landslide impacts on the south coast … to the ‘treacherous nature’ of the illawarra area and...

Australian Geomechanics Vol 47 No 1 March 2012 79

LANDSLIDE IMPACTS ON THE SOUTH COAST RAILWAY DURING THE 1988-90 EL NINO EVENT

Peter C Stone GHD Geotechnics, Locked Bag 2727, St Leonards, NSW 1590 Australia

57-63 Herbert Street, Artarmon, NSW 2064

ABSTRACT Major landslides occurred along a 25 km length of the railway connecting Sydney and Wollongong, on the south coast of NSW, as a result of significant and long duration rainfall during the El Nino event of the late 1980s. The impacts commenced with the tragic event at Coledale in April 1988, involving a major embankment failure with two fatalities, and culminated in over 100 individual sites being activated along the route. These sites were mainly embankment failures, but also included rock cutting instability, which were identified and treated progressively under a risk priority and safety management system. The repairs, which cost in excess of $70M, were implemented over a number of years, and included an intensive track closure (track possession) of the South Coast Railway during January 1990. The management system was developed, in close association with SRA personnel, with an over-riding early-warning system linked directly to the railway control co-ordinator in Wollongong.

The paper presents an overview of the instability experienced on the South Coast Railway, and its remediation at the time, the links to antecedent rainfall, and the philosophy of the risk management system. A selection of case histories is given, with their associated monitoring, and a listing of back-analysed geotechnical parameters is provided for reference purposes.

The paper serves to document a presentation given to the Sydney Chapter of the Australian Geomechanics Society in June, 1991 and is intended to be factual as at that time.

1 INTRODUCTION The South Coast Railway (SCR) was constructed as a single track service to Wollongong and opened in 1888. It is reported that the line suffered significant landslide activity during construction and shortly after its completion.

This paper deals with the instability issues which impacted the SCR a hundred years later during the 1988-90 El Nino period. More specifically, the subject area is the northern Illawarra region of the railway generally between Helensburgh (chn 47 km) and Bulli (chn 72 km). This 25 km section of line traverses an escarpment region which is renowned for its adverse topographical, geological and meteorological conditions. These conditions, in association with underground mining influences, plagued the South Coast line for those past 100 years.

The presentation is generally confined to rotational and translational earth slide issues but it does provide an example of the rockfall instability issues in railway cuttings. The extent of the site covered by the South Coast Railway is shown on Figure 1, where most of the 100 problematic sites were active in 1988-1990.

1.1 REGIONAL GEOLOGY The regional geology is indicated on Figure 1 and within Table 1. Bowman (1972) and Adamson (1974) provided significant insights into the regional geology of this area, with particular reference to its association and interaction with land instability.

By way of summary:

The upper (northern) section of the route traverses the Hawkesbury Sandstone formation in the Helensburgh area, near the contact with the Bald Hill Claystone sequence. Landslide issues were associated with this claystone unit.

The alignment then drops through the entire Narrabeen Group until the Scarborough fault (which is 40m downthrown to the north), being a feature at track chainage 64.8 km (north of Wombarra Station). In particular, the route closely follows the Bulgo Sandstone/Stanwell Park Claystone contact over much of this length. Major landslide sites were associated with this latter stratigraphic unit.

To the south of Scarborough, the route of the SCR is underlain by the Illawarra Coal Measures. Here, the alignment is situated stratigraphically between the Balgownie and Wongawilli coal seams until the Thirroul area. The geotechnical challenges in the Coledale area were associated with the Wongawilli Coal Seam.

LANDSLIDE IMPACTS ON THE SOUTH COAST RAILWAY DURING THE 1988-90 EL NINO EVENT PETER C STONE


Figure 1: Site setting of the South Coast Railawy depicting 100 sites demonstrating instability in 1988-1990



Table 1: Geological sequence within the Coalcliff area (Adamson, 1974)

Note: Thickness figures are indicative values only.

Interested readers can also refer to Leventhal & Flentje (2012) for more detailed stratigraphic columns and should refer to the Illustrative Sections through the Illawarra Escarpment therein for representative sections depicting the geologic setting and indicative landslide susceptibility of the geomorphic setting near Stanwell Park and Austinmer, which encompasses the SCR route at these two Illustrative Sections.

1.2 BACKGROUND The South Coast Railway was built as a single track service to Wollongong and opened in 1888. It is reported that very heavy rains caused difficulties during construction and shortly after completion. An early report by Shellshear (1890) relates the early history and the remedial measures undertaken to correct landslide challenges. In particular, his report refers to the ‘treacherous nature’ of the Illawarra area and the 'considerable trouble' with landslides in the cuttings during construction. The report then refers to the 'great rain storm' of May 1889 when some 500mm of rain fell over three days causing severe problems. It is thought that a major landslide event at Clifton, which resulted in the partial loss of the small township, occurred about this time.

Shellshear (ibid) reported on remedial works undertaken for a landslide in the Coalcliff area at that time. This area has been identified as the southern end of Stanwell Park, on the current alignment of Lawrence Hargrave Drive near the location of the remnant brick bridge pier footings. The works included the use of ash fill (as a light-weight fill) and an extensive system of subsurface drainage using mining techniques of the times. [During the construction of one of these drainage drives, landslide displacement was reported to have been occurring at 100mm per day.] The principles of these drainage works are not unlike those employed in current practice (deep trench drains). Shellshear (ibid) concluded that the landslide issues that impacted the line could not have been foreseen by the construction engineers since the “nature of (the) country cannot be judged at sight by the best of men”.

Various route deviations were constructed in about 1920 to improve the grades and to affect a dual track service. These changes occurred in the Helensburgh to Stanwell Park areas, including construction of the Stanwell Park viaduct structure. Sections of the current Lawrence Hargrave Drive follow the old railway alignment at Stanwell Park and Coalcliff (refer Figure 1).

Instability continued to plague the railway, particularly during heavy rainfall periods. The records of District Engineer TJ Smith provide detailed schematic observations of the study route for the 1950-64 period. It is noted that both these

Geological Unit Approximate

elevation (m, AHD)

Thickness (m) Description

Hawkesbury Sandstone +100m Massive quartz sandstone with minor shale

beds. Newport

Formation RL 250m 30m Sandstone and interbedded shale.

