed property.
Monitoring of cracks and level monitoring of the highway was started. Utility companies
attended the scene to make safe/cut off supplies to adjacent houses to prevent any
serious leaks due to the ground disturbance.
1.3 By Wednesday 12 April 2006, a hole, approximately 6m in diameter had appeared in the
front garden of No 122 Ridley Road. The sides of the hole were vertical for about 1.2m.
The base of the hole sloped at about 45 towards a deeper hole about 1.2 – 2.0m
diameter at a depth of about 5m below ground level. This hole was immediately beneath
the front wall of the house. Debris from the building partly obscured the hole, but it
appeared similar to a well hole. The subsoil in the area was noted as being grey gravel
and clay. The subsoil in the hole appeared dark grey in colour.
1.4 As the hole was deeper than the sewers, potential problems with local sewers were
unlikely to be the prime cause of the collapse. The most likely explanation was that the
hole was the result of the collapse of a cavity in the chalk leading to collapse of the water
bearing Thanet Sands into the cavity, followed by the collapse of the silts and clays of the
Woolwich and Reading Beds (Lambeth Series). Thames Water temporarily diverted the
flows in the foul sewer and undertook a CCTV survey of the sewer in the affected section
of Ridley Road. They confirmed there were no problems with the sewer.
1.5 It was also noted that Thames Water’s Shortlands Pumping Station was located nearby
(about 0.5km) and therefore any infilling of the hole was likely to require the agreement of
the Environment Agency as it could have an effect upon the deep chalk aquifer.
1.6 Over the next few days the adjacent properties of Nos 120 and 124/126 exhibited signs of
movement and were declared unsafe by Bromley Council’s Structural Engineers. Over
the weekend of 22/23 April 2006, the four affected houses of Nos 120 to 126 Ridley Road
were demolished following the granting of a court order.
1.7 In order to determine the cause of the hole (and the associated ground movements), and
to confirm if other collapses were likely, Faber Maunsell were requested to propose and
manage appropriate investigations. An initial Phase 1 Geophysical survey of the highway
comprising Ground Probing Radar and a Microgravity survey was inconclusive, but
appeared to rule out any major voids under the public highway. Following consultation
with Bromley Council and the Insurers for the affected private houses, it was agreed that
further investigations (on private land) were required to try and determine the cause of
the collapse and the risk of further ground movements occurring in the near future.
1.8 An investigation comprising further Micro-gravity surveys and intrusive deep boreholes
was envisaged. The objective of the Micro-gravity survey was to attempt to determine
how big the extent of the collapse was below ground, how deep into the ground it is, and
whether other similar features may exist in the locality. The Micro-gravity technique
requires successive refinement of the “model” using computer based methods to
eliminate known influences that affect the results. It was selected from a number of
available non-intrusive geophysical methods as being that most likely to achieve the
Faber Maunsell Highway Structures General Advice - Ridley Road Emergency Works 5
desired objective. However, it was always recognised that physical investigations using
boreholes would be needed to confirm the interpretation. Consequently these were
planned to be part of the overall investigation
1.9 This report provides a summary of the results of the Microgravity Survey, the Borehole
Site Investigation and Monitoring of Levels up to 6 September 2006, and provides an
interpretation of the results and possible mechanisms leading up to the collapse.
1.10 The report also provides details of works to reinstate the area of the “sinkhole” in the front
garden of No 122 Ridley Road, re-connect the drainage serving properties Nos 116 –
142 Ridley Road and treatment of a void found in one of the Boreholes.
1.11 A Location plan of the site is provided with the location of the four demolished houses
1.12 Thames Water were also contacted concerning the pumping station located at
Shortlands, several hundred metres from the site. They subsequently confirmed that at
the time of the collapse, the automatic recording equipment at the pumping station
showed a lowering of the water table compared to recent history. Similarly the records of
the clarity of the water showed slightly increased turbidity (microscopic particles in the
water) immediately after the initial collapse on 11 April.
