Hydromechanical constraints on piping failure of landslide dams: an experimental investigation
© Okeke and Wang. 2016
Received: 26 June 2015
Accepted: 23 March 2016
Published: 1 April 2016
Understanding the internal structure and material properties of landslide dams is essential for evaluating their potential failure mechanisms, especially by seepage and piping. Recent research has shown that the behaviour of landslide dams depends on the internal composition of the impoundment. We here present an experimental investigation of the hydromechanical constraints of landslide dam failure by piping. Experiments were conducted in a 2 m × 0.45 × 0.45 m flume, with a flume bed slope of 5°. Uniform dams of height 0.25 m were built with either mixed or homogeneous silica sands. Uniform-sized pebbles encased in a plastic mesh were used to initiate internal erosion. Two laser displacement sensors were used to monitor the behaviour of the dams during the internal erosion process while a linear displacement transducer and a water-level probe were deployed to monitor the onset of internal erosion and the hydrological trend of the upstream lake.
Five major phases of the breach evolution process were observed: pipe evolution, pipe enlargement, crest settlement, hydraulic fracturing and progressive sloughing. Two major failure modes were observed: seepage and piping-induced collapse. Majority of the dams composed of homogeneous material failed by seepage and downstream slope saturation, whereas dams built with mixed material failed by piping.
We found that an increase in soil density and homogeneity of the dam materials reduced the potential to form a continuous piping hole through the dams. Furthermore, the potential for piping and progression of the piping hole through the dams increased with an increase in the percentage of fines and a decrease in hydraulic conductivity. The rate of pipe enlargement is related to the erodibility of the soil, which itself is inversely proportional to the soil density. This study provides new insights into the governing conditions and breach evolution mechanisms of landslide dams, as triggered by seepage and piping.
Piping is an important hydrogeomorphic process which plays a vital role in the modification of the natural environment. Its widespread occurrence in all climates and in a wide range of soil types may be due to variations in the physicochemical conditions of the environment. Many geomorphological and hydrological studies have been made of the phenomenon of piping, due to its significant contributions to hillslope erosion processes such as gullying and landslides, as well as in the transport of solutes from regions of higher hydraulic head to regions of lower hydraulic head (Masannat 1980; Jones 1994, 2004; Faulkner 2006; Wilson 2011). Nevertheless, piping remains one of the major causes of failure of embankment dams. For example, the 1976 failure of the Teton Dam in Idaho (USA) was triggered by uncontrolled piping on June 3 during the initial filling of the reservoir. The dam failed two days later, claiming 11 lives, with an estimated total damage of about 1 billion dollars. Stene (1995) reported that over 300 million dollars was paid in claims for damage caused by the disaster. Similarly, piping is one of the common failure modes of landslide dams, although the phenomena is rarely observed, due to challenges posed by rugged terrain, which sometimes limits access to downstream areas. Several examples of the piping failure of landslide dams have been reported in the literature (Glazyrin and Reyzvikh 1968; Costa and Schuster 1988). The 1835 rock and debris avalanche dam that formed Lake Yashinkul on the Tegermach River in the present Republic of Kyrgyzstan is one historic example. This dam failed by piping in 1966, after a lifespan of 131 years (Pushkarenko and Nikitin 1988).
The internal structure of landslide dams plays a vital role in understanding the failure mechanisms of valley-confined deposits, and most importantly, for evaluating the mechanical resistance of landslide dams to failure by either piping or overtopping (Wassmer et al. 2004). Acquiring grain size data is critical for accurate assessment of landslide dams, but limited sedimentological data is available, due to crude sampling methods and the challenges posed by rugged terrain and poorly exposed deposits. Nevertheless, the study of sedimentological characteristics of natural river blockages is imperative for natural hazard assessment studies, as they have a major influence on the overall strength of dams, and control the rate of breach development (Fread 1988; Casagli et al. 2003). Grain size distribution analysis has been used to study the internal structure of some landslide dams (Crosta et al. 2006; Dunning 2006; Duman 2009; Dunning and Armitage 2011; Shugar and Clague 2011; Wang et al. 2013). Field and laboratory analysis of textural characteristics of several landslide dam materials show that they are mostly poorly to very poorly sorted, matrix- or clast-supported, finely skewed, brecciated, stratified to massive sediments, are usually armored with angular boulders, and are sutured with a matrix of very fine materials (Weidinger 2006; Capra 2007, 2011).
