EXPERIMENTAL STUDY ON RIPRAP PROTECTION OF VERTICAL- WALL ABUTMENTS

Transcription

1 1 EXPERIMENTAL STUDY ON RIPRAP PROTECTION OF VERTICAL- WALL ABUTMENTS Cristina Maria Sena Fael (1), António Heleno Cardoso (2) (1) Department of Civil Engineering and Architecture, Universidade da Beira interior, Edifício II das Engenharias, Calçada do Lameiro, , Covilhã, Portugal, phone: , fax: (2) Department of Civil Engineering and Architecture, Instituto Superior Técnico, Av. Rovisco Pais, , Lisboa, Portugal, phone: , ABSTRACT This study addresses the use of riprap as a countermeasure against scouring near verticalwall bridge abutments under clear water flow conditions. It deals with the diameter of riprap, D r50, and the layer thickness, t. It aims to confirm existing criteria and to define appropriate formulations, whenever pertinent. Experiments were performed on a rectangular sand-bed open-channel, by imposing different abutment lengths and by choosing three riprap stone sizes. It became obvious that i) the stone size depend on the ratio between the abutment length and the flow depth and that ii) winnowing can be avoided when the layer acts as a granular filter or the layer thickness exceeds 3D r50. Keywords: vertical abutments, riprap, matt 1 INTRODUCTION Scour around bridge abutments is widely recognized as one of the major causes of collapse of bridges. However, due to the complexity of the local scour phenomenon, there is a significant lack of guidelines for the design of countermeasures to mitigate scour. Research efforts are still needed in order to establish sounder guidelines. Two groups of countermeasures are referred to in the literature. The first group includes armoring countermeasures, where riprap mattresses are the most common; the second group includes flow-altering devices which are not in the scope of this paper. The riprap matting around abutments creates a physical barrier intended to resist to the eroding capacity of the flow. It is the most widespread countermeasure. However, studies concerning this countermeasure are comparatively rare and the existing guidelines are based on limited evidence. In practice, engineers frequently assume, for instance, that mats should cover the predictable scour area of the unprotected riverbed around the obstacle (bridge pier or abutment), ignoring that protection itself is a factor influencing scour. Other key aspects of mattress design are also poorly known or controversial. Scarcity of information justifies this new research effort aiming to confirm existing criteria and to define more appropriate formulations, whenever pertinent. Vertical-wall abutments were chosen for this study since they assumedly induce the most severe conditions concerning design criteria. Stone size and the thickness of mats operating without underlying filter cloths are the particular aspects under consideration in this paper. The study has adopted an experimental approach. Tests were carried out under clearwater flow conditions, i.e., conditions in which the mean velocity of the undisturbed approach flow is below or at the threshold velocity for the beginning of motion. The choice of these flow conditions is realistic since they represent the most common situation encountered in floodplains where abutments are the most frequently built.

2 2 2 LITERATURE REVIEW Bridge engineers often face the need to address the specification of riprap mats for bridge abutments, including, among other aspects, the median stone size and the layer thickness. The correct consideration of these design variables implies the understanding of riprap failure mechanisms. According to Eve & Melville (2000), the failure mechanisms of riprap are essentially the same for bridge abutments as for piers. For clear water conditions, Parola (1993), Chiew (1995) and Lauchlan (1999) identified three failure mechanisms: shear failure, winnowing failure and edge failure. These are the failure mechanisms the most probable close to bridge abutments built in floodplains, where clear water flow conditions are likely to occur. Shear failure is clearly linked with the riprap stone size; winnowing depends on the thickness of mats and on the gradation coefficient of the riprap stones; edge failure and plan dimensions of mattresses are clearly inter-related. For evaluating the riprap stone size, several authors have suggested empirical formulas. For vertical-wall abutments, the studies of Pagán-Ortiz (1991), Austroads (1994) and Richardson & Davis (1995) can be mentioned. The formulas found in the literature for this case can be arranged as follows: D r50 d ( s 1) r C n F m r = (1) where D r50 median riprap stone diameter; d flow depth; s r specific gravity of blocks; F r flow Froude number; C, m and n coefficients (cf. Table 1). Table 1. Coefficients C, m and n of Equation (1) for vertical wall abutments, according to different authors Authors C ( ) m ( ) n ( ) Pagan-Órtiz (1991) Austroads (1994) Richardson & Davis (1995) Another question related with stone sizing is obviously the gradation coefficient of riprap stones. Although this is not investigated in this paper, the criteria of Neil (1973), Gregorious (1985), and Brown & Clyde (1989) can be mentioned. Regarding the thickness, t, of the riprap layer, the Ministry of Works and Development (1979) recommended the value t = 2D r50, if it is placed on top of a suitably graded filter or filter cloth. Later, Lagasse et al. (1997) improved this criterion recommending that the riprap thickness should be at least 1.5D r50 or D r100. They also recommended that the riprap thickness should be increased by at least 50% when placed under running water, to allow for any uncertainties associated with the placement. No information was found in the literature regarding the thickness of riprap mattresses around abutments in the absence of filters of any kind. For cylindrical piers, experimental results presented by Parker et al. (1998), p. 242, seem to indicate that thicknesses of up to 12D r50 may be needed to completely arrest settling by winnowing, even in the absence of other failure mechanisms. For vertical wall abutments, where stronger wake vortices are to be expected due to the existence of vertical edges, mattresses may need to be even thicker. Next, the results of an experimental study on the sizing of riprap stones as well as on the thickness of mattresses without filter cloth are presented and discussed. It should be noted that