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2 Microgravity Survey
2.1 Zetica Limited were commissioned to carry out a Phase 2 Micro-gravity (MG) survey (to
extend the initial Phase 1 survey of the public highway) over an area of approximately
11,000 square metres. The results from the Zetica MG survey are reported in their
Report reference P0381/06/R1/A dated 12 June 2006. Figure 9 of their final report
including corrections for buildings and services, shows the Residual Complete Bouguer
2.2 The attached Figure 1 is based on the above map but also shows the lines of the main
sewers and the location of Boreholes that were subsequently drilled. The Residual
Complete Bouguer Anomaly is the result of various stages of post-processing of the
gravity readings taken on site over the study area. It takes into account the location of
known major sewers, the known variation in gravity for this location on the globe, the
calculated change in gravity to allow for the elevation (height above sea level) of each
individual reading, plus an allowance for the effect of the Railway embankment to the
south of the site and other known features (service company holes, depressions, etc).
2.3 The map shows two significant anomalies in the study area and two further more minor
anomalies. These are noted on the map as RG1 to RG4.
The two significant areas of “Residual Complete Bouguer Anomaly Lows” are:
• in the rear gardens of Nos 13 - 16 Marina Close and 1 – 4 Forstal Close, extending
beneath No 3 – 6 Forstal Close (RG1)
• in the rear gardens of Nos 122 – 134 Ridley Road (RG2)
The two minor areas:
• in the gardens between Nos 105b and 107 Ridley Road (RG3)
• in the rear garden of No 105 Ridley Road (RG4)
2.4 To help understand the physical form that the anomalies may take in the ground, the
Zetica report indicates the “signature” of certain types of anomaly. Essentially this is
related to the rate of change of gravity reading with distance from the feature. The
“signature” also helps in trying to distinguish between relatively small features near to the
ground surface or much larger ones at greater depth. To put this into perspective, the top
of the chalk – where solution cavities are likely to develop – is over 20 metres below the
ground surface at this location, i.e. the height of a six storey building. At this depth,
features may need to be of the order of perhaps 7 or 8 metres across to be readily
2.5 From our knowledge of the location, the minor anomalies may be related to the original
construction works of a 525mm diameter surface water sewer through the area (RG3),
and would be influenced by a shallow buried void such as a partially in-filled air raid
shelter or alternatively some other natural feature, such as an in-filled pre-existing
solution feature in the chalk.
2.6 Of the two more significant anomalies, whilst anomaly RG1 would appear to cover a
greater area, computer modelling by Zetica (following the results of the first boreholes) to
try and re-create this feature (along profile 1 – see figure 1) would appear to indicate that
the anomaly is not representative of a void or cavity, but instead could represent a
localised thickening of the upper layers of soil overlying the chalk bedrock deep below.
Possible explanations for this could be a geological feature such as an in-filled valley or
channel in the top of the chalk.
2.7 Computer modelling along profile 2 suggests that Anomaly RG2, located along the south
side of Ridley Road(in the locality of No 122), presents the most significant risk in terms
of the possible presence of further water filled voids existing in the chalk.
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3 Borehole Site Investigation
3.1 To assist us in determining suitable locations for the boreholes, prior to completion of the
computer modelling and analysis of the Micro-gravity results, Zetica provided a
Preliminary Interpretation Map that did not include final corrections for buildings and
3.2 Arrangements were made for Soils Ltd to sink a number of confirmatory boreholes. An
initial “control” borehole (BH1) was located at the end of Ridley Road, adjacent to No 10
Forstal Close and to allow more time for the interpretation of the Micro-gravity results.
This location was chosen as being away from any zones with anomalies. The driller’s
logs indicate Lambeth Group (Woolwich and Reading Beds) from 1.4 – 6.35m below
ground level, Thanet Sands from 6.35 – 24m and Chalk from 24 – 31m where the
borehole was stopped in firm chalk. At 23.5m there was a band of clayey sand. At 24m
the drilling tool dropped upon insertion from 24 to 24.95m indicating either very soft
material or a possible void or fissure. The top of the chalk was soft. A further void in the
chalk was noted between 29.5 and 30.5m.. The change in gravity due to a small fissure
at these depths would not be detectable by the micro-gravity measuring apparatus.
Water was encountered in the borehole at 24m and rose quickly to 12m below ground
level. Two standpipes were installed to measure ground water levels at depths of
between 3 – 15m and 27 – 30m (standpipes A and B respectively).