Numerous experimental methods have been used to simulate the development of internal erosion and piping in earth dams and landslide dams (Wit et al. 1981; Brauns 1985; Maknoon and Mahdi 2010). Hanson et al. (2010) analyzed variation in erodibility of different soil materials due to internal erosion in dams by conducting large-scale outdoor model tests. They observed that the rate of erosion in differing soil materials varied in orders of magnitude. Marot et al. (2012) used a triaxial cell device coupled with a hydraulic system to study the influence of angularity of coarse fraction grains on the internal erosion process. They concluded that the angularity of coarse fraction grains may increase the erosion resistance of the tested soils by a factor of five. Richards and Reddy (2012) performed laboratory experiments on the initiation of backward erosion in cohesive and non-cohesive soils using a triaxial test device called a true triaxial test apparatus. They observed that the primary mode of failure of non-cohesive soils was by backward erosion, which required a seepage velocity of 0.8 ~ 1.1 cm/s to initiate piping in uniformly graded sands. Ke and Takahashi (2012) studied the mechanical effect of internal erosion on gap-graded non-cohesive soils by conducting one-dimensional seepage tests using a fixed-wall permeameter. These authors concluded that the onset of internal erosion is indicated by loss of fine material, coupled with a significant increase in hydraulic conductivity.
Significant results have been obtained from experimental studies using triaxial cell chambers, constant-head permeameter, and other laboratory methods such as hole erosion tests (HET) and jet erosion tests (JET). However, none of these methods have simulated the potential failure mechanisms of landslide dams triggered by internal erosion and piping. This paper presents a series of experiments conducted to investigate the hydromechanical constraints of soils on the development of internal erosion and piping in landslide dams. The experiments were performed in a flume equipped with monitoring sensors and transducers capable of recording transient changes during the process. The two main objectives of this research are: (1) to identify the various failure mechanisms of landslide dams under varying hydromechanical properties of the materials forming the dams, and (2) to evaluate and infer the probable physical properties of dam materials and conditions most appropriate for long term stability of such dams. The research methodology aims at providing new ideas and insights on stability analysis of landslide dams, considering the sparse research to date on the material properties of landslide dams which are relevant in dam breach analysis and flood routing.
The experiments were carried out in a rectangular flume tank specifically designed for this study. Prior to the construction of the tank, the size, scale, and position of the dam model were considered to enable timely collection of accurate data. The flume tank was 2 m long, 0.45 m high and 0.45 m wide, and was made entirely of glass, with two 4 cm diameter drainage outlets at the downstream end to allow steady discharge of fluidized sediments. The flume was gently tilted to make a bed slope of 5°. The construction and modification of the flume tank considered the dimensions of other flume tanks used by previous studies, including the 1 m × 0.6 m × 0.45 m model used by Sidle et al. (1995), 5 m × 0.3 m × 0.5 m (Awal et al. 2009), 1.5 m × 1 m (Wilson 2009), 1.4 m × 1 m (Wilson 2011), and 0.5 m × 0.5 m × 0.5 m (Fox et al. 2014) flumes. The upstream lake was recharged from a drainage hose fed by a water tap. Pre-failure crest settlements associated with the onset of internal erosion and piping were measured with two CMOS multi-function analog laser displacement sensors attached to a wooden overboard. Concave-upward depressions observed during the initiation of internal erosion and piping were monitored by positioning one of the sensors (H d1) directly above the center of the dam crest, while the other sensor (H d2) was fixed 0.07 m from the central part of the crest. These measurements were made by allowing a maximum vertical distance of 0.25 m between the dam crest and the sensor heads. Lateral displacements and downstream transport of eroded soil particles were measured using a linear displacement transducer (LDT) with a maximum response speed of 0.2 m/s and a measuring range of 1.0 m. The transducer was fixed at the upper end of the flume tank, and a thin metallic wire attached to the sensor head was connected to a plastic ball of similar weight to the materials used in the experiments. The ball was buried at the center of the dam crest, while the outer end of the metallic wire, suspended by a known weight, passed through four stationary pulleys. Transient changes in the upstream water levels were tracked with a pore-water pressure sensor with a rated capacity of 50 kPa. The pore-water pressure sensor was fixed at a stationary position near the upstream dam toe to ensure accurate measurement and recording of data. All these sensors were connected to a standard real-time monitoring and recording unit comprised of a universal recorder (KYOWA EDX-100A) and a computer. The experimental setup was designed to simulate (1) the internal erosion process and piping development in relation to the nature of the material forming the impoundment; (2) the failure mechanisms of landslide dams made of materials of varying geotechnical properties; and (3) the rate of development of piping. Two digital video cameras were strategically positioned to record the failure sequence of the dams.
