Use of basic soil test data in internal erosion risk assessments

Transcription

1 Dams and Reservoirs (1), Keywords: dams, barrages and reservoirs/geotechnical engineering/risk and probability analysis ICE Publishing: All rights reserved Malcolm Eddleston Eur Ing Dr, BSc (Hons), DipEM, PhD, CEng, FICE, CGeol, FGS Technical Director, MWH, Birchwood, Warrington, UK Chi Fai Wan MSc, PhD, CEng, MICE, MIWEM, MIEAust Principal Dam Engineer, MWH, Melbourne, Australia Publication of the Environment Agency (EA) Guide to Risk Assessment for Reservoir Safety Management (Bowles et al., 2013) has increased the interest and awareness in undertaking risk assessments on dams in the UK. The guide introduces a three-tier approach to risk assessments. Tier 1 is qualitative and tier 2 introduces some basic quantitative tools. Tier 3 introduces more detailed quantitative methods and methods of dealing with uncertainty. A key part of undertaking successful tier 3 internal is gathering and interpreting the correct geotechnical information. The International Committee on Large Dams bulletin (Bridle, 2013) contains invaluable information taken from research around the world, which adds to our knowledge of the internal erosion processes via specialist test procedures and methods. On existing dams much of the information available is from historical site investigations using conventional investigation techniques and soil and rock testing. Many of the specialist tests used to evaluate the susceptibility of materials to internal erosion are not generally available in the UK and can be prohibitively expensive. This paper aims to make the most of commonly available standard classification test data to provide a means of obtaining information on the susceptibility of embankment materials to internal erosion. In many cases this may be sufficient to provide enough information for a reasonable tier 3 assessment to be made or identify areas where further information would be beneficial depending on the risk posed and potential consequences of dam failure. 1. Introduction Internal erosion is a common failure mode in embankment dams. However, it is difficult to analyse by conventional soil mechanics models or formulae. The process of internal erosion is generally represented in the form of event trees as indicated in Figure 1 (ICOLD, 2013). Methods are available to undertake quantitative internal using event trees, although the numerical outputs cannot be considered precise or accurate (Cyganiewicz et al., 2008; Engemoen et al., 2011; Fell et al., 2004, 2008a, 2008b; Foster et al., 2008; ICOLD, 2013; Scott, 2010; Scott and Fielder, 2010; USBR and USACE, 2012). Internal erosion can take place through the embankment, in the foundation (soil or rock) and at the embankment foundation contact. The detailed processes involved are described in the latest International Commission on Large Dams (ICOLD) bulletin (ICOLD, 2013). Failure mode analysis can develop event trees for each of these. As part of a risk assessment for each node of the event tree, it is possible to develop a list of the adverse factors that make each event tree node more likely and the favourable factors that make each event tree node less likely based on the key evidence and degree of belief that the likelihood is low or high. A key input to this relates to the properties of the watertight element of the dam and its shoulders and foundation. The object of this paper is to outline how basic soil classification parameters may be used to make these evaluations from available geotechnical reports or help to plan further investigations to gather more relevant data to make an informed risk assessment. 2. Initiation 2.1 General Internal erosion initiates when an unfavourable combination of material susceptibility, stress and hydraulic loading occurs as illustrated by Garner and Fannin (2010) in Figure 2. Garner and Fannin s work is related primarily to erosion of fine particles through a coarse and unstable granular matrix (suffusion/suffosion, see below for detailed definitions), but is equally applicable to erosion through concentrated leaks and contact erosion. 2.2 Erodibility The USBR best practice document (USBR and USACE, 2012) suggests that an early erosion resistance classification proposed 26