3.3 A second borehole (BH2) was sunk in the rear garden of No 122 Ridley Road, at an early
identified location of a “Free-Air Anomaly Low”. The drillers had problems drilling through
the Thanet Sands which “clamped” around the 150mm casing. This stratum is especially
dense near to the top of the chalk, at depths below 18m from the ground surface. This
difficulty was overcome by extending the 200mm casing before continuing with the
150mm casing. Soils Ltd completed the second borehole on Monday 5 June. The driller’s
logs indicate Lambeth Group (Woolwich and Reading Beds) from 1.3 – 3.7m below
ground level, Thanet Sands from 3.7 – 23.5m, Chalk from 23.5 – 30.5m. At 23m there
was a band of clayey sand. At 11.5m to 12m no material was recovered from the
borehole possibly due to the high water content of the sand at this level. The top of the
chalk was found to be soft. The borehole was terminated in firm chalk at 30.5m. Water
was encountered in the borehole at 8.2m. This borehole shows similar stratigraphy to
BH1, but did not show any presence of major voids or particularly loose ground,
notwithstanding the fact that no materials were recovered between 11.5 and 12m depth.
A further standpipe was also installed to monitor ground water levels in the chalk. The
apparent mismatch between the Anomaly and the borehole findings was later resolved,
and is discussed in Section 6 of this report.
3.4 A third borehole (BH3) was sunk in the rear garden of No 124 Ridley Road. This was
located following receipt of the Preliminary results of the refined micro-gravity survey map
and is located at a gravity contour low point, and above an area identified as a Residual
Bouguer Anomaly Low. The borehole revealed a void in the top of the chalk just at the
interface between the base of the Thanet Sand and the top of the chalk. The void is
between 22.5 and 24m below ground level, and the soil at the base of this void had a
high concentration of flints and some loose Thanet sand, although this may have
collapsed into the void by the action of breaking through by the drilling tool. The void is
about 10m below the ground water level which is currently at 11m below ground level.
The chalk below the void is quite soft with SPTs of about 12, thus we instructed the
drillers to proceed to below 30m below ground level to locate the top of competent chalk.
The borehole was terminated at 33m below ground level. The hole was cased with
150mm steel casing, and once finished we asked the drillers to extract the casing only up
to the top of the void and then leave this in-situ - to allow for possible filling of the void at
a later date.
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3.5 Based on the latest available information, a fourth borehole (BH4) was sunk towards the
rear south east corner of the garden of No. 126, this being the furthest east and closest
we could get to the railway without entering private land (other than that of the
demolished houses). The driller’s logs indicate Lambeth Group (Woolwich and Reading
Beds) from 0.85 – 3.9m below ground level, Thanet Sands from 3.9 – 24.8m, Chalk from
24.8 – 32m. The bore is recorded as damp from 8m below ground level, and wet from
11m. No voids or other major inconsistencies were noted.
3.6 The drilling rig was de-mobilised on 15 June, as the only practicable locations for further
boreholes were in private fenced gardens.
3.7 Following discussions between the Council, Insurers and homeowners, three additional
boreholes (Nos 5, 6 and 7) were subsequently sunk in the rear gardens of Nos 130 and
134 Ridley Road and in the rear garden of No 3 Forstal Close at locations indicated in the
Zetica report on the micro-gravity survey for further investigation, as shown on Figure 1.
Borehole No 5 (130 Ridley Road)
3.8 Sands and Gravels of the Lambeth Group from 1.1m – 5.5m, Thanet Sands from 5.5m –
26.7m and Chalk from 26.7m to the end of the borehole at 31.0m. The Thanet Sands
from 24.5m – 26.7m comprised clayey sand. The chalk from 26.7m for about 2m was
very soft. Water was encountered in the Thanet Sand at a depth of 10.1m and again in
Borehole No 6 (134 Ridley Road)
3.9 Sands and Gravels of the Lambeth Group from 0.65 – 5.9m, Thanet Sands from 5.9m –
25.5m and Chalk from 25.5m to the end of the borehole at 30.8m. The Thanet Sands
from 22.7m – 25.5m comprised clayey sand. Results of tests undertaken as the borehole
was driven indicate that from about 15m the Thanet Sand was noticeably harder and
therefore denser when compared with results from other boreholes at the same depth.
