Underwater detection of scour of abutment foundation and its influence on stability of bridge piers.

Li-Gang, Fang ; Liang-Liang, Duan ; Kai, Liang 等

Scour is one of the major causes for bridge failure. To prevent this type of failure, it is necessary to detect the scour of abutment foundation. As a dependable method to measure scour, the underwater detection technology has been used to determine the scour degree of abutment foundation of the Zishui bridge of Xiangqian railway in China, and the transverse and vertical displacements of the bridge piers were analyzed resulted from different scour degrees of the abutment foundation. The results showed that the maximum transverse and vertical displacement of the bridge piers depended mainly on the ultimate scoured area of foundation base.

INTRODUCTION

At the bridge site of a river, the scour around piers and abutments is a common occurrence and poses a challenging problem to the engineers due to its detrimental effect to the foundation of piers and abutments. The scour of foundations of bridges has caused unexpected problems, e.g. catastrophic. Scouring will often be most harmful around the upstream face of a pier, and may eventually cause the pier foundation to become undetermined. Abutments, located at either end of the bridge, help to transmit the weight of the bridge including traffic to the foundation bed. On the other hand, piers are located within the bridge span. Based on the supply of sediment by the approaching flow, localized scour can be classified in two ways: clear water scour and live bed scour. Clear water scour is the situation where no sediment is supplied by the approaching flow to the scour zone. Live bed scour, on the other hand, occurs where there is a supply of sediment by the approaching flow to the scour zone. The flow field around piers has been well researched. Therefore, it is very important to detect underwater conditions and study the influence of scour on stability of piers.

The Zishui Bridge located in Lengshuijiang to Xinhua section of Xiangqian railway was designed and constructed with 4x60m through steel truss beam; all the piers and bridge abutments were built in 1937 and still in use now. The spread footings were used for the bridge. It was a period for resuming in construction time of the new and old line of Hunan province from 1958 to 1960, during this period, the bridge was resumed and constructed, and installed No 1 and No 5 piers. Due to the desire of navigable fairway, the top of existed pier have been heightened for 2.45m. The crack of wing walls of two abutments was serious, and the crack also existed in abutment body, therefore, it was needed for removing and reconstruction according to design requirement.

Because of the impact of technology and economy at that time, detailed engineering investigation has not been done, and lower rock stratum and karst development condition of the foundation was not in detail. In order to ensure safety of Zishui Bridge at stake K198+475 of Xiangqian railway and safe traffic of the bridge, underwater detection and geologic examination were used for abutment foundation and sub-grade condition of the piers, scour and damage condition of sub-grade and foundation were known, and finite element calculation model and manual computation method both can be used for the calculation and analysis of compression stress of foundation base and displacement of the pier top under the influence of different scour degree, to determine the impact of scour on stability of the piers.

ENGINEERING GEOLOGIC CONDITIONS OF THE BRIDGE SITE

The site exploration was carried out using coring to determinate the locations and thickness of the rock strata below the piers. The exploration revealed the main revealed stratum of the working area includes Quaternary residual diluvial horizon and Liujiatan section and Zhongya section of Carboniferous system Yanguan Group, it is divided from new to old as follows now: Quaternary residual diluvial horizon (Qh): consist of clay, residual clay, sand and gravel stone, mainly distribute on both sides along the river and low-lying point of gully. Liujiatan section and Zhongya section of carboniferous system Yanguan Group (Cly): greyblack middle thick silllike argillaceous limestone with greyblack flaggy siliceous rock or firestone band. The thickness is 65.6m, this set of stratum are distributed in the range of the whole working area. There is no obvious big regional structure existing in the working area.

UNDERWATER DETECTION

Because of the use of this bridge for a long time, and the design and construction information about this bridge construction are incomplete, its abutment foundation are required to be detected to verify some concrete data and information about this bridge: the form and size of the foundation, actual state of scour for the foundation etc. In order to detect the foundation and its concrete conditions of scour, the method of underwater detection must be used. Therefore, the method that the divers perform underwater tracing and picture photography should be used.

The Canon digital camera S30 with a waterproof case was used for underwater detection.

