Overview of Design Concepts of Ground Improvement Techniques

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1 Technique: Mechanically Stabilized Earth (USA Mechanically Stabilized Earth (MSE) structures incorporate discrete layers of reinforcement, generally placed horizontally and attached to fascia panels, which are arranged between successive layers of fill during construction. Design Concepts and s A) EMPIRICAL AND ANALYTICA American Association of State Highway and Transportation Officials Allowable Stress Design (ASD), Latest and Last Edition 2002 American Association of State Highway and Transportation Officials Load Resistance Factor Design (LRFD), Third Edition with Interims The method typically addresses minimum factors of MSE section is treated as a block for evaluating safety based on a ratio of resisting forces/moments external stability, bearing pressure, global stability, divided by driving forces/moments. The method is drainage & seismic design. Internal stability is adjusted on the basis of the reinforcement type; evaluated for pullout & tensile resistance of inextensible (metallic strips & grids) and extensible reinforcements. (polymeric sheets, grids & straps). MSE section is treated as a block for evaluating external stability, bearing pressure, global stability, drainage & seismic design. Internal stability is evaluated for pullout & tensile resistance of reinforcements. The method typically addresses load factors and combinations assigned by defined service limit states, as well as reduction factors based on material types. The method is adjusted on the basis of the reinforcement type; inextensible (metallic strips & grids) and extensible (polymeric sheets, grids & straps). Use of ASD is being phased out by FHWA on public projects. Longer use of the method is expected on commercial projects. Use of the method has to be adjusted to the type of reinforcement used, which is why product-specific design is necessary from suppliers of the technology. Use of LRFD is being phased in by FHWA on public projects. The method is currently calibrated to give similar results as the ASD method. The rationale is to revise load & resistance factors based on performance data from projects designed using the methodology. Use of the method has to be adjusted to the type of reinforcement used, which is why product-specific design is necessary from suppliers of the technology References: See method descriptions. B) SPREADSHEET AND NUMERICAL (e.g. Finite Element ) / Advantages B1 Excel Spreadsheet Applications AASHTO code requirements are easily adapted to spreadsheet programs that that are tailored to specific MSE wall products. Output can be configured to address the length & number of reinforcements by elevation within the MSE block for both static & seismic evaluations. Most suppliers of the various MSE wall products use spreadsheets specific to their product. The spreadsheets developed by suppliers are generally not available to consultants or others outside the supplier companies. This leaves outside sources with the need to develop their own spreadsheets or depend on publicly available software for checking MSE wall designs. B2 Global Stability Programs The programs allow the reinforced soil volume to be Software programs such as PCSTABL from Purdue The evaluation of internal stability is limited to the input input with boundary conditions designated by x-y University and others are available, with provided on the reinforcement elements. Slope stability coordinates. The output provides analyses of adaptations allowing reinforcements to be added methods other than Modified Bishop may be appropriate potential slip planes through the reinforced block into the MSE block. The most common analysis and should be evaluated by a knowledgeable engineer that (internal stability), outside the reinforced block used in the programs is the Modified Bishop understands potential slope failure modes for MSE (external stability and composite combinations for. structures. internal/external stability. B3 The code requirements for AASHTO may be adapted to numerical modeling programs. However, the models are senstive to imposed When used correctly, numerical models are good for boundary conditions, requiring an experienced user identifying potential shortfalls in design of complex Finite element or finite difference modeling with input from intrumented MSE walls. The numerical models are more readily adapted to simultaneous evaluations of internal, external and global stability. applications and for simulating deflections for static conditions and seismic events. For knowledgeable users only. B4 MSEW Computer Program The program has a number of default values embedded The program analyzes a wide variety of MSE wall and difficult to identify for specific applications & Windows-based interactive program that attempts types. It combines all aspects of internal, external abd reinforcement types. The program uses a design to design and analyze all types of MSE applicationsglobal stability design. Its is a suitable analytical tool It follows AASHTO ASD and LRFD design methods. The program is sponsored by FHWA. for intial design purposes; however, spreadsheet programs specific to the product being designed should be used for final analysis. approach called the Simplified, which is less analytically rigorous and only used in the USA. Discrete reinforcing elements are not effectively modelled, since the program makes evaluations on the basis of continuous sheet reinforcements. References: See method descriptions.

