Title

Plaxis Bulletin Issue 33 / Spring 2013

Modelling Swelling Rock Behaviour in Tunnelling Editorial

South Toulon Tube: Numerical Back-analysis of In-situ Measurements

Stability Analysis of the Red River Dike: The Past to the Present Colophon Jori van den Munckhof den van Jori Design: Lengkeek Arny Beernink Erwin Brinkgreve Ronald board: Editorial subscribers Plaxis among worldwide distributed is and bv Plaxis of apublication is Bulletin Plaxis The Table of contents engineering practise and includes articles on the the on articles includes and practise engineering geotechnical in method element finite the of use the on focuses bulletin The (NL). association » as the Plaxis bulletin is printed in full-colour. full-colour. in printed is bulletin Plaxis the as encouraged, is photographs and figures in colour of use The pixels. 3mega of aminimum or dpi 300 least at of aresolution have they that ensure should author the used are figures ‘scanned’ or photographs If (eps,ai). format based a vector in text the from separately supplied be to have themselves figures The text. the in approximately placed be should they where indicated be should it text, the in used are figures case In reading. of ease for clearly written is article the that ensure should author The article. the of end the at listed be should they used, are references any If subsections. necessary, if and, sections appropriate into divided be should article the of body main The author(s). corresponding the for e-mail) (preferably information contact and authors the of name(s) the paper, the of title the include should It formatting. without text plain as formatted format, electronic an in submitted be preferably should manuscript The categories. these of any in fall that bulletin Plaxis the for papers of submission welcome editors The other. each with experiences and ideas share can PLAXIS of users where aplatform offers bulletin The PLAXIS. in implemented models the on backgrounds and studies case programs, PLAXIS the of application practical magazine of Plaxis bv and the Plaxis users users Plaxis the and bv Plaxis of magazine combined the is Bulletin Plaxis The The Netherlands The Delft AN 2600 572 Box PO Vogelezang Annelies c/o Bulletin Plaxis to: mail regular by or [email protected] to: e-mail by sent be can bulletin Plaxis the regarding correspondence Any

Page 22 Page 20 Page 14 Page 10 Page 5 14 10 04 03 22 05 20 Fax: +31 3107 257 (0)15 Fax: Tel: +31 251 (0)15 7720 www.plaxis.nl [email protected] Netherlands The Delft AN 2600 P.O. 572 Box bv Plaxis office: main Plaxis or agent local your contact PLAXIS about information For Present River Dike: to Past the The ofthe Red Analysis Stability Measurements In-situ Back-analysis of Numerical ToulonSouth Tube: New developments Editorial Recent activities Recent Behaviour in Tunnelling in Behaviour SwellingModelling Rock Volume Expansion Associated Ground and ofFrozen Modelling 3D Editorial

The Plaxis head office has moved and The third user’s article covers a stability analysis from the begining of this new year we have of the red river dike. The authors compare the started from our new location in Delft. We are very different levels of safety in the past with those »exited about the coming period for Plaxis, and of the present situation by using PLAXIS code expect to be able to provide the valuable products that includes several material models. In stability and services as we have been doing since the analysis, PLAXIS demonstrated successfully the founding of our company in 1993. With this move dike safety factor in flood waves with different we look to the future for 20 more years of success. construction stages.

In this issue of the Plaxis bulletin we have again In addition to the contributions by PLAXIS users, tried to collect interesting articles and useful there is an article on 3D modelling of frozen information for you. In the New Developments ground and associated volume expansion where column we take a look at PLAXIS facilities to deal PLAXIS Expert Services provided assistence with with pore pressure in saturated and unsaturated setting-up the models for analysis. conditions. These facilities will also be available in 3D with the relesase of the new 3D PlaxFlow We wish you an interesting reading experience module. The new module is scheduled for release and look forward to receive your comments on this together with PLAXIS 3D 2013 (expected summer 33rd Plaxis bulletin. 2013). The Editors The first user’s article involves the modelling of swelling rock behaviour in tunnelling. The article compares the results of a numerical back analysis of a constitutive swelling model to in-situ measurements, using swelling parameters derived from laboratory swelling tests.

