DYNAMIC BEHAVIOR OF BED MATERIAL AROUND BRIDGE PIER UNDER ABRUPT CHANGE OF WATER PRESSURE

 

 

MD. FARUQUE MIA* and HIROSHI NAGO**

 

*Graduate Student, Department of Environmental and

Civil Engineering, Okayama University Japan.

**Professor, Department of Environmental and Civil

Engineering, Okayama University, Japan.

 

 

ABSTRACT

Major failure of structures usually occurs during flood flows due to scouring, sinking and sliding. These types of failure are considered to be in close relation to the dynamic behavior of bed material around the structures. Failure of bridges due to the phenomenon of scouring around piers is a common, natural problem of river hydraulics. Experiments were carried out to investigate the characteristics of pore water pressure and effective stresses in the bed material around a circular bridge pier under abrupt change of water pressure. A laboratory model was used to clarify the effect of the variation of water pressure to determine the mechanism of the collapse of circular bridge pier during flood or storm waves. The experimental results showed that the bed material under sudden change in drop of water pressure near the pier was weakened by an increase in excess pore pressure, followed by a considerable quick removal of bed material.

 

Keywords: Pore pressure, Effective stress, Abrupt water pressure change, Scour, Bridge pier

 

INTRODUCTION

Most investigators have attempted to develop the relationships for the measurement of maximum scour depth for steady flow, which are used for design widely. However, the flow in a river during flood is unsteady with a rapid change in discharge and the scour action is especially strong during flood. Over the past 30 years, a numerous laboratory investigations of local scour around structure have been reported in the hydraulic engineering literature. Most of the theories were developed considering the scouring parameters as fluid, flow, bed materials and the geometry of the pier. Nevertheless, there is no unifying theory to use with confidence for safe and economic design yet.

Nago (1981) investigated the dynamic behavior of sand bed under oscillating water pressure with a view of the collapse of hydraulic structures during floods or storm waves. The pressures due to surface waves are transmitted into the bed and they give rise to horizontal and vertical pressure gradients, which encourage liquefaction (Zen and

Yamakazi, 1993). Nago and Maeno (1986) stated that the strength of the sand bed decreases notably and the unstable zone will occur during the crest or the trough being in front of the structures. Sakai et al. (1992) suggested that an important mechanism that makes the bed surface layers more susceptible to erosion is bed liquefaction. Liquefaction is considered important for estimating scour at, and hence the stability of, coastal structures (e.g. Zen and Yamakazi, 1993). A bed is in liquefied state when it has very low or zero shear strength. This has two effects: (1) it removes the capacity of the bed to support a normal load (zero bearing capacity) and (2) it makes the bed much more susceptible to erosion by waves and currents because of the reduced intergranular friction.

Under variation in the water pressure normal to the surface of the sand bed, the pore pressure changes with time, and excess pore pressure occurs. An increase in the excess pore pressure produces a decrease in the effective stress of the bed material (Nago and Maeno, 1987). Therefore, the determination of the dynamic mechanism of this decrease in the effective stress is considered very important to the design of hydraulic structures under variation in water pressure. An extensive scour around the marine structures such as platforms, bridges, subsea templates and so on may reduce its stability due to the action of waves and current, thus leading to its failure.

In this study, we approached a new theme to investigate the fundamental characteristics of the pore water pressure and effective stress in the bed material around bridge pier under the abrupt drop of water pressure using a laboratory model. This behavior can be considered as an important mechanism for the design of hydraulic structures against failure of scour.

 

EXPERIMENTAL WORKS

The experiments to study the effect of pore pressure and effective stress in the bed material during development of scour hole around circular bridge pier models were conducted in a flume 16 m long, 0.60m wide and 0.40m deep, located in the Hydraulics Laboratory of Okayama University, Japan (Fig.1 (a), (b)). Water is conveyed to the flume from an elevated tank by a pipe through an approach channel to measure the discharge by means of a sharp crested weir. The flow rate in the flume was adjusted by a valve in the pipe. Then the corresponding head on the sharp crested weir was measured for the supplied discharge value. The depth of water was being changed by controlling the tale gate. The working section, 1.0m long, 0.60m wide, and 0.57m deep was located 8.0m downstream from the entrance of the flume where the pier was located. This section was filled with the sediment below the bed level and the bed was flattened with the same size of the sediment used in the test section. Before the start of the experiment for the variation of scour depth with time, the working section and the bed was made level. The pier was placed centrally and vertically in the test section. Then the leveled area around the pier was covered with 3-mm thick acrylic sheet (Kothyari, Garde, and Ranga Raju, 1992). The valve was slowly adjusted without causing any disturbance to the bed material until the desired discharge was reached to the flume (Yanmaz, 1990) and the required depth was obtained by controlling the tailgate. When the expected flow conditions were established, the acrylic sheet was removed carefully that ensures no scouring occurred around the pier due to this operation. The scour depths were recorded from a reading scale attached to the wall of the pier relative to the initial bed against time. Six transducers were connected to the amplifier to record the digital data of pore pressure around the bridge pier. Five transducers were set directly to the wall of the pier and the rest was 25 cm away upstream to the pier (Fig.1(c), (d)). The average of at least 30,000 samples processed by a computerized data acquisition system at 50 Hz was taken by a digital recorder at each measured point.

The experimental conditions that were maintained in the laboratory can be summarized as follows:

1.      At first, steady flow conditions of clear water were established and the scour depths (ds) were recorded against time.