2 Technique: DEEP MIXING Design Concepts and s EuroSoilStab design guideline - ultimate limit state Stability is calculate on the base of slip surface calculations with averaged shea parameters, according to improvement area ratio. Full interaction between soil Stability of embankments and strip and columns is assumed. Hence, foundations. columns are just considered in active and direct shear zone and column design shear strength is restricted to low strength columns. Only for substantial axial loading considering low strenght columns. Creep in soil not considered. Spacing of columns is not considered. EuroSoilStab design guideline - serviceability limit state Group of columns model Assumption of same axial deformations in soil and columns. Settlement will be calculated in iterative procedure assuming a load distribution between soil and column. Soil material is assumed to be linear elastic, column material linear elastic perfectly plastic. Linear elastic constituents of reinforced soil. Arbitrary location of columns under the foundation. Settlement behaviour under oedometric loading conditions. Just oedometric loading conditions. Stress dependent stiffness of soil and column neglected. Time dependent (creep) settlements not considered. Lower bound of apparent Young modulus Circular and rectangular rigid foundations. of reinforced soil which give upper bound End bearing columns. settlement prediction (useful in practice). B) NUMERICAL (e.g. Finite Element ) B1 Homogenisation type 1 B2 Homogenisation type 2 B3 Full 3D model strength parameters based on experience. Works with any code. strength parameters based on an homogenisation procedure applying appropriate constitutive models for soil and column material. Needs special formulation of constitutive model and thus particular code. / Advantages loading conditions. Moderate loads. Easy to use. loading conditions. Moderate to medium loads. Improved modelling of interaction column/soil and of the elasto-plastic material behaviour of column and soil as compared to B1. No interaction column/soil is modelled thus not applicable for high loads when soil and/or columns are predominantly in the nonlinear (plastic) range of deformation behaviour. Same constitutive model for soil and column has to be assumed. when failure mechanisms have to be investigated. Requires substantial changes to code. Columns are individually discretized and Complex geometries, group of columns suitable constitutive models are used to supporting individual footings. Interaction Computationally demanding. Difficult to describe the mechanical/hydraulical column/soil can be modelled in detail and model problems with large number of behaviour of columns and surrounding complex constitutive models can be used columns. soil. Works with any code. for soil and columns. References: Canetta, G. & Nova, R. (1989). A numerical method for the analysis of ground improvement by columnar inclusions, Computers and Geotechnis, Vol. 7, Eurocode 7 (2005). ENV1997 part I - geotechnical design - general rules EuroSoilStab (2002). Development of design and construction methods to stabilise soft organic soils. Design guide soft soil stabilisation. CT Project No. BE European Commission. Industrial & Materials Technologies Programme (Brite-EU-Ram III) Brussels. Vogler, U. & Karstunen, M. (2007). Numerical modelling of deep mixed columns with volume averaging technique. In: (Eds.) Gian N. Pande and Stan Pietruszczak: Proc. of the 10th Int. Symposium on Numerical Models in Geomechanics (NUMOG X), Rhode, Greece, Taylor and Francis/Balkema publishers, Leiden, The Netherlands, M. Bouassida; A. Porbaha (2004). Ultimate bearing capacity of soft clays reinforced by a group of columns Application to a deep mixing technique. Soils and Foundations. Vol. 44, n 3, pp