The second user’s article is about the South Toulon Tube, and involves a numerical back- analysis on in situ measurements. The numerical model is described and the simulation is validated by comparing it to in situ measurements. The good fitting with the different measurements recorded in situ shows that the three-dimensional numerical modeling, with discretization of the inclusions, is a reliable tool to simulate the complex phenomenon of interaction between the excavation process, the reinforcements and the ground reaction.

www.plaxis.nl l Spring issue 2013 l Plaxis Bulletin 3 Title

New developments

Ronald Brinkgreve, Plaxis bv

An important principle in is the division of total stress into and pore pressure. PLAXIS fully supports this effective stress principle. It provides several facilities to deal with pore pressure in saturated as as unsaturated conditions. Pore pressure, in this respect, is generally , since air pore pressure is ignored. With the release of the new 3D PlaxFlow Module (expected summer 2013), all facilities to deal with pore pressure are available in 2D as well as 3D.

The new 3D PlaxFlow Module allows for Dynamic, Consolidation and Fully coupled flow- Backgrounds and further details about the various transient groundwater flow as well as fully deformation) to avoid the generation of suction in possibilities in the new 3D PlaxFlow Module coupled flow-deformation analysis. In the latter the unsaturated zone, equivalent with ‘classical’ are described in the corresponding manuals. »case, pore pressure can change in time as a result calculations based on Terzaghi’s effective stress. With the new implementation we have tried to of (undrained) loading as well as a (simultaneous) The Ignore suction option removes the need for make unsaturated behaviour, groundwater change in hydraulic conditions. This involves the different calculation modes. It simplifies the use flow and fully coupled analysis accessible and modelling of partially saturated soil behaviour in of calculation features and the interpretation of understandable for all geotechnical engineers. the unsaturated zone above the phreatic level. The results. We are very keen to know from you if we have behaviour in the unsaturated zone is defined by succeeded in reaching this challenging goal. the soil-water retention curve, which is included in the Flow tab of material data sets for soil & interfaces. Since in most geotechnical projects unsaturated soil data is not available, PLAXIS includes predefined curves for different types of soil.

In principle, the unsaturated zone includes suction (positive pore water stress as a result of capillary action). The amount of suction included in the active pore pressure (= effective suction) depends on the degree of saturation, Sr. The latter depends on the selected soil-water retention curve (Fig. 1). For very permeable , like coarse and , the degree of saturation decreases rapidly with the distance above the phreatic level, whereas in low permeable soils, like and , the saturation above the phreatic level remains high. Therefore, especially in low permeable soils the influence of suction can be significant.

In frictional soils, effective suction provides a kind of ‘artificial ’. In the new 3D PlaxFlow Module, suction can be taken into account when needed, but in many cases it might be desired to avoid the influence of suction on the results. Therefore, the option Ignore suction is available Figure 1: Example of a soil-water retention curve for any type of calculation (Plastic, Safety, y is the suction height above the phreatic level; Sr is the degree of saturation

4 Plaxis Bulletin l Spring issue 2013 l www.plaxis.nl Title

Foto by Christian Ammering

Modelling Swelling Rock Behaviour in Tunnelling

Bert Schädlich & Helmut F. Schweiger, Institute for and Engineering, Graz University of Technology, Graz, Austria Thomas Marcher, ILF Consulting Engineers, Innsbruck, Austria

Although a great amount of practical experience has been gained in the last decades, design in swelling rock is still a very challenging task, as the recent examples of the Engelbergtunnel in southern Germany and the Chienbergtunnel in Switzerland demonstrate. Reliable prediction of swelling pressures and swelling deformations especially in anhydritic rock is extremely difficult due to the heterogeneity of the material and the complexity of the involved transport mechanisms. However, modern design codes and engineering practice demand capacity checks for tunnel linings, which usually can only be provided by numerical analysis with an appropriate constitutive model. Such a constitutive swelling model, which adds swelling strains in dependence on the stress level and accounts for the time dependent evolution of swelling, has been implemented for Plaxis. This article compares the results of a numerical back analysis with this model to in-situ measurements, using swelling parameters derived from laboratory swelling tests.

