For this reason, a simpler approach has been attempted to understand the behavior of geosynthetics under the steady-short-term loading regime to demonstrate the conservatism of current design methods.
INTRODUCTION 1.1 General
Development of Geosynthetic Reinforced Soil Structures
Design Approaches and Computer Programs
In addition, when GRSS were designed and constructed based on LEA, a fundamental change took place in the design of building structures, bringing in the introduction of deformations and deformations as design criteria to be evaluated and controlled in an explicit way. This concept has been incorporated into the so-called limit state approach [LSA], where both failure conditions, ultimate limit states [ULS], and serviceability limit states [SLS] are analyzed. An additional feature of this approach is the introduction of risk factors, partial factors that replace the use of global safety factors.
For limit state design, Pradhan (1996), McGown et aI (1998) and Khan (1999) have suggested valid mechanisms involving ULS and SLS analyses. To date, their suggested approach can be considered to represent the only "true" limit state approach [LSA] applicable to the design of GRSSs, which is based on internal strains.
Actions and Design input parameters
- Soil Properties for Single-Stage Actions
- Geosyntbetic Properties for Multi-Stage Actions
- Reinforced Fill
- Retained Fill
- Foundation Soil
- Facing Units
- InextensibIe Reinforcements
- Extensible Reinforcements
- Rheological and Mathematical Modelling of Geosynthetics Behaviour Several attempts have been made in the past to simulate the behaviour of
- Esteve's Method
- Zener (Standard Linear Solid) Model
- Limit Equilibrium Approacb
- AASHTO Standard Specifications for Highway Bridges (1997)
- HA 68/94 Method (1997)
- A Model for Internal Ultimate Limit State Mechanism
- Sustained plus Cyclic loading
Typically, retained fill (fill behind the reinforced ground section) may be present in GRSSs, Figure 2.3(a). However, when the reinforcement breaks, the composite behavior reverts to that of the soil alone. Adopting the Performance Limit Strain [!>p] to define the reference strength [PRed for Ultimate Limit State design] can be seen as the rather conservative choice for the strength value, Khan (1999).
So the values of A2 depend on the type of soil and the type of geosynthetics. The length in the upper part of the structure is based on the so-called Tmax mechanism. Tc extrapolated the tensile strength (at 10% limiting strain) of the reinforcement at the design life and specified operating temperature.
Many tests have been carried out to identify the understanding of the behavior of geosynthetics against multi-stage loads in the past.
Research outputs! Case studies related to Multi-stage actions
If the change is significant, an iterative approach to determining the deformations in the soil and the reinforcements must be undertaken. Therefore, it is important to ensure that the duration of the strong motion is consistent within the design scenario. On the other hand, energy will decrease with distance due to attenuation of the motion.
Therefore, a short-term load duration of 20 seconds was chosen. In the second wall, there was uneven settlement of the ground and a large crack reached the base under the facing blocks due to liquefaction in the subsoil in front of the wall. Similarly, of the four GRS-RWs that experienced an earthquake, three were in an area where the (Japan Meteorological Agency seismic intensity) JMA scale was V or VI.
In two cases no deformation of the wall was observed, while in the third case the wall at the top of the cladding moved outward a maximum of about 2 cm. The bottom of the wall moved outward on average about S cm relative to the supporting subgrade of the foundation, pushing the subgrade laterally in front of the wall. The largest outward displacement occurred at the highest part of the wall, namely 26 cm at the top of the wall and 10 cm at ground level.
For most GRS-RWs built so far, to be conservative, some reinforcement layers were made larger than others at lower levels. This truncation may have reduced the seismic stability of the wall, especially in terms of overturning. If this were the case, considering the same design strength of the geosynthetics throughout the service life of the GRSS would likely be unsafe.
PI PI (b) Kelvin or Voigt model
I SPECIMEN 3 I
BEHAVIOUR OF MATERIALS UNDER DIFFERENT LOADING REGIMES 3.1 General
Perfectly plastic material 3.6.1 Perfectly elastic material
Tn Fig. 3.13 (b), the material receives an elastic load at point A to B that maintains corresponding to the sustained load P, as in Fig. 3.2 (b). In Fig. 3.14(a), the material assumes the load gradually, to to tl, point A to B, as long as the load PI is maintained. In fig. 3.14 (b) the material undergoes plastic strain gradually from to to tl, point A to B, as the strain is sustained.
From tl to t2, the strain due to the increasing traffic load Pt is added to that due to the steady load P, gradually, from point B to C, during the loading phase. The plastic deformation increases gradually from to to tI, from point A to B due to the sustained load P,. From t] to h, the stress due to the application of the short-term load P'hort-tenn is gradually added to that due to the permanent load P" point B to C.
