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This is the published version of a paper presented at 18th Nordic Geotechnical Meeting, 18-19 January 2021, Helsinki, Finland.
Citation for the original published paper:
Beijer Lundberg, A., Alderlieste, E., Spross, J. (2021)
Risk management and risk acceptance regarding the stabilityof slopes: Case study In: , #67
N.B. When citing this work, cite the original published paper.
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Risk management and risk acceptance regarding the stability
of slopes: Case study
A B Lundberg1, E A Alderlieste2, J Spross3, 1 ELU Konsult AB, Box 27006, 10251 Stockholm
2 Deltares, PO Box 177, 2600 MH Delft, The Netherlands
3 Division of Soil and Rock Mechanics, KTH Royal Institute of Technology, 100 44 Stockholm, Sweden
Corresponding author: anders.beijer@elu.se
Abstract. Most earthworks result in an alteration of the stress state in the ground, either through the excavation of soil or the placement of surcharge on the ground surface. These stress changes have the potential to cause a slope collapse or failure, often with large consequences for the construction project or the immediate environment. Slope stability calculations are therefore frequently carried out for various load and excavation stages in the design phase to reduce the risk in the construction phase. Nevertheless, slope collapses sometimes occur, both small ones with minor consequences, as well as larger ones with significant consequences for society. In the current paper, a case history of a slope from Scandinavia is examined to assess how the risk of slope stability was managed and allocated among the various agents, and how the slide could develop. Was the risk acceptable for society on an aggregate level? The soil conditions and the construction process for the case studies are elaborated in detail with respect to the risk management principles in ISO 31000 and the recently published guidelines from the Swedish Geotechnical Society. The risk acceptance and the concept of the risk owner, i.e. the entity that owns the specific risk, is discussed for the case study, as well as the consequences of the specific construction contract. The acceptable aggregate risk, i.e. the risk the local and national authorities can permit, is elaborated.
1. Landslides in soft clays
Natural clay slopes and excavations in clay in Scandinavia have long been associated with risk of landslides, which have occurred throughout recorded history [1]. Geotechnical design is consequently intensely concerned with risk management, especially related to the risk of slope collapses, as these can have very large consequences, especially if progressive failure of landslides occurs [2]. Different methodologies for risk management have evolved and guidelines have been proposed, see e.g. [3] and [4].
The legal consequences of negative outcomes can be specified in a legal contract or agreement, including third party liability. This frequently includes insurance requirements, which both specify and limit economic liability for legal persons [5]. Such contractual specifications are used for very large projects, e.g. [6], as well as for common day to day construction activities. There are, however, limitations to the possible economic liability and for the legal consequences of risks, in the case of small companies with limited economic resources, as well as the cost of civil law court process [5].
Since most construction companies are small and incorporated under limited liability [7], the legal system therefore cannot completely regulate the economic consequences of risks. The allocation of risk at agents (clients and contractors) within the legal system results in a residual economic risk for which the state or governmental body is responsible. This is a societal cost according to Coase [8] and is defined as an acceptable risk for society [9]. A conceptual illustration of the acceptable risk is presented in Figure 1, based on the formal definitions in [3]. The acceptable risk depends on both probability and the consequences on an event. The range of acceptable risk can be reduced, but this carries an aggregated cost for society in the form of regulation and the cost of judicial process [8]. The aim for regulation is to define the suitable risk level on the aggregate level. Note that also moral values and ethical matters are important, but these will not be addressed in this paper.
The current paper reviews a case study in which a small slide in soft clay was triggered by a modest surcharge load, the size (i.e. thickness of the layer) of which was acceptable according to the current legislation. The aim of this case study is to demonstrate the conflicts between intersecting construction activities and the risk of slide collapses in very soft clays. The risk could have been reduced by the geotechnical design procedure, consisting of in-situ testing and slope stability calculation [3]. Such measures, however, carry a cost, which for numerous existing similar cases could be very expensive for society [8]. The question is if the risk was acceptable, and how the regulatory regime should be designed to assess this risk.
