Desiccation fissuring induced failure mechanisms for clay levees

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Desiccation fissuring induced failure mechanisms for clay levees S. Utili 1,2, M. Dyer 2, M. Redaelli 2 & M. Zielinski 2 1 Corresponding author: e-mail: stefano.utili@strath.ac.

1 Desiccation fissuring induced failure mechanisms for clay levees S. Utili 1,2, M. Dyer 2, M. Redaelli 2 & M. Zielinski 2 1 Corresponding author: stefano.utili@strath.ac.uk 2 Department of Civil Engineering, University of Strathclyde, John Anderson Building, 107 Rottenrow, G4 0NG, Glasgow, UK. ABSTRACT: Desiccation fissuring is a phenomenon which got very little attention in the scientific literature until very recently. Nevertheless, many earth levees in Europe, USA and Asia are subject to strong fissuring for prolonged periods of time. Experimental evidence suggests that desiccation fissuring could be responsible of breach initiation in many cases. In this paper, firstly experimental data obtained from field investigations are reported, secondly small scale laboratory tests are described and thirdly a failure mechanism leading to breach initiation is postulated according to the gathered observational evidence. Field investigation consisted of walk-over surveys, trial trench excavations, measurements of suction profiles and double ring infiltrometer tests whereas small scale tests were ad hoc set up in the laboratory in order to measure the tendency of the levee material to crack. 1 INTRODUCTION Deterioration processes of earth levees are of particular concern because extreme flood events tend to occur more and more often. This, in turn, means that flood defense levees are subject to increasingly larger hydraulic loads. Desiccation fissuring is one of the deterioration processes known to have a significant adverse effect on the levee stability. Early work by Cooling & Marsland (1953) for the 1953 North Sea Flood identified desiccation fissuring as a major contributory cause for levee collapse. Also more recent work done by Dyer (2004, 2005) corroborates this conclusion. Therefore, field surveys have been undertaken to gather supplementary information about the extent and nature of desiccation fissuring and the role played by it in breach initiation. For general information, the main features of a typical UK flood defense levee are shown in Figure 1. The main features include (a) an embankment body that provides the mass obstruction against flood water, (b) the toe of the embankment on either the outward or inward embankment face, (c) the inward face of the embankment exposed directly to water, (d) the outward face on the landward side not normally directly exposed to flood water, (e) the crest at the top of the embankment that is typically flat and (ideally) several metres wide, (f) a drainage ditch also known as a soke or delph ditch excavated close to the inward toe of the embankment, (g) surface protection sometimes termed revetment in the form of vegetation (e.g. grass), manmade material (e.g. concrete) or a combination of different materials. Figure 1. Typical UK earth levee (after Morris et al., 2007). 1.1 Past evidence on levee failure Levee breach occurs rapidly and is difficult to predict. Not surprisingly, well-documented cases of levee breaches are rare. One of the few notable studies of flood levee failures is the paper on 1953 North Sea flood by Cooling and Marsland (1953) and the subsequent report by Marsland (1968) about the role of fine fissuring on levee collapse. In the case of the North Sea floods, Cooling and Marsland reported that the debris of a breach were often blocks of clay transported some considerable distance away from the levee, and in many cases the cause of failure was

