Bridge Scour in Nonuniform Sediment Mixtures and in Cohesive Materials: Synthesis Report

6. EFFECT OF COHESION ON ABUTMENT

SCOUR

In this chapter, effects of cohesion on local abutment scour are investigated experimentally for Montmorillonitic and Kaolinitic clay mixtures using larger scale, 1.2-m and 2.4-m wide test flumes at the Engineering Research Center, CSU. For cohesive soils with significant clay content (30 percent for the present mixtures), soil parameters such as compaction, initial water content, degree of saturation, shear strength, and type of clay mineral dominate the abutment scour. In this study these effects are quantified for flow conditions with Froude numbers ranging from 0.2 to 0.9. Equations relating flow and selected cohesive soil parameters to abutment scour were developed to explain the variability of abutment scour with cohesion properties. These equations express abutment scour in cohesive soils relative to clear-water scour in noncohesive material for the same flow and geometry conditions. Under the same geometric and flow conditions, measured scour in cohesive materials varies from 7 percent to 140 percent of that measured in medium sand.

6.1 GENERAL

Flow structure around abutments and the resulting local scour were studied in the past in numerous experimental, numerical, and analytical investigations for noncohesive materials. As a result, series of empirical and semiempirical prediction equations were developed to relate the scour depth at bridge abutments to different approach flow conditions, to sediment size and gradations, and to different abutment types and sizes.

The mechanism of cohesive-material scour is significantly different from scouring of alluvial noncohesive materials. The process involves not only the balancing of flow-induced shear stresses and the shear strength of soils to withstand scour, but also the chemical and physical bonding of individual particles and the properties of the eroding fluid. Cohesive materials, once eroded, remain in suspension. As a result, the phenomenon identified as clear-water local scour in noncohesive materials always prevails. Along with the eroding fluid properties, the scour process in cohesive soils is strongly affected by the amount of cohesive material present in the soil mixture as well as the type of mineral clay, initial water content, soil shear strength, and compaction of the clay. The objectives of this paper are to apply the knowledge gained in the past in cohesive material scour to study local scour around abutments and to analyze effects of compaction, initial water content, soil shear strength, and the approach flow conditions on abutment scour. For this purpose two different types of clay mixtures were used. The first cohesive mixture was a naturally occurring soil that contained 32 percent Montmorillonite mineral clay with almost equal amounts of fine sand and silt. The second cohesive mixture was prepared by blending 30 percent pure Kaolinite clay with medium sand. Experiments using the first soil were conducted under unsaturated and saturated soil conditions. The Kaolinite clay scour experiments were limited to unsaturated conditions where compaction and initial water content of the mixture were varied. As a result of this experimental study, empirical relationships were developed relating the scour in cohesive material to that observed in the noncohesive material that was used in preparing the mixtures under the same flow and geometric conditions.

6.2 EXPERIMENTAL SETUP AND MEASUREMENTS

The two categories of abutment scour experiments presented in this paper are classified as Montmorillonite and Kaolinite clay experiments. In each category of experiments, series of runs utilizing two experimental flumes were conducted by varying the clay properties under different flow conditions. A total of 126 experiments were conducted covering a wide range of flow and soil conditions. Details of the experiments are presented by Yakoub⁽²²⁾ and Molinas and Reiad ⁽⁵⁾.

Flumes

Experiments were conducted in two different larger scale test flumes housed at the Engineering Research Center at CSU to achieve desired flow intensities. These flumes are identified as the sediment transport flume and the steep flume. The sediment transport flume is 2.4 m wide by 60 m long, and the steep flume is 1.2 m wide by 12 m long. The flow depths used in the experiments varied between 0.12 m and 0.3 m, and approach Froude numbers ranged from 0.1 to 0.9.