Water was encountered in the Thanet Sand at a depth of 10.7m. The driller’s logs record
that while drilling through the Thanet Sands the water level in the borehole rose up to
levels between 1m and 4m below ground level.
3.10 No voids were found in either of the boreholes Nos 5 or 6.
Borehole No 7 (3 Forstal Close)
3.11 Lumps of concrete with occasional brick fragments and gravel from 0.8m – 2.9m, clayey
sand of the Lambeth Beds (2.9m to 5.0m) and Thanet Sand from 5.0m. Work on sinking
the borehole was significantly delayed by the difficulty of sinking the borehole through the
lumps of concrete. Chalk was not encountered until approximately 27m below ground
level. The borehole was stopped at 30.1m.
Ground Water Level Monitoring
3.12 Ground water levels have been monitored in the Thanet Sands and the Chalk through
piezometers installed in some of the boreholes. These have shown the general ground
water level in the Thanet Sands to be between 8.3m and 9.2m below ground level and
the general ground water level in the Chalk to be between 11m and 11.4m below ground
level. A table showing the recorded ground water levels is displayed below.
Ground Water Level (m)
Date Borehole 1 Borehole 2 Borehole 3 Borehole 4
17/07/2006 9.10 11.20 11.06 10.87 7.90
03/08/2006 9.22 11.32 11.34 11.02 8.28
16/08/2006 9.22 11.30 11.38 - 8.27
05/10/2006 - - 12.11 - 8.33
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3.13 A simplified geological section through No. 122 Ridley Road and the railway embankment
is presented as Figure 2, which also shows the scale of a typical house in relation to the
depth of the boreholes.
Factual Report on Ground Investigation
3.14 A factual report on the 7 boreholes sunk by Soils Limited is included in their report
reference J9428 dated 3 November 2006.
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4 Historical Data
4.1 Historical mapping data has been obtained and reviewed from the Council’s archives and
from local libraries and museums. This has currently ruled out the possibility of former
gravel quarries being located at the site (and in-filled with less dense material), as
records show the nearest of these to be located in the Westmoreland Road area. The
search has however revealed the location of old ditches and possible watercourses,
specifically along the line of the 525mm foul sewer between 105b and 107 Ridley Road,
which may account for the anomalies at RG3 and possibly RG4.
4.2 Residents have informed us that prior to the present development, watercress beds
occupied part of the valley, although no records have yet come to light that would confirm
4.3 No further evidence of previous uses of the Ridley Road area or explanation has been
found from searches of British Library Archives and a further search of the Bromley
Reference Library historical maps.
4.4 The evidence we have established is that the land was open land and then used as
allotments prior to the present development.
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5 Monitoring of Levels and Crack
5.1 Level monitoring of the site was started immediately following the initial collapse. Initially
this consisted of monitoring of levels of the highway along Ridley Road and Marina
Close. Following some cracking of the carriageway over the Easter weekend this was
extended slightly to cover a total of 36 points on the carriageway. Level monitoring points
were also subsequently established on several of the house elevations fronting Ridley
Road. These points are located on eleven properties; Nos 105a, 105b, 107, 114, 116,
118, 128, 130, 132 and 134 Ridley Road and No 16 Marina Close. Initially readings were
taken at daily intervals, gradually reducing to weekly and then monthly intervals. A final
set of readings was taken on 6 September 2006. Locations of monitoring points are
shown in Figure 3.
5.2 Initial downward movements were recorded over the Easter weekend and until 24 April,
since when the levels appear to have stabilised. The greatest movements recorded were
at monitoring points 15, 16 and 17, located on the carriageway of Ridley Road directly
outside the collapse site. The maximum overall movement recorded was a drop in level
of 27mm at monitoring point 16.
5.3 Downward movements were also recorded at the monitoring points along Marina Close.
The greatest downward movement recorded along Marina Close itself was 17mm at
monitoring point 22. This monitoring point is the closest point on Marina Close to the
collapse site, and the movement at this location is shown on Figure A3 in Appendix A.
5.4 A review of the results of the monitoring of levels on the highway and buildings indicates
no recent significant movements. In fact, following the initial movements over the Easter
weekend at the commencement of monitoring, there has been little if any change other
than a slight drop in levels of the carriageway immediately adjacent to the depression in
the front garden of No 122 during the demolition works. Bromley Council therefore
agreed that routine weekly level monitoring should cease from 21 June 2006.