2 attributed to desiccation fissuring of the levee that led to significant seepage of overflow flood water into the levee. Cooling and Marsland, attributed the detrimental effects of desiccation fissuring to increased seepage during overflow condition with a corresponding increase in pore pressure that led to slope instability of the landward face. In comparison a different mechanism leading to the levee breaching will be postulated in section 3. In 1996 a field survey of a flood levee was carried out by Meadowcroft et al. (1996) at Tollesbury Creek. Three clay levees along the Blackwater Estuary in Essex (UK) with an extensive crack pattern were failed in cofferdam tests. The experiment was notable for the observed high rate of seepage through the crest into the highly fissured clay in the landward zone of the levee. In one experiment water seeped out of the levee at the landward toe up to 7 m away from the overflow section which clearly indicated the presence of a well interconnected network of fissures within the levee. More recently greater attention has been given in the literature to the mechanisms responsible for fissuring of desiccated clay. For instance a notable study was carried out by Konrad and Ayad (1997). The authors presented the results of a desiccation test on an intact clay deposit at the experimental site of Saint-Alban, in Quebec (Canada). The study recorded an interconnected pattern of desiccation fissures forming hexagonal polygons as found in Thorngumbald (see next section) with the appearance of lateral cracks. 2 FIELD INVESTIGATIONS The field study reported in the paper was carried out at an historic and new flood levee at Thorngumbald near the city of Hull on the north shore of the Humber Estuary. The historic flood levee was replaced by a new flood levee in 2003 in order to create new salt marshes. The field study has involved the excavation of trial pits to expose desiccation fissuring beneath the crest and side slopes of the levees along with soil sampling for later laboratory analysis and on site double ring infiltrometer tests. The site visits to Thorngumbald were undertaken in 2003, 2005, and Observations from walk-over surveys and trial trenches Observations were made of desiccation fissures both on the surface and within shallow excavations. Four trial trenches were excavated along the crest and landward slope of the historic flood levee using a hydraulic back-actor. The final part of excavation was undertaken by a small spade to minimize soil disturbance. In addition, soil samples were taken using U100 sampling tubes pushed into the side and base of the trench and excavated. Visual inspection of the trenches revealed extensive fissuring of desiccated clay to a depth of approximately 600 mm below the crest height of the levee in each excavation. A photograph of one side of the hand excavated trench is shown in Figure 2. A Figure 2. Desiccation fissures photographed at Thorngumbald clay levee. The fissures extended approximately to 600 mm below ground level. The two identified zones (A and B) are indicated by the white dashed lines. It appears from the trial pits that the pattern of desiccation fissures can be divided into two zones. The zones are labelled A and B in Figure 2. In the upper zone A, there is a two dimensional array of fissures both perpendicular and parallel to the drying surface; similar to the pattern described by Konrad and Ayad (1997). In the lower zone B, fissures extend vertically from the horizontal fissures and terminate in intact soil. This simple classification of desiccation fissuring suggests that there is an upper surface zone of fill material that has been transformed into a rubblised soil structure, with a two dimensional network of fissures that significantly increases the mass permeability of the fill material and so allows water to flow through and along that portion of the levee. This pattern results in infiltration of water into the surface of the flood levee, which can potentially lead the landward slope to collapse. During the second visit in 2006, deep fissures were also observed in the historic levee to a depth of 1.0 m below crest level. The extent of the fissuring was so pronounced that the widths of the fissures were measured at 10 cm intervals. However, the network of fissures was different from the first survey. In this latter trial trench the fissures were not connected into a two dimensional network but instead tended to be single deep fissures that would allow the seepage of water into the depth of the levee but not lateral flow beneath the surface of the levee slope as it will be shown in the following section. In comparison field survey was carried at the new flood levee in The new levee was constructed B

The image shows a polygonal pattern of fissures characteristic of desiccation cracking.

3 in 2003 from locally excavated alluvial clays extracted from a borrow pit area of the new salt marshes. The field survey of the new levee in 2006 identified desiccation fissures along the crest of the levee as shown in Figure 3. The image shows a polygonal pattern of fissures characteristic of desiccation cracking. The width of the polygonal desiccation fissures varied from 5 to 25 mm and they were generally found in areas of poor grass cover. This particular shape of superficial fissures is similar to the patterns observed by Konrad and Ayad (1997). Figure 4. Double ring infiltrometer test. These observations concur with anecdotal evidence from full-scale cofferdam tests carried out by Marsland and Cooling (1958) where water overtopping the crest failed to reach the landward side because the water drained into the crest of the trial levee too rapidly, which led within matter of minutes to the progressive collapse of the landward face eventually resulting in breach. Figure 3. Desiccation fissures on the crest of the new levee two years after construction (2005). 2.2 Measurements of bulk permeability In addition to visual inspection of desiccation fissuring, double ring infiltrometer tests were carried out to measure the effect of fissuring on the mass permeability of the clay fill. The rings were driven approximately 10 mm into the crest of the new levee to provide a seal (see Figure 4). The results showed that water seepage occurred rapidly and the corresponding mass permeability is comparable to sand. Elsewhere on fissured sections of the new levee, the water drained away too rapidly to allow any meaningful readings to be taken. In comparison, the infiltration rate measured for a double ring infiltrometer test carried out on an unfissured section of the new levee was three orders of magnitudes higher than typical rates for clayey soils. This pronounced difference in infiltration rates is due entirely to the presence of desiccation fissures. The presence of fissures radically increases the mass permeability of the clay fill to that of a coarse grained soil. 2.3 Measured suction profiles In addition to the visual record of desiccation fissuring the material properties of the disturbed soil samples were measured to determine shrinkage, plastic and liquid limits and the in-situ moisture content as shown in Figure 5. The corresponding PL, SL, LL for the clay fill were determined to be 14, 25 and 49 % respectively. The shrinkage limit was determined measuring the volumetric change in a bath of mercury. The moisture content profile clearly shows that the highly desiccated clay fill within the top 500 mm has dehydrated towards the shrinkage limit of 14%. Instead, below 500 mm depth the in-situ moisture content varies between the shrinkage and plastic limits. This distinct change in moisture content agrees with the visual inspections of the trial trenches that show a well defined zone of desiccation. At greater depth the moisture content increases to approximately 30%. Similar moisture profiles were observed by Cooling and Marsland (1953) after the 1953 North Sea flood. The moisture content is close to the shrinkage limit in the upper part and then it varies between the shrinkage and plastic limits in the deeper layer.