3 Technique: Reinforced Soil structures using flexible low-extensibility reinforcements - France/European Union Design Concepts and s "Les ouvrages en Terre First Limit State format design for Vertical and near vertical retaining walls. Armée - Reinforced Soil structures, and probably Bridge abutments. Recommandations et for all geotechnical activities. Règles de l'art" - First Reinforced Soil Block is treated as a ed Reprint 1991 homogenous block providing the retaining structure. Internally the reinforcement resists the internal stresses generated by factored vertical stresses transformed into horizontal stresses by an active pressure coefficient. The increased vertical stress within the soil mass due to overturning is for the first time considered. Gives also details of conception and execution. Technology-specific recommendations : galvanised steel strips. French Standards NF- P , -1 and -2. Last edition : June Same method for internal and external Vertical and near vertical retaining walls. stability as in the 1979 document. Rules Bridge abutments. extended to all types of low extensibility reinforcements : strips, welded wire mesh, woven steel mesh. Introduction of global stability checking with Bishop method, as an additional requirement. No indication on execution and conception. No French standard for extensible reinforcements like geogrids. A new standard gathering all technologies + soil nailing, and compatible with the French annex to Eurocode 7, is under preparation, and should be ready by the end of A4 Comité Français des Global stability : Bishop Géotextiles "Recommandations External stability : sliding and pour l'emploi des overturning géotextiles dans le renfrocement des "Internal" stability : cinematic method ouvrages en terre" 1990based on Bishop's assumptions, but EN 14755: European standard for execution of special geotechnical works : Reinforced fills applying a virtual displacement and calculating the corresponding tensions in the geosynthetic layers according to their stiffness and their adherence in the soil. Not a design document - but gives excellent overview of the practice as far as systems, materials and execution are concerned. Steep slopes reinforced with geosynthetics (geogrids or geotextiles) All. No internal stability justification based on actual measurements. Complex "internal" method - no commercial software available - method hardly used by a few designers. Mostly informative. B) NUMERICAL (e.g. Finite Element ) Numerical methods are mainly used to check deformations and assess concepts, but not for the design of the reinforcement.

4 Technique: Reinforced Soil using Geogrids Design Concepts and s British Standard BS 8006 Tie-Back Wedge Limit State format design. Reinforced Vertical and near vertical retaining walls. Soil Block is treated as a homogenous block providing the retaining structure. Internally the reinforcement resists the internal stresses generated by factored vertical stresses transformed into horizontal stresses by an active pressure coefficient. Fill materials that satisfy particular criteria. Generally cohesionless fills or cohesive frictional fills. Maximum height undefined but will depend on facing type A4 A5 A6 A7 A8 UK Highways Agency HA 68 Bishop's Simplified method of Slices CUR 1989, Netherlands AS4678 Australia German Institut fur Bautechnik Public Work Research Center of Japan Railway Technical Research Institute - Japan Limit equilibrium two-part wedge Steep slopes of 70 degree face angle and Can only use one type of fill material design. Uses critical state friction angle less. and seeks equilibrium for both internal and external two-part wedge slip planes. throughout. Foundation is assumed to be competent to carry the steep slope, no slip surfaces through the foundation. Pore pressures modelled by pore pressure coefficients. No seismic. Slip Circle analysis used for steep Reinforced slopes with 70 degree and Can be used in most situations. reinforced slopes and external stability o below face angle. Can include many wall and bridge abutment structures different soil types, water regimes, seismic loading etc. Similar to BS8006, with different partial factors Single wedge design method similar to Walls and steep slopes including seismic the US NCMA, but Limit State. Soil parameters and risk factors characterised Limit equilibrium two-part wedge design Walls and steep slopes No seismic loading and no abutment loading This is the safety factor design method. Vertical or near-vertical wall constructed Two kinds of internal failure mode and for road or site preparation. How to four kinds of external failure modes are calculate the required tensile strength of considered. Required tensile strength of reinforcement is diffrenet according to the reinforcement is calculated by the better angle of facing. modified Bishop-method. This is the safety factor design method. Vertical or near-vertical wall constructed Two kinds of internal failure mode and for railroad or road two kinds of external failure modes are considered. Required tensile strength of reinforcement is calculated by the twopart wedge method. British Standard Code of Practice BS Strengthened / Reinforced Soils and Other Fills HA 68/94 UK Highways Agency. Design s for the Reinforcement of Highway Slopes by Reinforced Soil and Soil Nailing Techniques Public Work Research Center (pwrc) Design and Construction Manuals of Geotextile Reinforced Soil Structures (in Japanese) Railway Technical Research Institute (RTRI) 2001.Reinforced Railroad/ Road with Rigid Facing (RRR). (in Japanese)

