Proceedings of the 6 th International Conference on the Application of Physical Modelling in Coastal and Port Engineering and Science (Coastlab16)

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Proceedings of the 6 th International Conference on the Application of Physical Modelling in Coastal and Port Engineering and Science (Coastlab16) Ottawa, Canada, May 10-13, 2016 Copyright : Creative

MEHDI MOUSAVI 1, ABDOLREZA OSOULI 2, ERGYS BORAJ 3, FRANCISCO LAGUNAS 4 SINA NASIRI 5 1 Graduate research Assistant, Texas Tech University, U.S.A, m.mousavi-ghorabaee@ttu.

edu 4 Undergraduate research assistant, Sothern Illinois University Edwardsville, U.S.A, flaguna@siue.edu 5 Graduate research assistant, Sothern Illinois University Edwardsville, U.S.A, snasiri@siue.

1 Proceedings of the 6 th International Conference on the Application of Physical Modelling in Coastal and Port Engineering and Science (Coastlab16) Ottawa, Canada, May 10-13, 2016 Copyright : Creative Commons CC BY-NC-ND 4.0 Critical shear stress of silts and clays used in levees during floodwall overtopping S. MEHDI MOUSAVI 1, ABDOLREZA OSOULI 2, ERGYS BORAJ 3, FRANCISCO LAGUNAS 4 SINA NASIRI 5 1 Graduate research Assistant, Texas Tech University, U.S.A, m.mousavi-ghorabaee@ttu.edu 2 Assistant Professor, Southern Illinois University Edwardsville, U.S.A, aosouli@siue.edu 3 Undergraduate research assistant, Sothern Illinois University Edwardsville, U.S.A, eboraj@siue.edu 4 Undergraduate research assistant, Sothern Illinois University Edwardsville, U.S.A, flaguna@siue.edu 5 Graduate research assistant, Sothern Illinois University Edwardsville, U.S.A, snasiri@siue.edu ABSTRACT Prediction of scour in levees during the floodwall overtopping is critical. One of the key components in predicting the scour depth and erosion rate is the critical shear stress of the soil. Critical shear stress is the stress that if exceeded, the erosion starts. The resistance of soil is a result of friction and interlocking of particles, and possibly cementation or bonding at particle contacts. There are several empirical and analytical procedures to predict the critical shear stress of cohesive soils. The empirical methods usually consider the effect of each contributing parameter separately. The effect of saturation ratio of soils is usually neglected in determining erosion rates. Through a series of lab scaled levee tests, an equation is developed for predicting critical strength of the cohesive soils. The considered contributing factors in this study are plasticity index, compaction level, and median particle diameter together and saturation ratio. Results show that higher compaction level and higher plasticity will result in higher critical shear stress and higher saturation ratio decreases the critical shear stress. A method is proposed to identify the critical shear stress and verified by laboratory tests for levee applications. KEWORDS: Levee, Overtopping, Erodibility, Critical shear stress, Cohesive soil 1 INTRODUCTION Overtopping has always been of principal concern for coastal structures constructed to defend against flooding. A sample of these constructions is shown in Figure 1 (Guy, 2014). Similar structures may also be used to provide protection against coastal erosion. Other structures may be built to protect ship navigation or mooring within ports, harbours or marinas formed by breakwaters. Within river banks, harbours, or along shorelines, reclaimed areas must be defended against flooding and erosion. (William Allsop, 2008) Figure 1. Levee and floodwall system (Guy, 2014) 1