Thorngumbald Trial Trenches Moisture Content (%) Depth (m) 0.0 0.2 0.4 0.6 0.8 1.

Suction profiles achieved from trenches excavated below the levee crest and side slopes (2003).

4 Thorngumbald Trial Trenches Moisture Content (%) Depth (m) Trench No1 Crest Trench No1 Slope Trench No2 Crest Trench No2 Slope Trench No3 Crest Trench No3 Slope Trench No4 Crest Trench No4 Slope B 1.2 S.L. P.L. Figure 5. Suction profiles achieved from trenches excavated below the levee crest and side slopes (2003). 3 DESCRIPTION OF THE IDENTIFIED FAIL- URE MECHANISM The field observations and infiltrometer tests highlight the potential for desiccation fissuring to alter the fabric and texture of the fill material and increase the mass permeability by several orders of magnitude. This is caused by interconnected patterns of fissures that allow the rapid seepage of water into the desiccated zone. Based on these observations, Figure 6 shows a proposed failure mechanism where excessive internal seepage during overflow condition could lead to uplift of the desiccated clay blocks and progressive slope failure. It is proposed, that the network of shallow desiccation fissures could allow a critical hydraulic head to be developed beneath the outward slope leading to the uplift of the desiccated blocks of rubblised fill material. This failure mode depends on the hydraulic continuity between the levee crest and outward slope to allow a critical hydraulic head to be developed for uplift to take place. This scenario agrees with the observations reported by Cooling and Marsland (1953) for large scale cofferdam experiments. However, in the case of Cooling and Marsland the slope failure was attributed to an increase in pore pressure, that based on these new field investigations is unlikely to take place or be a primary factor in slope failure. A C a) b) Figure 6. Identified failure mechanism leading to levee breach: a) sequence of events taking place; b) identified uplifting of the clay blocks formed by desiccation fissuring. 4 SMALL SCALE TESTS Small scale laboratory studies have been undertaken to investigate the onset of desiccation fissuring. Fissuring was induced by allowing discs of soil to dry on both laboratory bench and inside a pressure plate under increasing soil suction. For the laboratory bench tests, soil samples were remoulded at the liquid limit and placed into cylindrical moulds with various diameters and thicknesses on a glass plate to compare the effect of geometry on the onset of desiccation fissures (diameter ranging from 66 mm to 122 mm). Inside the pressure plates, soil discs were prepared using 5 rubber 66 mm diameter sample rings; each one was numbered, weighed and measured. The samples were subjected to air pressures to create suction within them. This pressure was gradually increased to allow equilibrium of the samples pore-water pressure to the air pressure being reached. This allowed the gradual desaturation of the samples to occur. The pressure stages used were: 50

