Experimental study on the skewed integral bridge by using ... - umexpert

ICSE6 Paris - August 27-31, 2012

Dr. Shatirah Akib

ICSE6-276

Experimental study on the skewed integral bridge by using crushed concrete geobags as scour protection Shatirah AKIB1, Afshin JAHANGIRZADEH2, Lim Hong WEI3, Sharif Moniruzzaman SHIRAZI4, Sadia RAHMAN5 1

Department of Civil Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia Email: [email protected]

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Department of Civil Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia Department of Civil Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia 4 Department of Civil Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia 5 Department of Civil Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia 3

During flood, scour that occurs to any bridge structure may cause a bridge structure to fail. The scour that happens during a flood is one of the major reasons that cause a bridge to collapse. Currently, prediction of bridge scour is difficult due to the complexity of scour mechanism. Most of scour countermeasure design is based on formula formed during an experimental works. In this study, a physical modeling had been carried out to investigate the scouring mechanism on a skewed integral bridge during flooding condition. The possibility of utilizing crushed recycled concrete as filler to geobag as a countermeasure was discussed. Various experiments were carried out to investigate its possibility to reduce the impact of scour. Recycled crushed concrete geobags were proven to be an alternative countermeasure to reduce scouring impacts. Materials were cheaper compared to normal sand filler and green technology could be promoted.

Key words Skewed integral bridge, scour, scour countermeasure, crushed concrete, geobag.

I

INTRODUCTION

In Malaysia, the use of integral bridge has also dramatically increased for the recent years. This is because of the high maintenance of the conventional bridge leads to the preference of integral bridge. However, since the development of integral bridge is still new in Malaysia, factors that caused bridge failure other than loading must be investigated. The effects of natural disaster such as flood are one of the recent interests in integral bridge structure because it can cause scouring on the integral bridge [Akib et al., 2008; Akib et al., 2011; Fayyadh et al., 2011]. When flooding occurs, scouring will happen to the bridge structures and causing the abutment and the piers to be exposed. Scouring is a lowering level of the riverbed by water erosion to expose the foundation of the bridge. In such way, the piers and the abutment will lose their load carrying capacity which incorporates the skin friction capacity between the soil and also the structure. Thus, the expose of the structure will lead to the failure of the structure, either serviceability, or ultimately. The scouring actions that happen on the bridge are caused by the bridge opening. The embankment can also be damaged, and the soil will be removed from behind the abutment. In United States of America, about 60% of bridge scour are caused by hydraulic deficiencies [Richardson and Davis, 1995], including pier scour. The flow of individual streams exhibits a manifold variation and great disparities exist among different rivers. The alignment, discharge, cross section, and slope of a stream must all be correlated with the scour phenomenon, and this is in turn must be correlated with the characteristic of the bed material ranging from clays and fine silts to gravels and boulders. The effect of the obstruction itself (pier and abutment) must be assessed [Emmett, 1956]. The local scour is caused by the formation of vortices, the horseshoe vortex (Figure 1). The water at the upstream surface of the pier caused the acceleration of flow around the nose of piers. This vortex action removes bed material from around the base when the piers are in contact with the bed material. In addition, 1

Corresponding author

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the water surface rises upstream of the pier, forming an emerging circular profile known as bow wave [Grimaldi et al., 2006]. A scour hole formed when the transport rate of sediments removed from the base region is greater than the transport rate of sediment added into the base region. As the depth of scour continues to increase, the strength of the horseshoe vortex will be reduced, and the transport rate of sediment away from the base region will be reduced eventually. At one point, for live-bed scour, the equilibrium is reestablished between the material inflow and outflow and scouring ceases. For clear-water scour, the scouring ceases when the horseshoe vortex scour shear stress equal the sediment particle at the scour hole shear stress thereby reducing the transport rate from the base region [Lagasse et al., 2007; Richardson and Davis, 2001].

Figure 1: Schematic representation of scour at cylindrical pier [Richardson and Davis, 2001].