2 Levees are one of the earthen structures constructed across rivers or in the vicinity of lakes and seas to protect lands while floods, storm surges and hurricanes occur. Due to global warming and changes in sea level and weather, the number and the intensity of these natural disasters have increased demand for more stable and stronger protection structures on the bank of the rivers. Levee failures during hurricane Katrina in 2005 in New Orleans are the famous classical examples. (Mazdak Karimpour, 2014) Among various failure mechanisms that cause levee breaches, soil erosion is one of the most important factors (Kamalzare, 2013). Predicting soil erosion and scour generation when overtopping happens is important in order to design stable levee and floodwalls. One of the key components in predicting the scour depth and erosion rate is the critical shear stress of the soil. Critical shear stress is the stress that if exceeded, the erosion starts. The resistance of soil is a result of friction and interlocking of particles, and possibly cementation or bonding at particle contacts. There are several approaches for determining. Critical shear stress can be determined via flume tests using soil parameters such as particle size and soil specific gravity, measured in situ with a submerged jet test device, or it can be assumed zero. For cohesionless soils, Shields diagram provides estimates of critical shear stress based on particle size using a representative particle diameter and assuming no interaction among the sediment particles (Ing. A. Shields, 1936). The diagram then was extended to be used for fine grained materials but due to lack of enough experimental verification, its use can result in significant underestimation. Visually, is determined by measuring the shear stress at failure (L. A. Clark, 2007). The most widely used erosion measurement technique, particularly for cohesive soils, involves an open-channel flow test with soil forming the bed. The critical stress is then determined visually or graphically. Smerdon and Beasley (1961) conducted a flume study on eleven cohesive Missouri soils to relate basic soil properties to. The empirical relations developed between the soil properties and are shown by Equations 1 to 4 (L. A. Clark, 2007): =. ( ). 1 =. ( ). 2 =.. 3 =.. 4 Where = Critical shear stress (pa) = Plasticity index =Dispersion ratio =Mean particle size (m) =Percent clay by weight () Niel (1973) incorporated the depth of flow as well as soil properties and presented Eq 5 (L. A. Clark, 2007): Where = Critical shear stress (pa) = Specific weight of water (N/m3) =Specific Gravity of soil =Mean particle size (m) d=depth of flow (m) =. ( ) / / 5 In a more recent relevant study, Julian and Torres (2006) estimated critical shear stress based on the percentage of silt and clay (SC) (L. A. Clark, 2007): Where =. +. ( )+. ( ). ( ) 6 2

3 = Critical shear stress (pa) = Silt-clay content defined as particle sizes less than mm Submerged Jet Test is another way to calculate the critical shear stress. The jet scours the soil until the maximum scour depth is reached, and the rate of scour approaches to zero. At this equilibrium condition, the critical shear stress can be computed (Hanson, 1990). To consider all relevant soil properties (i.e., Plasticity Index (PI), Compaction Level (CL), and Saturation Ratio (SR), mean particle size) in critical shear stress estimation of fine grain soils, the erosion of levee soils were studied in a series of lab-scaled levee-floodwall simulation tests. A strain-measuring device has been developed to measure the applied stresses by freefall water from overtopped floodwalls. The results were used to develop a formula for based on all soil properties mentioned above from some of the conducted tests. Then, the developed equation was verified by using the other tests results which were not used in its development. At the end, the results from the verified equation was compared to other proposed equations from literature. 2 Methodology In this study the critical shear stress of levee soils due to overtopping was investigated by running lab scale models and using a strain measuring device. The 1 to 20 scaled physical models, constructed in laboratory, represent typical levees across Mississippi river. All the tests included a floodwall, which was represented by a wooden plate at the crest of the levee. A box with dimensions of 0.9 m (3ft) wide, 2.4 m (8 ft) long and 1.8m (6 ft) high was constructed using plywood and clear acrylic plastic material. The modeled levees inside this box were 0.23 m (9 in) wide, 0.18 m (7 ft) long and the top of the levee was 0.35 m (14 in) high. Figure 2 shows the constructed levee in the box and the scour width and depth definitions. Scour Width Scour Depth Figure 2. The simulator box used for constructing the levees Induced Stresses have been measured using a strain-measuring device, which was calibrated to measure the induced stresses of falling water on the soil surface. Figure 3 shows the strain-measuring device and the installation of the device inside the box. Figure 4 shows a sketch of the device during floodwall overtopping. The strain gauge is mounted on a cantilever beam and the stresses applied to the beam from water freefall is measured using the strain gauge. The cantilever beam was positioned at different elevations to measure the applied stresses induced by various height of water freefall. Since, during floodwall overtopping usually a scour hole is developed behind the floodwall, the freefall water impinges to a pool. In order to represent the difference of the pool depth above the soil surface during overtopping when stresses are measured using the device, a plastic container was attached on top of the beam to allow a formation of a pool. The strain gauge reading of the device was calibrated for the with pool and without pool scenarios. Different dead weights were hung from the tip of the beam and the generated strains were calibrated to develop a correlation of applied load versus strain reading. There, the force applied by the free waterfall could be estimated using the calibrated strain gauge. Once the loads acting on the beam is known, the applied stress will be calculated using equation 7: ( )= ( ) ( ) ( ) 7 3