Bald Hill Claystone RL 220m 15m Red-brown claystone, minor quartz lithic

sandstone. Bulgo

Sandstone RL 205m 120 Quartz lithic sandstone, minor shale and minor conglomerate.

Stanwell Park Claystone RL 85m 35m Red-brown and greenish claystone, minor

quartz lithic sandstone. Scarborough

Sandstone RL 50m 25m Quartz lithic sandstone, minor shale and minor conglomerate.

Wombarra Shale RL 25m 35m Grey shale with quartz lithic sandstone.

Tria

ssic

Nar

rabe

en G

roup

Coalcliff Sandstone RL -10m 10m Lithic quartz sandstone with minor shale.

Bulli Seam RL -3m 1 to 3m Coal.

Perm

ian

Illaw

arra

Coa

l M

easu

res

Illawarra Coal Measures

(below Bulli Seam)

Coal seams, carbonaceous shale, claystone, siltstone, sandstone, conglomerate.



(end) dates refer to wet years with severe rainfall events which affected the whole Illawarra region. Various remedial measures in the form of reconstructions, drainage systems and retaining walls were constructed in response to these difficulties, and were recorded by TJ Smith in his diaries.

Electrification of the railway was conducted in 1983/84 under a 'fast track' construction policy. To the author’s knowledge, geotechnical investigations for this work were generally limited to reports by McMahon Associates / SRA (1982), which listed possible problem locations, and to route reconnaissance notes. The electrification works involved widening of the alignment by both cutting and filling in certain areas, mainly for access purposes. There is minimal documentation available for the work-as-executed situation or for drainage and culvert details.

The Illawarra region is also an area of extensive coal mining activity over the years. Underground coal mining has occurred, both uphill and beneath the alignment and with various amounts of extraction. It is expected that these activities have initiated ground movements and caused changes to groundwater flows, thus exacerbating the difficulties of the geological setting in certain areas. It is understood that the Stanwell Park viaduct suffered severe crack damage in 1985, in all likelihood due to the phenomenon of valley closure associated with underground mining activities.

On the 30 April 1988, an embankment failure and debris (mudflow of ash) occurred at Coledale (chn 66.l km) leading to the death of two people. The tragedy coincided with a heavy rainfall event of 350 mm/day and the existence of a blocked drop structure / culvert. The rainfall for the month was about 700 mm. This same rainfall period caused problems elsewhere on the line and to surrounding roads and services. In particular, the RTA experienced a number of landslides in the Coalcliff to Scarborough area. These slides are indicated on the route drawing, Figure 1, adjacent to the Scarborough Fault. Many of the RTA slides were 'stabilised' using subsurface drainage measures.

Following the Coledale tragedy, and subsequent coronial enquiry, the SRA commissioned Longmac Associates to conduct an Engineering Study of the line in 1988. This work included: walk-over mapping surveys, collation of internal/external reports and track performance records, air photo interpretation and discussions with pertinent SRA personnel. The baseline survey (1984) of the overhead wire structures (OHWS) footings, and subsequent re-surveys, provided an important monitoring record for the study route. A risk assessment process was developed at this time, with Mr. D Christie (SRA Geotechnical Manager), and the work was documented in a report (Longmac, 1989) of which Figure 1 was part. A working set of larger scale drawings was also produced for the route containing the collated and assessed information, including the important culvert locations and characteristics.

Figure 1, and the associated site listing, was updated as the rainfall and instability events continued into 1989 and 1990.

In particular, the rainfall in the first half of 1989 has been rated as a near record for an extended event. The short term rainfall intensity was not excessive but the continuity caused an unprecedented number of geotechnical difficulties. Rockfalls and landslides occurred in over 50 locations along the line with a number of areas with major issues. The investigation and design of remedial works for these sites was an ongoing task. It is noted that a number of landslides also occurred in the adjacent Lawrence Hargrave Drive over the same period, which forced closure of that roadway.

2 GEOTECHNICAL ISSUES

2.1 IMPACT OF RAINFALL Rainfall is an obvious trigger to any landslide activity and the correlation of rainfall with such instability for any site, let alone geographic area, is difficult - due mainly to the lack of adequate objective data. Rainfall is also an indirect parameter given it is the recharge to groundwater within a susceptible landslide area, and the site specific generation of pore water pressure, which leads to the instability.

The problematic Illawarra area has been the subject of various studies into the relationship of rainfall to instability over the years. The landmark work of Bowman (1972) provided a collation of instability zones (and specific sites) superimposed over the background geology and topographic maps, albeit at a relatively small scale. However, Bowman also reviewed rainfall impacts. He found no correlation with daily rainfall events, but cited monthly rainfall as a better criterion. A threshold cumulative rainfall of 350mm per month was suggested by Bowman.

Young (1978) also conducted studies of rainfall impacts, which confirmed Bowman's work in terms of a monthly period. However, Young postulated 250mm as the threshold monthly rainfall to trigger instability.

A limited study of the rainfall impacts was also undertaken as part of this work for SRA on the subject instability events of 1988-90. These events generally relate to large scale, deep-seated, landslide behaviour rather than shallow, surficial landslides, earth flows and debris flows.

Long term rainfall records available for the area include the monthly data for Coledale Station (1943-1982), Woonona (1930-1990) and Mt Keira (1944-1989), being stations that were adopted to cover the likely range of rainfall over the



Illawarra escarpment region. In particular, the Coledale/Woonona and Mt Keira results could be expected to represent the lower and upper escarpment areas respectively over a 61-year period.

An analysis of the maximum monthly data for Coledale Station and Woonona for the common monitoring (45 year) period, and on an annual series basis, indicated little variation between the two stations. Conversely, an analysis of the Coledale Station and Mt Keira (monthly) data exhibits a considerable variation of results, as shown on Figure 2, with the rainfall of the lower escarpment about 75% of the upper area. This demonstrates the difficulty of assigning a suitable threshold for a large geographic area.

Figure 3 presents both the monthly and an extended 3-monthly rainfall for the Woonona/Coledale (lower escarpment) area, which covers records over the 1930 – 1990 period. It is apparent from this data that the large scale landslide activity has a poor correlation with monthly rainfall, given that the highly problematic periods of 1989 and 1990 rank only 26th and 9th respectively on this plot. This conflicts with the work of Bowman and Young, albeit noting that the size of the landsliding was not specified in their work. It is postulated that smaller volume, shallow colluvium landslides most likely correlate better with the short term (monthly) events.