In order to prevent the scour to be occurring, some scour countermeasure has been used to reduce the scour rate or elongate the scour time. Countermeasure methods for local scouring are a matter of concern for researchers as a preventive method. The reduction of the scour hole is the main purpose of scour countermeasures. The countermeasures are usually classified as those to protect the river bed or armoring countermeasures (e.g., riprap stones, mats and bags, Reno mattresses, concrete armors) and flow altering countermeasures, particularly aimed at influencing the horseshoe vortex (e.g., collars, slots, plates) [Parker et al., 1998; Zarrati et al., 2004; Melville, 2008]. Geo bag is a textile woven bag that is designed to be having enough strength and integrity to be filled with soil. A filled geo bag can be used for construction either shoreline structure or river structure. This geo bag is used to substitute other countermeasure such as rip-rap, gabion, or rock. In the industry, geobags are commonly used in coastal erosion prevention, but not in the river scour protection. Minimal number of research had been done on the possibilities of incorporating the abandoned concrete material as the filler inside the geobag as the countermeasures for scouring. It is economical when no suitable armor rock at the site. Geo bag is also allowing the utilization of finer materials to be used as filler material for construction of scour countermeasure. Many researches had been done to investigate the possibilities of the recycled concrete aggregate. Recycled aggregate can be generated from demolished construction structure which comprises of broken members or components such as slab, beam, brick wall and others. As for Malaysia government, the ninth Malaysian plan had encouraged the usage of recycled and reusable materials for the construction industry. However, the usage of recycled concrete aggregate in Malaysia is not common due to the abundant of resources for aggregate [Ismail Abdul Rahman et al., 2009]. There is a possibility of incorporating the recycled concrete into a geo-textile bag to stimulate a similar function, as a countermeasure as opposed to the conventional sand filled container. If this idea is a success, there are a lot of saving can be made since it is an alternative to the normal sand and the crushed concrete is free of charge. II

RESEARCH METHODOLOGY

The experimental work was constructed using a built up of skewed integral bridge model. The bridge model has a skew angle of 34 degree. This angle was chosen based on its superstructure of a single-span bridge with the highest allowable skew angle for integral bridge. The model was made using a polymer material. By applying the theory of geometry similarity, the prototype was distorted with a ratio of 1:75 to suit the condition of the hydraulic flume in the laboratory. The test flume is 16m long, 0.60m wide and 0.57m deep, and located at Hydraulic Laboratory in Department of Civil Engineering, University Malaya. The flume was using two fundamental elements to control the water depth and the water velocity, the water pump valve and the opening gauge. Thus, several pre-experimental works were done to get all the correct parameters for the valve turn and the gate opening height. This was to ensure the specific velocity at the upstream as well as the water level could be obtained before every experiment began. In order to simulate

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the flooding condition, the water level was set to be within the range of 200-205mm from the base of the flume for all experiment. The integral bridge had two sides, P and Q side (Figure 2). The piers and its corresponding abutment were labeled from P1-P8 and Q1 to Q8. The scour readings were taken by reading the vertical depth scales at the piers and the abutment. The vertical depth scales was a non-corrosive and transparent measurement tape. The model was constructed by using Perspex material.

Figure 2: Plan and Section view of the model integral bridge.

The skewed integral bridge was embedded into the floodplain. A flood plain was placed at the three quarter of the flume from upstream and to be modeled as a compound channel. Sand was poured into the flood plain until half of the abutment was covered. The main channel was poured with 5cm height of sand except at the middle of the floodplain. The sand was poured 50cm before and after the flood plain. The area was poured with 12cm height of sand to cover the piles of the pier. The sand was leveled to the required height and compacted. The scour depth was measured for 10 times at the interval of 10 minutes, followed by 5 times for 1 hour and 40 minute, and the last reading was taken after 24 hours. The scour measurement was taken at each side of the piles of abutment and pier (namely P side and Q side). In the experiment, the sand was used as the bed sediment with median particle size, d 50 = 0.13mm. The bed material specific gravity was 2.65. The Particle size: d50 = 0.8mm, d16 = 0.62mm, d84 = 1.04mm. Geometric standard deviation: σg= 1.29 (uniform bed) The final reading of the scour depth developed was read using a digital point gauge and the electromagnetic current velocity meter was used to measure the velocity of the water at each points

Figure 3: Digital point gauge (a) and Electric Current velocity meter (b).

