Highway Structures General Advice – Ridley Road Emergency Works

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					Highway Structures General Advice – Ridley Road Emergency Works
                                   Final Report on Site Investigations

        London Borough of Bromley Environmental Services Department
                                                   th
                                                 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


FIGURES

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


APPENDIX A
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

APPENDIX B
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                Introduction

                 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.
                                                                 o
                       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
                      shown shaded.
                 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.
Faber Maunsell   Highway Structures General Advice - Ridley Road Emergency Works                                           6




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
                           Anomaly Map.
                 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
                           discernable.
                 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.
Faber Maunsell   Highway Structures General Advice - Ridley Road Emergency Works                                           7




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
                       services.
                 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.
Faber Maunsell   Highway Structures General Advice - Ridley Road Emergency Works                                     8




                 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
                       the Chalk.
                 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
                                            A    B
                            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
Faber Maunsell   Highway Structures General Advice - Ridley Road Emergency Works                                   9




                 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.
Faber Maunsell   Highway Structures General Advice - Ridley Road Emergency Works                                    10




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
                       this.
                 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.
Faber Maunsell   Highway Structures General Advice - Ridley Road Emergency Works                                        11




5                Monitoring of Levels and Crack
                 Widths
                        Level Monitoring
                 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.
                                                                                                             th
                 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
                        expected.


                        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.
Faber Maunsell   Highway Structures General Advice - Ridley Road Emergency Works                              12




                 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.
Faber Maunsell   Highway Structures General Advice - Ridley Road Emergency Works                                       13




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
                       the Highway.
                 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).
                 Anomaly RG2
                 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
                       Zetica Ltd.
                 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.
                 Anomaly RG1
                 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.
                 General
                 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
                      below.
                 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
                      lorries.
                 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
                 No. 3
                 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
                       practicable.
                 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
                       7.
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               Conclusions

                 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.
FIGURES
APPENDIX A
                                                               Figure A1
                                               Ridley Road North Carriageway - Profile A-A

                    10.9


                    10.8


                    10.7


                    10.6
Reduced Level (m)




                    10.5                                                                                        12/04/2006
                                                                                                                21/04/2006
                                                                                                                21/06/2006
                    10.4                                                                                        06/09/2006


                    10.3


                    10.2


                    10.1


                     10
                           1



                               2



                                   3



                                       4



                                           5



                                                    6



                                                            7



                                                                    8



                                                                              9




                                                                                                     A



                                                                                                            B
                                                                                  10



                                                                                        11



                                                                                             12


                                                                                                  12



                                                                                                         12
                                                           Monitoring Point
                                                                                        Figure A2
                                                                        Ridley Road South Carriageway - Profile B-B

                    10.8




                    10.7




                    10.6
Reduced Level (m)




                                                                        Based on the reading                                               12/04/2006
                                                                        taken on 22 April.
                                                                                                                                           21/04/2006
                    10.5
                                                                                                                                           21/06/2006
                                                                                                                                           06/09/2006


                    10.4



                                Based on the reading
                                taken on 21 April.
                    10.3

                                                          Approximate
                                                       location of the hole

                    10.2




                                                                                                                                A




                                                                                                                                       B
                           13




                                      14




                                                       15




                                                                   16




                                                                                    17




                                                                                                   18




                                                                                                              19




                                                                                                                   20




                                                                                                                        21



                                                                                                                             21




                                                                                                                                    21
                                                                                               Monitoring Point
                                                      Figure A3
                                      Marina Close Centre Carriageway - Profile I-I

                     10.7




                    10.65




                     10.6
Reduced Level (m)




                    10.55
                                                                                                              12/04/2006
                                                                                                              21/04/2006
                     10.5                                                                                     21/06/2006
                                                                                                              06/09/2006


                    10.45




                     10.4
                                                                      Based on the reading taken on 24
                                                                      April.


                    10.35
                            26   25      24             23              22                         8     17
                                                  Monitoring Point
APPENDIX B
                                                     Distance (mm)
                         15
                            /0
                               4   /2
                                      00




       -0.100
                -0.050
                                             0.000
                                                          0.050
                                                                     0.100
                                                                             0.150
                                                                                     0.200
                         22              6
                            /0
                               4   /2
                                      00
                         29              6
                            /0
                               4/2
                                   00
                         06           6
                            /0
                               5/
                                 20
                         13         06
                            /0
                               5/
                                 20
                         20         06
                            /0
                               5/
                                 20
                         27         06
                            /0
                               5/
                                 20
                         03         06
                            /0
                               6/
                                 20
                         10         06
                            /0
                               6/
                                 20
                         17         06
                            /0
                               6/
                                 20
                         24         06
                            /0
                               6/
                                 20
                         01         06
                            /0
                                                                                                                  Figure A4




                               7/
                                 20




Date
                         08         06
                            /0
                               7/
                                 20
                         15         06
                            /0
                               7/
                                 20
                         22         06
                            /0
                               7/
                                 20
                         29         06
                            /0
                                                                                             Change in crack width at Point A No. 16 Marina Close




                               7/
                                 20
                         05         06
                            /0
                               8/
                                 20
                         12         06
                            /0
                               8/
                                 20
                         19         06
                            /0
                               8/
                                 20
                         26         06
                            /0
                               8/
                                 20
                         02         06
                            /0
                               9/
                                 20
                                    06
                                                      Distance (mm)
       15
          /0
             4   /2
                    00




                           0.000
                                   0.050
                                           0.100
                                                   0.150
                                                           0.200
                                                                   0.250
                                                                           0.300
                                                                                   0.350
                                                                                           0.400
       22              6
          /0
             4/2
                 00
       29           6
          /0
             4/
               20
       06         06
          /0
             5/
               20
       13         06
          /0
             5/
               20
       20         06
          /0
             5/
               20
       27         06
          /0
             5/
               20
       03         06
          /0
             6/
               20
       10         06
          /0
             6/
               20
       17         06
          /0
             6/
               20
       24         06
          /0
             6/
               20
       01         06
          /0
                                                                                                                        Figure A5




             7/
               20




Date
       08         06
          /0
             7/
               20
       15         06
          /0
             7/
               20
       22         06
          /0
             7/
               20
       29         06
          /0
                                                                                                   Change in crack width at Point B No. 16 Marina Close




             7/
               20
       05         06
          /0
             8/
               20
       12         06
          /0
             8/
               20
       19         06
          /0
             8/
               20
       26         06
          /0
             8/
               20
       02         06
          /0
             9/
               20
                  06
                                                   Distance (mm)
       15
          /0
             4   /2
                    00




                           0.000
                                   0.100
                                           0.200
                                                        0.300
                                                                   0.400
                                                                           0.500
                                                                                   0.600
       22              6
          /0
             4/2
                 00
       29           6
          /0
             4/
               20
       06         06
          /0
             5/
               20
       13         06
          /0
             5/
               20
       20         06
          /0
             5/
               20
       27         06
          /0
             5/
               20
       03         06
          /0
             6/
               20
       10         06
          /0
             6/
               20
       17         06
          /0
             6/
               20
       24         06
          /0
             6/
               20
       01         06
          /0
                                                                                                                Figure A6




             7/
               20




Date
       08         06
          /0
             7/
               20
       15         06
          /0
             7/
               20
       22         06
          /0
             7/
               20
       29         06
          /0
             7/
                                                                                           Change in Crack Width at Front of No. 128 Ridley Road




               20
       05         06
          /0
             8/
               20
       12         06
          /0
             8/
               20
       19         06
          /0
             8/
               20
       26         06
          /0
             8/
               20
       02         06
          /0
             9/
               20
                  06