“Swelling rocks” are geomaterials which the material itself, parameters determining the If necessary the time-swell behaviour can be related el increase in volume if water is allowed time-swell behaviour cannot be transferred from to elastic and plastic volumetric strains, ev and pl to infiltrate. The most prominent rock types laboratory tests to large-scale problems. ev , by using parameters Ael and Apl to define the exhibiting swelling behaviour are certain types time swelling parameter h : » q of claystone and anhydrite-bearing rocks, which Constitutive model can be commonly found in northern Switzerland The constitutive model used in this paper has (2) and southern Germany. Tunnelling in such been implemented by T. Benz (NTNU Norway) as materials is notoriously difficult: If a flexible a user-defined soil model for PLAXIS. The model Positive volumetric strains (loosening of the invert lining is installed, large invert heave employs four parameters for strength and stiffness material) result in faster approach of the final evolves after tunnel excavation. In case these and three parameters for swelling. f’ and c’ are swelling strain, while negative volumetric strains deformations are prevented by a rigid support the well-known Mohr-Coulomb angle and delay or may even stop the evolution of the swelling concept, large swelling pressures may develop cohesion, E and n are the isotropic elastic Young’s strains. This approach accounts for the dependency at the tunnel lining. It is well known that swelling modulus and Poisson’s ratio, respectively. Cross- of the swelling rate on the penetration rate of water. deformations – at least in claystone – reduce anisotropic elasticity can also be considered but is with the logarithm of stress, and that swelling not used in this study. The meaning of the swelling Construction of the Pfändertunnel deformations can be completely suppressed by parameters is shown in Figure 1 and Figure 2. The The 6.7 km long first tube of the Pfaendertunnel sufficiently high pressure. The chemical processes maximum swelling pressure sq0 is the axial stress near Bregenz (Austria) was constructed in 1976-1980 in anhydrite swelling, on the other hand, are beyond which no swelling occurs, the swelling according to the principles of the New Austrian completely different, and the semi-logarithmic potential kq gives the inclination of the swelling Tunnelling method (NATM). While top heading relationship between swelling strains and stress curve in semi-logarithmic scale and the parameter and bench excavation were carried out without level (Grob 1972) is not universally accepted for hq is related to the time until the final swelling major difficulties, significant invert heave of up these materials. Evolution of swelling with time strain has developed (Wittke & Wittke 2005). to 30 cm was observed after about 75% of the in both claystone and anhydrite depends on Cross-anisotropic swelling can be considered in tunnel length was excavated. These observations the availability of water, which is governed by the model, but again this feature is not used here. lead to detailed laboratory investigations of the the permeability of the material, layering of the swelling characteristics of the Pfaenderstock and the amount of water recharge. As (1) material, an extensive monitoring program and to some of these factors relate to characteristics of the installation of additional anchors in the tunnel the specific boundary value problem rather than invert.

www.plaxis.nl l Spring issue 2013 l Plaxis Bulletin 5 Laboratory swelling tests higher swelling potential, but lower maximum f’ = 34°, c’ = 1000 kPa) have been taken from John The Pfaenderstock consists of various layers of swelling pressures than the samples of series et al. (2009). Tunnel overburden is ~200 m above sandstone, conglomerate, claystone and marl, B. This notable difference was attributed to the tunnel crown, which is representative of the which are summarized as upper freshwater relaxation and swelling of the series B samples cross section at km 5+373. Linear elastic plate molasse. The marl (claystone) layers were before the samples could be tested. elements are used for the shotcrete lining, with E = identified as the rock type causing the swelling 7.5 GPa for the young and E = 15 GPa for the cured due to their high content of Montmorillonite For the back analysis two swelling parameter shotcrete. The final concrete lining is modelled (Weiss et al. 1980). Czurda & Ginther (1983) sets are considered, which represent the upper with volume elements assuming linear elastic distinguished between undisturbed molasse marl and lower boundary of the test results. The time behaviour and a stiffness of E = 30 GPa. The final