2 Reservoir rises Initiation Flaw exists (1) (2) Initiation Erosion starts Continuation Unfiltered or inadequately filtered exit exists (consider: no erosion/some erosion/excessive erosion/continuing erosion) Progression Roof forms to support a pipe Progression Upstream zone fails to fill crack Progression Upstream zone fails to limit flows Intervention fails Dam breaches (consider all likely breach mechanisms) Consequences occur (1) A flaw is a continuous crack or gap, high permeability or poorly compacted zone in which a concentrated leak may form. (2) For backward erosion piping (BEP) no flaw is required, but a continuous zone of cohesionless soil in the embankment or foundation is required or a cohesive confining layer is too thin to prevent blow out. Figure 1. Event trees representing process of internal erosion by Sherard (1953) is still useful in evaluating the likelihood of erosion. The classification is shown in Table 1. Examples of its application with basic soil classification for a typical Pennine dam clay core are shown in Figures 3 and 4. The Sherard boundaries are plotted directly from the plots in his PhD thesis Suffusion Seepage velocity Hydraulic gradient Pore pressure Critical hydraulic load Material susceptibility Internal instability Filter incompatibility Void space Free surface Low plasticity Suffusion Piping Contact erosion Boiling Hydraulic fracture Heave Soil distress Cracking Bridging Low stress Vibration Arching Critical stress condition Figure 2. Factors affecting the initiation of internal erosion (Garner and Fannin, 2010) and are tentative boundaries and in the case of particle size data have an element of overlap for the intermediate and least erosive soils. The plasticity plot in Figure 4 is similar to one published later by Gibbs (1962). The classification provides a useful guide to erosion properties of embankment materials based on simple soil classification data without the potential need to commissioning specialist erosion testing. However, the guidelines are not meant to be prescriptive. A closer look at the plots indicates the particle size data signify a high erosion resistance, whereas the plasticity data indicate a significant proportion of result with intermediate erosion potential. This illustrated the need to consider all readily available data in trying to predict the susceptibility of puddle clay to erosion such as the work of Moffat (1999, 2002). Other controlling parameters are: degree of compaction (93% or 95% compaction makes a big difference), the moisture content (relative to optimum moisture content) and the degree of saturation (the higher the degree of saturation, the higher the erosion resistance). These parameters should be available from a well-planned ground investigation and laboratory test schedule. The presence of cementing materials in the clay also has significant influence on erodibility. Erosion in rock will depend on the geological environment, topography, rock type, stress history and the nature and spacing of discontinuities and any infill material. 2.3 Concentrated leak erosion Where there is an opening through which concentrated leakage occurs, the walls of the opening may be eroded by the leaking 27

3 Sherard s piping category Soil properties Comments Greatest piping resistance category (1) Intermediate piping resistance category (2) Least piping resistance category (3) Plastic clay, (PI. 15), well compacted. Plastic clay, PI. 15), poorly compacted. Well-graded material with clay binder, (6, PI, 15), well compacted. Well-graded material with clay binder, (6, PI, 15), poorly compacted. Well-graded, cohesionless material, (PI, 6), well compacted. Well-graded, cohesionless material, (PI, 6), poorly compacted. Very uniform, fine cohesionless sand, (PI, 6), well compacted. Very uniform, fine, cohesionless sand, (PI, 6), poorly compacted Small and medium concentrated leaks will heal themselves or continue clear without an increase in quantity. Embankments may completely fail as a result of progressive piping caused by a large leak (defined arbitrarily as approximately K ft 3 /s or greater (14 l/s)). This will occur slowly and give plenty of time for remedial action to be taken. Safely resists saturation of the lower portions of the downstream slope indefinitely. May fail eventually as a result of erosion caused by a small concentrated leakage or progressive sloughing but only after a long period of time. If a large leak through the embankment developed in any manner piping causes failure in a short period of time. Usually fails completely within a few years of filling, if water finds its way to the unprotected downstream slope. May resist saturation of the downstream toe for many years, but this condition represents a dangerous situation. Any small concentrated leak on the downstream slope will probably cause failure in a short period of time. Table 1. Sherard s classification of piping potential of soils (Sherard, 1953) water. Such concentrated leaks may occur in various parts of the embankment, foundation and appurtenant works as listed below. Embankment & Cross valley differential settlement. & Differential settlement adjacent a cliff. & Cross-section settlement owing to poorly compacted shoulders. & Differential settlements in the foundation beneath the core. & Cross valley differential settlement. & Differential settlement causing arching of the core onto the shoulders of the embankment. & Small-scale irregularities in the foundation profiles under the core. & Cracking in the crest due to freezing. & Cracking in the crest due to desiccation by drying. & Poorly compacted or high permeability layer in the embankment. Foundations & Poorly compacted or high permeability layer on the corefoundation contact. & Cracks in a soil foundation. & Fissure, discontinuities and solution features in a rock foundation. Appurtenant works & Poorly compacted or high permeability layer around a conduit through the embankment. & Poor compaction around ducts for instrumentations. & Erosion into or along a (non-pressurised) conduit. & Poorly compacted or high permeability zone associated with a spillway or abutment wall. & Crack/gap adjacent to a spillway or abutment wall. & Differential settlement adjacent to a spillway or abutment wall. 28

