Experimental analysis of scour under circular pier

Rivers are natural flowing systems. Construction of a bridge pier across a river obstructs the flow of the river. The flowing water in the river erodes the sediment around bridge piers and all bridges and structures associated with waterways are potentially at risk of failure from hydraulic scour. Shirole & Holt (1991) stated that 60% of the 1,000-plus bridge failures occurring in the USA could be attributed to undermining of pier foundations following scouring or erosion of the channel bed. To protect bridges from scouring action, study needs to be carried out so that scour can be minimised and henceforth also the percentage of failures due to undermining.

‘Man, who overlooked water under bridge will find bridge under water’ (Neill 1973). This citation highlights the capability of water to erode the river bed and expose the foundation of bridge to scour. Scour is a natural phenomenon caused by the erosive action of flowing water on the bed and banks of alluvial channels. For economical design, problem scour around bridge piers is required to be controlled. A primary reason for concern about the stability of bridge foundations is the occurrence of scour around the pier. An experimental study has been under taken by Hamidifar & Omid (2017) to analyze the physics of scour holes for a sand clay mix bed that lies downstream of an apron. Sharafati et al. (2020) have used stochastic modelling to enhance the accuracy of scour prediction, whereas Pandey et al. (2018) experimentally investigated the temporal scour variation around bridge piers under clear water. Singh et al. (2020b) performed an experimental study to analyze the clear water scour under short contractions for sand beds. Illustration of the complex relation of flow and sediment with time and monitoring of the key consequences of bridge pier failure due to scour have been studied by Link et al. (2020).

Numerous researchers studied the effect of parameters that influence scour around a bridge pier for cohesive soils (Najafzadeh & Barani 2014; Devi & Barbhuiya 2017; Mahalder 2018). Estimation of the equilibrium scour depth is needed for economical and secure design of the infrastructural components of bridges Briaud et al. (1999). While underestimation of the scour depth leads to the design of too shallow a bridge foundation, on the other hand overestimation leads to uneconomic design (Ting et al. 2001). Khwairakpam & Mazumdar (2009) determined local scour around hydraulic structures. Singh et al. (2020a) reviewed the remedial techniques for local scour around bridge piers. A handy number of scour studies have been carried out with the aim of predicting equilibrium scour depth (Kothyari et al. 1992; Kothyari 2007; Jueyi et al. 2010; Masjedi et al. 2010; Fathi et al. 2011; Farooq & Ghumman 2019; Liang et al. 2020). Use of flow diverging devices to protect the bridge pier from scour has been studied by Zahedani et al. (2018). Memar et al. (2020) found a reduction in scour depth at couple of piers in a tandem arrangement due to the influence of collars. A number of studies has been carried out with a view to determining the equilibrium scour depth for clear water scour under steady flow condition. Pandey et al. (2017) evaluated the efficiency of the prevailing equations for scour around circular bridge piers. Numerous researchers have also presented equations to calculate equilibrium scour depth in the case of piers (Laursen 1958; Shen et al. 1969; Neill 1973; Breusers et al. 1977; Raudkivi & Ettema 1983; Yanmaz & Altinbilek 1991; Melville & Chiew 1999). Based on the studies of Laursen & Toch (1956), Breusers et al. (1977) established that the scour depths increase when discharge increases. The coarser sediment additionally produced a lesser scouring effect at the piers. Pandey et al. (2020) predicted the scour depth under a circular pier for a non-uniform gravel bed and validated the proposed expression.

Flow altering devices are used so that the shear stresses on the riverbed, in the vicinity of the pier, are reduced by altering the flow pattern around a pier, which in turn reduces the scour depth at the pier. The different flow altering devices, namely scour reduction using slots, scour reduction using collars, scour reduction using sacrificial piles, Delta-wing passive devices, and stone gabions with geo-textile filters have been used in the past to alter the flow and reduce scour (Beg & Beg 2013). The majority of the studies are devoted to determining the maximum depth of the scour around the bridge elements. A slot may be choked by floating debris and its construction is difficult as well. Sacrificial piles might become ineffective when the flow advancing on the piers changes its direction. Stone gabions and geotextile filters are likely to be clogged with the passage of time.

