Encyclopedia
Please note this is a comparison between Version 1 by Rita L. Sousa and Version 2 by Lindsay Dong.
Sinkholes are a significant underground hazard that threatens infrastructure and lives and sometimes results in fatalities. The annual cost of sinkhole damages exceeds $300 million, although this estimate is likely underestimated due to the need for national tracking. Sinkholes can also alter natural drainage patterns, leading to increased flood risk. While natural sinkholes occur, those in urban areas are predominantly manmade, caused by soil erosion from defective pipes, typically due to aging. Climate change, storm surges, and urbanization have accelerated subsidence in urban environments, posing greater risks to critical infrastructure and densely populated areas. Extensive research has focused on soil erosion in dams; however, this knowledge does not necessarily apply to erosion through orifices, where gravity and other factors play significant roles.
- internal soil erosion
- sinkhole
- defective pipes
- urban environments
1. Background
1.1. Internal Soil Erosion
The multifactorial nature of soil erosion certainly recalls a puzzle for the scientific engineering community. Indeed, soil erosion is commonly referred to as a complex natural process in which soil is carried away by physical forces such as wind and water, which can be both external or internal, i.e., it can occur at the surface or in the subsurface. External erosion is mainly attributed to surface wind loads and water flow [1]. This can be observed, for example, in desert settings or riverbanks [2]. Internal soil erosion is mostly due to water flow through the porous media.
Internal soil erosion happens when fine particles of soil migrate (or erode) through the porous domain due to seepage forces generated from water flow within a soil mass [3]. Internal soil erosion is a hazardous phenomenon since it often remains undetected until the final degradation steps and eventually failure, i.e., until it is too late to intervene [4]. Research that focuses on subsurface soil erosion (internal soil erosion) is disproportionally less than research that focuses on surface erosion processes (external soil erosion) [5]. Most subsurface erosion studies have concentrated on natural sinkholes primarily observed in soluble rocks [6][7][8][9][6,7,8,9] or on soil erosion through embankment dams [4][10][11][12][13][4,10,11,12,13]. Studies that focus on subsurface erosion in urban areas due to defective pipes are very rare.
(1)(1) Concentrated leak erosion Concentrated leak erosion happens when soil particles erode through an existing open space—e.g., cracks, gaps, animal burrows—due to seepage flow. Water flows within these open spaces freely and washes out the soil particles. This type of erosion mechanism is by far the most hazardous, which more frequently leads to the failure of embankment dams [4][13][4,13];
(2)(2) Backward erosion piping Backward erosion piping occurs primarily where a high hydraulic gradient exists beneath the embankments and downstream, and according to the U.S. Department of Interior [15], the seepage forces must be large enough to form a pipe in the direction of the seepage path;
(3) Contact erosion, also defined as ‘scour,’ refers to the gradual and selective erosion of fine particles that occurs at the contact zone between the foundation or core and filter during a flow regime parallel to the contact zone [13][16][13,16]. If this process remains undetected for a long period of time, backward erosion piping can occur as a result of this mechanism;
(4) Internal instability occurs when fine particles segregate or wash out from coarse particle mass due to the seepage flow. This type of erosion may occur with changes in volume (suffosion) or without changes in volume (suffusion), according to Fannin and Slangen [17]. Internal instability extensively happens in embankment dams. This type of erosion is one of the most studied by researchers, even though it is categorized as the least hazardous internal erosion mechanism.
It is undeniable that SEDP is a significant problem in urban areas, which can have fatal consequences, as demonstrated in previous case reports and investigations. This is a problem that is likely to become more pressing in modern cities because of increased urbanization and aging infrastructure. Despite the previous efforts to study internal erosion in embankment dams, these results cannot completely explain the mechanism of SEDP.
1.2. Internal Soil Erosion in Embankment Dams
Internal soil erosion in embankment dams has been extensively researched. Bonelli [14] and Robbins and Griffiths [13] classify internal erosion mechanisms in embankment dams into four groups: (1) concentrated leaks, (2) backward erosion piping, (3) contact erosion, and (4) internal instability, also known as suffusion and suffosion. These mechanisms (see Figure 1) may appear either separately or simultaneously depending on different factors that include the seepage flow direction, soil mass conditions, and soil particle distribution, among others.Figure 1. Mechanisms of soil erosion in embankment dams: (a) concentrated leak erosion, (b) backward erosion piping, (c) contact erosion, and (d) internal instability (suffusion/suffosion).