4 70 CE 60 CV Plasticity index: PI % CL Resistance category 2 Resistance category 1 CI CH MH MI Resistance ML category Liquid limit: LL % A-Line MV ME Figure 3. Piping classification of soils from plasticity data (Sherard, 1953) Resistance category 1 Passing percentage: % Resistance category 2 Resistance category Particle size: mm Fine Medium Coarse Fine MediumCoarse Fine Medium Coarse Clay Silt Sand Gravel Figure 4. Piping classification of soils from particle size distribution data (Sherard, 1953) 29

5 The ICOLD bulletin (ICOLD, 2013) uses the a classification based on the seepage and piping toolbox (SPT; Fell et al., 2008a) to classify soil resistance to concentrated leak erosion as indicated in Table 2. The likelihood of erosion initiating in a concentrated leak is a function of the erodibility of the soil and the average hydraulic gradient from the upstream to downstream of the core at the level of the assumed crack. The SPT (Fell, 2008a) presents a series of tables for different soil types relating the anticipated crack widths and hydraulic gradients to probability of erosion initiating. The Reclamation best practice guide (USBR and USACE, 2012) has developed these tables further using Monte Carlo analysis to allow for uncertainty. Table 3 gives the table for a low-plasticity clay typical of a UK puddle clay core for a Pennine-type dam using probability descriptors from Mason (2010) as opposed to probability values. A similar table can be produced for a variety of soil types. 2.4 Suffusion/suffosion Instability leading to erosion of fine particles through a coarse material has been termed both suffusion and suffosion in literature over many years sometimes interchangeably. To clarify the terms when reporting seepage-induced internal instability, Richards and Reddy (2007) and Moffat et al. (2011) clarified the terms as follows. & Suffusion: the finer material of an internally unstable soil moves within a coarser fraction without any loss of matrix integrity or change in total volume. & Suffosion: particle migration yields a reduction in total volume and consequent potential for collapse of the soil matrix. Sherard (1953) considered internally unstable soils as those where the coarser fraction of the soil does not filter the finer fraction. He suggested susceptible gradations limits for suffusion as shown in Figure 5. Reclamation s filter design standard (USBR, 2011) considers the slope of the gradation curve and defines a slope relating to the grading and susceptibility to suffusion as illustrated in Figure 6. If the grading curve is equal to or less than the slope defined by the line the soil is unlikely to be self-filtering. The ICOLD bulletin (ICOLD, 2013) using the Wan and Fell (2004, 2007) and Wan (2006) test data and interpretation of other researchers data with adoption of the Burenkova (1993) definition of soil grading parameters h9 and h0 taken from base grading curves as shown in Figure 6 to separate internally stable and unstable soils. Typical plot points derived from grading curves for a canal dam are also plotted indicating the materials to have low susceptibility to suffusion. Gap graded and coarsely graded soils with a flat tail of fines are also potentially unstable. 2.5 Backward erosion piping Backward erosion initiates at the downstream toe of the dam and progresses upstream under the dam. When it occurs, sand boils sometimes from near the downstream toe or sinkholes develop in the upstream shoulder. For backward erosion to progress, the soil above the developing erosion pipe must be capable of sustaining a roof below which the pipe develops. This usually requires cohesive soils above non-cohesive soils (usually sands and silts) and high hydraulic gradients, except where the embankment consists of internally unstable soil subject to suffusion. The ICOLD bulletin (ICOLD, 2013) notes that experience in USA and Europe is that backward erosion piping mostly occurs in the foundations of levees, dikes and dams where the eroding soil is fine to medium grain size sand, with a uniformity coefficient Cu, 3. Fell et al. (2008a) in the SPT have concluded that, based on the available data, the results of Wan and Fell (2004, 2007, 2008) tests on internal instability and their experience and judgement, soils with plasticity index. 7 should be considered not susceptible to backward erosion at the gradients experienced in dams and their foundations. The bulletin gives details of several research studies that have been carried out on a range of specific soils across the world. 2.6 Contact erosion Contact erosion occurs where a coarse soil such as gravel is in contact with a fine soil and flow parallel to the contact in the coarse soil erodes the fine soil. If this mechanism progresses then fine material is lost and the embankment settles and may be overtopped. 1. Extremely erodible SM with FC, 30% 2. Highly erodible SM with FC. 30%, ML, SC, and CL-ML 3. Moderately erodible CL, CL-CH, MH, and CH with LL, Erosion resistant CH with LL. 65 Table 2. Erosion resistance of soils from concentrated leaks (Fell et al., 2008a) 30