However, in terms of extended rainfall, the 3-monthly events provide a far better correlation with the known major landslide periods (refer to annotated years on Figure 3). The subject study years (1988-90) also have all their 3 month rainfall events in top 11 of the 60 years of historical records. It is apparent that the threshold 3-monthly rainfall is about 850 mm, which corresponds to a 6 – 7 year return period, for repeated activation of major landslide events.

It is also noted that the Engineering Study of the line, that followed the 1988 rainfall, led to a listing of 35 problem sites to be investigated and/or remediated. This listing had increased to some 100 sites by the end of the 3 year sequence of rainfall.

The probability for such a 3-year sequence is theoretically greater than 1 in 1000 years, assuming independence of the records. However, it is apparent that El Nino events do tend to extend over consecutive years thus negating to some extent the statistical relevance of the independence assumption.

Figure 2: Monthly rainfall records between upper Figure 3: Rainfall records on a monthly and and lower escarpment. 3-monthly basis.

2.2 GEOTECHNICAL RISK ASSESSMENT As mentioned above, a risk assessment process was developed, in close consultation with SRA, as part of our Engineering Study of the line in 1988/9. The process involved a relatively simple 5-category risk rating, as outlined in Table 2.

1974

1976 1989 (4 off)



The purpose of this process was to rank the sites in terms of risk so that appropriate safety management systems (refer Section 2.3) could be implemented while investigation and/or remedial works are undertaken. The assessment process was subjective and qualitative, and attempted to consider such factors as site features, geological setting, known site history, culvert adequacy and potential mode of failure, under adverse weather conditions. It is emphasized that the risk category was not changed with varying weather conditions in spite of obvious improvements to risk with drier weather. This approach allows the risk category to act as a ‘flag' for the potential problems at the site.

The risk assessment generally related to geotechnical situations but it did consider the presence of culvert facilities, where appropriate. It is noted that local, but potentially severe problems due to inadequate culverts (scour, flood flows), were a separate issue which received individual attention.

The assessment procedure adopted for the study referred to the risk and consequences of an 'event' (hazard) affecting the track and/or public safety. It did not relate to the risk or probability of a certain event actually occurring beyond the track.

The hazard referred to geotechnical issues such as rockfalls, landslides above/below the track and embankment failures. The mode of failure could vary from a fast, dramatic collapse (rockfalls and high, steep embankments) to a slow, self-stabilizing, slip-stick type of slip. The consequences of the former mechanism are obvious and involve the issues of safety, damage and disruptions to traffic/service. In particular, the Coledale-type debrisflow is considered to be a rare but dramatic failure which is difficult to predict in advance. It is clear that major culvert inadequacies are essential to activate this type of embankment collapse.

It is reiterated that the overall assessment concerned the perceived impact to the track [damage and disruptions] and/or public safety. It was thus possible to have an active landslip feature above the track with a 'low' [Category 4] threat to traffic due to a significant buffer zone. Similarly, a high probability rockfall situation may also be classed as a low risk to the track in such circumstances.

Table 2: Risk Categories

Category Risk Description

1 V High Track(s) closed due to geotechnical 'event' or very high risk of closure.

Remedial action required prior to re-opening track to normal traffic

2 High High risk to traffic operation/ safety and potential closure of track.

Mode of failure is a potential 'collapse' or rapid event, with minimal buffer zone to track

Early warning system (EWS) surveillance and/or speed restrictions required to maintain (short-term) track operations

3 Medium Medium risk to track/public safety

Perceived mode of failure is slow, such as (monitored) self-stabilizing slip.

Marginal buffer zone exists to (high risk) rockfall situation.

No history of problems in spite of perceived poor conditions (i.e. to be monitored).

Appropriate surveillance programme required to maintain track operation.

4 Low Substantial buffer zone exists for (high risk) cuttings or to small, side-fill experiencing movements.

Regular monitoring and inspections required prior to long-term remedial action.

5 Very Low Recent remedial works being monitored for effectiveness

The general SRA approach during these difficult and challenging times was to undertake urgently remedial works on Category 2 (high risk) sites, while accepting Category 3 sites in the short term, subject to appropriate safety action. The Category 4 site was adopted as the tolerable limit, with remedial works set for the longer term with on-going safety management.

It is noted that the risk assessment methodology used for this project has been updated significantly over the years by the RSA/RailCorp, and is thus reproduced herein in general terms only, for documentation purposes. The method was the forerunner of the more common matrix systems (likelihood verses consequence) developed by/for Councils, RTA and AGS.



2.3 SAFETY MANAGEMENT SYSTEM As mentioned above, the operations on the South Coast Railway were maintained during the extended wet period (1989-90) in spite of the large number of active problem sites affecting the track. This action was taken in association with the safety management system developed at the time. This system refers to such measures as track closures, speed restrictions, rail movement monitoring and various levels of surveillance. A progress reporting system of the major sites was utilised at this difficult time, which incorporated the safety management system under working conditions.

It is noted that the measures adopted for the safety system have been commonly used by the SRA, in part, for past problems on the line. The subject development acted to formalise the system and to apply it to potential problem areas, using the risk assessment ranking procedure.

The need and extent of the safety management system is a subjective decision made jointly by SRA operations staff and geotechnical personnel, with due consideration to track operations (SRA) and geotechnical matters. The procedure does carry some uncertainty but the intention is to manage the risk to an acceptable low level.

The safety management system must also be flexible in order to reflect changing conditions. In particular, the levels of surveillance and speed restrictions were relaxed during dry conditions, but upgraded with increasing rainfall.

The purpose of the safety management system was to provide for public safety while maintaining services through active problem areas. An important advantage of the system was that it provided the necessary lead time for investigations and/or monitoring in order to design appropriate remedial measures for the specific site. It thus facilitated remedial works targeted at the cause of the problem rather than its effect.

It is implicit that the risk to safety was at least low (Category 4 risk) while the safety system was in place. Conversely, the risk or probability of an event is not changed (nor the consequences of damage and/or service disruption). The fixing of the category thus acts as the flag for the future needs of the site.

