zswalls - zsoil€¦ · zswalls - user guide window 1-1: preselection of project settings z soil.pc...

ZSwalls�A standalone module of ZSoil.PC 2Dfor deep excavations and retaining walls

USER GUIDEEdition 2018, 2018-02-23

ZACE Services Ltd. © 1985-2018

Zace Services Ltd, Software engineering

P.O.Box 2, CH-1015 Lausanne

Switzerland

(T) +41 21 802 46 05

(F) +41 21 802 46 06

http://www.zsoil.com,

hotline: [email protected]

since 1985

Contents

Table of Contents 1

ZSWALLS 1

1.1 OVERVIEW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.2 FEATURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

GETTING STARTED 3

2.1 PRESELECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.2 MAIN WINDOW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

TUTORIALS 9

3.1 TUTORIAL 1 - DEEP EXCAVATION IN SANDY SOIL . . . . . . . . . . . . 9

3.1.1 Stratigraphy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

3.1.2 Preconsolidation history . . . . . . . . . . . . . . . . . . . . . . . . 12

3.1.3 Retaining system . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

3.1.4 Impermeable barrier . . . . . . . . . . . . . . . . . . . . . . . . . . 15

3.1.5 Excavation stages . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

3.1.6 Running calculations . . . . . . . . . . . . . . . . . . . . . . . . . . 18

3.1.7 Automatic reporting . . . . . . . . . . . . . . . . . . . . . . . . . . 20

3.2 TUTORIAL 2 - DEEP EXCAVATION IN CLAYEY SOIL . . . . . . . . . . . 21

3.2.1 Stratigraphy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

3.2.2 Retaining system . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

3.2.3 Buttress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

3.2.4 Excavation stages . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

3.2.5 Automatic reporting . . . . . . . . . . . . . . . . . . . . . . . . . . 30

EXCAVATION SUPPORT ELEMENTS 31

4.1 AVAILABLE ELEMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

4.2 TIEBACKS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

4.2.1 Anchors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

4.2.2 Nails . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

4.3 INTERNAL BRACING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

4.3.1 Rigid support . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

4.3.2 Fictitious spring . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

4.3.3 Bracing struts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

4.3.4 Pipes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

4.4 TOP/DOWN CONSTRUCTION . . . . . . . . . . . . . . . . . . . . . . . 37

AUTOMATIC REPORTING 39

REFERENCES 41

ZSWALLS

A simplified standalone version of ZSOIL.PC 2D [8] with a special reference to fast modelingof deep excavations and retaining walls. The program can alternatively be used as a pre-preprocessing template in ZSoilr in order to speed-up data generation.

1.1 OVERVIEW

ZSwalls� is a 2D deep excavation - retaining wall analysis software program. The programis based on the finite element method including coupled formulation for fully- and partiallysaturated two-phase media and advanced constitutive laws. ZSwalls� analyzes excavationsequences including intermediate stability checks.

In addition ZSwalls� v2018 offers:

• user-friendly graphical interface

• simplified overall input strategy

• automated and user-configured, ready to print reporting

ZSwalls�offers a variety of support elements which can be associated with different, typicallyapplied excavation methods by:

• retaining walls - diaphragm or sheet pile walls

• tiebacks - anchors, nails

• internal bracing - struts

• top/down technique - slabs

Finite element models which are generated by ZSwalls� , can be opened with the standardversion of Z Soilr(*.inp file). This allows the models to be upgraded in order to handle spe-cific conditions which are not offered in the template, and recalculated. Moreover, computedresults can be analyzed using ZSoilr post-processor whenever more demanding analyzes orspecific graphical outputs are needed.

ZSWALLS - USER GUIDE

Window 1-1: Preselection of project settings

Z Soil.PC

Window 1-1

1.2 FEATURES

ZSwalls� offers the following features:

• fast pre-processing of rectangular-shaped excavations based on automated finite elementmesh generation including locking-free finite elements and an advanced mesh tying tech-nique

• modeling of pile sheet and diaphragm walls, and reinforcement elements such as anchors,struts and nails

• accounting for surface loads

• advanced elasto-plastic constitutive model for soils, i.e. the Hardening-Soil model [5, 6],which is commonly recognized and accepted by the geotechnical community (refer to [3] forcomprehensive descriptions on theory, parameter selection and examples of case studies).The model is extended to account for small-strain stiffness which is observed in naturalsoils [1].

• time-dependent (consolidation) or steady-state analysis including partially-saturated effectsin soil (soil water retention curve model by van Genuchten [7]) - for the theory, refer to[8].

2

GETTING STARTED

2.1 PRESELECTION

The application starts with the Preselection dialog which allows you to configure initialproject setup before working with cases and detailed data definition. The dialog will alwaysshow upon startup as long as [Show this window on startup] is active �X.