(series A, Figure 4) and the fault zone material swelling parameters A0, Ael and Apl are calibrated lining thickness varies between 50 cm at the invert (series B, Figure 5). Series A samples showed to match the in situ time-swelling curve. and 25 cm at the crown. Swelling parameters are listed in Table 1. Sets 1a,

Numerical model and material parameters 1b and 2a only employ A0 for the time dependency The 2D finite element model used in this study of swelling, while in set 2b evolution of swelling is shown in Figure 6. Tunnel geometry and basic with time is entirely governed by elastic volumetric material parameters of the marl layer (E = 2.5 GPa, strains.

Fig. 1: Semi-logarithmic swelling law (Grob 1972)

Figure 3: Pfaendertunnel cross section 1st tube (after John & Pilser 2011)

Figure 2: Influence ofh q on evolution of swelling strains

Figure 4: Swelling test results, series A Figure 5: Swelling test results, series B

6 Plaxis Bulletin l Spring issue 2013 l www.plaxis.nl Figure 7: Development of invert heave with time Figure 6: Finite element model (dimensions in m)

Figure 8: Profile of vertical displacements, a) numerical analysis at t = 7180 d, b) measurements km Figure 9: Development of pressure on the lining (set 1b) 5+820 (after John 1982)

stops after activating the prestressed anchors. Parameter Set 1a Set 1b Set 2a Set 2b Increasing the maximum swelling stress by 50% (set 1b) yields ~50% more deformation and a

Swelling potential kq [%] 3.0 3.0 0.75 0.75 better match with the measurements. While such a significant influence may be expected, it should be Max. swelling stress s [kPa] 1000 1500 4000 4000 q0 noted that experimental results for these two sets

-3 -3 -3 plot so close to each other that either of the two A0 5.0e 2.5e 3.0e 0.0 parameter sets appears justified (Figure 4).

Ael 0.0 0.0 0.0 9.0 Surprisingly, sets 2a and 2b – which represent much smaller free-swell deformations – deliver A 0.0 0.0 0.0 0.0 pl more invert heave than sets 1a and 1b. This is a result of the higher maximum swelling stress in Table 1: Swelling parameters sets 2a and 2b, which activates swelling in deeper rock layers, yet with a small swelling potential. Swelling deformations are thus more widely Calculation phases Evolution of invert heave with time distributed with set 2a and 2b. After top heading / invert excavation (assuming Figure 9 compares the time-swelling curves pre-relaxation factors of 75% and 37.5%, calculated with the different parameter sets Modelling the evolution of swelling with time respectively), the concrete invert arch is installed. with the measured invert heave in km 5+373. entirely in dependence on elastic volumetric Swelling is confined in the model to an area of 15 The measurements plot close to a straight strains (set 2b) results in a slightly more prolonged m x 15 m below the tunnel invert. After a swelling line in logarithmic time scale, which cannot be time-swell-curve than using a constant value of A0 phase of 65 days, the final lining is activated, reproduced exactly by the exponential approach (set 2a). In set 2b the rate of swelling does not only followed by another swelling phase of 115 days. employed in the model. The match with the decrease due to the convergence with the final John (1982) reported that the decision on invert measured invert heave is, however, sufficient from swelling strain, but also due to negative elastic anchoring and pre-stressing was based on the a practical point of view. volumetric strains. The large positive volumetric swell heave deformations observed up to this strains after tunnel excavation are gradually point. In the cross section considered here this Set 1a delivers too little invert heave (10mm), and reduced in the swelling phases by the increasing resulted in an anchor pattern of 2.2 m spacing. the development of deformations completely swelling pressure.