5.5 A final set of level monitoring readings was taken on 6 September 2006. Figures
showing the carriageway profiles (along the sections shown on Figure 3) over the
monitoring period are shown in Appendix A.
5.6 It should be noted that the method of monitoring used has an error margin of
approximately +/- 5mm. Therefore minor level variations in the monitoring data are to be
Crack Width Monitoring
5.7 Demec studs were installed across four separate cracks on two properties: No.16 Marina
Close and No. 128 Ridley Road on 15 April 2006. There were two cracks monitored on
each property, with the locations of the cracks shown in Figure 3.
5.8 The cracks were measured using a Demec gauge, at the same intervals as the level
monitoring. Measuring of the crack at the rear of No. 128 Ridley Road was terminated on
23 April 2006 due to access no longer being available.
5.9 The crack on the front of No. 128 Ridley Road shows the largest increase in width, with
an increase of 0.5mm over the monitoring period.
5.10 The two cracks on the front of No. 16 Marina Close showed an initial minor increase in
width but there is no obvious continuing opening trend. Point B shows an increase in
width of approximately 0.3mm over the monitoring period, while Point A shows an overall
increase of approximately 0.16mm.
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5.11 A final set of crack width readings was also taken on 6 September 2006. Figures
showing the overall movement of the cracks at Points A and B No. 16 Marina Close, and
on the front of No. 128 Ridley Road are shown in Appendix B.
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6 Interpretation of Results
6.1 The results of the micro-gravity survey show that several areas of low density are located
beneath the surveyed area. The “low” gravity points can be caused by a small void at a
shallow depth, or a larger void at considerable depth or conversely a low density area
resulting from poorly compacted ground, peat layers or similar. The location of the most
significant anomaly shown on the Map (Figure 1) is located directly adjacent to the
houses that were required to be demolished. There are no areas of low readings under
6.2 As a result of the above, subject to maintaining the closure of the footway outside of No
122 until the “hole” and footway surfacing were made safe, the Highway was re-opened
to traffic, and the temporary fencing moved back to the southern kerb line of Ridley Road
in late June 2006.
6.3 The Microgravity Survey Report by Zetica Ltd identified two areas of anomalies RG1,
between Marina Close and Forstal Close, and RG2 in the rear gardens of Nos 122 – 134
Ridley Road. (See Figure 1).
6.4 The Zetica report gives no indication as to what size the "void" located in BH 3 extends
over. This could be a wide fissure although this was considered unlikely. Based on the
Micro-gravity results we would expect it to be a quite wide void, but certainly less than
20m in diameter, this being the spacing of the other two boreholes either side. The
natural "seal" at the base of the Thanet Sand/Chalk interface (as found at BH1) is not
present in this area and the "free" water table is at about 11m below ground level (rather
than being under artesian pressures in the chalk).
6.5 Based on the findings of our investigations, including experience and background
research, the ground collapse and void discovered in the Chalk at BH3 appear to show
the characteristics associated with the development of cavities in the chalk close to the
interface with the overlying Thanet Sands. Erosion of the chalk may be the result of a
combination of relatively high groundwater flows, fluctuating groundwater levels and
groundwater acidity. The overlying Thanet Sands would normally be affected to a much
lesser extent by these agencies. It is considered that a similar cavity existed at depth in
the chalk below the front garden of No 122 Ridley Road, and that collapse occurred
following progressive erosion possibly being triggered by the lowering of ground water
levels which reduced or removed hydraulic support to the roof of the cavity. Collapse of
the roof and infilling or partial infilling of the cavity with the overlying Thanet Sand Beds
could have given rise to the observed turbidity at the Shortlands Thames Water pumping
station, and the appearance of a classic “subsidence sinkhole” at the ground surface into
which the front of No 122 partially collapsed. Associated initial lateral surface ground
movements following the collapse of the sinkhole may then have subsequently caused
the structural damage and distress to adjacent properties. See Figure 4 showing
schematic features in chalk taken from the CIRIA Report C574 - “Engineering in Chalk”.