1.3. Soil Erosion in Urban Environments
One of the biggest threats stemming from soil erosion in urban environments is the formation of sinkholes. Sinkholes, cavities in the ground, are normally formed either by the dissolution of rock in karst environments or by internal soil erosion. Many researchers have studied the occurrence of sinkholes in karst terrains [6][7][18][19][20][6,7,26,27,28]. Soil erosion through dams is also relatively well understood due to the significant amount of research done in this field, as described in the previous section. However, in urban environments, deteriorating buried pipes are the main cause of SEDP. This type of internal soil erosion is initiated by the existence of defective buried pipes. Once the defect is present, if the water table is above the detective pipe, infiltration takes place, and the SEDP propagates due to the seepage forces induced by hydraulic gradients. As soil erosion progresses, a cavity right above the defect will form, which eventually may cause ground failure, typically causing a sinkhole [21][22][29,30]. This seems to be the most reported mechanism; however, if the defect is present in the lower part of the pipe, and the water table is below the pipe, exfiltration from the fluids inside the pipe may also cause soil erosion, creating a void right underneath the pipe. If it progresses, it may also lead to a collapse. Soil erosion in urban environments can have devastating consequences. Laura [23][31] reports a catastrophic sinkhole with damages of 7.7 million dollars in Tucson, Arizona, in 2002 due to soil erosion due to a broken sewer pipe. Also, in Syracuse, New York, a sinkhole with a diameter of 9 m and depth of 6 m collapsed because of the SEDP after a heavy storm in 2011 [24][32]. More recently 2017, another devastating sinkhole in the size of a football field appeared in Fraser, Michigan, causing 20 houses to sink into it [25][33]. A recent case of a large sinkhole happened in September 2021 near Hoboken in New Jersey, in the aftermath of hurricane Ida. The large sinkhole damaged the road and dragged two vehicles into it (see Figure 2).Figure 2. Sinkhole incident in NJ in the aftermath of tropical storm Ida in September 2021 (photo by Dr. Brunell, at Stevens Institute of Technology, NJ, USA).
2. Soil Erosion Due to Defective Pipes (SEDP): Experimental Studies
According to Kuwano et al. [26][36], embedded pipes may be defected due to several factors, including poor construction, aging process, environmental loads, external loads such as earthquakes and traffic loads, or a combination thereof. Kuwano et al. [26][36] reported that inadequate construction is the main reason for the origin of pipe defects, mainly due to human errors and accidents during construction. Indiketiya [27][35] describes defects in sewer pipes due to the construction of other nearby structures, such as piles, water pipes, and gas pipes. Indiketiya [27][35] stated that most of these defects are caused by human errors that can largely be avoided by proper due diligence during the planning phase of construction projects, which includes a detailed utility investigation. Utility investigations are fundamental in urban environments and densely populated areas with a complex subsurface utility infrastructure network (pipes, conduits, cables). It should include reviewing available public records and utility maps, contacting utility companies directly, when possible, and field inspection for evidence of subsurface utilities. Even though unintentional damage to pipes occurs during the construction of other structures, aging is likely to be the most influential factor causing the defect development in pipes. Pipe deterioration is intrinsically related to the type of pipe material, the environment surrounding the pipe, and the loads the pipe is subjected to during its life cycle. For example, ceramic-made and concrete-made pipes showed the highest deterioration proportion, and sewer pipes’ damage risk considerably increases when the pipe age is higher than 25 years [26][36]. Freeze-thaw cycles are an example of environmental loads which induce pipe defects [28][29][37,38]. Other external loads, such as traffic loads, adjacent construction of buildings, as well as earthquakes, can also cause pipe damage [30][31][39,40].2.1. Hydraulic Conditions
Based on the literature, hydraulic loading conditions are one of the most influential factors in the rate and volume of soil erosion. This makes sense since soil particle displacement initiates by seepage forces induced by a pressure difference resulting from the existence of pipe defect. Several researchers studied the effect of different types of hydraulic loadings on SEDP. Overall, three types of hydraulic conditions were used by different researchers to simulate real-life conditions: 1. monotonic water inflow (exfiltration) to simulate the pipe leakage. 2. monotonic water drainage (infiltration) to simulate leakage from surrounding media into the pipe. 3. infiltration-exfiltration cycles to study the long-term effects of leakage into and from pipes (see Figure 3 for details).Figure 3. Different types of hydraulic loading applications; (a) Monotonic water exfiltration (outflow), in which water supplies from the defect and drains from the top of the sample (b) Monotonic water infiltration (inflow), in which the sample is already fully saturated, and the water (rainwater) infiltrates through the defect, (c) Infiltration—exfiltration cycles where the water is supplied through the defect until it becomes fully saturated (exfiltration) and the supplied water discharges from the defect afterward (infiltration).