Measurements

Flow and sediment parameters measured in experiments were velocity distribution (vertical, longitudinal, and lateral), depth of flow, depth of scour, initial water content of soil, Torvane shear strength, and degree of compaction. Velocity measurements were carried out by the use of a magnetic flow-meter. In the experiments, approach velocity to each abutment was determined by depth- and width-integrated average of vertical velocity profiles. Similarly, the approach depth was determined from a width- and length-averaged value of water surface and bed elevation measurements. The bed elevations were measured along the flumes across the flow channel before and after each experiment. The depth of abutment scour was measured during and at the end of each experiment; it was determined by the difference between the minimum bottom elevation at the nose region of abutment and the maximum elevation away from the structure.

Cohesive soils

In this study, two cohesive soil mixtures were used. The first is a naturally occurring homogeneous soil containing 32 percent clay, 30 silt, and 38 percent fine sand particles. Utilizing the X-Ray Diffraction Test, the dominant clay mineral was found to be Montmorillonite. According to the unified soil classification system, the cohesive soil is classified as medium plasticity clay and the texture as clay loam. The second cohesive soil mixture was prepared by blending commercially obtained pure Kaolinite clay with medium sand. This mixture was composed of 30 percent clay and 70 percent sand. Montmorillonite clay experiments studied the effects of compaction, initial water content, and shear strength for unsaturated and saturated soil conditions. This was achieved by compacting the cohesive material placed around the abutments at various degrees of compaction and by using soils of different initial water content, whereas Kaolinite clay experiments investigated the effects of initial water content on unsaturated clay erosion by preparing mixtures with varying initial water contents. In these experiments compaction was also varied over a narrow range of conditions.

Abutments

In the experiments, rectangular vertical-wall abutments constructed of clear Plexiglas with protrusion lengths of 0.22 m and 0.11 m were used. Lengths of these abutments in the direction of flow were 0.44 m and 0.22 m, respectively. The scour depth development was measured against time utilizing three measuring tapes attached to the interior wall of each abutment and by the use of a small inclined mirror.

6.3 EXPERIMENTAL RESULTS

Results of abutment scour experiments in cohesive materials are presented in tables 29 through 32. These tables present the flow conditions and volumetric scour measurements along with cohesive soil properties and measured maximum local scour values. The clay mineral present in the cohesive soil mixtures used in set 1 and set 2 experiments (given in tables 29 and 30) was Montmorillonite. Tables 31 and 32 present results of experiments utilizing Kaolinite clay mixtures. The analysis that is presented in this study is aimed at quantifying cohesive material effects in terms of easily obtainable soil parameters. For this purpose, the parameters selected to represent the scour resistance of cohesive materials were limited to those given in tables 29 through 32. They were: clay content, compaction, initial water content, shear strength, and clay mineral. The effects of clay content on bridge scour are treated in a separate chapter in this report. In the following analysis, effects due to cohesive material properties other than clay content are investigated. In the past, erodibility of cohesive soils was also related to surrounding fluid, disturbed versus undisturbed soils, sodium adsorption ratio, plasticity, etc. Due to the complexities involved in quantifying these effects (a much larger variety of clays), these parameters were excluded from the present analysis.

Table 29. Results of Montmorillonite clay experiments conducted in the 2.44-m wide sedimentation flume using a 0.22-m abutment protrusion length.

Note:

Duration of experiments varied between 12 to 16 hours.

Table 30. Results of Montmorillonite clay experiments conducted in the 1.22-m wide flume using a 0.11-m abutment protrusion length.

Note:

Duration of experiments varied between 12 to 16 hours.

Table 31. Results of Montmorillonite clay experiments conducted in the 2.44-m wide flume using a 0.1-m abutment protrusion length.