In the case of step loading, fig. 3.15 (a), elastic strain is obtained at point A to B with the application of the sustained strain PI, followed by plastic strain from to to t], point B to C, as long as the strain is maintained. In Fig. 3.15 (b), at two, there is an assumption of elastic strain followed by plastic strain from to to t], points B to C, due to the sustained strain P,. In Fig. 3.16 (a), which is an illustration of the strain response of EVP material due to step loading under MSA, the material assumes an elastic strain at points A to B when the sustained load PI is applied.
The elastic load is recovered as the load Pshort-tennis released at tz, points E to F. In the case of step loading, fig. 3.17 (a), the material attains a recoverable strain ER upon application of the sustained strain PI at, points A to B. Similarly, Fig. 3.17 (c), which is the illustration for the case of continuous plus momentary loading, the recoverable strain ER is induced at !D, point A to B, which remains constant followed by the increase in the clamped strain EL from two to t], point B to C, due to the continuous load Ps.
IEp ('~ C (2)
I SPECIMEN 2 I
ISOTHERMAL BEHAVIOUR OF SOME GEOSYNTHETICS SUBJECTED TO SINGLE STAGE LOADING
General
Test set up and Procedure
The schematic of the test equipment used in the sustained load creep test is shown in Figure 2.14(a). Therefore, it becomes necessary to extrapolate the CREEP test data to the required design life of the structure. Similarly, it also becomes essential to extrapolate CREEP test data at higher load levels than those used during testing.
Two types of equations were found to fit the curves derived as total strain-time plots from the CREEP tests at different load levels, viz. So the parameters A and B were separated and plotted at different load levels where a linear fit was found to represent them. , Figures (a) and (b) of 4A and 4.5. From these linear fits, the parameters A and B were extrapolated at higher load levels required for the study, which when substituted into the above equations generate the required total strain-time curves at higher load levels.
Figures 4.6 and 4.7 are the total strain-deformation plots derived from the total strain-time plots (Figures 4.1 and 4.2) at 20 °C and different loading levels for SS2 and SR80 geogrids, respectively, following the procedure described in Chapter 3. in order to derive the load-locked strain curves, a set of data from unloading tests performed at 20 °C for the same geogrids at different load levels was collected and extrapolated at higher load levels, Figures 4.8 and 4.9, from which recoverable deformation can be found at any load level. To achieve a strain of 10%, the total strain time data were extrapolated at higher loads.
In the third chapter, it was established that geosynthetics would develop deformation under loading and that this total deformation ETat at any time comprises deformation components called. From these figures, it can be understood that at any time the strain components can combine uniquely to give a total strain. The total strain in geosynthetics under load consists of two components, namely the recoverable strains &R and the strains locked in &L.
Linear fit I
- Combined sustained-short-term loading test
- Discussion on test resuIts
So considering the loading period of the short-term load (Stage2) for 20 seconds is probably on the pessimistic side. Stage 2 represents where the short-term load is applied in addition to the self-weight of the structure. The application of the short-term load is arbitrarily chosen to be 100 hours from the assumption of the sustained load to allow for the time gap between the construction and the occurrence of an earthquake.
Stage3 again represents the loading condition similar to Stage 1, but after withdrawal of the short-term load, i.e. The loading scheme chosen for the tests is as shown in Figure 5.I. The Stage I Sustained Load [P,] was 25 kN/m which is the long-term Design strength of uniaxial geogrid B at 20°C according to BS8006 (1995). On the other hand, let it be assumed that the Stage2 load is applied to the same specimen after 1000 hours from the start of the sustained load.
As a result of the application of the sustained load, the sample would have assumed the same amount of ER (lOOhrs). Naturally, the available strain decreases with the age of the structure, because there is a continuous development of 'Locked-in Strain' in the geosynthetics due to the continued loading. Furthermore, the significance of the time of a seismic event for the lifetime of a GRSS was highlighted.
Based on the above, it has been suggested that geosynthetics are likely to be able to carry a much higher short-term load than long-term permanent load. From the interpretation of the test results, it can be observed that the material would likely exceed the limit strain of 10% even under an additional short-term load of 40 kN/m if the event occurred after 1000 hours. The relationship between the time of earthquake occurrence and the ability of GRSS to withstand additional short-term loading should be confirmed by further studies.
A Reassessment of the Design of Geosynthetic Reinforced Soil Structures, Ph. Thesis, University of Strathclyde, Glasgow, UK. The load-strain time behavior of Tensar geogrid. of the Conference on Polymer Lattice Reinforcement, London, 1984a, p. The behavior of reinforced earth walls constructed by different techniques.