6 Estimated likely crack width, Wc: mm Average hydraulic gradient, i ave 0?1 0?25 0?5 1?0 2?0 5?0 1 Highly unlikely Very unlikely Unlikely Possible Possible Probable 2 Very unlikely Unlikely Possible Neutral Probable Very probable 5 Unlikely Possible Neutral Probable Very probable Virtually certain 10 Possible Neutral Probable Very probable Virtually certain Virtually certain 25 Neutral Probable Very probable Virtually certain Virtually certain Virtually certain 50 Probable Very probable Virtually certain Virtually certain Virtually certain Virtually certain 75 Very probable Virtually certain Virtually certain Virtually certain Virtually certain Virtually certain 100 Very probable Virtually certain Virtually certain Virtually certain Virtually certain Virtually certain Table 3. Probability of initiation of internal erosion in a crack for CL or MH soils (adapted from Fell et al. (2008a)) According to ICOLD (Brauns (1985), Worman and Olafsdottir (1992) and Den Adel et al. (1994)) for D 15 /d 85 ratios less than 7?5 (where D 15 is particle size of the coarser soil for which 15% is finer and d 85 is the particle size of the finer soil for which 85% is finer), there is geometric filtration (regardless of hydraulic loading), so excessive contact erosion will not occur. 3. Continuation Erosion once initiated will continue unless the eroding forces are reduced or the passage of the eroded particles is impeded. The ability to restrict the passage of eroding material depends on the particle-size distribution between the base (core) material and the filters or adjacent materials controls if erosion will continue. Filter evaluation for older dams not meeting modern filter criteria is primarily based on the work of Foster and Fell (2001), who defined the following boundaries in relation to protection against continuing internal erosion. & No erosion the filtering material stops erosion with no or very little erosion of the material it is protecting. The Passing percentage: % Sherard's unstable band Stable Unstable USBR 4 slope line Particle size: mm Clay Fine Medium Coarse Silt Fine MediumCoarse Fine Medium Coarse Sand Gravel Figure 5. Sherard (1953) and reclamation suffusion boundaries 31

7 h' = d 90 /d h'' = d 90 /d 15 Figure 6. ICOLD method of assessing internal stability of granular soils (silt sand gravel soils and clay silt sand gravel soils of limited clay content and plasticity) increase in leakage flows is so small that it is unlikely to be detectable. & Some erosion the filtering materials initially allow erosion from the soil it is protecting, but it eventually seals up and stops erosion (self-healing). & Excessive erosion the filter material allows erosion from the material it is protecting, but the flows are self-healing. The extent of erosion is sufficient to cause sinkholes on the crest and erosion tunnels through the core. & Continuing erosion the filtering material is too coarse to stop erosion of the material it is protecting and continuing erosion is permitted. Unlimited erosion and leakage flows are likely. Relevant soil properties relating to the no erosion and excessive and continuing erosion for typical UK puddle clay cores are given in Tables 4 and 5. A recent example of the application of these boundaries to a typical British embankment dam is given by Bridle (2008, 2014) as illustrated in Figure 7. In this particular dam the D 15 shoulder fill is partly within the no-erosion filter (,0?7 mm) range of a Base soil category Fines content (1) Criteria for no erosion boundary 1 > 85% DF 15 # 9DB % DF 15 # 0?7 mm Note: The fines content is the % finer than 0?075 mm after base soil is adjusted to a maximum particle size of 4?75 mm. Table 4. No erosion boundary for the assessment of filters of existing dams (Foster and Fell, 2001) category 2 base soil, but the coarser grading encroaches into the excessive erosion zone. This implies that less filter protection would be afforded by the shoulder fill against internal erosion. For the purpose of risk assessment, we need to answer the question on how likely an erosion boundary is exceeded rather than saying Yes or No to whether the boundary is exceeded. Some guidance is given in the SPT for judging the probabilities that the filter/base falls into each of the No, Some, Excessive or Continuous erosion zones. The example in Figure 8 relates to the continuation of erosion of the clay core cut-off trench into an untreated granular horizon within the underlying glacial till. Foundation DF 15 values for No, Some, Excessive and Continuing erosion boundaries have been determined using methodologies detailed in Foster and Fell (2001). The probability of continuation is estimated using the chart for each branch of the event tree (No, Some, Excessive and Continuing erosion) based on the distribution of grading results along the DF 15 line. 4. Progression The progression stage of internal erosion is the process of development and enlarging of an erosion pathway through the embankment or its foundation. The SPT (Fell et al., 2008a and Bridle et al., (2007)) considers three separate progression processes, as follows & formation of a stable roof and/or side walls of the crack/ pipe through the core & potential limitation of flow by an upstream zone, concrete element or cutoff & the potential for an upstream zone to self-heal. The likelihood of soil being able to support a roof in relation to soil properties is given as shown in Table 6. Examples of flow limiters include sheet piled or concrete walls and grouting, all of which can improve the potential to limit erosion depending on the type used and the location and method of installation. The likelihood of various methods of flow limitation being successful are given in Table 7. The SPT (Fell et al., 2008a) relates the potential for self-healing being provided by an upstream zone to soil properties as shown in Table 8 with the influence of the material in the downstream shoulder given in Table Unsuccessful intervention The SPT (Fell et al., 2008a) outlines likelihood that a particular failure path can be detected, and if so, whether it is possible to intervene (e.g. by lowering the reservoir level), or carry out repairs to prevent the dam breaching. The likelihood of 32