6.6 However, it is not possible to rule out an alternative explanation in that localised erosion
commenced in the clayey layer at the base of the Thanet sands immediately overlying
the chalk, above an historic fissure in the chalk. The clayey layer elsewhere serves to
maintain artesian water pressure in the chalk, and if breached could lead to rapid
undermining of the Thanet sands. Arching would occur until the sand was no longer self-
supporting, resulting in the collapse. This explanation is considered less plausible. The
size of the cavity that would need to have developed in the Thanet Sand, to create the
surface depression that occurred, is unlikely to have been able to be self-supporting. In
comparison Chalk is a much stronger and competent material. Also, if the water flow was
sufficient to have removed fine material from the Thanet Sand it should still be continuing
to do so and the size of the depression should be continuing to increase. There is no
Faber Maunsell Highway Structures General Advice - Ridley Road Emergency Works 14
evidence that this is occurring, either here or elsewhere, and the microgravity and
borehole results support the presence of a cavity in the chalk, not the Thanet Sands. At
the control location, water levels in the Thanet Sands were found to be independent of
those in the chalk.
6.7 The issue of settlement, malfunction and damage to the sewerage pipe on the boundary
of No 122 with 124 Ridley Road is considered to be the result of progressive subsidence,
possibly as a result of leakage and erosion of surface subsoil. There may have been an
associated minor contribution to the main collapse, but given the geometry of the
situation, this is considered to be unlikely.
6.8 A longitudinal section (through the back gardens of Ridley Road along the line of Profile 2
on Figure 1) of the ground strata identified in boreholes Nos 1 – 6 has been prepared, as
shown in Figure 5. Densities of soils indicated by penetration tests in the boreholes have
been compared to consider possible explanations for the microgravity lows identified by
6.9 Computer modelling of the microgravity survey results by Zetica Ltd suggested the main
component of the anomaly could be explained by varying thickness of overlying strata, or
possibly relating to near surface voids or loosely compacted ground.
6.10 The results from Borehole No 5 plotted on the longitudinal section (Figure 5) show the
Thanet Sands to be relatively thicker at this location with the Chalk being encountered at
greater depth than at Boreholes Nos 1, 2 and 3. Also at Borehole No 5 the top 2m of the
Chalk was very soft.
6.11 Results from Borehole No 6 show similar geology to that of Borehole No 5, except that
the top of the chalk was relatively harder.
6.12 Within the broad microgravity low feature of RG2, Boreholes Nos 3, 4 and 5 all show low
strength chalk at the boundary with the Thanet Sands with voiding apparent in Borehole
No 3. In contrast Borehole No 2 which is outside this low has significantly more
competent chalk at this boundary.
6.13 Borehole No 6 which is positioned in a microgravity low, has competent chalk at the
boundary, but the chalk is shown to be softer below 26m. The presence of the railway
embankment to the south has limited the 3D modelling of the anomaly in this area, but
the benefit of Borehole No 6 is that a source of the anomaly within the study area has
been ruled out.
6.14 The results indicate that apart from the void at BH3, the remaining microgravity low points
identified in anomaly RG 2 appear to be explained by varying thicknesses of overlying
strata and softer material within the Chalk.
6.15 Computer modelling of the microgravity survey results by Zetica Ltd along a line between
Marina Close and Forstal Close (along the line of Profile 1 on Figure 1) suggested that
the anomaly may result from a localised thickening of lower density strata above the
Chalk, or possibly an area of lower density material within the sands and gravels of the
Lambeth Beds or the Thanet Sands.
6.16 Results from BH 7 confirm this hypothesis. Similarly, the presence of the concrete found
in the borehole is significant. The concrete appears to be concrete rubble and debris
tipped into the area. Within the borehole it has not been possible to determine how well
this was compacted, but it is most likely that there will be a number of small voids within
the deposited material. This factor combined with the top of the chalk being located some
5m lower that adjacent boreholes has obviously caused the microgravity low.
6.17 The boreholes have identified the top of the Chalk to be soft. This feature is not
uncommon at the top surface of the chalk where it is waterlogged, and is commonly
known as Putty Chalk.
6.18 The ground water levels recorded in the boreholes show that there are two separate
water tables, one associated with the Thanet Sands and one associated with the Chalk.
Faber Maunsell Highway Structures General Advice - Ridley Road Emergency Works 15
The ground water levels for both water tables fall towards the north, following the general
fall of the ground surface.