An early warning system (EWS) of movement monitors for landslide hazards was also developed during these times, as outlined below. These extensometer monitors provided continuous (automatic) surveillance of the selected sites on a fail-safe basis – in the event of displacement >35 mm, or loss of readings, trains were stopped within and from entering the rail section. The level of intensive manual surveillance of the line was thus able to be safely reduced.

A supplementary programme of automatic pluviometer stations was also implemented for the Illawarra line to record rainfall data. It was expected that these installations could provide an EWS warning of possible culvert overtopping issues and rockfalls. A set of lower-bound parameters was chosen for use in decision-making (consistent with a target 20% improvement in Factor of Safety), and the parameters were adjusted as experience developed over time.

It is noted that this risk management action was dependent on strict adherence to the safety system, track maintenance and, to some extent, on experience and judgment of personnel. In particular, decisions had to be made when to stop/start train traffic after the detection of (some) movement. This situation required close on-going liaison between operations and geotechnical staff. This communication had to be maintained over time in spite of staff changes and SRA policy (funding) variations. On balance, it was considered preferable that risk management should generally not be adopted as a long term remedial option of major infrastructure.

2.4 INVESTIGATION AND MONITORING

2.4.1 Methodology The investigations of any particular site generally involved a gambit of conventional techniques ranging from desktop reviews of site history through to drilling and monitoring of movements and groundwater. In brief, the following methods were employed:

• Review of site history (100 years), maintenance records; • Survey of OHWS footings (since 1983); • Vector peg survey (extent, direction of sliding); • Boreholes/test pits; • Inclinometers; • EWS monitors; • Piezometer standpipes; • Simple 'bucket-type' piezometers; • Analysis - generally the Sarma (1987) method was used and • Lab testing - minor, rely mainly on back-analysis for material shear strength assessment.



As mentioned previously, the availability of SRA records, such as the diaries of DE TJ Smith (1964), and past investigation reports (McMahon, 1982) provided valuable insights into past (and repeated) major problem areas. Moreover, the presence of the OHWS footings (with associated survey) provided 3D monitor virtually every 30m along the 25 km of railway track. The line was also walked for its entire length with observations mapped and risk ratings assigned, as part of our 1988 Engineering Study.

Drilling was used on selected sites to prove the provisionally assumed landslide model. In particular, the work was not confined to the rail corridor, due to the often large scale nature of the landslides – the largest failure was Site 15 at Coalcliff with a planar area some 600 m length by 150m wide, and 15m deep. This major site was the subject of a previous paper (Leventhal et al., 2000) which has not been included herein.

Table 3: Material Type Codes

South Coast Railway Material Type Codes Soil Type Origin Sub-group Code Prefix

Colluvium Ft Ripped sandstone Frs

Ash / coke Fa Fill

Slag

Fsg Hawkesbury Th Narrabeen Tn Colluvium derived from various rock

units Coal Measures Tm

Hawkesbury Sandstone H Garie Formation Ng

Bald Hill Claystone Nbh Bulgo Sandstone Nb

Stanwell Park Claystone Nsp Scarborough Sandstone Ns Wombarra Claystone Nw

Residual soil and weathered rock

Narrabeen

Coalcliff Sandstone Nc Illawarra Coal Measures M

Code suffix (where appropriate) Sandstone 1 Siltstone 2 Shale 3 Claystone / clay 4 Coal 5



Figure 4: Soil Classification Results for all samples (LHS) and claystone samples(RHS)

Figure 5: Laboratory Peak Strength Results (peak φ′ versus PI)



Samples were extracted for laboratory testing and inclinometers installed to measure the depth (and mode) of sliding. Standpipes, with bucket-type capsules, were installed to record rise in ground water between site visits. This approach, developed in Hong Kong, provided a rudimentary yet adequate means of recording such information, in lieu of the more complex electronic or pneumatic piezometers installations. [Note that at the time of this work, downhole water level data loggers were not commercially available.]

A significant reliance was placed on test pit and/or slot drainage mapping to gain appreciation of the soil profile. The exposure record for Metropolitan (Site 43), as outlined in case histories (Figure 11), demonstrates the typical localized nature of the slide surface, which could often be missed during conventional drilling and discontinuous sampling.

The vector peg approach, using star pickets and 3D survey, provided important data in defining the geotechnical model. The following case histories illustrate this point, with particular reference again to Site 43. In this case the combination of vector pegs, inclinometer, OHWS and mapping provide a definitive geometric model.

The hydrogeology of any site is critically important and measurements of ground water levels against movement monitoring often allowed threshold groundwater models to be developed (or inferred). One such major site at Clifton (Site 19) exhibited a very definitive water level threshold where movement stopped/started within only a few hundred millimeters. This situation was in spite of the depth of sliding at about 20m on a very flat basal plane in Stanwell Park Claystone.

A collection of laboratory test results is presented on Figure 4, with the nomenclature used for defining geological units as outlined on Table 3. Figure 5 provides a plot of the (peak) strength parameters against classification data and stratigraphic units. However, it is appreciated that the residual strength mainly governed, given the repeated nature and large strain conditions of the major landslides. A limited programme of such testing was conducted but the back-analysis results were considered more reliable (refer Table 4).

2.4.2 Early Warning System The various forms of monitoring employed for this project generally served the dual purpose of investigation (to define the geotechnical model) followed by safety management, particularly during the extended wet periods experienced during the work. However, in dry conditions a temporary stabilisation of most embankment sites was observed. Under these circumstances, the monitoring activities were often relaxed together with the surveillance requirements. In particular, the EWS monitors (refer Figure 6) was employed as the primary monitoring device, with most other labour intensive (survey and inclinometer) monitoring put on hold and/or used on a long term (bi-annual) reading basis.

Figure 6: View of Early Warning System (EWS) without electrical connections, at site near Stanwell Park Station (site SC6), 6 August 1990



Figure 7: Illustration of displacement results from monitoring of EWS (Site 61, Chn 61.200).

The EWS monitor was conceived and the prototype developed by the author, in association with Geotechnical Systems Australia and SRA personnel. As part of this project, the SRA developed the system, particularly in relation to complex logistics and polices of incorporating it into the signaling system of the South Coast Railway. SRA subsequently achieved a “highly commended” award for Engineering Products from Engineers Australia.