8 Base soil Criteria for continuing erosion boundary Criteria for continuing erosion boundary Soils with DB 95, 0?3 mm DF 15. 9DB 95 D 15 F. 9DB 85 Soils with 0?3, DB 95, 2mm DF 15. 9DB 90 Note: The fines content is the % finer than 0?075 mm after the base soil is adjusted to a maximum particle size of 4?75 mm. Table 5. Continuing erosion criteria (Foster and Fell, 2001) intervention being unsuccessful is considered in a sub event tree with the following nodes. & Likelihood of not observing the concentrated leak because it is not observable. & Likelihood that leak is observable but not detected. & Likelihood that the leak is observable and detectable but intervention fails. Information that relates to the possible rate of internal erosion is presented based on the work of Fell et al. (2001, 2003) primarily relating to breach by gross enlargement of a pipe. Qualitative terms are used to describe the times for development of internal erosion as follows. These should be used in conjunction with knowledge of the performance history of the dam as described in the Department for Environment, Food and Rural Affairs (Defra) report on the early detection of internal erosion (Defra, 2002) The Reclamation best practices and risk methodology (USBR and USACE, 2012) contains intervention tables which outline many factors related to detection and physical intervention actions including the following. Detection factors & Signs of internal erosion are detectable and recognisable & Evaluation of instrumentation data & Opportunity to observe signs of internal erosion ( eyes on the dam ) & Rate of erosion pathway development Physical intervention actions & Reservoir drawdown & Material erodibility (core and shoulders) Core gradings Shoulder fill grading Cumulative percentage passing: % DF15 10 No erosion filter zone Some erosion filter zone Excessive erosion filter zone mm mm mm Particle size: mm Figure 7. The use of Foster and Fells filter criteria to a typical British dam (Bridle, 2014) 33

9 Average core Cumulative percentage passing: % Average glacial sand and gravel NE SE EE CE Particle size: mm Figure 8. Assessment of erosion boundaries for a clay core cut-off trench into an untreated granular horizon within the underlying glacial till Soil classification Percentage fines Plasticity of the fines Moisture condition Likelihood of supporting a roof Clays, sandy clays (CL, CH,. 50% Plastic Moist or saturated Virtually certain CL-CH) Silts (ML or MH).50% Plastic or non-plastic Moist or saturated Virtually certain Sandy clays, gravelly clays 15 50% Plastic Moist or saturated Virtually certain (SC, GC) Silty sands, silty gravels, silty sandy gravel (SM, GM). 15% Non-plastic Moist Saturated Probable to virtually certain Neutral virtually certain Granular soils with some cohesive fines (SC-SP, SC-SW, GC-GP, GC-GW) 5 15% Plastic Moist Saturated Neutral virtually certain Possible Neutral Granular soils with some non-plastic fines (SM-SP, SM-SW, GM-GP, GM-GW) Granular soils (SP, SW, GP, GW) 5 15% Non- plastic Moist Saturated, 5% Non-plastic Plastic Moist and saturated Moist and saturated Highly unlikely Unlikely Highly unlikely Very unlikely Virtually impossible Highly unlikely to virtually impossible Notes: Lower range of likelihoods is for poorly compacted materials (i.e. not rolled), and upper bound for well compacted materials. Cemented materials give higher probabilities than indicated in the table. If soils are cemented, use the category that best describes the particular situation. Table 6. Probability of a soil being able to support a roof to an erosion pipe (adapted from Fell et al. (2008a)) 34