6.19 The two distinct water levels indicate that possibly the material that collapsed into the
original void beneath No 122 Ridley Road has formed a seal to the deep Chalk aquifer
Level and Crack Width Monitoring
6.20 The locations of level and crack width monitoring points are shown in Figure 3, with
graphical data in Appendices A and B.
6.21 Settlement was observed to the points located adjacent to the collapse hole on the south
side of the carriageway of Ridley Road outside No. 122. The maximum downward
movements recorded were 24mm, 27mm and 24mm at monitoring points 15, 16 and 17
respectively, with the majority of this movement taking place between 21 April and 25
April 2006 following demolition of properties and the associated movement of heavy
6.22 All other points show little movement, and due to the level of accuracy of the survey,
movement shown is not significant.
6.23 Crack widths measured on properties No. 16 Marina Close and No. 128 Ridley Road
showed an initial opening trend, but later readings show little overall movement. As
crack widths are also affected by other factors such as temperature and moisture in the
subsoil, small recent movements are not considered to be significant.
Faber Maunsell Highway Structures General Advice - Ridley Road Emergency Works 16
7 Treatment of void found in Borehole
7.1 Following consultation with Thames Water and Environment Agency a specification for
the treatment of the void found at Borehole No 3 was agreed, and the work was
completed by Soils Ltd, the borehole contractor, on 15 August 2006.
7.2 The void was filled with small sized gravel aggregate (pea shingle) and the top of the void
sealed with a “waterproof” bentonite cement grout. The borehole has been filled to the
surface with bentonite clay and the top capped with a metre of bentonite cement grout up
to about 0.5m below ground level. The volume of pea shingle used to fill the void was
approximately the equivalent of two one tonne “jumbo” bags as used by builder’s
merchants for delivery of aggregates.
Faber Maunsell Highway Structures General Advice - Ridley Road Emergency Works 17
8 Reinstatement of the “Sinkhole” at
No 122 Ridley Road
8.1 Again following consultation with Thames Water and Environment Agency a specification
for the reinstatement of the “sinkhole” at No 122 Ridley Road was agreed.
8.2 The sides of the depression (which was about 6 – 7m across) slope towards the centre to
form a conical shaped hole. The deepest part was about 3.2m deep, suggesting that
there is a “pipe” of potentially loose infilling rising about 20m above the top of the Chalk,
although collapsed Thanet Sands below the water table are likely to be relatively dense.
The depression appeared to be stable, with no signs of additional material being lost over
the first few months since demolition of the houses.
8.3 Thames Water and the Environment Agency had concerns about the sinkhole allowing
polluted surface water to enter the deep chalk aquifer. Our interpretation of the results of
the investigation are summarised in paragraphs 6.5 to 6.7 above. It is most likely that any
void in the chalk has been filled by Thanet Sand and that any original seal to the top of
the chalk strata provided by the clayey layers at the base of the Thanet Sand would have
been lost in the collapse. However, the feature is entirely the result of a "natural" event.
8.4 Work on the reinstatement of the “sinkhole” commenced on 22 August 2006. The loose
material in the sides of the depression was removed as far as possible using a long
reach excavator standing to the side of the depression. Infill material and soil around the
top of the central “pipe” was trimmed so that a concrete plug about 4m diameter and
900mm thick could be placed above the central “pipe”. The remainder of the depression
was then backfilled with readily compactable granular material with a slight clay content
to “bind” the fill. Figure 6 shows the typical section through the reinstated depression.
8.5 Compaction of the filling materials was undertaken using remotely controlled compaction
plant. For health and safety reasons it was not possible to enter the depression until it
was backfilled to a level not exceeding 1.2m below ground level. The option to batter the
sides of the depression to a safe slope prior to backfilling was not considered to be
8.6 The works were undertaken by London Borough of Bromley’s Term Contractor, O’Rourke
Construction and Surfacing Ltd. Reinstatement to just below ground level was completed
on 13 September 2006.
Faber Maunsell Highway Structures General Advice - Ridley Road Emergency Works 18
9 Reinstatement of Foul Sewer and
Surface Water Drainage
9.1 The greatest risk from pollution would appear to be due to possible future leaking of the
foul drainage system. Nos 116 – 142 Ridley Road are served by a private sewer to the
rear of the properties, with an outfall along the boundary between Nos 122 and 124
Ridley Road leading to Marina Close. Investigations have shown that generally this
sewer has been unaffected by the collapse except where it has fallen into the depression
formed by the sink hole.
