Rheological Behavior of Ultra-Soft Soil—Case Study: Urmia Lake Causeway Embankment Subsidence Ebrahim Ebrahimnezhad Sadigh MSc in Geotechnical Engineering, University of Tabriz, Tabriz, Iran [email protected] Dr. Tohid Akhlaghi Associated Professor, Department of Geotechnical Engineering, University of Tabriz, Tabriz, Iran. [email protected] ABSTRACT Rheology is a branch of continuum mechanics that studies material flows in non-Newtonian fluids, soft solids and wax like solids that traditional concepts of the elasticity and plasticity could not describe their behavior completely. Soil rheology is a branch of soil mechanics that studies origin and time dependent changes of stress and strain terms in soil. Application of rheology concepts for description of ultra-soft soils behavior after failure is a new idea that is introduced in this paper. General rheological equation of soil was analyzed by Bingham model for Urmia lake causeway rock fill subsidence and its results were compared with field measurements. The numerical results are consistent with the measured values and using rheological models is proposed as a suitable solution to solve complex geotechnical engineering problems in ultra-soft soils. KEYWORDS: Soil rheology, Ultra-soft soils, Subsidence, Urmia lake causeway INTRODUCTION Incomplete consolidation is generally associated with the existence of excess pore pressures in the soil, such that the in-situ effective stressσσ' = −u is less than the calculated effective overburden pressure γ 'z [1]. Sangrey (1977) identifies that rapid rate of sedimentation, gas in marine sediments, leakage from an artesian water or gas pressure source and wave-induced repeated loading are possible mechanisms which may result in the existence of excess pore pressure and "unconsolidated" sediments [2]. During rapid rate of sedimentation, the total stress increases as does the excess pore pressure, but dissipation of this excess pore pressure may be relatively slow. Sangrey et al. (1979) have reported a study in which rapid sedimentation in a sediment which was less than 10% consolidated, i.e. the effective stresses were less than 10% of the effective overburden pressures [3]. - 4211 - Vol. 22 [2017], Bund. 10 4212 This means that ultra-soft sediments mainly squeeze under loads without considerable consolidation and their behavior is same as a Bingham non-Newtonian fluid with high viscosity and yield stress [4]. Subsidence rate in such a continuum environment is a time dependent function with slowing rate and in some cases rate is fixed for a long time and slowly reduced. For example, we can refer to Urmia lake causeway project with 14 kilometers length that the resistance of the upper layers of soil is Φ ≈ 0̊ and C ≈ 1-20 kPa. Subsidence of the embankment on the lakebed has been measured over 25 meters. Since modeling in geotechnical softwares such as Plaxis, Geo-Office, Flac and etc is based on small displacement and the results are attributable before failure and the modeling of the embankment construction by "displacement filling" with common geotechnical softwares isn't possible. Large shear displacements with ultra-soft sediments failure during embankment construction are mainly subsidence or squeezing without consolidation of sediments (Figure 9). Thus, utilizing the rheological models with the Navier-Stokes and continuity equations can be a good idea to describe the behavior of soil squeezing in embankment construction by "displacement filling" method. Rheology The term “rheology” has been related to flowing media, since the main root of the word - rheo in Greek- means “to flow" [5]. Starting with Amenemhet’s need for a viscosity correction to improve the accuracy of his water clock in ~1600 BCE, rheology has primarily been concerned with solving practical problems. At the same time, the complexity of the issues involved both of physical and mathematical nature has attracted some of the finest scientific minds [6]. Rheology as an independent branch of natural sciences emerged more than 80 years ago [5]. The name “rheology” was proposed to describe “the study of the flow and deformation of all forms of matter” by E.C. Bingham and M. Reiner on 29 April 1929; Heraclitus quote “everything flows" was taken to be the motto of the subject [7]. Its origin was related to observation of “strange” or abnormal behavior of many well-known materials and difficulty in answering some very “simple” questions. For example: Clay looks quite like a solid but everybody knows that it can be shaped; it also takes the form of a vessel like any liquid does; if clay is a solid, why does it behave like a liquid? Rheology is a science concerned with mechanical properties of various solid-like, liquid-like, and intermediate technological and natural materials [5]. As per strict definition, rheology is concerned with the description of flow behavior of all types of matter. A useful engineering definition of rheology is the description of materials using "constitutive equations" between