10 Characteristics of upstream zone Flow limitation by an upstream zone Likelihood for no flow restriction No zone upstream of core (e.g. homogenous, earthfill with toe drain, earthfill Virtually certain with filter drains). High permeability zone (e.g. clean rockfill). Virtually certain Fill with. 15% cohesive fines, highly likely to support a roof, Highly probable to virtually certain mechanism causing cracking or flaw in the core is also likely to affect the upstream zone (e.g. common cause cracking). Fill with. 15% cohesive fines, highly likely to support a roof, features causing cracking or flaw in the core are not present below the upstream shell. defect Fill with 5% to 15% cohesive fines, likely to support a roof, Neutral to probable mechanism causing cracking or flaw in the core is also likely to affect the upstream zone (e.g. common cause cracking). Fill with 5% to 15% cohesive fines, likely to support a roof, features causing cracking or flaw in the core are not present below the upstream zone. defect and fines content Fill with,15% cohesionless fines, unlikely to support a roof, mechanism causing cracking or flaw in the core is also likely to affect the upstream zone (e.g. common cause cracking). upstream zone is, 1 Fill with,15% cohesionless fines, unlikely to support a roof, features Highly unlikely to unlikely causing cracking or flaw in the core are not present below the upstream zone. Fill with 15% to 30% cohesionless fines, may support a roof, mechanism causing cracking or flaw in the core is also likely to affect the upstream zone (e.g. common cause cracking). Fill with 15% to 30% cohesionless fines, may support a roof, features causing cracking or flaw in the core are not present below the upstream shell. Fill with. 30% cohesionless fines, may support a roof, mechanism causing cracking or flaw in the core is also likely to affect the upstream zone (e.g. common cause cracking). Fill with. 30% cohesionless fines, may support a roof, features causing cracking or flaw in the core are not present below the upstream shell. Upstream low permeability blanket (for internal erosion in the foundation). Table 7. Probability that flow in the developing pipe will not be restricted by an upstream zone, cut-off wall or a concrete element in the erosion path (adapted from Fell et al. (2008a) with the descriptors as described by Mason (2010)) Highly unlikely to unlikely depending on the confidence that there is not a common cause Very unlikely to possible depending on the confidence that there is not a common cause Neutral to very probable if gradient across upstream zone is. 1 Unlikely to possible/neutral if gradient across Possible to probable Highly unlikely to unlikely depending on the confidence that there is not a common cause defect Very probable to certain Highly unlikely to unlikely depending on the confidence that there is not a common cause defect Highly unlikely to unlikely depending on the extent of coverage of the piping soil layer & Potential erosion mechanism & Accessibility of downstream exit point & Adequate filter and drain material available & Ability to mobilise equipment and materials & Accessibility of upstream sinkhole or access point & Capability of low consequence intentional breach Fell et al. (2001, 2003) noted that in most cases it was not possible to identify precisely the time of initiation of erosion and the first signs of erosion occur with a concentrated, often muddy leak. Where it was possible, for example from increased seepage flows, the time for initiation was recorded. It should also be noted that for the incidents studied, the dam did not 35