Scour at the location of bridge piers founded on erodible beds is a major problem and concern. Foundation depth increases with depth of scour and hence the cost of bridge. Sometimes, the foundation of a bridge pier gets eroded/scoured due to more scouring by river than was anticipated at the design stage. In that case, scour depth can be minimized in existing bridges with an external device, such as by attaching a triangular prism. A very few studies on rectangular slots are available in the literature (Kumar et al. 1999) and research on reducing scour depth under circular piers with a triangular prism arrangement is very scanty. The triangular prism may be an effective arrangement to protect a circular pier from the scouring action of a river.

In the current study, the problem of scour around circular bridge piers has been addressed in order to meet the research gap discussed above. A triangular prism model has been used to understand the variation in scour depth. Laboratory experiments have been performed on the different prism arrangements at various values of discharge. The experimental results of temporal change in scour depths of a circular pier without and with a triangular prism (facing towards U/S and D/S) direction of flow have been compared. The effectiveness of triangular prism models with respect to simple circular piers has also been found. The experimental results of data measured for circular piers was compared with the equation proposed by Lacey for scour depth and was found to be unsatisfactory. Hence, a new relationship is proposed and then validated with observed data.

Three river sites were selected to take the bed sample for the experimental run; the sites are in the vicinity of Chandigarh, India. The first site is Ghaggar river, which is an intermittent stream in India, streaming amid the storms. It starts in the town of Dagshai in the Shivalik Hills of Himachal Pradesh and moves through Punjab and Haryana and weakens in Rajasthan, southwest of Sirsa, Haryana. This site was selected because Ghaggar river is known for its flashy floods in monsoons. Many times, the water of these floods reaches Fort Abbas in Pakistan. The sample was collected from a place with the co-ordinates 30°40′53″N-76°52′32″E.

The second site is Patiala Ki Rao, a seasonal stream that starts in Shivalik Range, Punjab, enters Chandigarh then onto Mohali and later converges into the Ghaggar River. The stream keeps running crosswise over Fateh Burj at Chappar Chiri. It is one of the seasonal rivulets amongst others in Chandigarh, which incorporates Sukhna Choe in the East and N Choe in the West. The sample was collected from a place with co-ordinates 30°47′01.6″N - 76°47′41.4″E.

The third site is a super-passage site near Kotla power house, situated between Kiratpur Sahib and Anandpur Sahib in Punjab, India, on Satluj canal with co-ordinates 31°12′41.8″N - 76°33′05.0″E, as shown in Figure 1.

Three soil samples were used in the present investigation from three different sites, namely Ghaggar, Patiala ki-Rao and Kotla super-passage. To find the type of soil, grain size analysis was carried out in the laboratory. Figure 1 represents the sites from where the samples were obtained and Figure 2 represents the grain size analysis of the three soil samples. The values of d₁₀, d₃₀ and d₆₀ were obtained from Figure 2 and then the coefficient of uniformity (C_u) and coefficient of curvature (C_c) were found, as shown in Table 1. Hence, Ghaggar soil was found to be uniform soil (poorly graded) and Kotla super-passage and Patiala-ki-Rao samples were found to be non-uniform soil (well graded).

The experimental setup consists of a rectangular hydraulic flume, gauges for the measurement of depth at various sections, a discharge measuring tank, different pier arrangements, a 36 HP motor and sump. The diameter (D) of the pier was taken one-tenth of width of flume (i.e. 35 mm) so that the effect of streamlines on pier could be neglected.

The pier material was a hollow steel pipe filled with concrete. The prism selected has its nose at 2.5D from the outer face of pier with a nose angle of 19̊12′. The triangular prism was welded with the piers, and both pier and prism had adequate strength to sustain wear and tear caused due to flow. The schematic view of the flume and discharge measuring tank in the Water Resources laboratory of Punjab Engineering College (Deemed to be University), Sector-12, Chandigarh, is shown in Figure 3.