2.2. Pipe Conditions: Pipe Defect and Pipe Depth
2.2.1. Pipe Defect Characteristics
The characteristics of the pipe defect, i.e., its size, shape and location, and orientation, seem to also play a significant role in SEDP. The most critical parameter appears to be the defect size, with shape, orientation or location playing a more secondary role [32][33][34][24,45,52]. However, the ratio between the soil particle and defect size is more important than the size alone.
Table 12 summarizes the most important studies that consider pipe defect characteristics as a variable parameter. The most common characteristics studied are the: 1. defect shape: square, rectangular, straight (i.e., strip), circular, or waist-shaped (Figure 46); 2. defect location and orientation: top and side of the pipe; transversal or longitudinal; 3. defect size: from 2 mm to 20 mm.
Figure 46.
Different shapes of studied defects; (
a
) radial, (
b
) straight, (
c
) rectangular, (
d
) circular (hole), (
e
) waist-shape, and (
f
) square.
Table 1. Parameters that effect SEDP pertaining to pipe condition.
| Researchers | Defect Size (mm) | Defect Shape | Defect Location | Pipe Depth 1 (mm) |
|---|---|---|---|---|
| Mukunoki et al. [32] | 5 × 5; 2.5 × 10; 5 × 50; 5 × 78.5 | Square, rectangle, straight | Top of the pipe, on the circumference of the pipe | 100 |
| Mukunoki et al. [35] | 2.1 and 5 | Straight | on the circumference of the pipe | 100 |
| Guo et al. [36] | 10 and 20 | Circular | Top of the pipe | 100, 200, 300, and 400 |
| Tang et al. [33] | 3 and 9 | Straight | Top, side and horizontal of the pipe | 250 and 300 |
| Indiketiya et al. [37] | 10 × 60; 20 × 60; 30 × 60 | Rectangle | Top of the pipe | 400 |
| Basak and Sarkar [38] | 3 and 5 | Straight | Top, side and horizontal of the pipe | 100 |
| Ali and Choi [39] | - | Artificially created crack with no particular shape or dimensions | Bottom and top | 40 |
| Zhang et al. [34] | 5 and 5 × 10 | Circular and waist-shaped | Top of the pipe | 500 |
1 pipe depth is equivalent to backfill height (hs).
2.2.2. Pipe Depth
The influence of pipe depth on SEDP was studied by Guo et al. [36][42]. According to Guo et al. [36][42], pipe depth has a remarkable direct effect on the eroded zone volume. This makes sense since if the backfill height (hs) is higher, the eroded zone will be larger, and consequently, if the eroded void reaches the surface, the sinkhole diameter will be larger. However, the shallower the pipe, the more likely a sinkhole collapse will occur for the same amount of eroded soil. Thus, pipe depth is an essential factor to consider when performing risk assessments, as is the probability of the consequences. It should be noted that having a large-scale model in the laboratory presents significant challenges, namely specimen preparation, monitoring, water supply, etc.