The most recent initial setup will be preserved if [Save this preselection as default] is active�X.

The Preselection window can always be called from the main toolbar of ZSwalls� .

Window 2-1: Preselection of project settings

Z Soil.PC

Window 2-1


The setup components of Preselection are as follows:

Wall type defines the type of retaining wall to be considered in the project

• [Sheet pile wall] - continuous wall constructed by driving steel sections into the groundbefore the excavation

• [Diaphragm wall] - continuous wall formed and cast in a slurry trench; diaphragm wallrefers to the final condition when the slurry is replaced by reinforced concrete thatacts as a part of a retaining system

• [Alterable] - wall type can be changed anytime during the project definition

Supporting system allows preferable supporting components to be used in the project

• [Anchors] - tie-back elements that consist of a pre-stressed cable (Free length), and apressurized concrete bulb in the soil (Bonded length); anchor’s bond length should beplaced behind the potential failure plane in the soil. For detail see Section 4.2.1.

• [Struts] - structural components designed to resist outward-facing support compressionwhich can be modeled by means of:

- [Rigid support] with an infinite Stiffness of the spring, i.e. k =∞- [Fictitious spring] defined by Spring stiffness: k and Prestress force F0:

- [Bracing struts] a sequence of non-prestressed steel pipes and are recommended formodeling relatively small-size excavations where an equivalent spring stiffness may de-pend on the position of the analyzed cross section with respect to the excavation corner

- [Pipes] a sequence of prestressed steel pipes with a given spacing (recommended forlarge-size or long excavations)

• [Slabs] - these support elements can be used to simulate the top/down method and repre-sent basement floors which are constructed as the excavation progresses; the top/downmethod can be applied for deep excavation projects where tieback installation is notfeasible and soil displacements have to be minimized.

• [Mixed system] - allows different reinforcement elements to be used, i.e. [Anchors],[Nails], [Struts] and/or [Slabs]; [Nail] elements consist of a steel reinforcing rod which isinstalled into a pre-drilled hole and grouted into place (bonded length)

Drainage conditions define the type of analysis to be applied during numerical simulationof excavation.

• [Single-phase] - ignores ground water presence in the soil; no partial saturation effect willbe considered in the simulation. This analysis applies whenever [Ground water table] isinactive.

• [Two-phase Steady state] - deformation of a soil is uniquely determined by the stressobtained for a fully dissipated pore pressure field meaning that the analysis is timeindependent (recommended for modeling the excavation in high-permeable soils, e.g.sands and gravels). A partially-saturated soil effect (suction) can be obtained only

4

2.1. PRESELECTION

above the ground water table behind the wall whereas inside the trench null pressuresare imposed at the soil surface of each excavation stage. The steady state analysis isavailable only if [Ground water table] is �X.

• [Two-phase Consolidation] considers the soil behavior that is strongly dependent ontime, especially for clay soils with low permeability. It accounts for undrained orpartially-drained conditions that can develop behind the retaining wall, as well aspartially-saturated zone at the bottom of excavation (effect of high suction built upbelow the excavation bottom in low-permeable soils; consider low values of saturationconstant α for silts and clays). The consolidation analysis is available only if the [Groundwater table] option is on activated �X.

� Quasi-undrained conditions can be obtained in low permeable soils (k = 10−9 −10−11 [m/s]) by setting fictitiously short time durations of excavation stages, i.e. 1day. In such a case, the undrained shear resistance Su will be obtained as a functionof the current stress magnitude, σ′ = (σ′v, σ

′h), effective strength parameters (friction

angle φ′, cohesion c), and finally, preconsolidation state.

Simulate impermeable barrier when this options is activated �X, it is possible to considerconstruction of a bottom seepage cap1 (below the excavation bottom) to prevent drainageof the ground water table into the excavation bottom. This is used to avoid:

- continuous water pumping when excavating in highly-permeable deposits (e.g sands)

- boiling of the trench bottom due to lowering of the water table within the trench supportsin saturated sands or gravels

- water contamination

FE mesh density allows you to adapt density of the finite element mesh with respect tosize of the analyzed boundary value problem

• [Automatic] - automatically finds an optimal element size with respect to the excavationdepth

• [Coarse] - with the element size of 1.0m in the zone of interest and 2.0m in the outerdomain

• [Medium] - 0.75 / 1.5m

• [Fine] - 0.5 / 1.0m

• [Very fine] - 0.25 / 0.5m

� Use [Coarse] mesh for preliminary analyzes of a project.� Applying [Very fine] mesh to relatively large models may lead to extended computingtime.

1In practice, bottom barriers can be realized by grouting techniques.