4 Figure 3. The strain gauge measurement device Figure 4. The strain gauge device under the floodwall during floodwall overtopping The soil used for these tests was classified as low plasticity silty soil (ML) and called borrow material in this paper. Also low plastic clay (CL) was mixed with silty soil to have variation in plasticity of levee soil. Therefore, besides the ML soils, two other soil mixtures were prepared. First mixture included 1 to 1 ratio of silty soil (ML) to clayey soil (CL), and the second mixture consisted of 1 to 2 ratio of silty soil (ML) to clayey soil, respectively. Therefore, total of three soils were used in levee tests. Sieve Analysis (ASTM D1921), Compaction (ASTM D698), Plasticity index tests (ASTM D4318) were performed on both of these soils and the mixtures. The plasticity of borrow material (ML) and the 1:1, and 1:2 ML to CH soil mixtures were 0, 6 and 9, respectively. The compaction tests were based on standard proctor test. The levee models constructed using 70, 80 and 90 percent compaction levels. Ten different lab scaled levee-floodwall erosion tests were conducted. These ten tests are specified in Table 1. In each test, the targeted compaction levee of the material was accomplished by the use of a hand tamper on several thin lifts. After compaction of each layer, 3 samples were taken from the compacted layer to verify the targeted compaction level. In order to run the tests, first the reservoir behind the levee was filled up to the top of the floodwall level. The flooding process started by pumping water to the reservoir. The resulted overtopping nappe initiated the scour generation. The overtopped water was drained at the downstream of the levee and was circulated to the reservoir. In order to monitor the erosion of levee material due to floodwall overtopping, three camcorders were used to monitor the generated scour from side, top and front angles. The vertical velocity of the water was calculated by using Equation 8 (free fall equation) (Johnson, 2012). = 8 Where w is the vertical speed at the point where water impinges the soil, g is the gravity acceleration and h is the distance from top of the floodwall to the levee surface. 4

5 Table 1. Tests used for creation and verification of the proposed equation Test Number Test Usage 1 Equation 2 Equation 3 Equation 4 Equation 5 Equation 6 Equation 7 Equation 8 Equation Verification 9 Equation Verification 10 Equation Verification Saturation Ratio (SR) () (mm) Compaction Level (CL) ( ) Plasticity Index (PI) () Results Figure 5 shows the erosion-applied stress results of the 7 tests. Using the results of this figure, the trend line of each data set could be obtained. The results show that with an increase of the plasticity index and compaction level the critical shear stress increases. The results also show that with an increase in the saturation ratio from Test 5 to 4, the critical shear stress decreases. Although the change of mean particle size had the smallest effect on the critical stress, it is observed that with an increase in the soil particle diameter there is a reduction in critical shear stress. The observations of the effect of PI and mean particle size on critical shear stress are in agreement with previous studies in this area (Kimiaghalam, 2014). Table 2 presents all the variable considered for developing the final equation with each test s trend line. A linear trend line was used for all the tests because of the good agreement with the test results. Trend lines then extended to intercept with applied stress axis to find the critical shear stress. Using the results of these tests, Equation 9 is proposed for determining the critical shear stress of the fine grain soils of levees during overtopping. The cumulative variance of the calculated and the measured from the tests is 0.14 pascal. Table 2 shows the calculations of the variance. Where PI= Plasticity index CL= Compaction Level D50= Mean Particle size (mm) SR= Saturation ratio =. ( ) ( ). (. ) (. ) 9 5