Experimental procedure and properties of the soils used
Mechanical and hydraulic properties of the dam materials D 50 is mean grain size, C u is coefficient of uniformity, C c is coefficient of curvature, ρ dry is dry bulk density, e o is the initial void ratio, n is porosity, and k is coefficient of permeability
D 50 (mm)
C u (mm)
C c (mm)
ρ dry (Mg/m3)
Dam mix A
3.9 × 10−6
Dam mix B
3.1 × 10−4
Dam mix C
4.8 × 10−5
Dam mix D
1.1 × 10−5
Dam mix E
3.2 × 10−4
Dam mix F
6.1 × 10−4
Dam mix G
8.2 × 10−4
Dam mix H
5.8 × 10−5
Summary of experiment carried out for dams composed of mixed materials
T e (s)
T b (s)
Dam mix A
Equal amounts of SS 4, 5, 6, 7 and 8
Piping and hydraulic fracturing
Dam mix B
SS-5 and SS-8 (70:30)
Piping followed by pipe roof collapse
Dam mix C
SS-6 and SS-8 (30:70)
Piping followed by pipe roof collapse
Dam mix D
SS-4 and SS-8 (30:70)
Well-defined piping hole which supported the pipe roof
Summary of experiments carried out for dams composed of homogeneous materials
T e (s)
T b (s)
Dam mix E
Downstream slope saturation;
Toe bulging followed by slope unraveling
Dam mix F
Downstream slope saturation;
Dam mix G
Poorly developed piping hole;
Downstream slope unravelling
Dam mix H
Formed well-defined piping role;
Supported the pipe roof
Results and discussion
Observed phases of the breach evolution process
Several phases of the breach evolution process were observed during repeated experiments. Qualitative assessments and observations, coupled with data obtained from precision sensors, helped to distinguish the various stages. The breach development processes observed included (1) pipe development, (3) pipe enlargement, (4) crest settlement, (4) hydraulic fracturing, and (5) progressive sloughing. While the breach development process followed the sequence listed above in some dams, others failed retrogressively, resulting in late-stage overtopping of the dams. Therefore, before describing the results, it is pertinent to define and describe the various failure phases observed in this study.
This process is related to the formation of a continuous piping hole, as a result of increased action of seepage forces through the soil micropores. The initial stage of this process starts with the initiation of internal erosion, which in this case, was enhanced by concentrated seepage through the artificial drainage channel. An abrupt drop in velocity of the seeping water at the opposite end of the drainage channel, about 5 cm before the downstream slope face, reduces the pressure of the seeping water through the soil micropores. However, the erosive cycle continues as a result of high pore-water pressure, leading to a partial reduction of the shear strength of the soil, as observed at the downstream slope face in the form of a ‘wet spot’. Summarily, the numerous complex mechanisms leading to the initiation of internal erosion and subsequent development of a piping hole include (1) generation of high pore-water pressure due to the incessant action of the seepage flux, which reduces the apparent cohesion of the soil, (2) increase in seepage forces through the soil micropores, which reduces the effective stress of the soil and produces drag forces sufficient for soil particles to be detached and entrained downstream, and (3) gradual evolution of the existing micropores, essentially caused by the hydraulic shear stress exerted by the seeping water. A continuous pipe is formed through the dam once appreciable aggregates of soil particles are removed and transported downstream by the flowing water. From observation during the experiments, it was noted that at the onset of the pipe development process, the initial diameters of the developing pipes were mostly smaller than or equal to the diameter of the artificial drainage channel.
