Geotechnics of Soil Erosion

By making use of the Shields diagram the paper illustrates that the commonly held belief that erosion of earth material results from the interaction between shear stress caused by flowing water acting on it is incorrect. It is shown that erosion results from shear stress under laminar flow conditions only. Fluctuating pressure is responsible for erosion when the flow is turbulent. This observation is particularly important when investigating erosion and scour of earth materials other than non-cohesive soils. For example, scour of rock formations and cohesive soils subject to turbulent flow can be satisfactorily explained only in terms of the effects of fluctuating turbulent pressures.

A number of research efforts since 1991 resulted in the development of technology that can be used to predict the erodibility and extent of scour in rock. This research indicated four potential failure modes; i.e. abrasion, block removal, brittle fracture and fatigue failure. Scour can be predicted by making use of semi-empirical and physically based methods. An empirical approach that was found to work well is known as the Erodibility Index Method. Physically based methods, known as the Comprehensive Fracture Mechanics Approach, which is used to predict scour by brittle fracture and fatigue failure, and the Dynamic Impulsion Method, which is used to predict scour by block removal, have been developed by Bollaert. Methods to predict scour by abrasion are not developed as yet. This paper summarizes current understanding and methods.

The US Bureau of Reclamation is responsible for the safe performance of many dams West of the Mississippi. Embankment dams are susceptible to uncontrolled seepage and possible failure by piping and internal erosion. Engineers need tests that will assist in evaluating risk of piping, and in the mid 1990's there was no commonly accepted test for evaluating piping and internal erosion potential. Recently, Reclamation's Dam Safety Office has funded several research programs to improve their ability to perform safety risk analysis. Reclamation along other agencies, funded research at the University of New South Wales (USNW), by Professor Robin Fell on piping and internal erosion. This comprehensive research was focused on determining all variables involved with piping and internal erosion. Part of the UNSW research involved slot and hole erosion tests. UNSW developed soil erosion indexes through extensive testing and factored those indices into overall risk evaluations. The Hole Erosion Test (HET) is a simple, low cost test but, it has difficulty because there are non-uniform flow conditions in the test. This paper presents a review of past Reclamation research on erosion, examination of other erosion tests currently available, which could be used for piping, a summary of UNSW research, and a summary of current Reclamation research on the HET. Reclamation's goal is to quantify the HET for standardization so engineers now will have a readily available test for piping and internal erosion for risk analysis.

This paper presents the initial results of an ongoing study at Texas A&M University on abutment scour in cohesive soil in the case of clear water scour. A wing-wall shape abutment and a rectangular channel were used in the laboratory experiments. Three different abutment lengths were used to investigate the effect of blockage ratio in this study. The scour depth comparisons with previous works made by others were made and show disagreements of results, and conclusion about clay and sand behavior during scour is presented. Two numerical models ranging from 2D to 3D were also used to compute the velocities as well as water level. Comparison of velocities and water level between the models and measurements is also presented.

Soil erosion by raindrop impact is an important source of soil erosion from slopes and other non-concentrated flow areas. Vegetation is one of the most effective ways for minimizing soil erosion by raindrop impact. Ground covers, such as mulches, rock, and rolled erosion control products (RECPs) are also effective in reducing the effects of raindrop impact by limiting the amount of soil exposed. This paper presents the results of a laboratory study that compares the rainsplash performance of 4 different RECPs (wood excelsior, coconut, jute, and polypropylene (PP)), with and without vegetation, using a rainsplash simulator. The results are used to evaluate the contribution of vegetation and the role of RECPs in minimizing soil erosion. Based on the results of 44 tests, it was found that the use of RECPs on bare soil slopes reduces soil losses by 74% to 91%. The use of vegetation in combination with RECPs further reduces soil losses by 95% to 99%. Rainsplash tests provide a useful venue for comparing the performance of RECPs and vegetation and the effects of different RECP index properties on soil loss results. It is recommended that rainsplash results be validated with large-scale ASTM testing and field results.

No-fines concrete is a pervious concrete obtained by eliminating the sand from normal concrete mix. Compared with conventional concrete, no-fines concrete has unique properties desirable for various applications. Because of the presence of large voids, no-fines concrete has lower density, cost and thermal conductivity, smaller dying shrinkage, no segregation, larger contaminant retaining capability, and reduced capillary movement of water. No-fines concrete is used for construction of pavement, storm water control utilities and green houses. This paper discusses the application of no-fines concrete as an ecology preservative method for stream bank erosion control. Soil erosion is an important factor that can trigger the instability of an embankment. A sustainable revetment should provide soil erosion protection without significantly changing the existing ecologic environment. The strength of no-fines concrete provides sufficient protection against scour of embankments. Although grasses are found to be hard to survive with ordinary types of bank revetment, especially when subjected to periodic inundation from river water level fluctuations, the large pore spaces of no-fines concrete protect grass seeds and provide an environment for grasses to grow. By installing artificial access holes in the no-fines concrete revetment, the ecologic conditions can be preserved to the maximal extent. The design criteria and durability of no-fines concrete revetments are discussed in this paper. This paper also provides an example application of a no-fines concrete revetment that achieved the desirable ecologic effects.