The maximum water depth is in the No.4 pier site with about 15m, all its foundation was flooded by water and the pier body was also flooded partly, this site is main navigation channel of this period now. And, there is rapidest current of water, the muddiest water quality, and relatively deep water depth in the No.4 pier site by observation, so its scour situation should be the most serious. In order to verify its foundation form and to measure and grasp scour situation, we should carry on careful underwater detection.

From the underwater photo, we could see that the foundation concrete on water face was scoured and destroyed, and the water face existed defect place, the foundation base was scoured seriously, and existed pore space.

For the condition of water return area in downstream of bridge foundation, the foundation concrete was kept well, and the foundation base was covered by silt.

The result of the detection is: though there was still 1.90m buried depth in some place of the No.4 pier foundation, all the upstream face and some downstream face surface have serious scour vugh, and the concrete of the foundation itself has presented the defect too.

THE INFLUENCE OF SCOUR ON STABILITY OF THE PIERS

The influence of scour on horizontal stability

When partial scour condition appear on horizontal bottom surface of the foundation resulted from scour (the upstream), as Fig. 1 shows, set horizontal size of the foundation as b, the size of scoured part is x, the bending moment of foundation base caused by horizontal wind force is M1, According to Fundamental code for design of railway bridge and culvert of P. R China, [K.sub.0] [greater than or equal to] 1.5, and then

[K.sub.0] = b - x/2/x/2 + [M.sb.1]/N [greater than or equal to] 1.5 (1)

There:

x [less than or equal to] 0.4(b - 2[M.sub.1]/N) (2)

[FIGURE 1 OMITTED]

It is obvious that when the scoured range of foundation base exceeds above-mentioned value, the pier foundation may be overturned. Even if no consideration of the influence of wind-force, scoured range of foundation base cannot exceed 40% of original size, and cannot exceed 20% of original of original size with consideration of wind load condition.

The influence of scour on the foundation base stress and eccentric examination

Because scour holes of piers are mostly formed at upriver, after scour of foundation base, the original foundation under unidirectional stress (straight line bridge) has became the foundation under bidirectional stress, and stressed area of foundation base has decreased, all above would cause the condition under stress of the foundation to be worsened which extent changes with scoured position and range of foundation base, so we should pay enough attention to these. In addition, scour caused the great mass of cobble used for protecting bridge foundation to be washed away, existing buried depth can not satisfied with the minimum request of specification.

THE INFLUENCE OF SCOUR ON STABILITY OF THE PIERS BY THE FINITE ELEMENT ANALYSIS

In order to analyze the impact of scour on stability of the piers, the finite element analysis is used, to calculate and analyze compression stress of foundation base of the piers and displacement of the pier top under the influence of all kinds of degree of scour separately. The scoured method of foundation base simulated by this model is changing support pattern of foundation base. If the scoured area is 10% of the area of foundation base along left part, the 10% of foundation base part will be dealt with hangup without any constraint form, but will only remain 90% of the area to restrain; displacement constraint will be adopted for the constraint form.

In order to analysis and calculation of the maximum transverse displacement of the top of pier caused by overturn of the pier body which resulted from scour for foundation base, calculation method under the condition of worst loading combinations will be adopted for this finite element method. At this moment, only the vertical and horizontal load combination will be considered, and for reaching most unfavorable combination, transverse wind load should be perpendicular to horizontal structure of the bridge, and contrary to the direction of current scour.

For scour condition of the pier with different degrees, in the course of this finite element analysis, the other analysis conditions (such as load, the pier entity's form) are all the same except support ways of the foundation bottom. This analysis has separately calculated compressive stress of foundation base and displacement of the top of pier under different scour degrees which scoured area of the pier are 0% (i.e. without scour), 10%, 20%, 30%, 40% and 50%.

The result and summarize of finite element structural analysis of No.4 pier are shown in table 1. Table 1.

According to calculation and analysis hereinbefore, the maximum compressive stress of foundation base at initial stage reduces slightly with the increase of scoured area with a little variety. When the scoured area is up to 20%, the maximum compressive stress of foundation base increases with the increase of the scoured area, and increment is relatively large. When the scour area reaches 50%, [[sigma].sub.MAX] = 1369.70kPa, has already exceed its allowable stress [[sigma]] =1080kPa. It can be calculated by linear interpolation that when scoured area is up to 44.5%, [[sigma].sub.MAX]=[[sigma]]=1080kPa, i.e. the ultimate scoured area should be 44.5%.

