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Publication Number:  FHWA-HRT-15-033    Date:  May 2015
Publication Number: FHWA-HRT-15-033
Date: May 2015


Scour in Cohesive Soils


Cohesive soils display a wide array of engineering properties, including erosion behavior, that are not easily derived from the physical properties of the soil. For noncohesive soils, the engineering properties, particularly the resistance to erosion, depend primarily on the soil structure and grain properties. The behavior of cohesive soils depends not only on these factors but also on the electrochemical attraction and repulsion characteristics of the soils. Further complicating any assessment is the possibility that these properties change when samples are removed from a site for testing or when samples are created in a laboratory. With respect to laboratory prepared soils, the soil structure may be influenced by the preparation technique and sequence.

This study had two objectives. The first objective was to introduce and demonstrate the effectiveness of a new ESTD that can mimic the near-bed flow of open channels to erode cohesive soils within a specified range of shear stresses. The ESTD employs a moving belt and a pump to generate a log-law velocity profile in a channel. The second objective was to develop a method for estimating the critical shear stress and erosion rates for a limited range of cohesive soils in the context of the HEC-18 scour framework. The method is based on using more easily obtained soil parameters to make the estimates so that direct erosion testing is not needed for many situations. General relations are proposed in this report for both best-fit and design applications.

An ESTD was designed to reproduce log-law velocity profiles in a test conduit to simulate open channel flow. The flow condition was achieved by propelling water through the test conduit with a moving belt and a pump. Cohesive soils with different percentages of clay, silt, and non-uniform sands were mixed and de-aired in a pugger mixer to prevent slaking. The erosion testing of these cohesive soil specimens revealed the following:

  1. The ESTD is capable of determining erosion characteristics of cohesive soils within the range of 0.063 to 0.31 lbf/ft2 (3 to 15 Pa). Its capability of directly measuring wall shear stresses enhances the understanding of the erosion process in cohesive soils.

  2. Preparation of erosion test samples by compaction usually leads to soil slaking, which cannot be tolerated to generate meaningful erosion function data.

  3. CL-ML soils are more erodible than CL soils (lean clays) because of the small PI values of CL-ML soils. For CL-ML soils, the erosion rate decreases as the PI increases.

  4. For cohesive soils, the erosion rate increases as the bed shear stress increases. The erosion curve generally follows a power law relation but can be approximated by a linear expression.

  5. At a given bed shear stress, the erosion rate generally increases with water content. The critical shear stress generally decreases with increasing water content.

  6. Cohesion, friction angle, and unconfined compressive strength are soil properties on a macro scale (engineering properties). Only unconfined compressive strength appeared to be useful for predicting erosion rates.

For more erosion-resistant soils and soft rock, other erosion testing devices are needed that will also simulate open channel conditions by creating a log-law velocity profile. The desired range for one or multiple devices is from 0.30 to 4.2 lbf/ft2 (15 to 200 Pa). Having a series of devices capable of testing erosion characteristics over a range of shear stresses using consistent rigorous methods is needed to understand the erosion characteristics of a full range of bed materials.

This study has resulted in quantitative guidance for predicting erosion in cohesive soils so that erosion testing is not required for every project. Estimates of critical shear stress are based on the water content, fraction of fines, PI, and unconfined compressive strength. In addition, an equation for estimating erosion rates when bed shear stress exceeds critical shear stress is proposed. For application, the designer must determine the critical shear stress of the soil, the unconfined compressive strength, and the PI.

The guidance may be used for engineering design within limits based on the range of values of the current data set and, to a lesser extent, the range from the Illinois field data. The Illinois field data were used to validate the methodology. Validation was also attempted using a field data set from Texas. However, all but four of the data observations were excluded from the analyses because of high values of PI (greater than 0.25) or high values of the fraction of fines (greater than 0.9).

The recommended limits for application of the proposed tools are as follows:

  1. The results only apply to fine-grained cohesive soils within the range of 4 to 25 percent for the PI and between 15 and 50 percent for the LL.

  2. The fraction of fines in the soil should be within the range of 10 to 90 percent.

  3. Soils should be saturated or nearly so. Even soils that are unsaturated (not below the normal flow water line) could become saturated during a flood event. These methods are recommended for soils with at least 90 percent saturation but can be applied to lower degrees of saturation.

  4. The proposed erosion rate relation with bed shear stress is limited to bed shear stresses less than or equal to 2.1 lbf/ft2 (100 Pa).

  5. Soils must be free of slaking. Soils exposed to wetting and drying cycles or located on the floodplain away from the water table may be subject to slaking, and, therefore, greater soil loss may occur than is estimated by the proposed tools. In such cases, use of these tools must be used with caution.

  6. The results presented here are based on one type of clay from the current laboratory tests and field clay types from the Illinois field data. Care must be taken when applying these methods to a broad range of clay types. Further investigation in this area is warranted.

A broad range of field and laboratory clays should be tested to compare their soil, engineering, and erosion properties with those described in this study. Although reasonable relations have been developed as part of this study, the soils tested represent only a small fraction of the universe that exists in the field.

Finally, critical shear stress is not a directly measurable parameter. As was done in this study and others, it is inferred by its behavior in one of many erosion testing devices with samples prepared in many different ways. Standard methods for sample preparation and testing should be adopted so that cross-study comparisons of data are more reliable.

This study provides insight into the erosion properties of a limited range of cohesive soils. Additional research in the following areas is recommended:

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