This phenomenon was observed in all the experiments, but the evolution process varied with the material forming the dam. In this case, crest settlement is related to the formation of a concave-upward depression at the center of the dam crest, as a result of internal erosion and piping within the material underlying the dam crest. This phenomenon is usually initiated by seepage forces through the dam, and can be associated with several other processes, such as soil arching, cracking, and hydraulic fracturing. The effect is more pronounced in low-density fine-grained soils and cohesionless soils with high void ratios, in which the development of very high pore-water pressure conditions leads to the reduction of the effective stress of the soil. Crest settlement was more evident in dams built with homogeneous materials than in those built with reconstituted materials, excluding dams containing a significant amount of fines, such as dam mixes D and H.
This failure mechanism is common in dams built with reconstituted materials. The hydraulic fracturing process is initiated by differential settlement, arising from the different compressibilities of the soils, coupled with uneven compaction. This leads to the development of tensile stresses in weak or soft zones as pore-water pressure increases through the dam. Observations during the experiments found that as soon as the upstream lake level reached the tip of the encased pebbles, seepage forces converged into the pebbles and any other hydraulically weak zone, leading to erosion of soil particles along the developing conduit. The crack formation can be related to increased pore-water pressure, which reduces the minor effective principal stress across the plane of the crack (σ 3 < 0). This further implies that hydraulic fractures occur once the pore-water pressure in the dam is greater than or equal to the total stress σ 3, or equal to the tensile strength of the soil, σ t (Mattsson et al. 2008).
This type of failure was observed in dams built with very loose cohesionless materials (dam mixes E, F, and G), and to a lesser extent in dams built with dam mixes A and B, but was rarely seen in dams built with very fine sand and silt. The process is often triggered when the seepage forces are less than or equal to the shear strength of the soil. Thus, the inability of the seeping water to produce sufficient drag forces needed to dislodge and entrain the soil particles and form a continuous piping channel leads to gradual seepage flow towards the downstream slope. The saturation of the downstream slope due to seepage leads to the reduction of the effective stress of the soil and subsequently causes very small slumps and slides in the form of cantilever failures to occur at the toe of the slope. This, in turn, leaves very steep faces which fail under increased pore-water pressure. This cycle of failure continues until the exposed section of the dam yields to the effect of increased pore-water pressure, and slides downstream, leading to the partial breaching of the dam.
General description of the experiments
The initial condition set for the experiments assumed that either the dammed lake is of low discharge or the location where the landslide blocked the valley is relatively ‘dry’ (Korup 2004). Hence, no tailwater was present at the downstream area, since the slope of the flume tank was 5°. After building the dam model, the upstream lake level was gradually increased until the water reached the tip of the encased pebbles. The hydrological trend and failure mechanisms of the dams were largely controlled by the hydromechanical properties of the soil materials. The two major failure modes observed were seepage and piping. While piping failure was dominant in dams composed of very fine materials or mixed soils with an appreciable amount of fines, seepage, and downstream slope saturation were dominant in dams built with homogeneous cohesionless soils.