Being similar with the analysis result of the maximum compressive stress of foundation base, the analysis curve of the maximum transverse, longitudinal and vertical displacement of the top of No.4 pier have similar condition. When the scoured area reaches 20%, displacement value increases rapidly.

THE INFLUENCE OF SCOUR ON STABILITY OF THE PIERS BY THEORETICAL CALCULATION

The horizontal displacement of the top of the pier includes elastic displacement of the pier itself and the displacement of inclination of the pier caused by differential settlement of the foundation which can calculated from the angle of inclination.

The theoretical calculation result of transverse displacement of the top of the NO.4 pier resulted from inclination of the pier is shown in Table 2.

Based on Fundamental code for design of railway bridge and culvert and Eq. (1), transverse dimension of the foundation is 13.53 m, bending moment of foundation base caused by transverse wind force is 7268.65 kN x m, vertical load N is 24394 kN, by calculation, under the action of eccentric moment of centroid of area and wind load, limiting state of equilibrium appears when the dimension of scoured part is 5.17m, it will be calculated from this that the pier will be overturned when scoured area of foundation base is up to 36.8%.

CONCLUSION

When situation of scour is that the scoured area is 0 %, (i.e. the condition without scour), the calculation result according to theory of elasticity is similar to the result of the finite element analysis, and the result of the finite element analysis is greater than theoretical calculation result. The different conditions that the scoured area increases by 10% will be calculated separately by the same method till the scoured area reached 50%, at this moment, the maximum compressive stress of foundation base has already been greater than allowable compressive stress of foundation base at this moment, i.e., [[sigma].sub.MAX] >[[sigma]]. It indicates that bearing capacity of sub-grade at this moment is not enough, and sub-grade would be destroyed. When the scoured area is up to 44.5%, [[sigma].sub.MAX]=[[sigma]]=1080kPa, i.e. the ultimate scoured area should be 44. 5%.

When scoured area of foundation base is 20%, various kinds of deformation and stress of the foundation change violently, it can be concluded that the foundation will be failure.

REFERENCES

Li Ke-chuan, Luo Xue-shu (2000). "Foundation Engineering", 2nd edition, Beijing: Railway Press

Liu Cheng-yu (2000). "Soil Mechanics", 2nd edition, Beijing: Railway PressQiu Bo-yong, Qiao Jiang-dong, Sheng Xin-wang, Wen Yu-song (2000). "Bridge Engineering". Beijing: Railway Press

FANG LI-GANG

School of Civil Engineering and Architecture, Central South University, Changsha, Hunan, China

DUAN LIANG-LIANG

School of Civil Engineering and Architecture, Central South University, Changsha, Hunan, China

LIANG KAI

School of Civil Engineering and Architecture, Central South University, Changsha, Hunan, China

Table 1. Maximum compressive stress of foundation base and displacement
of the top of pier of No.4 pier.

Items/ Maximum Maximum
scour compressive transverse
 stress of displacement
 foundation base [[DELTA]x.sub.MAX]
 [sigma] MAX (kPa) ([10.sup.-3]m)

0.0% 608.26 0.341
10.0% 584.77 0.656
20.0% 555.91 1.600
30.0% 671.19 4.657
40.0% 864.19 10.029
50.0% 1369.7 20.875

Items/ Maximum Maximum
scour longitudinal vertical
 displacement displacement
 [[DELTA]y.sub.MAX] [[DELTA]z.sub.MAX]
 ([10.sup.-3]m) ([10.sup.-3]m)

0.0% 0.959 3.202
10.0% 0.955 3.281
20.0% 0.943 3.591
30.0% 1.256 4.483
40.0% 1.612 6.075
50.0% 2.227 10.804

Table 2. Theoretical calculation result of transverse displacement
resulted from inclination of the pier.

Scour 10% 20% 30% 40% 50%

Maximum transverse 2.22 3.52 5.76 10.26 18.12
 displacement
 ([[DELTA]s.sub.MAX]
 ([10.sup.-3])