3 3 the study is still in progress. Consequently the results and conclusions can not be regarded as final; they will be updated in the future. 3 EXPERIMENTS Experiments were carried out in a 28.0 m long, 4.0 m wide and 1.0 m high concrete flume. The right lateral wall of the flume is made of glass panels, which make it possible to observe the flow in the reach where countermeasures were tested. This reach includes a 3.0 m long, 4.0 m wide and 0.6 m deep recess box, starting at 13.9 m from the flume entrance. Vertical wall abutments were placed on the bottom of the recess, at its mid cross-section (at 15.4 m from the flume entrance), protruding at right angle from the glass wall. The recess was practically filled with one of two different natural quartz sands, depending on the tests. For each test, one of three different types of riprap stones was placed around the used abutment, on top of the pertinent quartz sand. The grading curves for the quartz sands and for the riprap stones are shown in Figure 1 and their properties are listed in Table 2. This table includes the gradation coefficient, σ, that was calculated from 1/2(D 84.1 /D 50 + D 50 /D 15.9 ). The bed and riprap materials can be considered to be uniform, since σ < Riprap Sand #1 #2 #3 #1 #2 70 Percentage passing (%) Diameter (mm) Figure 1. Sand and riprap grading curves The abutments were simulated by 140 mm wide, vertical wall, parallelepiped perspex boxes. The top of the perspex boxes was kept open, allowing the handling of a video camera to record images of the experiments from inside. Abutment lengths equal to 0.30 m, 0.51 m, 0.72 m, 0.93 m and 1.13 m were used. This way, L/d varied between 2.46 and 9.42.

4 4 Table 2. Sand and riprap properties Material D 15.9 D 50 D 84.1 σ D s or s r (mm) (mm) (mm) ( ) ( ) Sand # Sand # Riprap # Riprap # Riprap # The study was done for a reasonably high and practically constant flow depth (d 0.12 m). During the experiments, the water was pumped from a reservoir and the discharge could vary continuously from 0.0 m 3 /s up to 0.18 m 3 /s. Just upstream the flume entrance, a diffuser pipe has been installed to ensure the uniform flow distribution along the flume width. At the downstream end of the flume, a hand-operated tailgate made it possible to regulate the water level. The water falls from the tailgate into the mentioned reservoir, closing the water circuit. The flume is equipped with a moving carriage used to install measuring equipment. Two sets of experiments were carried out. Prior to these two sets of experiments, some preliminary tests were run so as to determine the critical flow velocity, U c, for the beginning of motion of quartz sand #1 and the three riprap stones. The first set of tests aimed at evaluating the critical flow velocity for scour inception, i.e. the approach flow velocity above which scour develops close to the abutment. Fifteen short duration tests were carried out. For each type of riprap, quartz sand # 1 was placed in the recess box, entirely covered with a filter cloth and one upper layer of riprap. The thickness of this layer, levelled with the concrete flume bottom, was 3D r50. Tests started with a very low flow velocity. Velocity was successively increased while the flow depth was maintained constant by handling the downstream tailgate. The procedure was repeated until riprap stones began to move close to the abutment. The second set of tests was designed so as to further address the thickness of the mattresses. Thirty-one tests were performed without filter cloth: 18 for sand #1 and 13 for sand # 2; only riprap # 2 and riprap # 3 have been used in this set. Tests were accomplished for L/d = 9.42, 7.75, 6.00, 4.25 and As for the first set of tests, the sand recess box was covered by a riprap layer levelled with the adjacent concrete flume bottom. No filter cloth was used. Two riprap layer thicknesses, t, were tested per setup, i.e. per combination of L/d and type riprap stones: t = D r50 and t = 2D r50 or t = 2D r50 and t = 3 D r50. Approach flow velocities were kept equal to 90% of the corresponding critical values of scour inception, as defined in the first set of tests. A rule was fixed to the transparent wall of the abutment and scouring was monitored with the video camera placed inside the abutment until equilibrium could be practically guaranteed. 4 DISCUSSION 4.1 PRELIMINARY TESTS ON THE BEGINNING OF MOTION The beginning of sediment motion is difficult to identify in the laboratory, because this is not a neat, unambiguous phenomenon. It rather depicts a random character, of which the visual evaluation is somewhat subjective. Therefore, two values of U c critical velocity for the beginning of motion were retained: a lower limit, where motion is about to but still does not exist, and an upper one, corresponding to a very weak sediment motion (incipient motion). Table 3 presents the observed values of U c, together with the results obtained through the equations of Neil (1967) and Garde (1970), as well as with the results of Shields diagram.