The EWS monitor proved to be a most versatile instrument, acting as both a continuous (automatic) early warning installation and an accurate movement monitor, in association with manual surveillance. The EWS is a fail-safe device which activated the track signals to stop all trains in the rail section in the event of activation, and also in the event of loss of monitoring data. The early warning function was tested successfully in situ at Wombarra Station where major scour damage was caused to the line during the record rainfall of June 1991.

In terms of movement monitoring by the EWS, an accuracy of about ±lmm was recorded by track patrol personnel on a regular basis, under (minimum) surveillance conditions. This result provided invaluable information on the response of the various sites to specific rainfall events. A typical result is shown on Figure 7 for Stanwell Park Station (Site 61) at chn 56.200 km. This plot shows the results of vector pegs, inclinometers and the EWS monitors in relation to rainfall. Clearly it is only the EWS data that facilitates any reliable relationship to be developed against rainfall – this site appears to respond rapidly to a short term, high intensive event.

2.4.3 Analysis Once the geological model was defined, the back-analysis was generally carried out using the technique of Sarma (1987), which is ideal for this predetermined model situation. A typical Sarma summary plot for Metropolitan Crossover (Site 50) is given in the following case histories (Figure 15). Various options of treatment were examined, which generally related to the controlled reduction of groundwater, which was identified as the main cause of distress.

A collection of selected back-analysis results, with location, landslide size/geometry, mode of distress, and basal geology is presented in Table 4. This table also indicates the range of remedial actions undertaken, albeit that control/reduction of groundwater was often the main approach.

Table 5 provides a summary of strength parameters from this analyses, with due regard to the repeated failure, large strain, residual shear strength, nature of the failure that was frequently the situation.

It is noted that the target Factor of Safety for any remedial action was adopted as a 20% relative improvement on the failure situation, particularly when the failure model was well defined. The relatively low value was also considered



reasonable given the lower bound residual strength nature of the failures (together with acknowledgment of the basic definition of Factor of Safety in terms of reduction of shear strength to instigation limiting equilibrium), the need to spread the repair funding over many sites and that monitoring was continued on all repair sites to verify the measures taken.

It was also appreciated that the main objective of the (drainage) remedial work was to manage the instability so that rapid movements were avoided or totally negated. Conversely, ongoing creep-type movements could not be totally eliminated during adverse rainfall conditions, due to the discrete (slot) nature of the drainage systems.

These investigation activities are briefly discussed in association with the geotechnical drawings and photographs for the various selected sites, in the following Case Histories.

Table 4: Back-analysis results

3 CASE HISTORIES The selected case histories below provide an insight into the challenges faced by RSA during the turbulent El Nino period of 1988–90. The content is brief, by necessity for this paper, but the modes of distress and the choice of remedial action is outlined.

3.1 HELENSBURGH (SITE 2) CHN 48.250 KM The site is a 40m high side-fill embankment located between the Helensburgh and Metropolitan tunnels – refer Figure 1. The embankment was the scene of a 'burning ash' problem (1968) and previous instability (from at least 1964). It is noted that ash was often used in the early steam locomotive era to in-fill depressions/scours and/or landslide displacement, and thus its presence can be indicative of repeated problems. A significant increase in movement



occurred in April 1989 during the heavy rains at that time, resulting in tension cracking along the crest (Figure 8) with movement and dislocation extending under both tracks.

The landslide model was determined as a large-scale inclined planar feature extending over 80m in width along the track, which was underlain by the Bald Hill Claystone. The influence of groundwater was considered to be mainly via a softening influence along the inclined slide-contact.

A major buttress structure was selected for the remedial works, with gabion wall toe support, which in-filled the small valley at the bottom of the embankment (refer Figure 8). The buttress was constructed during the 1990 close-down possession using slag materials in lieu of the planned coal waste from the nearby Metropolitan Colliery.

The remedial works have performed well to date with no significant movement recorded from the installed instrumentation. An EWS was installed as a fall-back measure (for safety) until the performance was validated

Figure 8: View north of primary slip scarp beside OHWS 48. 259 km (18 May 89), and embankment remediation

works looking (UP) northward (26 Jan 1990)

3.2 METROPOLITAN (SITE 43) CHN 48.750 KM This site is again located on a side-fill embankment, within a gully feature, between Metropolitan Tunnel and the colliery siding (refer Figure 9). A major landslide movement occurred on 1 May 1989 which caused closure of the DN track. The total movement recorded on an affected OHWS was some 700mm up to mid-July 1989 (onset of dry weather).

The landslide was a rotational/flat planar feature (active-passive wedge model) extending 19 m deep below the track (and 3 m below a lower access track) and along the Bald Hill Claystone/Bulgo sandstone interface (refer Figure 10). As mentioned previously, the geometry of this failure is very well defined by the inclinometers and vector peg measurements. Moreover, the nature of the localized slide plane gouge, from mapping of the excavation exposure, illustrates the importance of this type of verification process and the potential limitations of conventional borehole sampling (refer Figure 11).

This event was also thought to be a 'first-time' failure (given the lack of historic maintenance), which was triggered by a temporarily high groundwater level due to failure of the upper drainage diversion system.

The remedial work involved an UP side cut-off drain and major slot drainage extended under the tracks. The toe was supported by a supplementary reinforced earth buttress (gabion and mesh) keyed into the underlying bedrock (refer Figure 12). The ongoing discharge from the drainage outlet pipe illustrates the success of the works.



The work was carried out during the 1990 close-down.

Figure 9: Plan view of failure with investigation layout

Figure 10: Idealised inferred section at Site 43 – (see also Leventhal & Flentje (2012) Figure IIS-1A)



Figure 11: Tension crack on DN-side, Metropolitan (SC43), 3 May 1989 (left) and exposure of failure surface during

remediation construction 5 Jan 1990 (right).

Figure 12:

Construction of gabion wall (left) and dry weather drainage pipe outflow (right) as seen on 8 Jan 1990 (Site SC43)



Figure 13: View eastward over completed works (15 May 1990) at Site SC43.