www.plaxis.nl l Spring issue 2013 l Plaxis Bulletin 7 Modelling Swelling Rock Behaviour in Tunnelling

Figure 10: Variation of maximum swelling stress Figure 11: Variation of time swelling parameters (set 1b)

Figure 12: Variation of rock stiffness (set 1b) Figure 13: Variation of stress pre-relaxation

-3 Distribution of swelling strains over depth swelling pressure does not occur at the tunnel to 2000 kPa (A0 = 2.5e ). Results indicate a linear

The proportion of the rock mass which is affected invert but at a distance of ~3.8 m. increase of invert heave with sq0 (Figure 10). This by swelling depends primarily on the maximum is primarily the result of the increasing depth of

swelling stress. For set 1b (sq0 = 1500 kPa) the Anchor prestressing increases the normal stress the swelling zone below the tunnel invert (Figure swelling zone is confined to about 2 m below the on the lining by about 90 kPa. The difference to 8), and not so much due to higher swelling strains tunnel invert, which matches well with the sliding the distributed prestressing force of (0.8*640 directly underneath the tunnel invert. micrometer measurements in the neighbouring kN / 2.2 m / 2.2. m) = 106 kN/m2 is a result of the cross section km 5+820 (Figure 8). The swelling already closed final lining, which distributes part Influence of other material parameters

zone with set 2a (sq0 = 4000 kPa) is much deeper of the applied load in circumferential direction. The influence of other material parameters on due to the higher maximum swelling pressure, Comparing the increase in pressure to the swelling swelling deformations is limited. As expected, the

even though similar invert heave is obtained with line of set 1b at 200-300 kPa (Figure 4) explains the time swelling parameter A0 has a notable influence both parameter sets. These results indicate that limited influence of prestressing in the numerical on the evolution of swelling deformations, but the maximum in situ swelling pressure is rather calculations. Even though anchor prestressing not on final deformations (Figure 11). Varying the in the range of 1000-2000 kPa than close to the increases the pressure by ~45%, reduction of final elastic rock stiffness had virtually no effect on in-situ stresses. swelling strain is only about 18% due to the semi- swelling deformations after tunnel excavation logarithmic swelling law. Additionally, the effect (Figure 12), but naturally changed deformations Swelling pressure of prestressing diminishes rapidly with increasing during tunnel excavation. Variation of the 2D Figure 9 shows the distribution of swelling distance to the tunnel, and the deeper rock layers pre-relaxation factors – which in most practical pressure on the tunnel invert lining for different remain virtually unaffected. cases are an educated guess rather than a stages in time for parameter set 1b. The thoroughly derived parameter – also had no circumferential distance L is measured from the Variation of maximum swelling pressure notably influence on swelling deformation (Figure tunnel invert, such that L = 0 m is directly at the The calculated invert heave is notably sensitive 13). As no temporary invert lining was installed

invert and L = 5 m is the end of the swelling area. to the maximum swelling pressure sq0 assumed after top heading excavation, stresses in the rock No pressure measurements are available. in the numerical analysis. As the variation of this mass at the tunnel invert drop to ~0 during tunnel parameter in the laboratory swelling tests is excavation, independent of the pre-relaxation Due to the stiffer support provided to the tunnel rather large– albeit concealed by the logarithmic factors applied in the excavation phases.

lining at the sides of the tunnel, the maximum stress scale – sq0 has been varied from 500 kPa