2.3. Soil Properties
The backfill properties are some of the most influential factors in SEDP. In fact, how the backfill affects SEDP is the most studied factor by several researchers. The most studied properties are the Particle Size Distribution (PSD), relative density, and soil type. Mukunoki et al. [35][41] used three types of sands with a maximum grain size of 0.85, 2, and 4.75 mm to study the effects of PSD and the ratio between size defect and maximum grain size on SEDP. In their experimental work, the defect was radial. According to the results obtained by Mukunoki et al. [35][41], increasing the ratio between defect size and maximum grain size increases the cavity size, which potentially leads to a ground collapse. More importantly, for a given ratio, the SEDP rate is not constant, and the cavity volume varies depending on the backfill soil PSD curve; both the uniformity coefficient and the curvature coefficient of the soil play a significant role in SEDP. For example, for a constant ratio of B/Dmax = 1.1 (B is the defect width and Dmax is the maximum particle size) for two different backfills, the one with a greater uniformity coefficient and curvature coefficient is more susceptible to erosion.2.4. Other Influencing Factors
2.4.1. Nearby Pipes
Wang et al. [40][65] studied the effect of other nearby existing pipes and dynamic loads on SEDP. During the experiments, pipe depth, the relative distance between other pipes and the defective one, and the dynamic load amplitude were the varied parameters. It was observed that when the dynamic loading amplitude increased, the ground subsidence significantly increased (while the other parameters were constant). In addition, they observed that the location of the existing non-defect nearby pipes affected both the cavity formation and its shape. Sato and Kuwano [41][66] conducted several small-scale tests to investigate the effect of other buried structures on the cavity formation induced by erosion due to a defective pipe. They used rectangular wooden blocks in different positions to simulate other existing buried structures. They observed that the block position and orientation changed the cavity size and shape due to the hydraulic gradient changes near the defect. They also observed an increased soil loss when the block was located right above the defect, making this configuration the most vulnerable one in terms of soil erosion.2.4.2. Fluidization
Fluidization is a phenomenon that can occur when a defect is present on a pressurized pipe that can promote or cause the occurrence of SEDP. Fluidization occurs when the granular soils transform into a fluid-like state after being pressurized by a liquid, mostly water. This can happen when a pressurized buried pipe leaks within a granular backfill [42][67]. There are a few relevant studies that elaborate on the effect of pipe defects on the backfill disturbance area and shape, as well as the parameters that most influence the onset of fluidization [42][43][44][45][67,68,69,70]. Most of these studies focus on determining the critical flow rate for full fluidization; however, there are no studies that investigate the coupling between fluidization and soil erosion. It is worth highlighting that this mechanism is similar to the sand boiling in the embankment internal erosion mechanism.3. Soil Erosion Due to Defective Pipes (SEDP): Numerical Studies
Numerical simulation is an important approach to studying soil erosion; however, numerical simulations that concentrate specifically on modeling SEDP are rather scarce. Internal soil erosion is a multiscale problem, i.e., it occurs in dimensions of different orders of magnitude. Macroscale applications are normally modeled using continuous numerical models such as the finite element method (FEM) [46][47][48][71,72,73] and the finite difference method (FDM) [49][50][74,75]. The limitation of these models is that, due to their continuous nature, they are not able to model the changes in soil particle distribution during the erosion process (which occurs at the microscale). To address this limitation, many scholars developed numerical simulations to model the micromechanics of internal erosion [50][51][52][53][54][75,76,77,78,79]. Many recent studies use the discrete element method (DEM) since this particle-based method can take into consideration the effect of every particle [55][56][80,81], making this method ideal for micro-scale applications. Nevertheless, the method is not suitable for modeling large-scale systems due to its high computational consumption. Others use numerical methods such as the Boltzmann method [54][79] and the material point method [52][77] to model internal erosion at the micro- or mesoscale. There are also many numerical studies that use Finite Element Method (FEM) models to look into the effects of voids on buried structures like tunnels and pipes [57][58][59][60][84,85,86,87]. These studies focus on determining the stresses and strains caused by the voids on the existing buried infrastructure, but they do not model the processes by which these voids were formed (initiated and progressed), and they do not necessarily pertain to voids