The crushed concrete had been obtained from the dump area of University Malaya concrete laboratory. It was crushed to become smaller pieces using a crushing machine. The crushed concrete had been sieved according to BS1377: Part 2: 1975 with sieve range in 20mm to 75μm in order to measure the proportion of the crushed concrete content. The Geobag chosen was a mechanically woven bag. The bags can be made from various materials, but the most common bag is woven polypropylene (Figure 4). The size was 20 mm X 80mm X 50 mm. The size of the Geobag was calculated by using the design method proposed by Pilarczyk (2000).

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Figure 4: Geobag with fine crushed concrete and coarse crushed concrete.

III

RESULT AND DISCUSSION

The experimental work was conducted to study the effect of scour due to various factors such of time, velocity, side of the integral bridge, and countermeasure imposed. Its maximum scouring depths were observed over time for various velocities. The scouring depths were compared between without Geobag and with Geobag experiment. Fig. 5 showed the influence of time on local scouring depth of integral bridge for P side and Q side. P side and Q side had different scouring depth over time. P side always had higher scour development compared to Q side for 35cm/s and 45cm/s but not 25cm/s. The maximum scouring depth for 25cm/s was the same for P and Q side, both exhibits similar scouring depth over time. This is due to the low velocity of the flow which had no tremendous effect on the scouring depth development. From Table 1, the approach velocity at P side was 15.1cm/s, and Q side was 14.2cm/s. The minor velocity different did not impose much different on scouring depth due to the sediment coarseness. Average velocity, V (cm/s) 25 35 45

Approach velocity (cm/s) P side (Vp) Q side (Vq) 15.1 14.2 26.8 24.9 37.6 36.2

Table 1: Average velocity and its corresponding approach velocity for P and Q side.

For 35cm/s velocity, the maximum scouring depth for P side was greater than Q side, which was 29mm against 24mm. This was due to the difference of approach velocity. The difference of approach velocity caused the scouring depth to be greater at P side. For 45cm/s velocity, the maximum scouring depth at P side was the same as Q side. The sand was scoured till up to the bottom of the flume. The scouring depth could go even deeper if the flood plain was deeper. Thus, from the observation of the previous scouring depth (before 24 hours), the trends of scouring at P side had higher scouring depth compared to Q side. Figure 5 shows that scouring phenomenon occurred at the flood plain. Scouring depth was decreased at 400 mins for P side and around 600 mins for Q side for 45cm/s velocity. This was due to the armoring effect that occurred. The flood plain in front the scour holes area was almost flushed out at the time of 400 mins. The depth of water increased and decreased the approach velocity for this time. However, as time passed by, the sediment could not hold its position from the attack of the current on its face. The reason that this phenomenon happened for P side at 400mins and Q side at 600mins was that the P side was nearer to the current due to the bridge skewed. Regardless of it was with Geobag or without Geobag, the scouring depth was developed with time. Decreasing of scouring depth may happen sometimes but often increased over longer period. Despite having a fall of local scouring depth at some times, the overall of the graph of scouring depth was increasing for all cases with and without Geobag as a countermeasure.

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Figure 5: Influence of time on local scour of integral bridge for P and Q sides for with and without Geobag as countermeasure.

From Figure 6, 7 and 8, the graph shows that regardless of the velocity of the flume current, the scouring depth was decreasing by the implementation of Geobag as a scour countermeasure. For the velocity of 25cm/s, the maximum scouring depth had been decreased from average 9 mm from both P and Q side to average 4.5mm from both P and Q side. The Geobag had successfully decreased the depth of scour by 50%. For velocity of 35cm/s, the maximum scouring depth had been decreased from average 26mm from both P and Q side to average of 5mm from both P and Q side. The Geobag had successfully decreased the depth by 21mm, which was around 81%. For velocity of 45cm/s, the maximum scouring depth had been decreased from average 180mm from both P and Q side to average 72.5mm. The Geobag had successfully stopped the flow of 45cm/s from continuing eroded the flood plain to the flume bottom, which was 180mm. The Geobag delayed the catastrophic disaster to the integral bridge, which was the whole floodplain being flushed out as in 45cm/s without any countermeasure. After 24 hours, the Geobag was no longer in its original position and had switched its position.

