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.
Pipe development
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.
Pipe enlargement
The mechanism of pipe enlargement can be related to the effect of the hydrodynamic forces produced by the flowing water on the hydromechanical properties of the soil under varying physicochemical conditions. The evolution of the pipe through the dam changes the dynamics of the seeping water from low-pressure flow through the soil micropores to high-pressure flow through the enlarging pipe. At this stage, the enlargement of the pipe and subsequent progression of the breaching process is usually rapid, and thus depends on several properties of the soil, including the interlocking effect, the shear strength, and density of the soil, as well as the energy of the flowing water. The tractive force theory based on the bed load formula suggests that the amount of sediment transported per second per unit width of a conduit q
s
is a function of shear stress τ (Singh 1996):
$$ {q}_s=f\left(\tau \right) $$
(1)
Thus, the erodibility of the soil at the periphery of the flow path and the hydraulic shear stress are two key factors which determine the rate of erosion and the time of progression through to completion of the breaching process. The complexity of the pipe enlargement process with respect to sediment transport mechanics has been described by the excess shear stress equation:
$$ \varepsilon ={k}_d{\left({\tau}_a-{\tau}_c\right)}^a $$
(2)
where ε is the sediment transport rate (m/s), k
d
is the erodibility coefficient (m3/N-s), τ
a
is the hydraulic shear stress on the soil boundary (Pa), τ
c
is the critical shear stress (Pa), and a is an exponent, usually assumed to be 1 (Hanson and Cook 1997; Fell et al. 2003). Enlargement of the pipe depends on the ability of the material to support the pipe roof. Hence, observations from the series of experiments showed that most of the homogeneous dams failed by progressive saturation of the downstream slope. In contrast, well-defined piping holes were formed in dams composed of mixed materials with a higher piping tendency, as evident in dam mixes D and H, where the pipe roofs survived for a relatively longer time.
Crest settlement
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.
Hydraulic fracturing
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).
Progressive sloughing
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.
The first physical defects observed during the experiments were the formation of longitudinal cracks at the upper part of the upstream slope, as the level of the upstream lake rose with time. This was usually followed by the settlement of the entire dam, and subsequent formation of concave-upward depressions at the center of the dam crest. The rapid increase in seepage gradient along the encased pebbles enhanced initiation of internal erosion at the boundary between the dam material and the encased pebbles. Progression of internal erosion through the dam was manifested by the appearance of a wet spot on the downstream slope (Fig. 7a). Generally, two primary transverse cracks are formed near the left and right banks of the dam. With time, these transverse cracks were crosscut by miniature cracks from which small slumps and slides may occur. Evolution of the piping hole at the downstream slope could be rapid or may collapse instantaneously, depending on the shear strength of the dam material (Fig. 7b). The breaching process continued with enlargement of the piping hole due to steady saturation, and subsequent collapse of the pipe wall through existing cracks and weak zones on the slope (Fig. 7c). The piping hole diameter increased with corresponding increase in the discharge through the dam, causing a rapid drawdown of the upstream lake. The pipe roof collapsed once pore-water pressure was greater than or equal to the effective stress of the dam material (Fig. 7d). The collapse of the pipe roof led to a gradual rise in the upstream lake until the energy of the eroding medium was able to dislodge and transport the collapsed sediments downstream. The experiments ended by the formation of a wide breach channel, with base width ranging from 0.1 m to 0.35 m (Fig. 7e).
Failure mechanisms of dams built with mixed materials
Four representative experiments were carried out to assess the potential for piping and failure of dams composed of mixed materials of varying physical properties (Table 2). The characteristic physical properties of the reconstituted materials influenced the stage hydrographs and the deformation behaviour of the dams. The potential for formation of a continuous piping hole through the dams increased with an increase in percentage fines content. The mechanism of failure of the dams was primarily initiated by piping, although the likelihood of the developing pipe to form a uniform cylindrical hole varied with the density and magnitude of shear stress exerted by the seeping water. Similarly, the ability of the dam material to support the roof of the piping hole varied with the amount of fines in the soil. Visual observations show that the dam mix D material (Run series 4) manifested higher tendency of sustaining the roof of the piping hole in comparison with other materials, where the roof of the poorly developed piping holes collapsed under steady propagation of the wetting front. Hydraulic fractures caused by internal stress redistribution increased with an increase in heterogeneity of the materials. Figure 8 shows the hydrological trends and mechanisms of breach evolution in dams composed of mixed materials. A steady rise in upstream lake level initiated a hydraulic head gradient that produced seepage forces through the dams. This consequently led to the formation of concave-upward depressions, mostly at the center of the dam crest. The formation of these depressions on the dams may be attributed to internal instability caused by suffusion which triggered the development of extensive cracks and soft zones (Fig. 9). The rate of change of volume (settlement) of the unsaturated dam materials and the decrease in shear strength may be attributed to the relationship between wetting front propagation and the hydromechanical properties of the materials. The onset of internal erosion is marked by buckling of the ball target due to loss of tension induced by the shear stress of the seeping water. It is interesting to note that the progression of seepage and formation of a piping hole coincided with lowering of the upstream lake level (as a result of steady discharge through the piping hole), and retraction of the ball target attached to the linear displacement transducer. The results indicate that the progression and enlargement of the piping hole were controlled by cohesion, grain size distribution, particle density and energy of the eroding medium. The general deformation behaviour of the dams composed of heterogeneous and anisotropic cohesionless materials was characterized by the formation of cracks (longitudinal and transverse) aligned perpendicular and parallel to the dam axis. The final stage of the breach evolution process was marked by formation of a wide breach channel with base width ranging from 0.1 m to 0.25 m.