Run ID	Flow Discharge Q (l/s)	Percent Clay Content CC (%)	Initial Water Content IWC (%)	Dry Density dry (t/m3)	Compaction C (%)	Torvane Shear Strength S (kPa)	Flow Depth Y (m)	Approach Velocity U (m/s)	Scour Depth Dsc (m)	Scour Volume Vsc (l)	Flow Temp. T (oC)
8-80-MA	356.79	32	46.7	1.226	70.9	0.979	0.177	0.664	0.113	4.899	20.0
8-81-MA	240.69	0	4.0	1.758	91.4	3.655	0.155	0.503	0.253	21.011	18.3
8-81-MB	240.69	5	6.9	1.363	69.7	1.957	0.148	0.433	0.210	5.097	18.3
8-81-MC	240.69	10	11.4	1.463	71.9	7.829	0.155	0.384	0.131	0.963	18.3
8-81-MD	240.69	13	11.0	1.254	63.5	7.829	0.146	0.366	0.140	1.161	18.3
8-82-MA	172.73	0	4.0	1.758	91.4	3.655	0.152	0.360	0.158	16.990	18.9
8-82-MB	172.73	5	4.3	1.389	71.1	1.957	0.148	0.308	0.104	3.596	18.9
8-82-MC	172.73	10	10.1	1.449	71.2	7.829	0.158	0.277	0.073	1.104	18.9
8-82-MD	172.73	13	5.6	1.329	67.3	7.829	0.155	0.250	0.067	1.246	18.9
8-83-MA	133.09	0	4.0	1.758	91.4	3.655	0.143	0.280	0.113	6.513	19.4
8-83-MB	133.09	5	4.2	1.300	66.5	1.957	0.145	0.229	0.094	1.897	19.4
8-83-MC	133.09	10	8.5	1.320	64.9	7.829	0.148	0.204	0.067	0.850	19.4
8-83-MD	133.09	13	9.9	1.335	67.6	7.829	0.146	0.177	0.052	0.453	19.4
8-84-MA	300.16	0	4.0	1.758	91.4	3.655	0.155	0.680	0.274	63.996	20.0
8-84-MB	300.16	5	6.2	1.289	66.0	1.957	0.151	0.591	0.262	32.989	20.0
8-84-MC	300.16	10	11.3	1.193	58.7	7.829	0.151	0.533	0.152	16.990	20.0
8-84-MD	300.16	13	9.8	1.286	65.1	7.829	0.149	0.454	0.165	19.001	20.0

Note:

Duration of experiments varied between 12 to 16 hours.

Table 32. Results of Kaolinite clay experiments in the 2.44-m wide flume, with 0.22-m abutment width.