3.3 METROPOLITAN CROSS-OVER (SITE 50) CHN 48.900 KM This site is located 150m south (DN track) from Site 43 and in a similar topographic and geological setting. A significant slide failure occurred on 20 April 1990 causing subsidence (dip) of both tracks – refer Figure 16. The site had been subject to past instability and track alignment issues, which were mainly associated with local instability of the over-steep batter. The 1990 movements had in fact ripped though these earlier remedial works.

The geometry of the slide was inferred by the investigation (refer Figure 14) but no movement had been detected since installation of the inclinometers (by mid 1990), which coincided with a dry period. It is thought likely that the failure was triggered by temporary rises in groundwater and that some (temporary) self-drainage had occurred since initial displacement, due to significant distortions of the side-fill. A typical Sarma analysis of the site is given on Figure 15, for illustrative purposes. This approach allows the relative Factor of Safety improvement to be determined, for changes in drainage and/or support conditions. This approach served as a useful tool in evaluating and discussing various options for treatment.

Figure 14: Inferred Geotechnical Section for Site 50



A slot drainage system was implemented for the remedial works, and was subject to strict environmental controls. A risk management approach was employed (with EWS) prior to implementation of the remedial works.

Figure 15: Typical Instability analysis summary (Site 50)

Figure 16: Site 50, looking north (UP) - note deformed tracks (on 2 April 1990) (left) and scarp of failure surface on lower access road (right) on 9 May 1990.



3.4 BALD HILL CUTTING (SITE 33) CHN 54.950 KM This site is situated on the UP-side approach to Bald Hill tunnel (refer Figure 17). The cutting is approximately 20 m high and is composed of (blocky) Bulgo Sandstone. A significant rockfall occurred during the wet summer of 1989.

The site was assessed as Risk Category 2 and a speed restriction imposed due to the poor sight distances and proximity to the tunnel portal..

A programme of de-vegetation, scaling and limited bolting was undertaken in March 1989. This work was extended during the 1990 close-down to include 4 m rock bolting with shotcrete/mesh, as shown on Figure 17. A catch fence was also installed above the remedial works and an associated EWS was installed.

Figure 17: Bald Hill Cutting (SC33) following initial scaling operations on 1 March 1989 (left) and view of mesh and

rockbolt installation 17 January 1990 (right) prior to coverage with shotcrete.

3.5 STANWELL PARK (SITE 35) 56.300 KM This site includes a large side-fill embankment with a history of instability issues documented from 1950 (by DE TJ Smith). Re-constructions of the embankment were undertaken in 1987 and 1988, but displacements were again experienced in April 1989 that resulted in major subsidence of the access road (refer Figures 20 and 21) and severe distortions of the slag embankments (Figure 22).

The investigations, particularly targeted in the lower valley, demonstrated that the instability involved a major landslide which extended over the whole lower valley down to Stanwell Creek itself (refer Figure 18). The slide was an inclined/flat planar mechanism with the lower slide plane some 50m below track level at the colluvium/Stanwell Park Claystone contact (Figure 19). The “land crack” was evident on a private access road below the rail corridor, and where an inclinometer was sheared at about 30 m depth. The movements were triggered by extended rainfall periods.

It was accepted that the railway was in fact riding on the top end of the large landslide and it was impractical to attempt to control the entire landslide footprint. The case also illustrated the need to define the failure model, rather than remediate its effect (as previously done by reconstruction of the side-fills). It was noted that the access road beside the track services the DN track viaduct, and thus was needed on a regular basis.

A discrete anchored pier wall solution was adopted to isolate the track from the adjoining landslide. It was accepted that the access road could be temporarily lost during major events. The pier wall remediation work was mainly carried out during the 1990 close-down with the anchor system installed the following year.

There is another major landslide site, known as Seabank (Site 14), situated directly opposite Site 35 on the other side of Stanwell Creek. This similar historic slide was a primary subject of a Master’s thesis (Pitsis, 1992) and will not be repeated herein, other than as summarized in the back-analysis results (refer Table 3). However, the presence of this slide, again failing upon Stanwell Park Claystone at significant depth, indicates the problematic nature of this geological unit. [In subsequent geological mapping and landslide inventory work, Flentje (pers. comm, 2012) has noted across the Wollongong City Council Local Government Area that the Stanwell Park Claystone and the Wombarra Claystone have 10% and 9% respectively of their subcrop affected by landslides.]



Figure 18: Site plan for Site 35 (top) and annotated with displacement vectors (bottom)



Figure 19: Inferred geotechnical cross-section for Site 35

Figure 20: View south (DN) of well-defined headscarp feature on access track (27 April 1989) – Site 35.



Figure 21:

View north (left) of land crack with slag embankment in background (amongst the trees) (19 May 1989) with close-up view of crack scarp (above).

Figure 22: View east of large-scale tension cracking/shearing in embankment below the track.



Figure 23: View west over pier wall and subsided access road (10 August 1990), prior to installation of anchors.

3.6 CLIFTON CRACK (SITE 19) 61.600 KM This site consists of a relatively small embankment over moderately sloping terrain that falls toward Lawrence Hargrave Drive (refer Figure 24). Movements of the OHWS footings were detected over a 5-year period with some track maintenance undertaken. There was a history of movement at the adjoining level crossing which was thought to be associated with the Clifton Fault which passes near this region. This landslide, adjacent to and potentially related to the fault, was activated locally through high pore pressures in 1988, causing disruption to the road (refer Figure 27).

The apparent extent of the landslide (Figure 24) was first detected via routine surveillance, which noted tension cracking in an RTA stockpile area situated well above the site. Subsurface investigations were then instigated. The landslide geometry was established by inclinometer and vector pegs as a flat planar feature some 20 m below track level (Figure 25). Sliding occurred, in a retrogressive fashion, at the colluvium/Stanwell Park Claystone contact, triggered by well-defined high pore water pressures. As mentioned previously, a threshold piezometric head could be established in this case where movements only occurred above this level.

An adjacent landslide experienced by the RTA (now RMS) downhill is unrelated to the SRA landslide, apart from the proximity to the fault and the likely consequential concentration of groundwater in this area. This downhill slide is an inclined planar feature underlain by Scarborough Sandstone bedrock.

Various options were proposed for the SRA landslide, with risk management adopted in the short/medium term due to the slide’s perceived slow mode of deformation (and thus Category 3 risk). A borehole EWS approach was adopted for this management purpose.