the stress history and the strain history [8]. Rheology is a branch of Continuum mechanics that deals with the deformation and flow of matter, especially the non-Newtonian flow of liquids and the 'soft solids' or solids under conditions in which they respond with plastic flow rather than deforming elastically in response to an applied force [5]. Soil Rheology Soil rheology is a branch of soil mechanics investigating the origin of, and the time-dependent changes in the stressed and strained state of soil. Although the rheology of clay soils has sprouted into Vol. 22 [2017], Bund. 10 4213 a self-contained branch of knowledge quite recently, the rheological properties of these soils have been given very careful consideration for a long time [4]. Referring to the data on long-term tests of clay, Terzaghi (1925, 1943) has pointed out that soils explicitly exhibit the properties of elastic after-effect (denoted as creep) [9]. Flow is also a term frequently used to describe the secular deformation of rock mass, glacier movements, etc. It is also popular in soil mechanics, being used there as a synonym of the term "creep". However, if we use creep to denote, as settled above, the time-dependent deformation of all kinds, then, strictly speaking, flow is a special case of creep, referring to that stage when deformation is developing at a constant rate. In the theory of creep, plastic flow is understood to be a viscous flow induced by a load exceeding a certain limit (the so-called Bingham limit of plasticity); quite frequently it is referred to as viscoplastic flow [4]. The importance of taking into account the property of flow in soils was stressed by Puzyrevsky (1934) [10]. Gersevanov (1937) wrote that soil is likely to develop Bingham's flow which is a state in which a body, subjected to a certain stress, begins to unceasingly change its shape, passing into a viscous condition like a viscous fluid [11]. Rheological models for soft soils can roughly be categorized as one of the following types (Yuan Jing 2001): (1) elementary rheological models, (2) yield surface model, (3) endochronic plasticity model, (4) empirical model [12]. although soil rheology was used for describing time dependent phenomena such as creep, relaxation and the deterioration of strength due to a long-term load application, but using of rheology concepts to describe ultra-soft soils flow after the failure is new ideas that is presented in this paper. Rheology of Ultra Soft Soils or Mud Cohesive sediments or mud are common in marine, estuarine and coastal environments [13-14- 15-16-17 and 18]. The term mud commonly describes a complex mixture of fine-grained mineral sediment (clay and silt), small amounts of sand, water and organic material of diverse nature [19]. During formation of mud deposits, several mud states may appear. Such as dilute suspension, fluid mud and semi to fully consolidated mud beds. Appearance and composition of cohesive sediments are highly variable [20]. In general, rheological studies have been performed on artificial cohesive sediment suspensions (as reviewed by Coussot 1997), although some rheological characterization of natural mud has been done as well (e.g. O’Brien and Julian 1988; Aijaz and Jenkins1994; van Kessel and Blom 1998; Faas and Wartel 2006). During formation of fluid mud, the rheological properties are changing from Newtonian to non-Newtonian, i.e. non-linear response to the shear rate [21-22-23]. Currently, the vast majority of study was limited to the macro level, microscopic level and micro level were hardly ever investigated, and nanoscopic level study have not been actually carried out. Some studies have performed at microscopic level in recent years and the concept of “fluid rheology” and “rheological phase material” proposed [24]. The transition from a loosely packed layer of fluid mud to a compacted mud occurs due to further self-weight consolidation. Porewater is driven out of the flocs and the space between the flocs caused by the submerged weight of subsequently depositing particles which lead to an increase of density and a decrease in void ratio and permeability through time [25]. Vol. 22 [2017], Bund. 10 4214 There is good relevance between the model parameters and rheological phase material and it indicates that the proposed model may provide a new way for preliminarily establishing the relationship between macro behavior and microscopic physical mechanism of fluid rheological properties of soft soil [26]. Soil Constructional Equations as a Viscoplastic Material At an early stage, attempts were made to apply the Newtonian mechanism of ideally viscous flow to soils. For example, Hvorslev (1937, 1939) treated soils on these lines. In the 1940s, Haefeli and Schaerer (1946), pointed out that in the general case the flow rate-stress relation is non-linear in soils. Approximately at the same time, Casagrande and Wilson (1950) succeeded in establishing the fact of clay failure due to a long-term creep from uniaxial compression.