11 Time for erosion in the core of the embankment or in the foundation Soil classification Gradient along pipe 0?2 Gradient along pipe 0?5 SM with, 30% fines Very rapid Very rapid SM with. 30% fines Very rapid Very rapid SC with, 30% fines Very rapid Very rapid SC with. 40% fines Rapid Very rapid ML Very rapid to rapid Very rapid CL-ML Rapid Very rapid CL Rapid Very rapid to rapid CL-CH Rapid Rapid MH Rapid Very rapid to rapid CH with liquid limit, 65% Rapid to medium Rapid CH with liquid limit. 65% Medium to slow Medium Table 8. Rate of erosion of the core or soil in the foundation breach, so the time estimates used are the times from when progression of the piping began, to when the piping incident ceased, either by self-healing or intervention. The Reclamation best practices and risk methodology (USBR and USACE, 2012) and Engemoen and Redlinger (2009) note that only one failure (Teton) out of 101 total incidents reported (although there may be more unreported) on US Bureau of Reclamation (USBR) dams suggests only about a 1 per cent probability that the initiation of internal erosion will lead to complete dam breach. This is attributed to a number of factors involving continuation and progression of erosion and full development of a breach, but two factors appear to particularly stand out. In most cases, signs of the actual initiation of internal erosion (such as sinkholes, sand boils and excessive seepage) were observed and necessary remedial actions were quickly taken. A recent UK example of an internal erosion incident at Upper Rivington illustrated what actions could be taken in the event of the detection of a serious internal erosion incident (Charles, 2005; Gardiner et al., 2004). The embankment dam was a 12 m high Pennine embankment dam built in Minor leakage was observed through the walls of the culvert, which passes through the body of the dam, over a number of years. However, on Wednesday 9 January 2002, at about 13:30, a water company operative driving across the top of the dam as part of a 48-h inspection regime, noticed that the flow of water coming from the downstream end of the culvert had increased and was clouded. On entering the culvert a jet of water was observed issuing from one of the half-brick openings in the wall (see Figure 9). The reservoir supervising engineer arrived on site about an hour and a half after the problem was first noticed and contacted a panel all reservoir engineer, for advice who advised that the reservoir should be lowered immediately and the hole plugged. The panel engineer then travelled through the night to be on site the next day. He advised that the embankment was to be inspected three times per day, that the culvert was to be checked for structural defects once a day and that the drawdown should continue with auxiliary pumping to a level 7 m below top water level (TWL). This level was reached on Friday 18 January. Material description Likely breach time Coarse grained rockfill Soil of high plasticity (plasticity index. 50%) and high clay size content including clayey gravels Soil of low plasticity (plasticity index, 35%) and low clay size content, all poorly compacted soils, silty sandy gravels Sand, silty sand, silt Slow medium Medium rapid Rapid very rapid Very rapid Table 9. Influence of the nature of the material in the downstream section of the embankment on the likely time for development of the breach 36

12 5.1 Breach Breach occurs when either the failure mode is not detected or intervention is not attempted or is unsuccessful. The type of breach depends on the internal erosion mechanism being considered, embankment type and the specific failure mode being considered. According to Fell et al. (2008a), there are four breach mechanisms typically considered & gross enlargement of a pipe or concentrated leak & sloughing or unravelling of the downstream face with downstream slope is over-steepened to the point of instability & sinkhole development & slope instability, which is considered to be a very likely breach mechanism for most dams. Figure 9. Internal erosion incident at Upper Rivington (Gardiner et al., 2004) The incident shows that internal erosion incidents can develop quickly, and reinforces the need for on-site emergency plans. The two factors in preventing a disaster were early detection of the new leak and rapid lowering of the reservoir. There are also a number of instances where it appears that self-healing or collapse of developing internal erosion took place and either stopped the process or provided warning such that intervention could take place. This episodic nature of internal erosion incidents, which can lead to these failure mechanisms taking decades to progress (or initiate in some cases), has been demonstrated in all categories of internal erosion, particularly in those involving foundation materials, conduits or drains. The SPT (Fell et al., 2008a) includes a procedure for screening of potential breach mechanisms, which recognises that gross enlargement of a pipe is usually the most critical mechanism in puddle core and homogenous dams. It generally relates the likelihood of breach by enlargement of a pipe to soil classification, assuming there is no restriction on flows and no intervention to lower the reservoir level, as detailed in Table Conclusions Internal erosion risk assessment forms part of a proactive approach to risk assessment and management for embankment dams on an ongoing basis, rather than through reliance solely on safety recommendations from ten yearly statutory inspections or a reactive approach based upon visual inspections. The process inherently requires engagement of ground engineering and dam engineering professionals, detailed geotechnical desk study and investigation, instrumentation along with expert elicitation. This leads to a significant increase in hazard Characteristics of core material Soil classification SM, SC, ML, dispersive soils CL, CL-CH, MH or CH with liquid limit, 65% CH with liquid limit. 65% CH with liquid limit, 65% Or CH with liquid limit. 65% Time for reservoir level to fall below the invert of the pipe. 2d 1 2 d, 1d. 2 weeks 1 2 weeks, 1 week Likely to self-limit Probability of breach by gross enlargement (Mason, 2010) Virtually certain Very probable Probable Neutral Very to highly probable Possible to probable Unlikely to possible Very unlikely to unlikely Table 10. Probability of breach by gross enlargement of the pipe (adapted from Fell et al. (2008a)) 37