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2.2 MAIN WINDOW

Window 2-2: Main screen of application

Z Soil.PC

Window 2-2

MENU BAR consists of the following drop-down menus:

File : standard file operations such as [Open], [Save], [Save as]

� Project data are saved in *.inpw files and can also be modified with a text editor.

Preselection : calls the setup of basic components which are required in the project, Win2-1

Settings : allows the analysis to be configured with:

- [Type of analysis] ( [Consolidation] / [Steady-state] )

- [Duration of excavation stages after wall installation]

- [Duration of consolidation after completing the excavation]

- [Number of computational steps for each excavation stage]

- Enabling [Impermeable barrier]

- [Depth of impermeable barrier with respect to excavation bottom]

- [Mesh density] ( [Automatic] / [Coarse] / [Medium] / [Fine] )

6

2.2. MAIN WINDOW

For example, refer to Win. 3-2.

Run analysis : automatically generates FE model and runs ZSoilr calculation module

Report : automated and user-configured [Report content], ready to print reporting; theproject details, as well as the logo of company can be specified in [Report Settings]

� With [Run analysis], ZSwalls� stores the project data in *.inpw file and generatesa finite element model which is saved in the standard ZSoilr format - *.inp file. Itmeans that the model can be opened, modified, and recalculated with the standardversion of Z Soilr. Moreover, ZSoilr can be used anytime for data post-processing ifmore demanding analyzes or graphical outputs are required.

Help : quick access to this User Guide

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MAIN SCREEN of the application (Win.2-2) consists of the following functional elementswhich allow the user to:

Borehole : define soils layers, ground water table level, soil constitutive model, and modelparameters

Sheet pile wall / Diaphragm wall : choose type of the retaining wall and define its ma-terial and geometrical characteristics

Loads : specify [Dead load] or [Live load] which will be acting on the surface terrain

� [Dead load] can be used to simulate moderately inclined terrain or an embankmentbehind the wall; therefore it exists already in the initial state, whereas [Live load] isgradually applied starting from the initial state until wall installation; it can be used tosimulate extra loading due to construction works.

+ Anchor : add new Anchors, tie-back support elements, and define their characteristics

+ Nail : add new Nails, auxiliary tie-back support elements, and define their characteristics

+ Strut : add new Struts, outwards-facing support elements, and define their characteris-tics

+ Slab : add a new Floor slab, to simulate top/down excavation technique, and define itscharacteristics

+ Buttress : add and define characteristics of an outwards-facing elastic slab which isintroduced at the excavation bottom right behind the excavation front that advances inthe out-of-plane direction

Exc. : define the level of excavation bottom

L : define height of the wall

H , B1, B2: define dimensions of the model

Timeline : [STAGE: N] and duration of stages

Timeline slider : interactively preview of excavation stages

� EXC.END - END stage is meaningful and visible only for [Consolidation] analysis.

8

TUTORIALS

3.1 TUTORIAL 1 - DEEP EXCAVATION IN SANDY SOIL

You can open the first tutorial file via [File][Open tutorials]: Tut1-Excav-Sand.inpw

In this example, a boundary value problem of a deep excavation in sandy soil is presented. Thegoal of this example is to illustrate a [Steady State] simulation in a high-permeable subsoilincluding an application of [Impermeable barrier] below the excavation bottom (marked by athick blue line).

Window 3-1: View of project configuration

Z Soil.PC

Window 3-1

The project consists of a 16.8m deep excavation supported by three rows of prestressedanchors. Assuming a central plane-of-symmetry, half of the 60m wide trench is modeled.The external boundaries of the model are set sufficiently far away from the simulation regionto reduce their influence on the results.


Window 3-2: Settings

Z Soil.PC

Window 3-2

3.1.1 Stratigraphy

Borehole dialog allows the user to define the soil stratigraphy (Win.3-3). [Soil layers] areorganized in columns whereas [Model parameters] in rows; the table is scrollable.

[Soil layers] can be described by two commonly used models applied by geotechnical profes-sionals:

• [Hardening Soil model] (HS) - describes many macroscopic phenomena that are exhibitedby soils for complex stress paths that are encountered during deep excavations. Therefore,it is recommended for simulation of excavation problems. In this model, the soil strengthresistance is defined by the Mohr-Coulomb criterion.

• [Mohr-Coulomb] has limited ability to reproduce soil behavior for serviceability state analysismainly due to a unique stiffness modulus which controls loading and unloading modes

Double-clicking inside a unit weight cell calls the Unit weights calculator (Win.3-4), whichallows the user to calculate the [Dry unit weight] γD and [Initial void ratio] e0 based on theproperties typically measured in the laboratory, i.e. natural moisture content wn, unit weightof skeleton γS, and apparent unit weight γ.

� [Apparent unit weight] γ is used only for [Single-phase] simulations. It is inactive aslong as [Ground water table] is activated �X.