5 Technique: STONE COLUMNS Design Concepts and s A4 Priebe Balaam, Booker Goughnour, Bayuk Bouassida, de Buhan, Dormieux Unit cell approach. Linear Elasticity in the soil. Column yielding. K0=1. Depth-dependent improvement factor. (1,2) Widely accepted design method. Unit cell approach. Linear Elasticity. Uniform vertical strain. No radial movement/no shear stresses. Rigid raft. (3) extension of (3): Column yielding (MC). K0=1. (4) With extension similar results to Priebe. Iterative porcedure for solution with simplified graphical presentation. K0<1. Time dependent analysis. Column Yielding (MC). (5) Unit cell and group of columns (6). Yield design theory. Settlement improvement: Extensions for floating columns and single/strip foundations available. Evaluation of bearing capacity. Settlement improvement. Moments and Shear forces in rigid rafts. Time dependent behaviour. Settlement improvement. Bearing capacity predictions as a function of improvement area ratio; Experimental validation. Increase of bearing capacity. Simplified assumptions with respect to soil behaviour. Assumption of K 0=1? Only one soil layer. End bearing columns. Only one soil layer. End bearing columns. Conservative prediction for group of columns. References: (maximum 2 per described method) (1) Priebe H. J. (1975) Abschaetzung des Setzungsverhaltens eines durch Stopfverdichtung verbesserten Baugrundes. Die Bautechnik, 5, (2) Priebe H. J. (1995) The design of vibro replacement. Ground Engineering, 28, No. 10, (3) Balaam, N. P.& Booker, J. R. (1981), Analysis of rigid rafts supported by granular piles. Int. J. Numer. Anal. Meth. Geomech., Vol. 5, (4) Balaam, N. P.& Booker, J. R. (1985), Effect of stone column yielding on settlement of rigid foundations in stabilized clay. Int. J. Numer. Anal. Meth. Geomech., Vol. 9, (5) Goughnour, R.R., and A.A. Bayuk (1979). Analysis of stone column soil matrix interaction under vertical load. Int. Conf on Soil Reinforcement, (6) M. Bouassida; P. de Buhan; L. Dormieux (1995). Bearing capacity of a foundation resting on a soil reinforced by a group of columns. Géotechnique, Vol. 45, n 1, pp B) NUMERICAL (e.g. Finite Element ) B1 Homogenisation type 1 B2 Homogenisation type 2 B3 B4 Full 3D (2D)-model Unit cell model strength parameters based on experience. Works with any code. strength parameters based on an homogenisation procedure applying appropriate constitutive models for soil and column material. Needs special formulation of constitutive model and thus particular code. Columns are individually discretized (equivalent "walls" in 2D-analysis) and suitable constitutive models are used to describe the mechanical/hydraulical behaviour of columns and surrounding soil. Works with any code. Elastoplastic behaviour incorported including primary consolidation effect due to columns installation. / Advantages loading conditions. Moderate loads. Easy to use. loading conditions. Moderate to medium loads. Improved modelling of interaction column/soil and of the elasto-plastic material behaviour of column and soil as compared to B1. Complex geometries, group of columns supporting individual footings. Interaction column/soil can be modelled in detail and complex constitutive models can be used for soil and columns. ology for prediction of improved Young modulus and extent of zone of improved soft soil. No interaction column/soil is modelled thus not applicable for high loads when soil and/or columns are predominantly in the nonlinear (plastic) range of deformation behaviour. when failure mechanisms have to be investigated. Requires substantial changes to code. Computationally demanding, in particular 3D models. Group of columns installation, assumed in short term conditions, is in progress. References: Lee, J.-S. & Pande, G. N. (1998). Analysis of stone-column reinforced foundations. Int. J. Numer. Anal. Meth. Geomech., Vol. 22, Z. Guetif; M. Bouassida; J. M. Debats (2007). Improved Soft Clay Characteristics Due to Stone Column Installation. Computers and Geotechnics. Vol 34 n 2; pp

