Computer Simulation of Levee Erosion and Overtopping

Improved computer models of erosion have been developed, considering soil hydraulic conductivity. The models deal with erosion of levees, dams and embankments due to overtopping. The simulations trace the formation of rills and gullies, beginning with initial overtopping and continuing to final breaching. Physical models performed at 1-g and high g using a geotechnical centrifuge have been used to calibrate the models. Previous models did not consider soil hydraulic conductivity, and although results were quite good for the formation of rills and gullies and sediment quantities, breach times were underestimated. Essentially the water flow was treated as if passing over a solid surface, not entering the soil, and the total water flow was available for erosion. Thus, breach times were underestimated. Soil erodibility parameters had to be adjusted in order to achieve good agreement with breach times. The new models developed consider soil hydraulic conductivity, and produce good agreement with the performance of the physical modeling, including breach times and the use of proper soil erodibility parameters

Measuring terrain distances through extracted channel networks

This paper initiates a forensic analysis of the causes of levee failures by analyzing and extracting information from a sequence of elevation data. This is a crucial step in bettering the design and construction of levees and dams. (Fully diagnosing failures usually requires knowledge beyond the geometry of the levee, such as weather conditions and material properties). We use results from computer simulations of levee overtopping for training data. The simulations use smoothed particle hydrodynamics coupled with a well-known erodibility model. Using the sequential nature of our data, we extract important channel networks that form as the soil is scoured away. We present a series of metrics to measure the distance between channel networks to assist in determining the critical threshold value used to extract important channels from the flow network. Methods for determining this ideal threshold have gone mainly unexplored, and so we present a comparison of various threshold values and how closely they identify matching channel networks on sequential terrains. These threshold values allow us to identify important properties of the terrain that form its fingerprint, a way of characterizing the geometry of the terrain. Our method for fingerprinting terrain is an important step toward the diagnosis of levee failure from digital elevation data

Validation of Erosion Modeling: Physical and Numerical

The overall intent of this research is to develop numerical models of erosion of levees, dams and embankments, validated by physical models. The physical models are performed at 1-g and at high g\u27s using a geotechnical centrifuge. The erosion is modeled in detail, from beginning to end, that is from the time the levee is overtopped until the levee is breached. Typical quantities measured as a function of time are the depth, width and volume of rills, number of junction points, are the rills straight or meandering, sediment transport quantities, and finally breach. This data can be obtained from the numerical modeling, but is difficult to obtain from the physical modeling. Video images indicate the physical modeling agrees quite well with the numerical modeling. A comparison has also been done between observed breaching width and the FEMA new formula for both 1-g and centrifuge tests

COMPUTER SIMULATION OF OVERTOPPING OF LEVEES

There have been many cases of earth embankment failures, for example, Hurricane Katrina in 2005, where breaching occurred and devastated the surrounding population. Levee failures are preventable by a better understanding of the ways in which these embankments are designed and fail. The objective of this research is to protect levees against future failures. This paper studies various overtopping quantities and durations to represent the same level of levee erosion hazard. This study is based on experimental results of steady flows on the land side of a levee. The effect of water flow has been investigated and a comparison has been done between rills formations and erosion time for various water flows. Results showed that the pictures of digital simulations and real photographs which have been taken during tests in the laboratory are in a good concordance. Ha habido muchos casos de fallos de terraplén, por ejemplo, el huracán Katrina en 2005, en el cual se produjo una ruptura, devastando la población de los alrededores. Las fallas de diques se pueden prevenir, y es un objetivo de esta investigación alcanzar una mejor comprensión de las maneras en que estos diques se diseñan y fallan, a fin de poder protegerlos contra futuros fallos. Este documento desarrolla y recomienda equivalencias preliminares de combinaciones acumulativas de varias cantidades de desbordamiento y las duraciones asociadas que representan el mismo nivel de riesgo de erosión del dique. Las metodologías se basan en los resultados experimentales de flujos constantes en el lado seco de un dique. El efecto del flujo de agua se ha estudiado específicamente en esta investigación, y se ha hecho una comparación entre las formaciones de surcos y el tiempo de erosión para distintos flujos de agua