The soil properties were studied, and respective silt factors were calculated for the three soil samples collected from the three-river site in the vicinity of Chandigarh (India). The soil samples were then placed one by one on the hydraulic flume with adequate compaction. Simultaneously, the pier arrangements were set up. Three pier arrangements were selected and placed in the hydraulic flume at 1.22 m, 3.05 m and 4.87 m from the inlet, respectively. The first pier, which was 1.22 m away from the inlet, was a simple circular pier. The second pier, which was 3.05 m from the inlet, was a circular pier with a triangular prism facing upstream attached to it; and, thirdly, the last pier, which was 4.87 m from the inlet, was also a circular pier attached to a triangular prism with the prism facing downstream. Discharge was measured in the discharge measuring tank with the help of a ball gate valve. The readings of scour depths were measured during the experimental run at 15-minute intervals using an inverted pointer gauge. Each soil sample was laid 4–5 times in the flume and readings of scour depth were obtained for different discharge values using an inverted pointer gauge that could measure precisely up to 0.5 mm. Figure 4(a) shows the arrangements of pier and triangular prism, Figure 4(b) shows the circular pier before the experimental run and Figure 4(c) shows the final scour in a circular pier after the experiment.

The readings of scour depth observed after two hours in every experimental run have been tabulated in Table 2, were compared with Lacey's scour depth and plotted against each other, as shown in Figure 5(a). The observed readings of scour depth under the circular pier were much less as compared to Lacey's scour depth, as shown in Figure 5. It was observed that Lacey's values were overestimated and a calibration is required. After calibration, a new relationship was proposed between scour depth and (q²/f) value, as shown in Figure 5(b).

• q – Discharge per unit width

• f – Silt factor

A relationship between scour depth and (q²/f) was proposed, as given in Equation (2). The relationship was obtained with three values of scour after two hours from each soil sample. This relationship was required to be validated. The values left were used in validating the relationship, the model was validated by putting the values of (q²/f) in the proposed relation and comparing those values with observed values. The comparison brought satisfactory results with minor variation of ±20% from observed values, as shown in Figure 7.

It was observed from Table 3 that scour depths obtained at a certain discharge are in the following sequence in all the experimental runs: simple circular pier > circular pier with prism at D/S end > circular pier with prism at U/S end.

This study is important in finding the scour depth around circular piers in the vicinity of Chandigarh city; scour depth can be minimized for the existing bridges with an external device by attaching a triangular prism. The study also helped to analyse the effect of discharge and different arrangements of the circular pier with triangular prism on the maximum scour depth. Experiments were performed in a rectangular hydraulic flume with three different soil samples from North India. For each soil sample, experiments were run for varying discharge values for three different arrangements. The existing relationship for scour depth proposed by Lacey was not found to be satisfactory for the present study. Hence, a new relationship for these was derived and validated. The efficiency of the triangular prism in U/S direction of flow was found to be maximum and the amount of scour reduction is of the order of 61–68% for the Ghaggar soil sample, 54–62% for the Patialaki-Rao soil sample and 53–68% for the Kotla super-passage soil sample for various discharge values. The efficiency of the triangular prism in the D/S direction of flow is lower than that of the U/S case. The amount of scour reduction is of the order of 20–32% for the Ghaggar soil sample, 18–33% for the Patialaki-Rao soil sample and 13–27% for the Kotla super-passage soil sample for varying discharge values. The experimental data did not comply with the calculated by Lacey's formula. The comparison of the observed value of scour depth with computed values clearly shows that all the data points are underestimated by ±70%. The comparison brought unsatisfactory results and hence a site specific relationship was proposed and validated for the present study for the region around Chandigarh city from where the samples from the river bed were collected.

None.

All relevant data are included in the paper or its Supplementary Information.