6 Technique: VERTICAL DRAINS Design Concepts and s Hansbo (1981) Based on the Terzaghi's onedimensional consolidation theory and Barron's equal strain theory, a closed Settlement of embankments and strip formulation for vertical drains improved foundations. soil was proposed taking into account drain spacing, drain resistance and soil disturbance Just for substantial axial loading. The method considers radial drainage only. However, a combined vertical and horizontal drainage can be readily analysed using Carillo's equation. B) NUMERICAL (e.g. Finite Element ) B1 Matching type 1 B2 Matching type 2 B3 Matching type 3 B4 Full 3D model type 1 B5 Full 3D model type 2 / Advantages Using a matching technique (geometric matching, permeability matching, or a mix of geometric and permeability matching) to simplify an axisymmetric vertical drain system for a plane strain Matching may need to be checked for vertical drain system by setting the layered compressible soil. The modification of average degree of consolidation on a loading conditions. Moderate loads. Easy the conductivity of vertical drains are not horizontal plane at any time and any to use. taken into account. depth for a plane strain condition Uh_pl equal to Uh_ax for an axisymmetric condition. A factor will then be obtained to increase the subsoil permeability drain spacing. As the same as B1, additionally, take directly into account the effect of smear in the plane-strain approach. loading conditions. Moderate loads. Easy to use. No theoretical basis for assuming the smear zone to map directly from axisymmetry to plane-strain. Computationally heavy. The modification of the conductivity of vertical drains are not taken into account. An equivalent value of vertical hydraulic conductivity (kv_e) of PVD-improved subsoil can be estimated by a simplified formulation based on the equivalent onedimensional consolidation degree equal to the average consolidation degree taking into account the vertical and loading conditions. Moderate loads. Easy Modelling of smear zone may be problematic. radial directions due to the vertical to use. drains. The subsoil can then be analyzed in a similar manner to that of non-treated natural subsoil just with the kv_e value for vertical permeability instead of natural kv. Vertical drains are individually discretized and modelled as draining boundaries. Suitable constitutive models are used to describe the hydraulical behaviour of drains and surrounding soil Complex geometries, group of vertical drains installing individual footings. Computationally demanding.difficult to model Interaction drain/soil can be modelled in problems with large number of drains. detail and complex constitutive models can be used for soil and drains. Vertical drains are modeled as a Advantages over B4:additionally the material, individually discretized and mechanical behaviour and the change of suitable constitutive models are used to the conductivity of drains can be taken into describe the mechanical/hydraulical account. behaviour of drains and surrounding soil Computationally demanding.difficult to model problems with large number of drains. References: Chai J.C., Shen S.L., Miura N., Bergado D.T. (2001). A simple method of modeling PVD improved subsoil. Journal of Geotechnical and Geoenvironmental Engineering, 127 (11), p Hird, C.C., Pyrah, I.C., Russell, D., Finite element modelling of vertical drains beneath embankments on soft ground. Geotechnique 42 (3), Hird, C.C., Pyrah, I.C., Russell, D., Cinicioglu, F., Modelling the effect of vertical drains in twodimensional finite element analyses of embankments on soft ground. Canadian Geotechnical Journal 32, Hansbo, S. (1981). Consolidation of fine-grained soils by prefabricated drains. Proc., 10th Int. Conf. Soil Mech. and Found. Engrg., Vol. 3, Indraratna, B.; Redana, I.W. (1997). Plane-strain modeling of smear effects associated with vertical drains. Journal of Geotechnical and Geoenvironmental Engineering, v 123, n 5, p

Geoguide 6 The New Guide to Reinforced Fill Structure and Slope Design in Hong Kong

Geoguide 6 The New Guide to Reinforced Fill Structure and Slope Design in Hong Kong Geotechnical Engineering Office Civil Engineering Department The Government of the Hong Kong Special Administrative Region