9.2 For practical reasons of maintaining adequate falls, the 150mm private foul sewer
between the manhole in the back garden of No 122 Ridley Road and the pipe crossing
Ridley Road was reinstated on its original line along the boundary between numbers 122
and 124 Ridley Road. To minimise the number of joints, resist further settlement and
potential leakage in the future, the reinstated sewer was constructed using PVC-U pipes.
9.3 The works to reinstate the foul sewer were completed on 19 September 2006.
9.4 The existing surface water system serving Nos 120 -134 Ridley Road was disturbed by
the collapse of the sinkhole. Approximately 5m of both the inlet to the manhole in the
front garden of No 124 Ridley Road and the outfall in the highway were exposed and
replaced as necessary and the manhole rebuilt to correct levels
9.5 Again these works were undertaken by London Borough of Bromley’s Term Contractor,
O’Rourke Construction and Surfacing Ltd, with the works completed on 23 September
2006. A drawing showing the extent of the drainage replacement work is shown in Figure
Faber Maunsell Highway Structures General Advice - Ridley Road Emergency Works 19
10 Consultation with Network Rail
10.1 Following an initial suspension of Eurostar and other rail services immediately following
the collapse early on the morning of 11 April, services resumed as normal straight away
following an inspection of the railway adjacent to Ridley Road. Network Rail have been
monitoring the tracks adjacent to Ridley Road on a weekly basis and have found no
movement to date.
10.2 Network Rail have been kept informed of the further investigations and the proposals for
treatment of the void in Borehole No 3 and the reinstatement of the sinkhole.
10.3 Network Rail have also been informed that all works relating to the treatment of the void
in Borehole No. 3 and the reinstatement of the sinkhole have been completed, including
the reinstatement of the foul and surface water drainage.
Faber Maunsell Highway Structures General Advice - Ridley Road Emergency Works 20
11 Recommendations for further work
and remediation of site
11.1 Copies of the Ground Investigation report containing factual in-situ and laboratory test
data from the Ground Investigation contract is available to be supplied to Insurers. This
may be used to aid the design of new foundations should the decision be taken to re-
build the demolished properties. Consideration should be given to founding new
properties on a reinforced concrete raft foundation, depending on the degree of success
achieved in infilling the void and stabilising the surrounding ground. The alternative of
piles would need to be over 30m long to found into competent chalk.
11.2 After review of the final level monitoring data taken on 6 September 2006, it is considered
that further monitoring of the highway is no longer necessary. Level monitoring of the
individual properties is now the responsibility of the respective insurers, should they
consider this necessary.
Faber Maunsell Highway Structures General Advice - Ridley Road Emergency Works 21
12.1 The investigations undertaken to date identified a small void at the interface between the
Chalk and Thanet Sand strata at the location of Borehole No 3. This has now been filled.
No other significant voids have been identified. The boreholes show variations in the
depth of the Chalk/Thanet Sand interface which is a typical feature of this interface.
12.2 The further investigations completed to date would appear to support our interpretations
that the most likely cause of the collapse of the ground at 122 Ridley Road was
associated with the development of a cavity in the Chalk close to the interface with the
overlying Thanet Sands followed by progressive erosion of the Thanet Sands.
12.3 The investigations have not found evidence of further cavities in the Chalk similar in size
to the cavity that must have existed beneath No 122 Ridley Road and there is probably
therefore a low risk of a similar collapse occurring in the study area.
Ridley Road North Carriageway - Profile A-A
Reduced Level (m)
Ridley Road South Carriageway - Profile B-B
Reduced Level (m)
Based on the reading 12/04/2006
taken on 22 April.
Based on the reading
taken on 21 April.
location of the hole
Marina Close Centre Carriageway - Profile I-I
Reduced Level (m)
Based on the reading taken on 24
26 25 24 23 22 8 17
Change in crack width at Point A No. 16 Marina Close
Change in crack width at Point B No. 16 Marina Close
Change in Crack Width at Front of No. 128 Ridley Road