2.      To investigate the effect of pore pressure and the effective stress in the bed material around the pier, the depth of flow was risen relative to the normal depth.

3.      The sudden drops were allowed at a stage when the equilibrium local scour around the pier was almost reached.

4.      In order to investigate the effect of pressure drop size, the experiments were conducted with sudden pressure drops of 5 cm, 10 cm and 15 cm respectively.

5.      Uniform bed materials were used with the mean particle size of 0.25 mm and the porosity was assumed as 0.40.

6.      The size of the pier was used 6 cm in diameter (D).

7.      Bed materials were placed as a 3-cm thick layer in the flume bed with a bed slope of 0.002.

 

FUNDAMENTAL EQUATIONS

The motion of the water and the sand in the layer were analyzed by the same method as for the ground water problems in the elastic acquifer. The relation between the compressive stress and the pore water pressure can be equated to the downward acting force on the plane of contact of the sand ( Nago, 1981). That is,

 

(1)

in which, pore water pressure rgh and the weight of the sand column above the plane of the contact gsz can be represented as follows,

(2)

(3)

 

where,

 

h : pore water pressure in head (variation from hydrostatic pressure

relative to mean water level)

hs : variation of water pressure acting on the surface of the bed

h' : excess pore water pressure

z : depth of the sand layer measured from the top of the sand

surface as datum

r : density of water

rs : density of sand

g : acceleration due to gravity

lw : porosity of water part

l : porosity of the sand column

( l=lw+la , la: porosity of air part )

 

Substituting equations (2) and (3) into equation (1), and considering lw@l,

 

constant (4)

from which the condition of liquefaction factor can be stated as,

 

(5)

 

EXPERIMENTAL RESULTS

Fig.2 shows the change of water surface variation relative to the initial water level (h0) of steady flow with time. In this paper, the effects of this fluctuation with a maximum rise of 10 cm water head on bed material around a circular bridge pier are considered for discussion. Fig.3 indicates the time history of the distribution of pore water pressure in each point considered in the model. It is clear that the pore water pressure increases with the rise of water head and decreases with fall of water head. Pt.2 was observed in water inside the scour hole. Pt.3, Pt.4, Pt.5 and Pt.6 were into the bed material. The pore water pressure in the level of Pt.3 and Pt.4 of bed material reached almost equal to that of Pt.2 but not hydrostatic. Fig.4 represents the effective stress of different levels of bed material around the pier. The effective stresses decrease gradually with increase of pore water pressure. In case of the level of Pt.3, the effective stress decreases sharply during the sudden drop of water pressure and liquefaction factor becomes less than one which indicates unstability of the bed layer. The liquefaction factor also indicates a value of less than one in other cases. Fig.5 demonstrates the removal of bed material during the corresponding phenomenon. A notable scouring was observed for the sudden drop of water pressure. This is happened due to the considerable decrease of the effective stress of bed layer around the bridge pier.

 

CONCLUSIONS

In this study, the dynamic behavior of bed material around a circular bridge pier under the action of abrupt change of water pressure was discussed based on the experimental results. The results showed that the effective stress in the bed material decreases notably as the pore water pressure increases and causes unstability of the bed around the pier due to the pressure change in drop. Because of the weakening of the bed, a significant amount of bed material removed out quickly in the presence of pressure drop and the local scour around the pier increased by 10% more than that of steady flow. Based on this investigation, the dynamic behavior of bed phenomenon can be performed for near future study on rational design method of hydraulic structures against failure and the estimation of local scouring around the bridge pier.

 

ACKNOWLEDGEMENT

This research was supported by a Grant-in-Aid for Scientific Research (C) (1998) from the Japanese Ministry of Education, Science, Sports and Culture. We are grateful for this support.

 

REFERENCES

kothyari, C., Garde, J. and Ranga Raju. G. (1992). "Temporal variation of scour around circular bridge piers." J. Hydr. Div., ASCE, Vol. 118, No.8, pp. 1091-1105.

Nago, H. (1981). "Liquefaction of highly saturated sand layer under oscillating water pressure," Memoirs of the School of Engrg., Okayama Univ., Japan, Vol.16, No.1, pp.91-104.

Nago, H and Maeno, S. (1986). "Dynamic behavior of sand bed around structure under wave motion." Memoirs of the School of Engrg., Okayama Univ., Japan, Vol.21, No.1, pp.81-91.

Nago, H and Maeno, S. (1987). "Pore pressure and effective stress in a highly saturated sand bed under water pressure variation on its surface." Natural Disaster Science, Vol.9, No.1, pp.23-35.

Sakai, T., Hatanaka, K. and Mase, H. (1992). "Wave-induced stresses in sea bed and its momentary liquefaction." Proc. Am. Soc. Civ. Engrs, J. Waterways, Port, Coastal and Ocean Engrg, Vol.118, (WW2), pp.202-206.

Yanmaz, A. M. and Altinbilek, H. D. (1990). "Study of time-dependent local scour around bridge piers." J. Hydr. Div., ASCE, Vol. 117, No.10, pp.1247-1268.

Zen, K. and yamakazi, H. (1993). "Wave-induced liquefaction in a permeable sea bed." Report of the Port and Harbour Research Institute, Japan, Vol.6, pp.155-192.

 

 

 

Fig 1. Experimental model

 

 

Fig 2. Variation of water surface profile

 

 

Fig 3. Pore water pressure

 

 

Fig 4. Effective stress

 

 

Fig 5. Excess scour developed due to pressure drop