The site was subsequently remediated in 1995/6, using an innovative large diameter well system interconnected by a basal gravity feed to a central outlet shaft. The interconnected pipes and outlet pipe to the lower road culvert were constructed by directional drilling techniques from the deep shaft structure. The design work for this scheme included 3D modeling of the site together with in situ pump testing, with observation holes, to derive appropriate groundwater parameters.



Figure 24: Site plan (Site 19, Clifton crack) with lateral movements annotated.

Figure 25: Inferred geotechnical model for analysis.



Figure 26: View north over single track at Clifton (Site 19 – 30 Nov 1988). Site appearance does not demonstrate major landslide feature failing at 20 m depth.

Figure 27: Separate features with landslide affecting Lawrence Hargrave Drive left) on 26 April 1989 with land crack uphill from this event (right) on 26 May 1988. Both features are immediately below Site 19.

.Figure 28: View downslope of independent landslide below Lawrence Hargrave Drive on 26 May 198, demonstrating problematic nature of this area.



3.7 CLIFTON TOPS (SITE 22) 62.300 KM This site is situated on the western side of the rail line between chn 62.160 and 62.300 km, UP track from Scarborough Station (refer Figure 29). The geomorphology includes a geologically controlled terrace with a mantle of colluvium. The resultant landform consists of a benched area of colluvium overlying a high cutting of Scarborough Sandstone. The benched area has been disturbed by (full extraction) underground mining resulting in ponded water and local (edge) instability of the colluvium. This instability occurred often as debris flows into the lower UP cess drainage and onto the track during periods of heavy rainfall, particularly in 1990 (Figure 31 and Figure 32).

The investigations included: review of available information, survey and mapping of the landform, vector peg monitoring and selected drilling to establish the bedrock profile and thereby confirm the landslide mechanics.

This portion of the rail line is the original alignment built in 1889 as a single track. It was changed to double track in 1920 by excavations into the upside cutting. Further excavation work was done for electrification in 1983/84.

The main technical challenge at the site, apart from difficult access, was the localised instability along the high cliff edge, triggered by perched groundwater within the colluvium. A remnant layer of Stanwell Park Claystone exists between the colluvium and Scarborough Sandstone, thus promoting ponding (refer Figure 30)

A staged method of remedial action was undertaken, which involved local re-grading and slot drainage in the edge area, combined with surface drainage/lining within the benched area (refer Figure 29).

Figure 29: Site plan for Site 22



Figure 30: Inferred geotechnical section (Site 22)

Figure 31: Clifton (SC 22) 9 April 1990. View north (UP) over cutting with debris flow "mud runs” down the face of the cut above the track.



3.8 COLEDALE (SITE 27) 65.900 KM This is a 5 m high side-fill embankment situated above Squires Crescent, Coledale (refer Figure 33). The site had a history of movements dating back to the early 1930's. In addition, movement was detected in April 1988 resulting in a detailed investigation by the SRA. The site is some 200 m UP track from the site of the tragic embankment failure at Chn 66.100 km.

The slide geometry is an inclined (flow) planar feature which involved the UP track and extended almost to the roadway below (refer Figure 37). The embankment consisted of ash fill, thus indicative of past restoration works, and cumulative movements of up to 3 m. A high groundwater condition existed in the toe with perched water recorded entrapped within the ash.

A slot drainage approach was adopted for the remedial works with berms and an UP side cut-off drain. The disturbed nature of the subgrade was observed during slot drain excavations (refer Figure 36). The works were constructed in May 1989 (Figure 35) with some buckling distress caused to the lower roadway (refer Figure 37). This buckling appeared to relate to stockpiling during excavation, which was quickly removed but again demonstrates the sensitive nature of the area.

The completed works performed well during the subsequent record extended rainfall event of 1990, which would have been expected to trigger failure without remediation. Post-remediation, the northern sub-surface slot drain exhibited constant flow even in dry weather (refer Figure 38).

Figure 32: Clifton (SC 22) 9 April 1990 show tension cracking above the cutting.



Figure 33: Site plan for Site 27 depicting schematic locations of piezometric and vector peg monitoring.

Figure 34: Inferred geotechnical cross-section for Site 27



Figure 35: Coledale (SC 27) 9 May 1989 Slag backfill into major slot drain beneath tracks

Figure 36: Coledale (SC 27) 10 May 1989 Trench box support for slot drains construction.

.



Figure 37: Coledale (SC 27) 11 May 1989 View from below the track showing the extent of the works and the buckling of the gutter in Squires Crescent that occurred during the remedial works

Figure 38: Coledale (SC 27) 12 January 1990. Continuous discharge from northern drain notwithstanding dry weather

4 CONCLUSIONS 4.1 LANDSLIDES

• The sizes of landslides impacting the railway were often 100m wide (along track), but even up to 600 m and up to 15-20 m in depth – there was a need to think 'big' in terms of possible extent.

• Mode of Failure – the majority of instability was in or upon claystone units, often with flat planar failure surfaces,

• No 'run-away' situations were experienced, apart from the tragic Coledale (ash) event. The measured rates of movement were variable for the sites studied but up to 300 mm per event.



• Basal shear strength properties were related to the geology and direction of sliding often associated with white clay gouge material (10-20 mm thick). It is noted that laboratory testing can be misleading unless the localised gouge material is sampled.

• Colluvial material generally has higher strength, due to its mixed nature and geological source, while low residual shear strength (particularly for the claystone units) conditions often governed instability considerations (whilst noting that failure surfaces could involve both materials).

• Ash was present in most SCR sites which was an indication of past landsliding and/or scour treatment.

• Typical back-analysed residual strength parameters from 17 major events are summarised in Table 5.

Table 5: Summary of back-calculated residual shear strength parameters.

Basal Geology Strata unit No of slides Typical φr Bald Hill Claystone Nbh4 3 25° - 30°

Bulgo Sandstone Nb2 1 25° Stanwell Park Claystone Nsp4 7 12°- 15° gouge

20° – 25° inclined Scarborough Sandstone Ns2 2 20° - 25° Wombarra Claystone Nw4 1 as per Nsp4 Coal measures clay M4 3 15°

[Where cr = 0] 4.2 TRIGGERS

• Rainfall/groundwater recharge is the primary trigger, which is case specific.