5 5 Table 3. Critical velocity for the beginning of motion of sand and tested riprap stones Uc (ms-1) Material Tested Neil (1967) Garde (1970) Shields lower upper Sand # Riprap # Riprap # Riprap # Next, the following values of U c will be used: sand #1, 0.36 ms 1 ; riprap #1, 0.53 ms 1 ; riprap #2, 0.72 ms 1 ; riprap #3, 1.05 ms 1. Apart from the intrinsic value of the measured values of U c for the subsequent discussion, it can be conclude that, within the experimental range of this study, the equations of Garde (1970) and Neil (1967) lead to better predictions than Shields diagram. 4.2 SIZE OF RIPRAP STONES, D r50 As for the beginning of motion, the determination of critical flow velocity for scour inception is subjective; it also led to the definition of intervals. The values reported below correspond to the centre of such intervals. The results of the fifteen tests carried out on this issue could have been used to check the applicability of Equation (1) and/or to obtain a new equation with a similar structure. Instead, an alternative approach to access D r50 was adopted. This alternative approach is based on the definition of the critical value, I c, of the approach flow intensity, I = U/U c, below which scour does not show up, for a given value of L/d. Obviously, when no obstacle exists in the flow, the beginning of motion can be predicted through formulations like those used in 4.1. It is also consensual that the presence of obstacles increases, in their vicinity, the susceptibility for the motion of particles. In practice, this means that, even for flow intensities less than 1, scour may occur close to obstacles. Some controversy exists on the lower value of I, i.e. on the value of I c, below which no scour exists. Some authors have adopted Chabert & Engeldinger s (1956) result for piers: no scour would occur as long as I < I c = 0.5. Cardoso et al. (2002) corroborated this conclusion for comparatively short abutments (L/d < 7). On the contrary, Melville (1992) and Melville (1997) implicitly suggested the safe value I c = 0. Hager & Oliveto (2002) have proposed an equation to calculate I c as a function of the flow contraction, L/B, where B = channel width. According to these authors, L 1 B I c = (2) 6 For wide open channels where scour is not influenced by flow contraction, Fael et al. (2006) suggested one equation that can be rewritten as L 1 d I c = (3) 8 It should be emphasized here that Equations 2 and 3 were established for sand bed material. A priori, they may or may not directly apply to the design of riprap stones. Figure 2 compares the results of these equations with the observed values of I c. The observed values of I c were calculated from the values of the measured approach flow velocity corresponding to scour inception. It is clear that I c depends on L/d. As L/d increases, there is an increas-

6 6 ing susceptibility for scour inception. It seems that the equation of Fael et al. (2006) fits the data better than the equation by Hager & Oliveto (2002). Still, it should be noted that the observed values of I c might be smaller than those issued from the equation of Fael et al. (2006), particularly for the coarser riprap. For L/d = 9.42, I c may be as low as In practice, it is clear that the widespread criterion of value Hanco (1971) I c = 0.5 leads to unstable riprap stones, as soon as L/d > 3 ~ 4. I c 1.0 Riprap #1 Riprap #2 Riprap #3 Hager & Oliveto (2002) 0.5 Envelope curve Fael et al. (2006) 0.0 L d Figure 2. Variation of I c with L/d for sand and for riprap stones Apparently, I c depends also on d/d r50. For the data obtained in this study, the dependence of I c on both L/d and d/d r50 is given by the following regression equation (R 2 = 0.803) d Dr L d I = (4) c Equation (4) could be suggested for design purposes. However, for safety reasons, since the equation was obtained from a comparatively short data set, the lower envelop curve (see Figure 2), L I c = 1, (5) 5 d is suggested instead. Taking advantage of Equation (5), let us assume a 20 m long vertical wall abutment inserted in a 4 m deep flow, of which the approach velocity is 2 ms 1. According to Equation 8, I c = This means that the riprap stones around the abutment must be designed as if they were to remain stable for a fictitious flow velocity U = I c 1 x2 ms 1 = 5.2 ms 1 in the bottom of the approach channel. From Neil (1967) and Garde (1970) equations, the stone size of stable riprap, D r50, would then be around 0.4 m. 4.3 RIPRAP LAYER THICKNESS, t The main objective of this section is to assess the minimum mattress thickness, t, needed to avoid failure due to winnowing of the underlying sediment particles through the voids of the riprap stones in the absence of filter cloth. Since experiments were run for approach flow intensities below I c of scour inception, shear failure was neither expected nor observed; as the entire recess box was covered with riprap, edge failure was not possible either. Thus, only failure due to winnowing was expectable. Table 4 summarizes the results of the second set of experiments described in chapter 3. In the table, N number of equivalent layers, such that t = ND r50, T test duration. One rele-