In this example the [Ground water table] is active �Xand its level is located 3.0m below theterrain surface.

10

3.1. TUTORIAL 1 - DEEP EXCAVATION IN SANDY SOIL

Window 3-3: Material parameters

Z Soil.PC

Window 3-3

Window 3-4: Unit weights calculator

Z Soil.PC

Window 3-4

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3.1.2 Preconsolidation history

The stress history in HSM, can be defined in two ways (Win.3-5):

• through the [Overconsolidation ratio] to obtain a constant OCR profile:OCR = σvc/σ

′v0 where σvc is the vertical preconsolidation stress and σ′v0 denotes the

effective vertical stress.

• through the [Preoverburden pressure] to obtain a variable OCR profile (OCR decreaseswith increasing depth, e.g. Win. 3-6):qPOP = σvc − σ′v0

� The preoverburden pressure is useful to describe varying OCR profile for superficiallayers of natural soils subject to mechanical overconsolidation or dessication.

With regards to the initial stress state, the coefficient of in− situ earth pressure at rest canbe defined ([Initial Ko State]) with the following methods:

• Manual

• Automatic if on �X[Auto evaluation K0]

Window 3-5: Preconsolidation historyZ Soil.PC

Window 3-5

[Auto evaluation K0] uses the commonly applied relationship from [2], which implies that K0

in natural soils increases with overconsolidation:

K0 = KNC0 OCRsinφ′ (3.1)

The maximal admissible value of K0 is limited by the coefficient of passive earth pressure Kp:

Kp = tan2

(45o +

φ

2

)=

1 + sinφ

1− sinφ(3.2)

In this example, the preconsolidation pressure in the first layer is defined by the preoverburdenpressure qPOP and the automatic evaluation of the initial K0 implies a variable profile of K0

(Win.3-6). For the second layer, it was assumed that from a depth of 20.0m the soil isnormally consolidated (OCR = 1) and K0 was automatically evaluated equal to KNC

0 .

12


Window 3-6: Preconsolidation history and K0 profiles

Z Soil.PC

Window 3-6

3.1.3 Retaining system

The retaining system consists of a 32m long PU25 sheet pile wall and three rows of prestressedanchors for which the characteristics are depicted in Win.3-7 and Win.3-8, respectively.

� Free length for anchors, Lf , is represented by a truss element for which longitudinalstiffness k is given by:

k =EA

Lf(3.3)

with: A - cross section area of free length, E - Young’s modulus

� Prestressing force F0 is released in the next time step following installation of ananchor. � Bond length is internally divided in a few equally long segments for which theresistance is computed with:

Ri = πDdhliqs (3.4)

with: Ddh - diameter of drill hole, li - length of a segment, qs - external shear resistance

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Window 3-7: Sheet pile wall characteristics

Z Soil.PC

Window 3-7

Window 3-8: Anchor elements settings

Z Soil.PC

Window 3-8

14


3.1.4 Impermeable barrier

An impermeable barrier appears in the simulation with the retaining wall installation. It isintroduced to prevent the ground water draining into the excavation trench. The option isactivated in Win.3-2.

Technically speaking, the flow discontinuity is modeled by means of interface elements forwhich continuity applies only to displacement field, i.e. no fluid exchange across the interface.

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3.1.5 Excavation stages

Window 3-9: Control of excavation stages

Z Soil.PC

A.1

-� endbeginning A.2

Window 3-9

Each intermediate excavation stage can be configured by clicking [STAGE:N] button, see

Win. 3-9 A.1 . In the setting dialog, you can modify:

Offset between anchor and platform which defines a required distance between anchorhead level and installation platform, e.g. typically 0.5m to 1.0.

Delay of excavation beginning which allows you to postpone the beginning of excavationin order to account for 3D excavation effect - time required for realization of out of planeanchors at the same excavation level, or simply, time required for grout maturation beforetendon prestressing ; this option is meaningful for real-time consolidation analyzes

Number of computational steps for this excavation step which allows you to controlthe number of time increments to be simulated between the beginning and the end of astage

Time step amplification factor which is the multiplier that controls the rate of time incre-ment increase (meaningful for consolidation analysis in order to gradually increase lengthof time steps); the value of 1.0 results in constant time increments.

Finally, at the end of the stage, you can apply [Safety factor analysis] (tanφ − c reductionalgorithm) in order to evaluate the global safety factor. During the analysis, strength param-eters of soil are gradually reduced until the global system equilibrium is lost, or [Target safetyfactor] - maximal reduction factor to be considered during the safety analysis - is reached.