Run ID	Flow Discharge Q (l/s)	Percent Clay Content CC (%)	Initial Water Content IWC (%)	Dry Density dry (t/m3)	Compaction C (%)	Torvane Shear Strength S (kPa)	Flow Depth Y (m)	Approach Velocity V (m/s)	Scour Depth Dsc (m)	Scour Volume Vsc (l)	Flow Temp. T (oC)
8-71-MA	157.16	30	14.8	1.661	84.1	30.337	0.177	0.308	0.015	0.453	19.4
8-71-MB	157.16	30	13.4	1.672	84.7	41.101	0.173	0.262	0.000	0.000	19.4
8-71-MC	157.16	30	13.3	1.684	85.3	42.080	0.168	0.238	0.055	1.303	19.4
8-72-MA	421.92	30	15.4	1.707	86.4	23.487	0.191	0.811	0.030	0.793	19.4
8-72-MB	421.92	30	14.7	1.758	89.0	41.101	0.183	0.820	0.037	0.963	19.4
8-72-MC	421.92	30	15.4	1.707	86.4	16.636	0.175	0.960	0.070	1.897	19.4
8-73-MA	286.00	30	20.1	1.741	88.2	7.829	0.159	0.637	0.021	0.651	18.9
8-73-MB	286.00	30	26.5	1.621	82.1	3.914	0.157	0.527	0.040	1.048	18.9
8-73-MC	286.00	30	29.7	1.523	77.1	1.957	0.165	0.457	0.037	0.991	18.9
8-74-MA	351.13	30	18.8	1.732	87.7	10.765	0.165	0.719	0.030	0.850	20.0
8-74-MB	351.13	30	23.4	1.641	83.1	4.893	0.155	0.686	0.046	1.189	20.0
8-74-MC	351.13	30	27.5	1.541	78.0	2.936	0.139	0.777	0.058	1.388	20.0
8-75-MA	237.86	30	21.8	1.695	85.8	4.893	0.158	0.512	0.000	0.000	20.0
8-75-MB	237.86	30	25.1	1.641	83.1	1.957	0.161	0.460	0.000	0.000	20.0
8-75-MC	237.8	30	29.7	1.532	77.6	0.979	0.170	0.439	0.000	0.000	20.0
8-76-MA	489.88	30	21.8	1.695	85.8	4.893	0.193	0.817	0.037	0.963	20.0
8-76-MB	489.88	30	25.1	1.641	83.1	1.957	0.177	0.808	0.067	1.812	20.0
8-76-MC	489.88	30	29.7	1.532	77.5	0.979	0.167	1.201	0.158	9.486	20.0
8-77-MA	170.75	10	15.5	1.695	85.8	2.936	0.152	0.030	0.146	8.014	19.4
8-77-MB	170.75	20	17.1	1.741	84.5	3.914	0.158	0.223	0.012	0.396	19.4
8-77-MC	170.75	50	16.6	1.552	85.2	45.016	0.173	0.195	0.094	3.511	19.4
8-77-MD	170.75	50	27.1	1.478	81.1	2.936	0.174	0.171	0.000	0.000	19.4
8-78-MA	235.03	10	13.5	1.692	85.6	2.936	0.155	0.512	0.229	13.989	19.4
8-78-MB	235.03	20	14.7	1.518	73.6	3.914	0.155	0.460	0.046	0.481	19.4
8-78-MC	235.03	50	15.7	1.335	73.3	45.016	0.168	0.439	0.192	6.513	19.4
8-78-MD	235.03	50	25.8	1.586	87.0	2.936	0.171	0.372	0.000	0.000	19.4
8-79-MA	288.83	10	16.2	1.841	93.2	2.936	0.162	0.582	0.256	21.011	20.0
8-79-MB	288.83	20	16.8	1.775	86.1	3.914	0.161	0.479	0.052	1.359	20.0
8-79-MC	288.83	50	15.7	1.335	73.3	45.016	0.174	0.424	0.177	12.006	20.0
8-79-MD	288.83	50	30.9	1.455	79.9	2.936	0.174	0.357	0.000	0.000	20.0
8-80-MB	356.79	20	16.8	1.775	86.1	3.914	0.170	0.607	0.152	10.987	20.0
8-80-MC	356.79	50	15.7	1.335	73.3	45.016	0.176	0.539	0.226	18.010	20.0
8-80-MD	356.79	50	30.9	1.455	79.9	2.936	0.168	0.472	0.000	0.000	20.0

Note:

Duration of experiments varied between 12 to 16 hours.

6.4 ANALYSIS

The functional relationship between the maximum depth of abutment scour and the parameters defining the soil, fluid, and the geometry of the abutment is derived through dimensional analysis. The depth of abutment scour in cohesive material (D_sc) is expressed as a function of the following independent variables:

(36)

in which D_s = depth of abutment scour in noncohesive material for conditions corresponding to D_sc; Y = depth of approach flow; a = abutment protrusion length; L = length of abutment in the direction of flow; V = velocity of approach flow; IWC = initial water content; C = compaction related to the optimum compaction; CC = clay content; S = Torvane shear stress, Mn = type of clay (e.g., Kaolinite, Illite, Montmorillonite); T = water temperature; t = duration of experiment; g = gravitational acceleration, = angle of attack; = abutment shape factor; = fluid density; and L = kinematic viscosity of fluid.