Figure 6: Effect of time on local scour of integral bridge for both P and Q side for with and without Geobag as a countermeasure at 25 cm/s.

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IV

CONCLUSION

In this paper, the scouring of an integral bridge had been studied and a better solution to design a scour protection already achieved. Physical modeling of bridge clarified the rate of scour increased with the increased of velocity regardless of the existent of countermeasure. Crushed concrete as Geobag filler was proven to be an alternative countermeasure to normal sandbag for use as one of the best factors of bridge scour countermeasure. It decreased the scour rates for each velocity and there was no restriction in designing dimension. During the experiment, it was recognized that the finer the mixture of crushed concrete of the cement paste part, the more flexibility was. However, the coarser bed sediments produced slower scour rate. Furthermore, this crushed concrete was crushed from recycled concrete which promoted environmentally friendly, economical, and yet an effective solution to design scour protection. Furthermore, it could benefit the economic sectors because the wastes are turned into something useful. V

FUTURE STUDY

It is recommended that the geotextile filter have a lifetime of at least 20 years without decay when placed on the bed of a natural river in the vicinity of a bridge pier [Parker et al., 1998]. The scope of this study was limited to the experimental investigation on the skewed integral bridge by using crushed concrete geobags as scour protection only. Further research on the life time durability and materials of geobag as a bridge countermeasure should be undertaken. VI

ACKNOWLEDGEMENTS AND THANKS

Financial support by the High Impact Research Grants from the University of Malaya (UM.C/625/1/HIR/116) is gratefully acknowledged.

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VII REFERENCES Akib, S., Fayyadh, M. M., and Othman, I. (2011). – Structural Behaviour of a Skewed Integral Bridge Affected by Different Parameters, Balt. J. Road. Bridge. Eng., 6(2): 107-114.

Akib, S., Othman, F., and Othman, I. (2008). – Scour Behaviour on singly and doubly row pile integral bridges. Proceeding of United Kingdom Malaysia Engineering Conference, University College London. Fayyadh, M. M., Akib, S., Othman, I., and Razak, H. A. (2011). – Experimental investigation and finite element modelling of the effects of flow velocities on a skewed integral bridge. Simul. Model. Pract. Theory., 19(9): 1795-1810. Grimaldi, C., Gaudio, R., Cardoso, A. H., and Calomino, F. (2006). – Local scouring at bridge piers and abutments: Time evolution and equilibrium. Proceedings of 3rd International Conference on Fluvial Hydraulics, 1657–1664, Lisbon, Portugal, 6-8 September, 2006. Lagasse, P. F., Clopper, P. E., and Zevenbergen, L. W. (2007). – Countermeasures to Protect Bridge Piers from Scour. NCHRP Report 593, Transportation Research Board, National Academies of Science, Washington, D.C. Laursen, E. M., and Toch, A. (1956). – Scour around Bridge Piers and Abutments. Iowa Institute of Hydraulic Research, State University of Iowa. Melville, B. W.( 2008). – The physics of local scour at bridge piers. 4nd International Conference on Scour and Erosion, Tokyo, Japan, Parker, G., Toro-Escobar, C., and Voigt, R. L. (1998). – Countermeasures to protect bridge piers from scour. Final Report, National Highway Research Program, Transportation Research Board, National Research Council, University of Minnesota, Minneapolis. Pilarczyk, K.W. (2000). – Geosynthetics and Geosystems in Hydraulics and Coastal Engineering. A. A. Balkema Publications, Rotterdam. Rahman, I. R., Hamdam, H., Ahmad Zaidi, A. M. (2009). – Assessment of Recycled Aggregate Concrete, Mod. Appl. Scie., 3(10). Richardson, E. V., and Davis, S. R. (2001). – Evaluating Scour at bridges (4t ed.), Federal Highway Administration Hydraulic Engineering Circular No.18, FHWA NHI 01-001. Richardson, E. V., and Davis, S. R. (1995). – Evaluating Scour at Bridges. Third Edition. US Department of Transportation, Publication No FHWA-IP-90-017. Zarrati, A. M., Gholami, H. and Mashahir, M. B. (2004). – Application of collar to control scouring around rectangular bridge piers, J. Hydraul. Res., 42(1): 97-103.

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