16


Window 3-10: Stage setup

Z Soil.PC

Window 3-10

Excavation stages can easily be previewed and verified by moving the bottom slider - 3-9A.2 - in the horizontal direction. Win.3-11

Window 3-11: Preview of excavation stages

Z Soil.PC

Window 3-11

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3.1.6 Running calculations

Once the excavation problem has been defined, the calculations can be carried out. [Runanalysis] automatically generates the FE model (Win.3-12), and simulates the excavationsteps with the aid of the genuine ZSoilr calculation module.

Window 3-12: Automatically generated FE model

Z Soil.PC

� Stress and displacement compatibility between fine and coarse parts of FE model arehandled by an advanced mesh tying technique.

Window 3-12

The status of computing can be followed on the screen (Win.3-13). In the case of a divergencefor a given solver, the module automatically switches to another available solver1. The solvercan also be manually switched by the user using the button [Skip to next nonl. solver], if theprevious computation showed that the current solver is not effective.

You can manually switch between solvers by disabling maximal iteration limit: �[Automaticallyincrease MAXIT].

In some difficult-to-converge cases, a modification of the tolerances for right-hand side(TOL RHS) and energies (TOL E) can be made on ongoing computations (e.g. due tooscillations). However, it is not recommended to increase TOL RHS over 2%, or TOL E over0.2%.

1The available solvers: Newton-Raphson, BFGS, Initial Stiffness, Accelerated Initial Stiffness, ModifiedNewton-Raphson

18


Window 3-13: Computation progress

Z Soil.PC

Window 3-13

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3.1.7 Automatic reporting

At the end of computing, an analysis log shows the computational status for each excavationmacro stage. Typically, three status statements can be expected:

Computation completed which means that the system finished in equilibrium and compu-tation was successfully accomplished

Computation not completed due to divergence of computation (e.g. excessive soil/walldeformations or failure of anchor’s bond length)

Computation interrupted by the user if the computational module was stopped by theuser.

Window 3-14: Log of accomplished analysis

Z Soil.PC

Window 3-14

In the analysis log dialog, you can configure the [Report content], or directly [Open report].For detailed automatic reporting refer to Section 4-10.

Now, you can open an example of the automatically generated report for Tutorial 1.

20

3.2. TUTORIAL 2 - DEEP EXCAVATION IN CLAYEY SOIL

3.2 TUTORIAL 2 - DEEP EXCAVATION IN CLAYEY SOIL

You can now open the second tutorial file via [File][Open tutorials]: Tut2-Excav-Clay.inpw

A deep excavation in clayey soil is presented in the following example. It illustrates a real-timesimulation in quasi-impermeable soils using the [Consolidation] approach. Consolidation isunderstood as a coupled, hydro-mechanical analysis. A non-linear consolidation algorithmsimulates the transient behavior of the two-phase medium. This approach is more suitableto analyze transient states during excavation in cohesive soils.

Window 3-15: View of project configuration

Z Soil.PC

Initial state configuration

Excavation completed

Window 3-15

In this example, the excavation support consists of internal bracing which transfers the lateralearth pressure between opposing walls through compressive, pre-stressed struts. A 11.4m deepexcavation is supported by two rows of prestressed struts and a [Bottom buttress] is introducedto prevent excessive wall deformations at the final excavation stage (refer to Sec.3.2.3).Assuming a central plane-of-symmetry, half of the 56m wide excavation is modeled. The

171201 21


external boundaries of the model are set sufficiently far away from the simulation region toreduce their influence on the results.

[Dead load] is used to simulate moderately inclined terrain or an embankment behind thewall. It already exists in the initial state configuration, whereas live loads are applied afterthe wall installation and are marked in green, cf. Win.3-15. Surface loads can be definedthrough [Loads] and the dialog Win.3-16.

Window 3-16: Surface loads

Z Soil.PC

Window 3-16

Moreover, this tutorial illustrates the use of the [Automatic parameter estimation] to make afirst-order guess for parameters based on a general description of the soil behavior type.

� Automatic parameter estimation relies on statistical data and empirical correlations.It is the user’s responsibility to verify the suitability of parameters for a given purposeand adjusting them according to parameter determination based on laboratory and in situresults.

3.2.1 Stratigraphy

In this example, it is assumed that stratigraphy consists of a superficial coarse-grained soil- gravel, and underlying clays. The ground water level is located in gravel at 3.1m belowterrain surface. ZSwalls� offers a simplified automatic [Parameter estimation] which is basedon Virtual Lab [4] - a module for advanced parameter determination for soils in ZSoilr .This option can be invoked from [Borehole] dialog window, by clicking on [Set params], seeWin.3-19. Soil parameters can be roughly estimated based on a general soil description whichis defined in [Parameter estimation] dialog, see Win.3-17.

