Long-term reinforcement strains for column supported embankments with viscous reinforcement by FEM

doi:10.1016/j.geotexmem.2017.04.003

Geotextiles and Geomembranes

Volume 45, Issue 4, August 2017, Pages 307-319

https://doi.org/10.1016/j.geotexmem.2017.04.003 Get rights and content

Abstract

The time-dependent performance of deep-mixing-method column supported embankments reinforced by viscous reinforcement is investigated for different long-term (at 99% degree of consolidation) reinforcement strains using fully coupled three-dimensional finite element method. The influence of long-term reinforcement strains on long-term net embankment height, maximum crest settlement, maximum differential settlement at the crest and horizontal toe movement of embankments numerically constructed over two soft foundations is explored with the consideration of viscosity of two reinforcement products. Based on a series of numerical simulations, an approach to controlling the deformations of column supported embankments to modest levels while maximizing their long-term service heights is proposed. Also, a correlation between long-term reinforcement strain and end-of-consolidation reinforcement strain is suggested for the studied cases.

Introduction

Columns installed by the deep-mixing-method (DMM) have been widely acknowledged as a cost-effective and speedy means of supporting roadway embankments constructed over soft soil (e.g., Bergado et al., 1999, Lin and Wong, 1999, Igaya et al., 2011, Chai and Carter, 2011, Jamsawang et al., 2011, Voottipruex et al., 2011a, Chandra, 2012, Dahlström and Wiberg, 2012, Kamruzzaman et al., 2012, Bruce et al., 2013, Chai et al., 2015, Liu et al., 2015). To minimize column construction costs and improve embankment stability, geosynthetic reinforcement is increasingly used as basal reinforcement in combination with columns to control embankment deformations including both vertical and horizontal displacements (e.g., Forsman et al., 1999, Parmantier et al., 2005, Lai et al., 2006, Pooranampillai et al., 2012, Borges and Gonçalves, 2016, Chen et al., 2016, Zhang et al., 2016).

It is well known that geosynthetics typically made of polyethylene (PE) and polypropylene (PP) are prone to experience creep/relaxation (e.g., Bathurst and Cai, 1994, Leshchinsky et al., 1997, Jones and Clarke, 2007, Kongkitkul et al., 2007, Yeo and Hsuan, 2010, Franca and Bueno, 2011, Miyata et al., 2014). Some researchers (e.g., Li and Rowe, 2001, Li and Rowe, 2008, Rowe and Taechakumthorn, 2011, Karim et al., 2012, Taechakumthorn and Rowe, 2012a, Taechakumthorn and Rowe, 2012b, Taechakumthorn and Rowe, 2012c) have demonstrated that reinforcement viscosity is of importance to the interpretation of time-dependent behaviour of basally reinforced embankments without columns. Also, a few recent studies (e.g., Ariyarathne et al., 2013, Liu and Rowe, 2015a, Liu and Rowe, 2016) have shown that it is necessary to consider the viscous nature of reinforcement for reinforced embankment supported by rigid piles and or semi-rigid DMM columns.

Geosynthetic reinforcement has been reported to play an important role in improving the performance of DMM column supported embankments, particularly reducing the embankment's lateral spreading through some numerical analyses (e.g., Huang and Han, 2010, Yapage and Liyanapathirana, 2014, Liu and Rowe, 2016). Two mainstream design codes (BSI, 2010; EBGEO, 2011) suggest that a practical upper limit of 6% reinforcement strain be imposed for reinforced and pile/column supported embankments although the measured maximum reinforcement strains in some laboratory tests (e.g., Van Eekelen et al., 2012a, Van Eekelen et al., 2012b) and calculated maximum reinforcement strains in many numerical analyses (e.g., Huang and Han, 2010, Yapage and Liyanapathirana, 2014) were all lower than 3%. There is currently no publication investigating how the choice of maximum reinforcement strain can influence the behaviour of embankments supported by columns. There is a need to improve understanding of this issue. Furthermore, although it is common practice to design geosynthetic reinforced embankments based on short-term (end-of-construction) analyses some have proposed use of long-term reinforcement strength and evaluated embankment performance with the concept of long-term service height suggested by recent publications (e.g., Rowe and Taechakumthorn, 2011, Taechakumthorn and Rowe, 2012a, Taechakumthorn and Rowe, 2012b, Taechakumthorn and Rowe, 2012c). Therefore, the objectives of this paper are: (i) to explore the suitable selection of long-term (at 99% degree of consolidation) reinforcement strain for DMM column supported embankments with viscous reinforcement constructed over two soft foundations so that embankment deformations can be controlled to modest levels while their long-term service heights can be maximized; (ii) to investigate the correlation between maximum end-of-construction and long-term reinforcement strain for aforementioned cases using the fully 3D coupled finite element method (FEM) that has been calibrated by successfully modelling the performance of reinforced and piled embankment in the field with respect to piles and soil settlements, pressures, excess pore pressure, and reinforcement strains (Rowe and Liu, 2015).

Section snippets

Mesh discretization details

The granular roadway embankments examined in this paper had 2:1 (horizontal: vertical) side slopes with 56 m width at the base and were numerically constructed over two types of 15 m-thick soft clay underlain by the rigid and permeable layer, which are similar to those studied for basal reinforcement only by Rowe and Taechakumthorn (2011).

Although the reinforced embankment with a lateral slope supported by a square grid of DMM columns is fully 3D, to minimize the computational cost, it can be

Constitutive models and model parameters

The constitutive models for all materials including geosynthetic reinforcement, subsoil, DMM columns, embankment fills and associated interfaces related in this paper are listed in Table 1.

Results and discussion

Both the British design code (BSI, 2010) and the German design code (EBGEO, 2011) suggest that the reinforcement strain should be controlled within 6%. To explore a suitable selection of long-term reinforcement strain for DMM column-supported embankments, a series of G1-reinforced embankments supported by 2 m centre-to-centre spacing columns installed in Soil A (Scenario I) were numerically constructed up to heights providing maximum long-term reinforcement strains occurring at the edge of

Conclusions

The influence of the selection of maximum long-term strain in two viscous geosynthetic reinforcements (G1: polypropylene geotextile; G2: High density polyethylene geogrid) on the time-dependent behaviour of DMM column supported embankments over two foundation soils (with and without a crust) was investigated by a series of 3D coupled elasto-plastic Biot consolidation finite element analyses. By taking account of the creep strain in the reinforcement developed after embankment construction, this

Acknowledgements

The first author would like to acknowledge the funding support from the National Natural Science Foundation of China (Grant No. 51608461), the Fundamental Research Funds for the Central Universities (Grant No. 2682016CX019), the State Key Laboratory for GeoMechanics and Deep Underground Engineering, China University of Mining & Technology (Grant No. SKLGDUEK1726). Also, a grant (A1007) from the Natural Sciences and Engineering Research Council of Canada awarded to the corresponding author is

List of notations

DMM: deep mixing method
HD: High density polyethylene
PP: Polypropylene
EOC: end-of-construction
RS: reinforcement strain
$ε_{i j}$: total strain tensor for reinforcement model
${\dot{ε}}_{i j}$: total strain rate tensor for reinforcement model
$ε_{i j}^{e}$: elastic strain tensor for reinforcement model
${\dot{ε}}_{i j}^{e}$: elastic strain rate tensor for reinforcement model
$ε_{i j}^{v}$: viscoelastic strain tensor for reinforcement model
${\dot{ε}}_{i j}^{v}$: viscoelastic strain rate tensor for reinforcement model
$E_{0}$: individual spring modulus for reinforcement model
$E_{1}$: reference spring

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