8 Plaxis Bulletin l Spring issue 2013 l www.plaxis.nl Concluding remarks References This article presented the results of a back • Czurda, K. A., and Ginther, G. (1983) “Quellver- analysis of measured swelling deformations in the halten der Molassemergel im Pfänderstock bei Pfaendertunnel (Austria). A constitutive model Bregenz, Österreich“, Mitt. österr. geolog. Ges., based on Grob’s swelling law and exponential 76, pp. 141-160. convergence with final swelling strains over time • Grob, H. (1972) “Schwelldruck im Belchentun- was used for the numerical calculations. Input nel“, Proc. Int. Symp. für Untertagebau, Luzern, swelling parameters were derived from laboratory pp. 99-119. swelling tests. Due to the large variation of • John, M. (1982) “Anwendung der neuen öster- laboratory test results, the sensitivity of model reichischen Tunnelbauweise bei quellendem predictions on the input swelling parameters was Gebirge im Pfändertunnel“ Proc. of the 31st investigated. Geomechanik Kolloquium, Salzburg, Austria. • John, M., Marcher, T., Pilser, G., and Alber, O.

Different sets of swelling potential kq and (2009) “Considerations of swelling for the 2nd maximum swelling stress sq0 delivered very bore of the Pfändertunnel”, Proc. of the World similar swelling deformations at the tunnel Tunnel Congress 2009, Budapest, Hungary, pp. lining, as increasing sq0 is roughly equivalent 50-61. to increasing kq. However, good match with • John, M., and Pilser, G. (2011) “Criteria for the measured displacement profile below the selecting a tunnelling method using the first tunnel invert was only obtained with sq0 = 1500 and the second tube of the Pfänder tunnel as kPa, which represents the upper edge of the example”, Geomechanics and Tunnelling, 4(11), experimental results on undisturbed molasse pp. 527-533. marl. Using higher values of sq0 (and lower values • Weiss, E. H., Müller, H. M., Riedmüller, G., of kq) delivers too large swelling zones. The invert and Schwaighofer, B. (1980) “Zum Problem heave measurements plot close to a straight line quellfähiger Gesteine im Tunnelbau“, Geolog. in logarithmic time scale, which cannot be exactly Paläont. Mitt. Innsbruck, 10(5), pp. 207-210. reproduced by the exponential approach of the • Wittke, W., and Wittke, M. (2005) “Design, con- constitutive model. The match with the measured struction and supervision of in swelling evolution of swelling, however, is sufficient from a rock”, Proc. 31st ITA World Tunnelling Congress practical point of view. 2005, pp. 1173-1178.

www.plaxis.nl l Spring issue 2013 l Plaxis Bulletin 9 South Toulon Tube: Numerical Back-analysis of In-situ Measurements

J.P. Janin, H. Le Bissonnais, A. Guilloux, TERRASOL, Paris, France, D. Dias, University Joseph-Fourier, Grenoble, France, R. Kastner, LGCIE, INSA de Lyon, Lyon, France, F. Emeriault, INP, Grenoble, France

Full face excavation with ground reinforcement has become a common technique to build large tunnels in soft rock or hard soil. Nevertheless, at the design phase, it remains difficult to assess the effect of the different construction and reinforcement elements on the control of the ground movements and settlements. In order to improve the understanding of ground response to this tunnelling method, a monitoring section has been installed during the construction of the south Toulon tunnel (France). An important database was obtained and subsequently used for numerical back-analysis. A 3D FE calculation (PLAXIS 3D v.2010), modeling the real pre-reinforcements system and workflow steps, permitted to simulate the in situ measurements.