22


Window 3-17: Parameter estimation setup Gravel and Clay 1 NC

Z Soil.PC

Gravel Clay 1 NC

Window 3-17

Window 3-18: Soil description setup for parameter estimation

Z Soil.PC

Soil behavior typedescription

Clay 1 NC Clay 2 NC Stiff clay OC

Plasticity Medium Low Low

Consistency Medium Medium Stiff

Stress history Normally-consolidated Normally-consolidated Overconsolidated

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Window 3-19: Material parameters

Z Soil.PC

Window 3-19

� Saturation constant α can be taken as the inverse of the approximated height of the capillary rise.The smaller saturation constant, the higher suction (apparent cohesion), e.g. for α = 2 (sands) →maximal suction = 5 kPa, α = 0.3 (clays) → maximal suction = 33 kPa. The maximal suction can be

easily calculated for null residual saturation degree (Sr = 0) and p→∞ as S · p =γFα

with γF being

the water unit weight.

24


3.2.2 Retaining system

The retaining system consists of a 28m long diaphragm wall which is embedded in overconsolidated clays,its characteristics are given in Win.3-20.

Window 3-20: Characteristics of diaphragm wall

Z Soil.PC

Window 3-20

Two rows of prestressed struts are represented by [Pipes] - elements representing a sequence of prestressedsteel pipes with a given interval and a buckling length of 40m, see Win. 3-21. In the model, due to planesymmetry of the boundary value problem, the total length of pipes equal to 56m is considered. The propertiesassumed in this boundary value problem are given in Win.3-22. The assumed pipe spacing is one strut perdiaphragm wall segment - a typical width of 6.0m.

Window 3-21: Scheme of retaining system

Z Soil.PC

Window 3-21

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Window 3-22: Characteristics of struts

Z Soil.PC

Window 3-22

26


3.2.3 Buttress

Finally, we introduce a horizontal [Buttress] in order to limit excessive wall displacements at the final exca-vation step. This element consists of an outwards-facing horizontal slab (elastic beams with impermeablecontact elements between soil and bottom side). Typically, realization of the bottom buttress closely followsthe excavated berm in the out-of-plane direction as illustrated in Win.3-23. Therefore [Unloading rate atinstallation] allows you to control the unloading of passive earth pressure at the moment of buttress instal-lation. With [Relative installation time] you can define the time of installation with respect to the start andthe end of the final excavation stage.

Characteristics of the buttress are given in Win.3-24 where we assume that E = 10GPa corresponds to youngpervious concrete. The unloading rate 50% means that reactive forces obtained from the previous excavationstage were unloaded only to 50% even though the last excavated layer was removed from the model. Thedefined relative installation time 0.5 means that the buttress will appear at day 97.5, half way between day90 and day 105.

� [Unloading rate at installation] can be defined between 0 and 100%, where 0% means that thebuttress is hypothetically introduced before the excavation, whereas 100% corresponds to realizationafter complete excavation of the berm in out-of-plane direction.

Window 3-23: Bottom buttress

Z Soil.PC

Window 3-23

Window 3-24: Characteristics of buttress

Z Soil.PC

Window 3-24

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3.2.4 Excavation stages

Window 3-25: Preview of excavation stages

Z Soil.PC

B.1B.2

B.3

-� endbeginning B.4

Window 3-25

Since time is an important factor in the case of an excavation in low-permeable soils, the analysis type is setto [Consolidation], see Win. 3-26.

The excavation is carried out in three macro stages (30, 30 and 40 days, respectively), and is followed by anextra post-excavation stage (120 days) which is available only in [Consolidation] analysis (Win.3-27 - Laststage). This stage allows to trace post-excavation effects related to dissipation of excess pore water pressure(typically, decreasing suction).

Window 3-26: Analysis settings

Z Soil.PC

Window 3-26

Notice that excavation in Stage 2 and 3 is postponed to 7 days in order to account for the time whichis required to install and prestress the out of plane struts at the same excavation level, see Win.3-27 -Intermediate stage.

28


Similarly to the [Steady state] analysis (cf. Win.3.1.5), each intermediate excavation stage can be specifically

configured by clicking [STAGE:N] button, see Win.3-25 B.1 . In this case study, all stages are divided in twosub-stages to simulate excavation between support layers by removing two equally thick soil layers stages.

In the last stage with excavation works, [STAGE:3], the safety analysis is activated SF , cf. Win.3-25. This

option allows you to examine the global safety factor at the end of excavation by a gradual reduction oftanφ− c parameters. Such an analysis can also be carried out for any excavation stage.

Window 3-27: Stage setup

Z Soil.PC

Intermediate stage (B.1 in Win. 3-25)

Last stage (B.3 in Win. 3-25)

Window 3-27

� Simulation of undrained soil behavior can be obtained by running [Consolidation] analysis withfictitiously short excavation time periods, say 1 day per excavation state. Note that the effective param-eters have to be used so that the undrained resistance will depend on the current effective stress stateand excess pore water pressure.