Vpliv geosintetičnega armiranja na strižno trdnost dvoslojnih tal

Due to the low bearing capacity of soft clayey soils in places that because of economic, military or geological conditions we are oblige to build a structure on, geosynthetics will be used to reinforce the soil and improve its bearing capacity. Particularly, A good example is roadways, where geosynthetics are placed between the interface of the granular materials and the soft-soil sub-grade to improve the bearing capacity of the composite layers. In previous research the behavior of one-layer soils that were reinforced with different kinds of geosynthetics were studied by experimental and analytical methods and some numerical models have been developed. In this paper the behavior of two-layer soils (granular base and clayey sub-grade) that were reinforced with some geosynthetics are investigated. Large-scale direct shear tests were performed on unreinforced and reinforced samples with different geosynthetics. The results show that depending on the characteristics of the geosynthetics, the inclusion of these materials may increase or decrease the shear strength parameters of the interface of two-layered soils. It implies that the geosynthetic-reinforced soils in the sub-base layer of roads are so sensitive to the characteristics of geosynthetics and will perform better than non-reinforced soils and consequently the load-carrying capacity of the basement will improve only if the appropriate geosynthetics are used. However, geogrid shows more reinforcement efficiency under higher vertical stresses. Increasing the relative density of the clayey sub-grade would also cause the geogrid reinforcement to be more effective.Kljub slabi nosilnosti mehkih glinastih tal smo zaradi ekonomskih, vojaških ali geoloških pogojev prisiljeni graditi na takšnih tleh, zato za učvrstitev tal in izboljšanje nosilnosti uporabljamo geosintetiko. Za izboljšanje nosilnosti kompozitnih plasti na cestah namestimo geosintetiko na vmesno ploskev med zrnastim materialom in mehkimi tlemi. V prejšnjih raziskavah smo s poskusnimi in analitičnimi metodami preučevali obnašanje ene plasti tal, ki je bila učvrščena z različnimi tipi geosintetike, ter razvili nekatere numerične modele. V tem prispevku podajamo rezultate raziskav odziva dveh plasti tal (zrnato osnovo in glineno plast), ki sta učvrščeni z geosintetiko. Testi strižne trdnosti so izvedeni na nearmiranih in armiranih vzorcih z različnimi geosintetikami. Rezultati kažejo, da lahko vključevanje teh materialov poveča ali zmanjša parametre natezne trdnosti vmesne ploskve med dvema plastema tal, odvisno od značilnosti geosintetike. Iz tega vidimo, da so tla, učvrščena z geosintetiko v spodnjih osnovnih plasteh cest tako občutljiva na značilnosti geosintetike, da bodo delovala bolje kot neučvrčena in da se bo posledično izboljšala nosilnost temeljev samo, če uporabimo primerno geosintetiko. Geo-mreža je veliko bolj učinkovita za učvrstitev pri večjem vertikalnem pritisku. Tudi povečanje relativne gostote glinenih spodnjih plasti pozitivno učinkuje na geo-mrežo

Vpliv plastičnosti in normalne napetosti na nedrenirane strižne module glinenih zemljin

The shear modulus, known as Gmax, is a key parameter for predicting the static and dynamic behavior of soils. Its value decreases by increasing the shear strain. This is because of reducing the soil’s stiffness as a result of increasing the shear deformation. The increasing of the shear modulus by increasing the shear strain is affected by some of the soil properties, such as the Void ratio (e), the Over consolidated ratio (OCR), the Normal stress (σ), the Plasticity index (PI), the Water content (ω%), the Shear strain rate, the Soil structure, and the Loading history, etc. In this paper, undrained, direct shear tests were conducted to study the effect of the plasticity index (PI) and the normal stress (σ) on the shear behavior and the shear modulus of remolded clays. The results show that the normalized shear modulus at a constant strain will generally increase as the σ and PI increase, and the common empirical equations for undisturbed soils at γ = 0~0.1 might be applicable for the disturbed soils too.Strižni modul, znan kot Gmax, je ključni parameter za napovedovanje statičnega in dinamičnega obnašanja zemljin. Njegova vrednost se zmanjšuje s povečanjem strižne deformacije. To je zaradi zmanjšanja trdnosti zemljin kot posledice povečanja strižne deformacije. Na povečanje strižnega modula s povečanjem strižne deformacije vplivajo nekatere lastnosti tal, kot so količnik por (e), prekonsolidacijsko razmerje (OCR), normalna napetost (σ), indeks plastičnost (PI), vsebnost vode (w%), strižno deformacijsko razmerje, struktura tal, zgodovina obremenjevanja, itd. V članku so prikazani rezultati direktnih nedreniranih strižnih preiskav porušenih glin, ki so bile izvedene z namenom proučevanja vpliva indeksa plastičnosti (PI) in normalne napetosti (σ) na njihove strižne lastnosti in strižni modul. Rezultati kažejo, da normalizirani strižni moduli pri konstantni napetosti v splošnem naraščajo z naraščanjem indeksa plastičnosti in normalne napetosti in da je lahko splošna empirična enačba za intaktne zemljine pri γ = 0~0.1 uporabna tudi za porušene zemljine

Energy Dissipation in Soil Structures During Uniform Cyclic Loading

Characterization of soil response under cyclic loading is one of the major challenges in evaluating liquefaction triggering. In this paper, we have performed numerical simulations to study dissipated energy and accumulated damage in soil structure at onset of liquefaction. For this purpose, at first, we validated Plasticity Model for Sands (PM4Sand) in capturing soil cyclic response with findings in experiments. Thereafter, the model was utilized to simulate soil behavior during uniform cyclic loading under controlled boundary conditions and stress paths. Simulations were performed on soils with different relative densities and under different confining pressures. The results of this study indicate that energy dissipation is directly related to PWP generation, and is independent of the amplitude, form and frequency of loading. Dissipated energy can be utilized as a versatile metric to characterize soil strength degradation and liquefaction triggering during cyclic loading