Figure 1. An idealized illustration of risk allocation in society, acceptable and unacceptable risk. 2. Soft soils in Sweden
The geological conditions in Sweden are characterized by very soft clays and very hard Precambrian rock. During the Weichselian glaciation, the sedimentary soil layers were eroded, and in the subsequent deposition process basins of clays were deposited in a marine environment [10]. In the eastern part of the country, areas were still submerged in water as late as year 1200 A.D. In these areas large organic and sulphide clays are common, resulting in a soil profile which often is highly heterogeneous [11]. The soft clays are frequently very sensitive, resulting in significant sample disturbance during sampling [12]. The geotechnical design process consequently requires a careful assessment of the natural soil properties during site investigation and design parameter selection for slope stability assessments [13].
3. Case study
The current case study concerns a landslide which occurred in the coastal suburbs of Stockholm. Figure 1 depicts the area where the slide occurred, being a minor bay along the North-Eastern suburbs of Stockholm, bordering the Baltic sea. The area is highly developed and along the shore of the bay shown in Figure 2, most of the shoreline is filled with berthage and mooring facilities for private boats. The (red) circle in Figure 2 shows the location where the slide occurred, which took place along the northern
end of the private villa which was under construction at the time. The slide occurred after the contractor for the villa needed to dispose superfluous gravel and rock masses from the construction works. A layer of around 300–400 mm of gravel fill was placed on the natural ground. That same week the slide shown in Figure 2 occurred, partly displacing the berthage also shown in Figure 2 and Figure 3.
Figure 2. The location of the slide shown in the red circle.
No one was injured during the slide. The sports boat club quickly restricted the access to the ramp and bridge to the boats. The slip surface continued to creep during the following weeks, thereby gradually pushing the berthage into the Baltic sea, as shown in Figure 4.
Figure 3. The outline of the slide immediately after it occurred, also showing the placement of fill along the shore.
Figure 4. The landslide area around a month later. 3.1. Back-analysis of the slide
A back-analysis of the slide was carried out to assess how the change in topography before the slide had influenced the stability of the slope. An exhaustive site investigation was carried out for reconstruction of the quay along the Northern part of the bay, elaborated in [14], which was used as the basis for the back-analysis. The clay layers were assumed to be very soft, since the clay was deposited in marine conditions along the shore. The calculation was carried out with the program Slope/w [15], using the Morgenstern–Price method [16]. The soil parameters were adapted from [14], and the undrained shear strength of the clay was adjusted to find a critical factor of safety. A thickness of 300 mm of fill was used in the numerical analysis.
Figure 5 shows the results of the back-analysis using the soil parameters shown in Table 1, resulting in a total factor of safety of around 1. The undrained shear strength was found to be as low as around 4 kPa, which was around the lower boundary of the values found through measurements with the field vane test in [14]. However, the height of the fill was not measured exactly, and a critical value of around 5 kPa for the undrained shear strength seemed reasonable considering the local soil conditions.
Table 1. Soil parameters used in the back-analysis of the slide, with the different layers shown in Figure 5.
Soil layer Bulk unit weight
(kN/m3) friction (°) Angle of intercept (kPa) Cohesion Undrained shear strength (kPa)
Fill 18 33 1 -
Clay 14 - - 4,2
Figure 5. A back-analysis through slope stability calculation of the slide with an undrained shear strength of 4,2 kPa, resulting in a safety factor of around 1.
4. Remediation of the slide and actions to reduce the risk in the future
The municipality and the sports boat club had to agree to a suitable method of remediation after the slide occurred. Different options including onshore excavation and placement of fill along the bottom of the bay were considered, and a solution involving both these measures was eventually agreed upon.
Afterwards, the municipality evaluated the responsibility between the various actors, and investigated whythe slide could occur. The disposed fill was in fact very limited, around 300–400 mm. Notably, according to the Swedish planning law, the topography can be adjusted by less than 500 mm without any permit required. The action that triggered the clay was consequently legal. The regulations are similar in Norway, where slides are frequently caused by private construction activity making changes to the topography, see e.g. [17].