5 kpa, 100 kpa, 200 kpa, 300 kpa, 400 kpa and 500 kpa. Each pressure stage was applied for two days to the samples to ensure balance of the pore pressures. The samples were weighed between successive pressure stages in order to calculate the water loss. Once each stage had been completed, the 5 samples were dried in the oven and weighed to determine the soil water characteristic curve (SWCC). 4.1 Critical cracking ratio Cracking was observed for laboratory bench tests by progressively reducing the thickness of the soil disc. The geometry and relative proportion of the soil disc were observed to influence the development of desiccation fissures and in particular the development of a pronounced crack across the diameter of the soil sample. The effect of geometry and relative sample thickness can be characterised by considering a critical cracking ratio (CCR), first termed by Dyer (2005), which is the ratio of the diameter to the depth of the sample when one crack extends diametrically across the sample as shown in Figure 7. A unique value of critical cracking ratio was determined for each clay type tested. halfway between liquid and plastic limits is shown in Figure 8. The results indicate a relationship between plasticity index and critical cracking ratio. In addition, the graph shows a significant increase in the critical cracking ratio when samples were laid in a drier condition. This suggests that the extent of cracking and the propensity of fissures to form decreases with lower initial soil moisture contents. Critical Cracking Ratio Critical Cracking Ratio vs Plasticity Index Decreasing initial moisture content Plasticity Index (%) Laid at L.L. Laid at P.L. + P.I./2 Figure 8. Critical cracking ratio vs. plasticity index, for samples at L.L. (liquid limit), and midway between the plastic and liquid limit (after Coulson, 2003). Figure 7. Photograph of a whole crack running from edge to edge of the cylindrical specimen used. The presence of minor fissures parallel to the major one is also visible (after Coulson, 2003). To investigate the effect of the initial moisture content on the onset of cracking, soil samples were prepared with a different initial moisture content chosen to be halfway between liquid limit and plastic limit (ie. P.L. + ½ P.I). A comparison between the samples prepared at the liquid limit and those at 4.2 Comparison between small scale and field tests The soil water characteristic curves (SWCC) determined from the pressure plate tests allowed a link to be established between the onset of desiccation cracking of the soil discs in the pressure plates and the corresponding soil suction. The soil discs in the pressure plates were constructed with CCR=12. Desiccation fissuring was observed to take place at a matric suction of 300 kpa. Based on the SWCC, the soil suction corresponded to a moisture content of approximately 16%. The result is in good agreement with the moisture content determined for the disturbed soil samples taken from the desiccated zone in the Thorngumbald flood levee. The laboratory and field studies suggest that the susceptibility of different clayey fill to desiccation fissuring can be replicated in the laboratory for small soil discs with geometry corresponding to the CCR; and the moisture content or soil suction at which the material is likely to develop significant desiccation fissures in the field can be predicted and hence monitored. 5 CONCLUSIONS Field observations show that desiccation fissuring can result in two main patterns of fissures. These are either individual fissures that can extend to considerable depth (up to 100 cm) or a network of fissures

6 that effectively transform the fill material into a rubble that markedly increases the mass permeability of the fill material. This latter pattern of fissuring is the more critical condition that allows high quantities of flood water to seep into the levee and flow through the structure all be it at a shallow depth of typically 30 to 60 cm. This critical condition can result in different adverse effects. The first and perhaps most obvious is erosion of the fill material. A second is the development of a critical hydraulic head within the desiccated surface layer of the flood levee particularly along the side slope. The hydraulic head could potentially uplift the rubblised layer of fill and initiate slope instability as illustrated in Figure 6. This would result in progressive collapse of the side slope as observed by Cooling and Marsland (1053) for the 1953 North Sea Floods. But in the case of the studies by Cooling and Marsland, the seepage of flood water was interpreted as an increase in pore pressure and corresponding decrease in effective stress leading to slope instability. This earlier explanation now seems less plausible given that desiccation has been shown to transform the soil into a rubblised material. As a result it is proposed that seepage through the rubblised fill could result in uplift or wash out of blocks of clay fill triggering a progressive slope failure and hence breach initiation when the collapse extends up to the crest of the levee. This is a new and alternative interpretation of the mechanics for breach initiation due to desiccation fissuring that needs to be proven in model tests. If proven to be correct it would be relatively simple to characterize it using either limit equilibrium methods or a numerical method (e.g. DEM) and determine design charts. KONRAD, J.M. & AYAD, R Desiccation of sensitive clay: field experimental observations. Canadian Geotechnical Journal, 34, MEADOWCROFT, I.C., MORRIS, M., ALLSOP, N.W.H. & MCCONNELL, K Tollesbury managed set back experiment. HR Wallingford, Wallingford, R&D Report TR 5. MARSLAND, A The shrinkage and fissuring of clay in flood banks. Building Research Station, Internal report No. 39/68. MARSLAND, A. & COOLING, L.F Tests on full scale clay flood bank to study seepage and the effects of overtopping. Building Research Station, Internal report No. C562. MORRIS, M., DYER, M., SMITH, P., FALKINGHAM, J. & SIMM, J Management of flood defences. DE- FRA/EA, R&D Report FD ACKNOWLEDGEMENTS The financial support of the EPSRC Flood Risk Management Research Consortium (FRMRC) and of the Environment Agency is gratefully acknowledged. 7 REFERENCES COOLING, L.F & MARSLAND, A Soil Mechanics studies of failures in the Sea Defence banks of Essex and Kent. Proc. of the ICE Conference on the North Sea Floods of 31 January / 1 February London, pp COULSON, B The effect of fine fissuring of clay on the stability of flood defence embankments. MEng Final Year Report, University of Durham, Durham. DYER, M Performance of flood embankments in England and Wales. Proc. of ICE Water Management, 157, No 4, DYER, M Further tests on the fissuring of clay fill at the Thorngumbald flood embankment. Proc. International symposium on advanced experimental unsaturated soil mechanics (TARANTINO A., ROMERO E. and CUI Y. J. (eds)). Taylor & Francis Group, London.

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