171201 29


3.2.5 Automatic reporting

Once the computation has been carried out, the report can be configured and generated (cf. Section 4-10).Sometimes, it can be useful to examine and present the maps of displacements, displacement vectors, andpore pressure and suction distribution, see Win.3-28 (NB. these results are not active in a default configuredreport). The evolution of suction zones for each stage in the analysis is illustrated in the automaticallygenerated report for Tutorial 2.

Window 3-28: Report content - activating color mapsZ Soil.PC

•••

Window 3-28

Window 3-29: Color maps - distribution of suction S · pZ Soil.PC

Window 3-29

30

EXCAVATION SUPPORT ELEMENTS

4.1 AVAILABLE ELEMENTS

ZSwalls� offers a variety of support elements (Win.4-1) to associate with different excavation methods:

• tiebacks (anchors, nails) - Section 4.2

• internal bracing (struts) - Section 4.3

• slabs for top/down technique - Section 4.4

Window 4-1: Available support system elements

Z Soil.PC

RETAINING WALLS

Flexural stiffness Normal stiffness Stress resultants

EI EA M , N , Q

Diaphragm wall by wall width by wall width per linear meter

Sheet pile wall by sheet by sheet cross section area per linear meter

TIEBACKS

Spring stiffness Prestressing force Normal stress resultant

k F0 Nx

Anchor per anchor per anchor per anchor

Nail per nail - per nail

INTERNAL BRACING (Struts)


Rigid support infinity - -

Fictitious spring per strut per strut per strut

Bracing struts per linear meter -

Pipes per strut per strut per strut

Buttress per linear meter - per linear meter

TOP/DOWN CONSTRUCTION (Slabs)


Slab per linear meter - per linear meter

Window 4-1


4.2 TIEBACKS

Tiebacks (or anchors) are structural elements which work in tension and are attached in sufficiently resistantand rigid earth or rock. Tiebacks help to eliminate obstructions inside the excavation inherent in struts.

The retaining system consists of the subsoil which provides the ultimate support to the system and a tensionmember which transfers the load from the retaining wall to the soil. Anchors are prestressed.

4.2.1 Anchors

[Anchors] are tie-back elements that consist of a pre-stressed cable (Free length), and a pressurized concretebulb in the soil (Bonded length); anchor’s bonded length should be behind the potential failure plane in thesoil.

Free length for anchors, Lf , is represented by a truss element for which stiffness k is given by:

k =EA

Lf(4.1)

with: A - cross section area of free length, E - Young’s modulus.

Prestressing force F0 is released in the next time step following installation of an anchor.

Bond length is internally divided in a few segments of equal length for which the resistance is computedwith:

Ri = πDdhliqs (4.2)

with: Ddh - diameter of drill hole li - length of a segment qs - external shear resistance

Window 4-2: Anchor element

Z Soil.PC

Window 4-2

32

4.2. TIEBACKS

4.2.2 Nails

[Nail] element consists of a steel reinforcement rod which is installed into a pre-drilled hole and grouted intoplace (Bonded length)

Rod and bond length are internally divided in a few segments of equal lenghts for with resistance:

R = πDdhliqs (4.3)

with: Ddh - diameter of drill hole li - length of a segment qs - external shear resistance whereas longitudinalstiffness of rod is given as follows:

k =E · πD2

rod/4

li(4.4)

with: E - Young’s modulus for rod Drod - diameter of rod

Window 4-3: Nail element

Z Soil.PC

Window 4-3

171201 33


4.3 INTERNAL BRACING

Internal or cross-lot bracing transfers the lateral earth pressure between opposing walls of the excavation,through compressive, pre-stressed struts. ZSwalls�offers a number of different elements to model the internalbracing, cf. Win.4-4. The calculators for spring stiffness k are automatically called when creating a newstrut, and can be reopened by a double-click inside the cell [Stiffness of spring] , see Win.4-4 .

Window 4-4: Different types of internal bracing elementsZ Soil.PC

Window 4-4

4.3.1 Rigid support

Rigid support allows the user to model a fixity with an infinitely rigid spring.

Window 4-5: Rigid supportZ Soil.PC

Window 4-5

4.3.2 Fictitious spring

Fictitious spring allows the user to introduce a support with a given Spring stiffness k, as well as Prestressforce F0 per spacing unit. The results are computed per spacing unit.

Window 4-6: Fictitious springZ Soil.PC

k = EA/L with L = 1.0 m

Window 4-6

34

4.3. INTERNAL BRACING

4.3.3 Bracing struts

Bracing struts consist of a sequence of non-prestressed steel pipes. This option is recommended to modelrelatively small-size excavations where an equivalent spring stiffness may depend on the position of the ana-lyzed cross section with respect the the excavation corner.