7 7 vant result for the analysis is the equilibrium scour depth, d se, generated by winnowing, as measured at the end of each experiment. Table 4. Scour depth due to winnowing versus riprap layer thickness Test Sand L L/d Layer variables T d se (m) ( ) riprap N ( ) t (mm) (h) (m) E1 # # E E3 # E E # E6 # E E E # E10 # E E # E13 # E E # E16 # E E E19 # # E E21 # E E E E E # E E # E E # E31 # E # E Several criteria can be found in the literature to check this hypothesis. Those of Terzaghi- Vicksburg (1943), Brown & Clyde (1989) and Richardson et al have been considered. The most exigent one is the criterion of Terzaghi-Vicksburg (1943). According to this author, the effect of filter will be present as soon as the following conditions are simultaneously verified: D r15 /D 85 < 5; 5 < D r15 /D 15 < 20 D r50 /D 50 < 40

8 8 The ratios D r15 /D 85, D r15 /D 15 and D r50 /D 50 of the materials used in this study are summarized in Table 5. It is clear that riprap #2 acts as a filter both for sand #1 and sand #2 while riprap #3 does not show such a behavior. This should be the reason why, according to Table 4, scour is nonexistent for riprap #2 on sand #1 almost regardless of the layer thickness. The exception is test 1 (L/d = 9.23; N = 1), where a 2 cm deep scour hole was observed at the end of a 5-days run. The same result is observed for riprap #2 on sand #2 as soon as t 3D r50. Table 5. Verification of the design criteria of granular filters Sand #1 Sand #2 Conditions Riprap #2 Riprap #3 Riprap #2 Riprap #3 D r 15 /D D r 15 /D D r 50 /D For riprap #3, which should not act as a filter according to the criterion of Terzaghi- Vicksburg (1943), scour is practically absent when it is placed on sand #1 as soon as t 3D r50 but some experiments seem to indicate that even t = 2D r50 may also be sufficient. However, when it is placed on the finer sand #2, thicknesses as high as t = 6D r50 may be needed to arrest winnowing. It should be noted that the deepest scour depths for the combination (sand #1; riprap #3) were observed at the shorter abutment (L/d = 2.46) for a given N value. This result is somewhat surprising since the wake vortices at the edge of longer abutments are expectably stronger and should render winnowing stronger too. Since this is not confirmed, further experiments are needed with the simultaneous measurement of the flow field to obtain an insight into the physics of phenomena. However, this result should be confirmed too, since, according to Parker et al. (1998), p. 242, winnowing can act down to depths of about 12D r50 around cylindrical piers. 5 CONCLUSIONS The use of riprap protection of vertical-wall abutments under clear-water flow conditions was addressed in this study. Abutments protruded at right angle from the wall of a rectangular, sand-bed channel. For the experimental range of this study (2.46 L/d 9.42), the following conclusions can be drawn: i) Regarding the design of stable riprap stone sizes, it is clear that characteristic diameters depend on the Froude number as well as on L/d. The influence of L/d is not taken into consideration by several formulations selected in the literature. ii) The concept of critical flow intensity, I c, for scour inception applies to the design stone sizes. Equation 8 is suggested, which, leads to increasing stone sizes as L/d increases. iii) Scour due to winnowing did not develop when the riprap layer acted as a granular filter. Scour seems negligible when, in the absence of filters of any kind, the riprap thickness is such that N 3. ACKNOWLEDGMENTS The writers wish to acknowledge the financial support of the Portuguese Foundation for Science and technology, through POCI/ECM/59544/2004.

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