Failure mechanisms of dams built with homogeneous materials
The failure mechanisms of dams composed of homogeneous and isotropic cohesionless materials were assessed with four different soils (Table 3). A steady rise in upstream lake initiated a hydraulic head which enhanced concentrated seepage through the artificial drainage channel. Propagation of wetting front was observed to occur at rates higher than those observed in dams built with mixed materials. Steady propagation of wetting front within the unsaturated cohesionless dam materials resulted in a decrease in matric suction (negative pore-water pressure) which caused a reduction in interstitial voids, as evidenced by the formation of concave upward depressions at the central part of the dam crest. Figure 10 shows the hydrological trends and failure mechanisms of dams built with homogeneous and isotropic cohesionless materials. The onset of internal erosion and mobilization of the soil particles adjacent to the drainage channel coincided with buckling of the ball target and formation of cracks, thereby triggering initial deformation of the dam crest. Internal redistribution of stresses initiated by intense seepage led to several processes including hydraulic cracking, dam crest settlement, downstream face saturation and toe bulging, and downstream slope unraveling. The majority of these processes were apparent in experiments conducted with dam mixes E, F, and G, where steady seepage through the dams resulted in exfiltration, sapping erosion, undercutting and sloughing of the partially liquefied soil. In contrast, the experiment conducted with dam mix H revealed a well-formed piping hole that lasted for a relatively longer time (Fig. 11). The rapid drawdown of the upstream lake level could be related to the rate of enlargement of the piping hole due to the erosive forces of the seeping water, which continued until the material supporting the pipe roof collapsed into the channel. The results indicate that the potential to form a piping hole through the dams decreased with an increase in density and hydraulic conductivity.
Effect of density on erodibility of the dam materials
Figure 12 shows the relationship between the time of collapse of the dam crest, T
b
and dry bulk density of the various soils composing the dams. Variations in hydromechanical behaviour were more evident in dams composed of soils of lower density than in those built with soils of higher density. An exception to this case was dam mix D (T
b
, 184 s), where the interlocking bonds between very fine particles of silica sand 8 and coarser particles of silica sand 4 at optimum water content seemed to be stronger than in other soil samples. Previous studies of internal erosion and soil erodibility observed that progression of the piping hole and erodibility of material at the periphery of the piping hole depended on the compaction density and water content (Hanson and Robinson 1993; Fell et al. 2003). Field erodibility tests conducted by Chang et al. (2011) on two landslide dams triggered by the 12 May 2008 Ms 8.0 Wenchuan earthquake in Sichuan Province of China showed that an increase in bulk density was inversely proportional to the coefficient of erodibility with depth. Furthermore, large-scale physical tests carried out by Hanson et al. (2010) in their investigation of the impact of erosion resistance on internal erosion in embankment dams identified that erosion resistance for the same embankment material increased with an increase in compactive effort and water content. Observations made during the experiments in our present study yield similar results. Similarly, a comparison between the failure process of run series 4 (dam mix D) and run series 2A (dam mix F) shows that even at a higher erosion rate q
s
, the breach evolution process of the two dams varied greatly. Figure 13 shows the relationship between the initial void ratio of the dam materials and the time of collapse of the dam crest (T
e
and T
b
). The time of onset of internal erosion, T
e
, and the time of dam crest collapse, T
b
, reveals the effect of internal instability caused by suffusion. This led to early initiation of internal erosion in dams composed of mixed materials, even at lower void ratios, due to selective removal of fines from the soil matrix and subsequent destabilization of the soil structure. This result indicates that void ratio and other physical properties such as permeability, percentage fines content, and density affect the initiation and progression of internal erosion in dams. Furthermore, the relationship between initial void ratio and the dimensionless settlement index S
I
(Fig. 14), clearly shows the behaviour of the dam materials as pore-water pressure and seepage gradient increased through the dams:
$$ {S}_I=\frac{s}{1000\times H\kern0.5em \log \left[\frac{{\mathrm{t}}_2}{{\mathrm{t}}_1}\right]} $$
(3)
where s is crest settlement in mm between times t
1 and t
2, and H is the dam height in meters (Charles 1986). The general deformation behaviour of the dams shows that settlement increased with an increase in void ratio. Low density cohesionless soils with high void ratios, such as silts, are generally brittle, and thus are prone to cracking during differential settlement. This process is usually associated with the formation of tension cracks and other weak zones of low stress condition. A typical example is the Red Willow Dam in southwest Nebraska, USA. The dam is a 38.4-m high homogeneous embankment made up of low plasticity silts. Emergency investigations conducted by the US Bureau of Reclamation discovered sinkholes at the downstream face, whereas cracks appeared above the outlet works conduit and other locations near the right abutment. The brittle nature of the embankment material coupled with the low density and plasticity index of the soil led to the settlement of the crest to about 1.2 m below the original height, which caused the reservoir level to be lowered.
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.