Calculation of the equivalent stiffness k can be carried out with the aid of stiffness calculator which alsoaccounts for flexural stiffness of a continuous wale, see Win.4-7. In the FE model, truss elements are createdwith unitary spacing, and results are computed per linear meter.

Window 4-7: Equivalent stiffness calculator for bracing struts

Z Soil.PC

Window 4-7

171201 35


4.3.4 Pipes

Pipes elements consist of a sequence of prestressed steel pipes with a given spacing. These elements arerecommended to model large-size or long excavations where the results are sought for each prestressedmember instead of for each effective meter.

Calculation of the equivalent stiffness for a pipe, k = EA/B1 can be carried out with the aid of stiffnesscalculator which ignores strut spacing and stiffness of wale, see Win.4-8. In the FE model, truss elementswhich represent the pipes are created with given spacing, and results are computed per pipe element.

Window 4-8: Equivalent stiffness calculator for pipes

Z Soil.PC

Window 4-8

36

4.4. TOP/DOWN CONSTRUCTION

4.4 TOP/DOWN CONSTRUCTION

Slabs can be used to simulate the top/down method and represent basement floors which are constructedas the excavation progresses; top/down method can be applied for deep excavation projects where tiebackinstallation is not feasible and soil displacements have to be minimized.

The equivalent stiffness is calculated automatically for the scheme illustrated in Win.4-9. The calculatorfor spring stiffness k is called when creating a new slab and can be reopened with a double-click inside thecell [Stiffness of spring], see Win.4-10. The solution for k is obtained with the displacement method whichassumes a cantilever beam fixed at both ends.

Window 4-9: Slabs

Z Soil.PC

Equivalent stiffness for the analyzed section:

k = −6EI 6b2 − 8ac

4a(1)

with:

I =t ·B3

12, a =

1

x+

1

(L− x), b =

1

x2+

1

(L− x)2, c =

1

x3+

1

(L− x)3(2)

Window 4-9

171201 37


Window 4-10: Settings for slabs

Z Soil.PC

� A double-click inside the [Stiffness of spring] cell calls the spring stiffness calculator.

Calculator for stiffness of spring

Window 4-10

38

AUTOMATIC REPORTING

Results are synthesized in a automatically-generated report. The contents can easily be configured using[Report content] dialog. The ready-to-print document consists of:

- the graphical representation of geometry and the FE model

- general settings of project including summary of applied elements

- the properties of materials and members

- user-configured results, cf. Win. 5-1

Examples of automatically-generated reports can be found in Tutorial 1 and Tutorial 2.

Window 5-1: Report content

Z Soil.PC

Window 5-1


� It is recommended to print color maps of displacements and displacement vectors for preliminarycalculations in order to verify model kinematics, correctness of domain size (preferably null displacementsin outer boundary elements), or length of tie-back support elements (anchor’s bonded length outside ofa potential slip surface), etc.

Author’s name, company’s name and logo can be defined through [Report settings] (Win. 5-2) and will besaved for following projects. This data is then printed in the report headers, e.g. Win.5-3.

Window 5-2: Report settings

Z Soil.PC

Window 5-2

Window 5-3: Report header

Z Soil.PC

Window 5-3

40

References

REFERENCES

[1] T. Benz. Small-strain stiffness of soils and its numerical consequences. Phd, Universitat Sttutgart, 2007.2

[2] F. Kulhawy and P. Mayne. Manual on estimating soil properties for foundation design. Technical report,Electric Power Research Institute (EPRI), 1980. 12

[3] R. Obrzud and A. Truty. The hardening soil model - a practical guidebook. Technical Report Technicalreport ZSoil.PC 100701, Zace Sevices Ltd, 2012. 2

[4] R. Obrzud, A. Truty, and K. Podles. Virtual lab. Technical Report ZSoil.PC 120201, Zace Sevices Ltd,2016. 22

[5] T. Schanz. Zur modellierung des mechanischen verhaltens von reibungsmaterialien. Mitt. Inst. fA¼rGeotechnik, Universitat Stuttgart, 1998, 45, 1998. 2

[6] T. Schanz, P. Vermeer, and P. Bonier, editors. Formulation and verification of the Hardening Soil model.Beyond 2000 in Computational Geotechnics. Balkema, Rotterdam, 1999. 2

[7] M. van Genuchten. A closed-form equation for predicting the hydraulic conductivity of unsaturated soils.Soil Sci. Soc. Am. J, 44:892–898, 1980. 2

[8] ZSoil. User manual ZSoil.PC v2018, Soil, Rock and Structural Mechanics in dry or partially saturatedmedia. ZACE Services Ltd, Software Engineering, Lausanne, Switzerland, 1985-2018. 1, 2

zswalls - zsoil€¦ · zswalls - user guide window 1-1: preselection of project settings z soil.pc...

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