that were formed by SEDP. It should be noted that the number of numerical studies directly related to SEDP is low. Most of the studies listed above either study internal soil in general terms or study it in the context of other applications, such as landslides or embankment dams. None of the current numerical methods mentioned above can model the complex multiscale mechanics of the initiation and evolution of SEDP. The current studies are quite simplified; they either focus on the macroscale neglecting the small-scale grain interaction and changes in the particle size distribution during internal erosion, or focus on the microscale only and are unsuitable for modeling real-life problems. Certainly, numerical modeling to simulate SEDP (initiation and progression) requires further in-depth investigations, particularly when it comes to the development of efficient multiscale approaches.4. Discussion on the Potential Mechanisms of SEDP
Internal erosion is a significant concern in both embankment dams and defective pipes, and understanding the underlying mechanisms is crucial for mitigation and prevention. While there are similarities in the principles of internal erosion, distinct factors contribute to erosion in each scenario. In embankment dams, internal erosion occurs when eroded soil particles within the dam structure are transported by flowing water, compromising the dam’s stability over time. Figure 58 illustrates what scholarrs believe to be the sequential steps involved in SEDP and its correlation with the occurrence of sinkholes. Sinkholes are a direct outcome of the progression through steps 3, 4, 5, and 6. These steps encompass the development of a cavity resulting from hydraulic loading on a defective pipe (step 1), followed by the stabilization and expansion of the cavity under the influence of hydraulic loads (steps 2 and 3). As the cavity expands, the surrounding soil’s cohesion is compromised, leading to instability (step 4). Consequently, the soil above the cavity collapses, perpetuating the process of SEDP (step 5). This ongoing erosion ultimately leads to an enlargement of the defect size, referred to as the new defect size (step 6). The repetition of steps 3–6 eventually culminates in the occurrence of a sinkhole (step 7).Figure 58.
SEDP progression and sinkhole occurrence.
By examining the mechanisms and previous research on SEDP, it becomes evident that the application of hydraulic loads initiates contact erosion, also known as scour. This initial erosion process subsequently triggers other internal erosion mechanisms, such as concentration leak erosion and internal instability. Therefore, it can be hypothesized that SEDP follows a sequential pattern involving the formation and expansion of cavities, loss of cohesion, collapse of soil, and the enlargement of the defect size. These mechanisms are supported by existing studies on SEDP, which highlight the role of hydraulic loading in initiating erosion and the subsequent development of sinkholes.
5. Gaps in Current Knowledge and Needed Research
5.1. Lack of Extensive Studies and Repeatability
Most of the existing studies do not look at the particular parameter effect on SEDP in an extensive manner. They often tend to try to study several parameters in one experimental campaign ending up not varying each parameter in a systematic and extensive way. There is also a lack of repeatability in the results since, often, only a few experiments have been carried out without any tests performed under the same conditions for verification.5.2. SEDP Mechanisms
Most existing research focuses on one specific internal erosion mechanism: internal instability (suffusion/suffosion), where finer soil particles are eroded from within a matrix of coarser soil particles. However, from studies on soil erosion in dams, it exists different mechanisms that can lead to soil erosion. One of these mechanisms is leakage erosion, which is characterized by soil particle loss through cracks or gaps. Statistics of failures collected by Foster et al. [4] and the National Performance of Dams Program (NPDP) at Stanford University [61][90], and the ERINOH database [62][91] clearly show that the most dangerous mechanism of all initiating mechanisms of internal erosion causing failures in dams is concentrated leaks.5.3. SEDP Scenarios
Most of the research focuses on studies where the defect is at the top of the pipe. Some studied other locations, such as the sides. If the defect is present in the lower pipe part, and the water table is below the pipe, exfiltration from the fluids inside the pipe may also cause soil erosion, creating a void right underneath the pipe. It can then progress and lead to a collapse (Figure 69). Only Ali and Choi [39][50] performed limited small-scale tests in which the lower part pipe defects were considered. However, their results are inconclusive, and the effect of hydraulic conditions, e.g., the water table below the pipe, was not well considered.Figure 69. Two possible extreme scenarios for SEDP occurrence. The defect is located at the (a) top, with the water table above the pipe and (b) bottom, with the water table below the pipe.