13 understanding and confidence regarding safety and performance; providing improved ability to maintain reservoirs at fullest capacity. The process relies on expertise from the geotechnical and reservoirs panel community along with the collection and rigorous interpretation of the best available geotechnical data from historic and contemporary investigations. Acknowledgement The authors gratefully acknowledge the permission of Rod Bridle, Dam Safety Ltd, Amersham to present Figure 7. REFERENCES Bowles DS, Brown A, Hughes AK et al. (2013) Guide to Risk Assessment for Reservoir Safety Management. Environment Agency, Bristol, UK. Brauns J (1985) Erosionsverhalten geschichteten Bodens bei horizontaler Durchstromung. WasserWirtschaft 75: (in German). Bridle RC, Delgado F and Huber NP (2007) Internal erosion: continuation and filtration: current approaches illustrated by a case history. In Assessment of the Risk of Internal Erosion of Water Retaining Structures: Dams, Dykes and Levees. Intermediate Report of the Working Group of ICOLD Deutsches Talsperren Komitee, Technical University of Munich, Munich, Germany, Nr.114. Bridle R (2008) Assessing the vulnerability of a typical British embankment dam to internal erosion. In Ensuring Reservoir Safety into the Future (Hewlett H (ed.)). Thomas Telford, London, UK, pp Bridle R (2013) ICOLD Bulletin 164: Internal erosion in existing dams, levees and dikes, and their foundations. British Dam Society Meeting, Institution of Civil Engineers, London, 19 May. Bridle R (2014) Some findings from the ICOLD Bulletin on internal erosion in existing dams. In Dams and Extreme Events Reducing Risk of Aging Infrastructure under Extreme Loading Conditions. US Society on Dams, Denver, CO, USA, pp Burenkova VV (1993) Assessment of suffusion in non-cohesive and graded soils. In Filters in Geotechnical and Hydraulic Engineering (Brauns J, Heibaum M and Schuler U (eds)). Balkema, Rotterdam, the Netherlands, pp Charles JA (2005) Use of incident reporting and data collection in enhancing reservoir safety. Dams and Reservoirs 15(3): Cyganiewicz J, Sills G, Fell R et al. (2008) Seepage and piping toolbox overview. In The Sustainability of Experience Investing in the Human Factor. US Society on Dams, Denver, CO, USA, pp Defra (2002) Defra Research Contract: The early detection of Internal Erosion. Department for Environment, Food and Rural Affairs, London, UK. Den Adel H, Koender MA and Bakker KJ (1994) The analysis of relaxed criteria for erosion-control filters. Canadian Geotechnical Journal 31(6): Engemoen WO, Fiedler WR, Osmun DW and Scott GA (2011) Shedding some light on this thing called risk assessment, part III putting it all together. Journal of Dam Safety 9(4): Engemoen WO and Redlinger CG (2009) Internal erosion incidents at Bureau of Reclamation dams. In Managing Our Water Retention Systems. US Society on Dams, Denver, CO, USA, pp Fell R, Wan CF, Cyganiewicz J and Foster M (2001) The Time for Development and Detectability of Internal Erosion and Piping on Embankment Dams and their Foundations. School of Civil and Environmental Engineering, The University of New South Wales, Sydney, Australia, UNICIV Report No. R-399. Fell R, Wan CF, Cyganiewicz J and Foster M (2003) Time for development of internal erosion and piping in embankment dams. ASCE Journal of Geotechnical and Geoenvironmental Engineering 129(4): Fell R, Wan CF and Foster M (2004) Methods for Estimating the Probability of Failure of Embankment Dams by Internal Erosion and Piping Piping through the Embankment. The University of New South Wales, Sydney, Australia, UNICIV Report No. R-428. Fell R, Foster M, Davidson R et al. (2008a) A Unified Method for Estimating Probabilities of Failure of Embankment Dams by Internal Erosion and Piping. The School of Civil and Environmental Engineering, the University of New South Wales, Sydney, Australia, UNICIV Report No. R-446. Fell R, Foster M, Davidson R et al. (2008b) Seepage and piping toolbox initiation of internal erosion. In The Sustainability of Experience Investing in the Human Factor. US Society on Dams, Denver, CO, USA, pp Foster M and Fell R (2001) Assessing embankment dams, filters which do not satisfy design criteria. Journal of Geotechnical and Geoenvironmental Engineering, ASCE 127(4): Foster M, Fell R, Vroman N et al. (2008) Seepage and piping toolbox continuation, progression, intervention and breach. In The Sustainability of Experience Investing in the Human Factor. US Society on Dams, Denver, CO, USA, pp Gardiner KD, Hughes AK and Brown A (2004) Lessons from an incident at Upper Rivington reservoir - January Dams and Reservoirs 14(2): Garner SJ and Fannin RJ (2010) Understanding internal erosion: a decade of research following a sinkhole event. 38

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