A standard geotechnical risk assessment including the principles specified in [4], [18] and [19] would certainly have indicated the risk of a (potential) landslide. This is, however, typically not standard practicefor the construction activity at such a site (rebuilding of a private house) and would most likely be prohibitively expensive for most house owners. In the next section, the issue of effective geotechnical risk management targeting landslides involving by small clients and contracting firms is discussed. 5. Suggestions for risk management and risk acceptance
5.1. Municipality as risk owner
Considering the potentially large consequences associated with slope failures in urban areas, such risks need to be carefully managed. Normally, geotechnical risks are allocated to the client or contractor of a construction site [4]. This follows from the concept of being the risk owner – the party responsible for the construction works is also responsible for the associated risks. The risk owner is normally identified as the party that carries the financial risk, which makes the risk owner responsible for decisions regarding risk reducing or mitigating actions (risk treatment). However, as the awareness of geotechnical risks generally is low among the general public, it may be unreasonable to put the responsibility of identifying, assessing and treating such risks on the individual landowner. The municipality may, therefore, be considered the natural risk owner, if slope failures in urban areas (or within municipality zones) are seen as a societal issue. In contrast to small land and private home owners, the municipality
normally does have the technical capacity and financial means to reduce the general geotechnical risk level, and can cost-effectively do so over larger areas.
For the municipality to be(come) a suitable risk owner, however, it needs to establish and maintain a risk aware culture in its organization. The risk management methodology developed by the Swedish Geotechnical Society [20], which is based on the ISO 31000 [18], proposes four basic requirements that need to be fulfilled to achieve this:
1. “The scope and object of the risk management shall be established. 2. The decision maker (risk owner) shall subscribe to the concept of risk.
3. Engineers with formal responsibilities shall have essential knowledge of risk management. 4. A system for communication and transfer of risk-related information shall be established.” For a municipality, the third requirement may be the biggest challenge, as municipalities rarely employ geotechnical engineers While this certainly is the case in Sweden, the same situation is common for other European countries including The Netherlands. For a municipal development and planning office, an important first step is to create awareness of risks related to landslide, and account for them in preparations of local plans. One way to raise the awareness is to let a geotechnical engineering consultant conduct a risk screening with respect to potential areas for landslides in the municipality. Based on this, detailed risk assessments through slope stability analyses can be made, resulting in local loading restrictions and other risk treatments to reduce the risk for slope failures. A similar approach is taken for Dutch dikes and levees along the major rivers crossing the country. Based on preliminary findings more detailed analyses can be performed.
Regarding the fourth requirement, it is essential to communicate knowledge of potential landslide risks to people living and doing business in landslide susceptible areas, as a means to reduce the likelihood of causing landslides in small, private construction sites.
5.2. The Dutch experience in a Swedish context
An underlaying question that needs to be raised involves the (societal) consequences and responsibilities. In the Dutch system, where dikes and levees are constructed to control and regulate rivers and canals, water boards have the task of monitoring and assessing the safety of these dikes and levees with certain intervals. Water boards report to provincial and national governmental bodies. Furthermore, research is conducted by engineering and consultancy firms as well as research institutes as Deltares, with the aim of improving design guidelines as well as failure calculation (methodologies) to allow for assessing more complex scenarios and cases. In Sweden, a framework agreement between a municipality and one or more engineering or consultancy firms specialised in slope failure assessments could be considered. The aim should be an overall risk and cost reduction regarding landslide hazards.
In line with the case reported earlier in this paper, a (minor) slope failure during the construction of a project in The Netherlands can be handled by the contractor, if consequences are marginal or do not affect a location outside the project area and the final project result is not jeopardized. For more severe consequences, typically, the legal framework steers the discussion as is the case in Sweden. And should no mutual understanding or agreement to the solution of the dispute found, court cases may follow. The latter is often not desired both resulting from the economic as well as the reputational cost.