Applying the dimensional analysis using D_s, V, and D as repeating variables, and using appropriate simplifications, the following set of dimensionless parameters can be obtained:

(37)

In the derivation of equation 37, the CC was eliminated as a variable since, as shown in Chapter 4, the effects of this parameter was found to be an independent factor up to 12 percent clay content. In the cohesive abutment scour experiments, the clay content was kept above 30 percent. In abutment scour experiments the variation of scour with time was measured. This relationship was shown by Molinas and Reiad to be an asymptotic function with a sharp initial scour development followed by a gradual increase⁽⁵⁾. The initial rate of scour hole development is generally controlled by the nature of the clay mineral and other cohesive material parameters such as compaction, initial water content, etc. Experimental and theoretical studies have shown that the velocity at the nose region of a vertical wall abutment is amplified by a factor of 1.2 to 1.8 times the approach V. According to Molinas, Khereldin, and Wu, bottom shear stresses that are related to V² are also amplified (by up to 11 times) and cause local scour in the affected zone⁽²⁶⁾. If approach velocities are increased beyond a threshold value defined as critical velocity, the entire approach channel bottom becomes subject to general scour in addition to the local scour. Under these conditions, the scour hole development process continues until equilibrium slopes are attained for the entire reach and may last indefinitely. The experimental study presented in this report limited itself to conditions in which the oncoming flows do not scour the approach reach (clear water conditions). Under these conditions, as soon as the scour hole reaches a depth where the shear stress within the base becomes equal to the critical shear stress of the cohesive material, local scouring ceases. The duration of experiments in the study were long enough to maintain the equilibrium condition for at least 4 hours. The final scour depth values obtained under these conditions are independent of time, and therefore in deriving equation 37 the time parameter, t, is eliminated. Additionally, the dimensionless S can be related to C and IWC, further reducing the number of parameters.

The local abutment scour analysis in this study was conducted separately for the two clay types. This distinction is made since the type of clay mineral (Mn) was found to have a dominant effect on the scourability of bed material.

Montmorillonite Scour

The measured values of (D_sc /D_s) were regressed against the remaining dimensionless groups in equation 37 using nonlinear multiple regression analysis. The best-fit regression equations resulting from the statistical analysis of experimental data were reported earlier by Molinas, Reiad, and Jones⁽²⁷⁾ and are given below.

For unsaturated clays with initial water content less than 25 percent:

(38)

where the IWC and C are in percent.

For saturated clays with initial water content in the range of 28 to 45 percent:

(39)

In deriving equations 38 and 39, IWC ranged from 12 to 45 percent, and C ranged from 58 to 89 percent. Equations 38 and 39 were developed based on laboratory tests in which Froude numbers ranged from 0.1 to 0.6 and soil shear strength values ranged from 0.1 to 0.63 kg/cm². The plot of equations 38 and 39 with observed data is presented in figure 36.

Figure 36. Computed and measured relative abutment scour for Montmorillonite clay.

Figure 37. Computed and measured relative abutment scour for Kaolinite clay.

Kaolinite Scour

For unsaturated Kaolinite clay soils, the basic form of equation 37 was retained. For the cohesive soil consisting of 30 percent Kaolinite clay and 70 percent medium sand with a median diameter of 0.81 mm, the best-fit regression equation from the statistical analysis of experimental data is:

(40)

Scour initiating velocities for the Kaolinite clay mixture were experimentally determined to be 0.6 m/s. For velocities smaller than this value, the scour ratio is zero. The plot of equation 40 with observed data is presented in figure 37.

6.5 CONCLUSIONS

Abutment scour in cohesive soils shows a wide range of variability depending on the properties of soils. In the experiments, under the same geometric and flow conditions, measured scour in cohesive materials varied anywhere from 7 percent to 140 percent of that measured in medium sand. This is due to initial water content, soil shear strength, degree of compaction, and type of clay mineral present in the soil. For Montmorillonitic soils, the relative scour depth is expressed in terms of initial water content and degree of compaction. For unsaturated Montmorillonitic mixtures, abutment scour depth decreases as the compaction and initial water content increases. For saturated cohesive soils, however, the scour depth is mainly a function of initial water content and is inversely proportional to initial water content. For unsaturated Kaolinitic mixtures the initiation of scour takes place under higher flow intensities, and under same flow conditions, the total scour may be up to 80 percent smaller than the corresponding scour encountered in noncohesive material.

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