Failure mechanisms of dams built with mixed materials
Failure mechanisms of dams built with homogeneous materials
Effect of density on erodibility of the dam materials
Natural analogues of seepage and piping in landslide dams
As mentioned before, the potential for seepage and piping failure of landslide dams can be attributed to the textural and sedimentological properties of the impoundment material, as well as the hydrological characteristics of the upstream lake. From a sedimentological perspective, the likelihood of piping and seepage failure of rock avalanche dams is high in comparison with landslide dams that preserved the original stratigraphy of their source rock. A vast number of rock avalanche dams are comprised of fragmented materials and are mostly characterized by a binary internal structure consisting of: (1) a highly pulverized and matrix-supported basal layer which is very erodible but has low permeability because of its low void ratio, and (2) an upper layer dominated mostly by a coarse blocky carapace of disjointed angular boulders, with large void spaces that support internal erosion (Davies and McSaveney 2011; Strom 2013). Similarly, Dunning et al. (2006) identified three distinct sedimentological facies in rock avalanche dams: the Carapace, Body and Basal facies. The high hydraulic conductivity of the Carapace facies and the relative nature of its unstructured comminuted mass serves as a channel for seepage erosion and piping. This phenomenon is analogous to the 1992 failure of the Rio Toro landslide dam in Alajuela Province of Costa Rica. The dam failed by seepage and piping when the upstream lake level reached an elevation of 934 ~ 936 m and seeped into the upper pervious and blocky carapace layer, resulting in progressive failure and undermining of the downstream slope (Mora et al. 1993). Another similar event is the 2004 failure of the Tsatichhu landslide dam in Bhutan, which failed by a combination of dam face saturation and progressive seepage through the upper Carapace facies (Dunning et al. 2006). The failure mechanisms of these two natural analogues displayed similar characteristics to the results of our present study.
A comprehensive experimental investigation was conducted to study the hydromechanical constraints of soils in the initiation and development of piping in landslide dams, with emphasis on the potential failure mechanisms of dams composed of various soil materials. Landslide dams were built in a flume tank, and internal erosion within the dams was initiated using an artificial drainage channel composed of uniform pebbles encased in a plastic mesh. Initial condition assumed that the upstream lake was ‘dry’. Thus, the upstream lake was recharged at a steady state flow rate of 1.2 × 10–4 m3/s.
The various phases of the breach evolution process observed in this research included pipe development, pipe enlargement, crest settlement, hydraulic fracturing and unraveling of the downstream slope. The potential for formation of a continuous piping hole in dams composed of homogeneous soils decreased with an increase in bulk density and hydraulic conductivity. Dams composed of homogeneous soils failed mostly by seepage and downstream slope saturation (Dunning et al. 2006), whereas piping holes were formed in dams built with mixed soils, depending on the percentage fines content and the interlocking bonds between the soil particles.
This study did not measure erodibility coefficients of the dam materials, as it can be obtained from hole erosion tests (HET) (Wan and Fell 2004). However, it is evident from the results obtained here that the time of failure and the potential breach evolution mechanisms of the dams were controlled by soil density, including other factors such as void ratio, permeability and the degree of compaction, except in a few cases such as dam mix D (run series 4), where the interparticle bonds between the soil particles and low permeability of the material affected soil erodibility. Early onset of internal erosion was observed in dams built with mixed materials, especially in the case of dam mix A. This condition may be attributed to suffusion, which caused adverse changes in permeability, porosity, void ratio and pore-water pressure distribution in the soil. Conversely, the time of onset of internal erosion in the homogeneous dams increased with an increase in permeability.
Crest settlement and associated features such as cracking and hydraulic fracturing occurred in all the experiments, but were characteristically pronounced in the homogeneous dams, mostly in dams built with dam mixes F and G. The mechanism of dam crest collapse in dams built with mixed materials originated from two primary transverse cracks running parallel to the downstream slope, whereas the collapse mechanism of the homogeneous dams began from a longitudinal crack perpendicular to the downstream slope.
Although the breach evolution process observed in this study and the hydromechanical behaviour of the dams were constrained by space and time scales adapted in the flume experiments, the results provide valuable insights into the factors controlling the potential for seepage and piping failure of landslide dams. However, hole erosion tests (HET) need to be conducted on the finer materials used in these experiments to further evaluate the relationship between soil erodibility and hydraulic shear stress.
This investigation was financially supported by JSPS KAKENHI Grant Number A-2424106 for landslide dam failure prediction. We thank Prof. BP Roser (Shimane University) for his helpful suggestions and editorial assistance. The authors would like to thank the anonymous reviewers for reviewing the draft version of the manuscript.
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