6. Concluding remarks
A case study of a small landslide has been presented along with an extended discussion regarding acceptable risk for society and how it should be managed. In the case of the slope failure discussed in this paper, the municipality had not considered the landslide risk within its planning system when the construction permit was granted, and as a result it occurred. If the municipality or the landowner had performed a simple slope stability analysis, the conclusion probably would have been that mitigating measures would need to be taken. The slope failure would likely have been mitigated and damage would have been minimized. However, this would have resulted in additional cost for society in the form of a
site investigation program and the subsequent slope stability analysis. The landowner would likely not know whether such activities were needed, and the contractor would not consider it necessary since it was not legally required for the specific construction work. Due to the large number of small private construction projects, the aggregated cost of such extensive site investigations and slope stability analyses would be quite significant and would also reduce the incentive to conduct rebuilding and extension of existing buildings, with a negative benefit for society. The local and national authorities consequently need to balance the need for risk mitigation in the planning system and the cost of such mitigation. The risk acceptance of society forms the basis of such a strategy, where the acceptance of the aggregated individuals governs the suitable strategy, involving the cost of risk mitigation from multiple areas in society. The local and national authorities organize their activities based on a formal or informal acceptance for a variety of different risks, the resulting exposure to these risks follow from this prioritization.
As lessons learnt and for educational purposes, it would be interesting to study and assess similar cases, particularly when adequate soil data is available. An accurate procedure on how to deal with comparable projects can then be devised as well as assembling a dataset to evaluate the suitable the risk management strategy for the local and national authorities.
References
[1] Bjerrum L 1955 Stability of natural slopes in quick clay Geotechnique 5(1) 101-119
[2] Bernander S 2011 Progressive landslides in long natural slopes: formation, potential extension and configuration of finished slides in strain-softening soils Doctoral dissertation, Luleå tekniska universitet
[3] Fell R, Ho K K, Lacasse S, Leroi E 2005 A framework for landslide risk assessment and management In Landslide risk management pp 13-36 CRC Press
[4] Spross J, Olsson L, Stille H 2018 The Swedish Geotechnical Society’s methodology for risk management: a tool for engineers in their everyday work Georisk: Assessment and Management of Risk for Engineered Systems and Geohazards 12(3) 183-189
[5] Sappington D 1983 Limited liability contracts between principal and agent Journal of economic Theory 29(1) 1-21
[6] Flyvbjerg B, Bruzelius N, Rothengatter W 2003 Megaprojects and risk: An anatomy of ambition Cambridge University Press
[7] Hillebrandt P M and Cannon J 1989 The management of construction firms: Aspects of theory London: Macmillan
[8] Coase R H 1988 The firm, the market, and the law University of Chicago press
[9] Fell R 1994 Landslide risk assessment and acceptable risk Canadian Geotechnical Journal 31(2) 261-272
[10] Larsson R and Åhnberg H 2005 On the evaluation of undrained shear strength and preconsolidation pressure from common field tests in clay Canadian Geotechnical Journal 42(4) 1221-1231
[11] Lundberg A B and Alderlieste E A 2018 Some considerations related to the interpretation of cone penetration tests in sulphide clays in eastern Sweden In Cone Penetration Testing 2018 CRC Press
[12] Lundberg A B 2017 Sample disturbance in deep clay samples In Landslides in Sensitive Clays pp 133-143 Springer Cham
[13] Lacasse S 2017 Parameters for soft clays In Soft Soil Engineering pp 25-36 Routledge
[14] Lundberg A B and Li Y 2015 Probabilistic characterization of a soft Scandinavian clay supporting a light quay structure Geotechnical safety and risk V 170-175 IOS Press Amsterdam
[15] Geo-Slope 1991 Slope/W Geo-Slope International Ltd Calgary Alberta Canada
[16] Morgenstern N U and Price V E 1965 The analysis of the stability of general slip surfaces. Geotechnique 15(1) 79-93
the partial collapse of Mofjellbekken bridges in Norway In Landslides and Engineered Slopes Experience, Theory and Practice pp 1091-1097 CRC Press
[18] International Organization for Standardization 2009 ISO 31000:2009: Risk management: Principles and guidelines International Organization for Standardization Geneva
[19] Purdy G 2010 ISO 31000: 2009—setting a new standard for risk management Risk Analysis: An International Journal 30(6) 881-886
[20] SGF 2017 Risk management in geotechnical engineering projects – Requirements: methodology Report 1:2014E. 2nd ed. Swedish Geotechnical Society, Linköping