• Extended rainfall (3-monthly) appears to correlate with the repeated activation of deep-seated landslides. Shorter duration rainfall (monthly or shorter) would be considered applicable for small volume, shallow (predominantly colluvial) landslides.

• Groundwater parameter (ru) was back-figured to be generally 0.30 - 0.35 at the time of failure.

• 1988, 1989, 1990 (summer) 3-monthly rainfall total depths all rank in the top 11 events (of the previous 60 years). A 3-monthly rainfall threshold of about 850mm to trigger major landslide events is indicated.

• It is anticipated that re-activations of such events have now been largely negated by the capital works programme conducted by SRA (and RTA).

4.3 REMEDIAL WORKS • Attention to surface and sub-surface drainage was the key to solving most problematic sites on the South Coast

Railway.

• The approach was philosophy similar to that used successfully by Shellshear (1890), over 100 years ago.

• A relative Factor of Safety improvement of at least 20% was adopted as a balance between the geotechnical conditions (defined nature of the landslide, lower bound properties) and spreading of finite funding resources across the project.

• Recent advances in equipment technology allowed slot drains to some 15 m depths and large diameter wells, with gravity drainage installed via directional drilling equipment.

• Drains should extend to and below the slide plane to intercept recharge that occurs via fractured rock.

• The primary purpose of drainage remedial works was to control and/or negate the impact of accelerated movements upon the railway due to the rainfall events. In additional, it was accepted that a degree of creep-type movements during such times could not always be avoided, inter alia due to the discrete nature of the drainage systems.

• Other remedial solutions used for the SCR included: pier wall isolation, embankment buttress, reinforced soil buttress berms, and regrading.

• Risk management (employing an EWS) was a key element in the management of instability issues over the subject length of the South Coast Railway during the difficult and challenging times, and for verification of the works thereafter. Remedial works were adopted as long term solutions for this major element of rail infrastructure serving the Illawarra.

• The works completed during the 1989/90 close-down performed well during record 1990 rainfall events.



5 ACKNOWLEDGEMENT The author acknowledges the (then) State Rail Authority for permission to present this information in June 1991 to a technical meeting of the Sydney Chapter of the Australian Geomechanics Society, followed by presentation to SRA personnel at Coniston. The continued support of the SRA in the development of solutions to managing track safety of the line during this particularly challenging period, is acknowledged. In this regard, the author wishes to recognise the invaluable and on-going support during this major project provided by Messrs M Kerr, M Hickey and R Ford, with particular reference to SRA Geotechnical Services Manager, Mr David Christie.

The author recognises the contribution of many during the intensive investigation and geotechnical design period, and particularly the contribution of long-time colleague Laurie de Ambrosis as a “sounding board” for technical issues and Greg Kotze in the geological interpretation during the Engineering Study and, though not discussed herein, the management of rockfall issues within the railway cuttings.

The author wishes to thank colleague Andrew Leventhal for his perseverance in encouraging him to prepare this paper for publishing. His contribution in preparation of this paper is gratefully acknowledged, as without it the paper would not have materialised.

6 REFERENCES Adamson CL (1974), "Geological Report on the stability of the Western Gully Site", report prepared for Longworth &

McKenzie Pty Limited, June 1974. Bowman HN (1972) “Natural slope stability in the City of Greater Wollongong”, Records of the Geological Survey of

New South Wales, Volume 14, Part 2, 29/9. Leventhal AR , Stone PC and Christie HD (2000), “Landsliding of the South Coast Railway – The Coalcliff Slide”.

Proc. GeoEng2000, An International Conference on Geotechnical and Geological Engineering, Melbourne, 19 – 24 November 2000, Technomic.

Leventhal A & Flentje P (2012), “Illustrative sections depicting landslide susceptibility of the Illawarra escarpment”, Australian Geomechanics V47N2, March 2012 – a companion paper in this issue.

Longmac Associates (1988), “Engineering Study of South Coast Railway, Helensburgh to Thirroul Stations ,Stage 1” (Ref AGT5014) 15 Decemeber 1989

McMahon Associates & State Rail Authority NSW (1982), “Illawarra Electrification Project, Catalogue of Geotechnical Problem Sites, Seabank to Scarborough area”.

Pitsis SE (1992), “Slope Instability along the Illawarra Escarpment”, Master of Engineering Science thesis, University of New South Wales.

Sarma SK (1987), “A Note on the stability analysis of slopes”, Geotechnique Vol 37, pp 107-111 Shellshear W (1890), “On treatment of slips on the Illawarra railway at Stanwell Park”, J. Proc. Royal Soc. NSW, Vol

24 No 1, pp58-62. Smith, TJ (1964), “Illawarra Line Drainage Plans 1950-1964, Helensburgh to Thirroul”, (SRA records) Young ARM (1978), “The Influence of debris mantles and local climatic variations on slope stability near Wollongong,

Australia”, Catena, Braunschweig 1978 Vol 5, pp 95-107.

7 BIBLIOGRAPHY AGS (2000) “Landslide risk management concepts and guidelines”. Australian Geomechanics Society, Australian

Geomechanics, Vol 35 No 1, March 2000, reprinted in Vol 37 No 2, May 2002. AGS (2007a) “Guideline for landslide susceptibility, hazard and risk zoning for land use planning”. Australian

Geomechanics Society, Australian Geomechanics, Vol 42 No 1, March 2007. AGS (2007b) “Commentary on guideline for landslide susceptibility, hazard and risk zoning for land use planning”.

Australian Geomechanics Society, Australian Geomechanics, Vol 42 No 1, March 2007. AGS (2007c) “Practice note guidelines for landslide risk management”. Australian Geomechanics Society, Australian

Geomechanics, Vol 42 No 1, March 2007. AGS (2007d) “Commentary on practice note guidelines for landslide risk management”. Australian Geomechanics

Society, Australian Geomechanics, Vol 42 No 1, March 2007. AGS (2007e) “Australian GeoGuides for slope management and maintenance”. Australian Geomechanics Society,

Australian Geomechanics, Vol 42 No 1, March 2007.

landslide impacts on the south coast … to the ‘treacherous nature’ of the illawarra area and...

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