6 Erosion Rate (mm/hr) PI=0 CL=80 SR=19 PI=0 CL=90 SR=36 PI=6 CL=70 SR=19 PI=6 CL=80 SR=30 PI=6 CL=90 SR=47 PI=0 CL=80 SR=75 PI=0 CL=90 SR= Applied stress (Pa) Figure 5. Erosion rate-applied stress results of the tests used for creation of the proposed equation Test Number (SR) (mm) Table 2. Variances of the outcome of the figures vs. the proposed equation (CL) (PI) Erosion rate (ER)- trend line equation from Fig. 5 (Pa) From Eq. 9 (pa) Variance (Pa) ER=51.425τ ER=60.707τ ER=42.624τ ER=42.624τ ER=47.774τ ER=52.437τ ER=98.492τ Equation 9 was verified using 3 other levee-floodwall tests shown in Table 3. Equation 9 was used to predict the and the results were compared with the calculated from the trend lines. As the results show there is a good agreement between calculated critical shear stresses from equation with the measured ones from Figure 5. The biggest difference was observed on the 2CL: 1ML soil combination, which was about 1 Pa lower than the trend line. This is probably because no tests with PI of 9 was used in development of Eq 9. Test Number (SR) (mm) (CL) Table 3. Detailed information of verification tests (PI) Erosion rate (ER)- trend line equation from trend line (pa) from equation 9 (pa) Variance (Pa) ER=109.72τ ER=143.15τ ER=25.92τ At the end for comparison with other proposed equations, results from Equation 9 were compared with the results from Equation 1, 3 and 5. A mean depth of flow of 1 inch that represented the test conditions for the levee-floodwall systems was 6

7 considered in Equation 5. Results are shown in Table 4 with all the participating variables. All equations except Equation 9 are insensitive to soil properties. It is also worth noting that the only equation that considers almost vertical impingement of water freefall is Eq 9. The other equations assume horizontal flow of water on the soil. Test Number (SR) (mm) Table 4. Comparing the results of different equations (CL) (PI) Eq 1 (pa) Eq 3 (pa) Eq 5 (pa) Eq 9 (pa) Figure 6 shows the results of proposed equation comparing to other equations. It can be seen that the proposed equation is sensitive to change in alteration in soil properties and provides an interval of critical shear stress from 0.52 Pa to 3.66 Pa. Equation 1 which only considers plasticity index provides the lowest critical shear stress of The average critical shear stress from Equation 1 is 0.53 Pa. Equation 3 provides larger critical shear stress comparing to the other equations with a mean of 3.54 Pa. Equation 5 provides a mean of 1 Pa for critical shear stresses. Finally, the mean critical shear stress based on Equation 9 for the seven tests is 1.9 Pa. Critical Shear stress (Pa) Test number Proposed Equation Eq1 Eq3 Eq5 Figure 6. Comparing results of proposed equation vs. other equations 4 Summary and Conclusion The goal of this study was to propose a new equation for predicting critical shear stress of the silty and clayey soils used in levees during floodwall overtopping. The proposed equation unlike previous equations takes into account the plasticity index, saturation ratio, mean particle size and compaction level of the soil in predicting critical shear stress and is more appropriate for erosions due to free fall water. Levees with larger compaction levels and percentage of plastic material will have higher resistance against shear stresses induced by water overtopping from floodwall while increase in saturation ratio and mean grain size results in reduction of critical shear stress of the levee s soil. 7

8 REFERENCES Guy, A. (2014, September 17). Climate London. Retrieved from Climate London: Hanson, G. J. (1990). Surface erodibility of earthen channels at high stresses: Part II. Developing an in situ testing device. Transactions of the ASAE, Ing. A. Shields, W. P. (1936). Application of Similarity Principles and Turbulence Research to Bed-load Movement. Soil Conservation Service Cooperative Laboratory, California Institute of Technology. Johnson, E. B. (2012). Levee scour from overtopping storm waves and scour counter measures. Ocean Engineering, Kamalzare, M. H. (2013). Computer Simulation of Levee Erosion and Overtopping. Geo-Congress at Stability and Performance of Slopes and Embankments. San Diego. L. A. Clark, T. M. (2007). Methods for determining streambank critical shear stress and soil erodibility: implications for erosion rat e predictions. American Society of Agricultural and Biological Engineers, Mazdak Karimpour, A. O. (2014). Levee erosion and scour potential due to floodwall overtopping. XV Danube. Vienna. Navid Kimiaghalam, S. P. (2014). An experimental study on the effects of physical, mechanical,and electrochemical properties of natural cohesive soils on critical shear stress and erosion rate. International Journal of Sediment Research. William Allsop, T. B. (2008). Direct hazards from wave overtopping the forgotten aspect of coastal flood risk assessment? Flood and Coastal Management Conference. Manchester. 8

SCOUR CHARACTERIZATION DUE TO WATER FREE FALL

SCOUR CHARACTERIZATION DUE TO WATER FREE FALL Abdolreza Osouli, Corresponding Author, Ph.D., P.E. Assistant Professor of Civil Engineering, Southern Illinois University at Edwardsville, Engineering Building,