The south Toulon tunnel will connect extensometer on the tunnel axis and 3 surface ), average ground properties of the motorways A50 and A57 from Nice to target prisms (X, Y, Z) close to the previous different strata were proposed at the detailed Marseille. It is an urban shallow tunnel, 12 m in instruments. In addition, 4 radial extensometers, design phase (see Table 1). »diameter and 1820 m in length, excavated through 6 vibrating wire strain gauges placed on the steel very difficult heterogeneous soils and with a rib, 5 pressure cells and convergence targets were 2.3 Excavation method limited overburden (maximum of about 35 m). The installed from the tunnel. The south Toulon tunnel was excavated on the construction method used is full face excavation basis of the so-called “ADECO-RS” method and ground reinforcement ahead of the tunnel 2.2 developed by Lunardi [2008]. According to this face by pipe umbrellas (long forepoling) and The geological profile of the section has been method, pipe umbrellas (6° or 14° of inclination) face bolting. The construction sequences and drawn (see Fig. 1) based on the and horizontal face bolts were installed. The the amount of reinforcement were continuously investigations. They showed that the geological excavation process progressed by 1.5 m steps and adapted to the overburden, the soil conditions stratigraphy is generally horizontal and the after each step one HEB 180 steel rib was installed. and the measured settlements (Janin et al [2011]). degree of alteration of the phyllitic bedrock In this zone, the tunnel invert (HEB 220 + concrete) In addition to the regular settlement measures, a is considerably high. Despite the variations was realized with a distance to tunnel face of specific monitoring zone was set up to improve the of materials characteristics (especially in the about 39 m. understanding of ground response and to collect precise data for validating a 3D numerical model. The complex phenomenon of interaction between the excavation process, the reinforcements and the ground reaction demand in fact a 3D approach. In this paper, the numerical model is described and the simulation is validated by comparing it to in situ measurements.

2 Presentation on the Analised Section The monitoring section is situated in “Alexandre Ier” garden on the west side of Toulon at the excavation progress PM 882. In this zone the cover depth was about 25 m.

2.1 Instrumentation The instruments set up in the analyzed section are presented in Figure 1. The instrumentation from the surface is composed of 2 on both sides of the tunnel, one vertical Figure 1: Three-dimensional model

10 Plaxis Bulletin l Spring issue 2013 l www.plaxis.nl Eref = Eref Eref = 3.Eref Depth (m) Soils g(kN/m3) 50 oed ur 50 c’ (kPa) j’ (°) y (°) v K (MPa) (MPa) ur 0

0 to 3.5 Fill 19 1.6 4.8 2 20 0 0.2 0.5

3.5 to 5.9 Colluviums 20.8 40 120 10 30 0 0.2 0.5

Below 5.9 Bedrock 24.2 240 720 40 25 0 0.2 1

Table 1: Main geotechnical parameters

25 m. A three-dimensional non uniform mesh with Support type Support description E (GPa) Thickness (m) smaller elements around the excavation is used. Finally, the model contains 158000 tetrahedral Walls’ tunnel HEB 180 (spacing 1.5 m) + 25 cm shotcrete 13.5 0.25 elements and 247000 nodes. All movements are fixed at the bottom of the model and horizontal Tunnel invert HEB 220 (spacing 1.5 m) + 30 cm shotcrete 14 0.3 displacements are blocked in model’s lateral Tunnel face 15 cm shotcrete 10 0.3 faces.

Table 2: Mechanical parameters of tunnel support 3.2 Mechanical parameters and simulation of the excavation process The ground is represented by the non linear 3 Three-dimentional Back-analisys elasto-plastic HS model (Hardening Soil Model) A three-dimensional model was realized to implemented in the PLAXIS code. Hejazi [2008] simulate the tunnel excavation process of the showed, in a tunnel excavation study, that this area where the monitoring zone had been placed. model might produce ground deformations The analysis was carried out by means of the that better fit with in situ measurements than three-dimensional finite element code PLAXIS 3D using the linear elastic/Mohr-Coulomb model. (version 2010). The geotechnical parameters considered in the simulation are listed in Table 1. The initial stress 3.1 Geometry field is considered as isotropic (in conformity with Considering the geometry symmetry, only half the studies previously realized on Toulon soils by of the entire domain is taken into account in the Constantin [1988] and Dias [1999]). analysis as shown in Fig. 2. The real shape of Toulon tunnel with a cross section area of 120 m2 is In the numerical model, the shotcrete at tunnel imported from CAD. The tunnel is then modeled face, the excavation support and the tunnel invert by extruding the surface in X direction. The tunnel are modeled by “plate” elements with a linear temporary support and the pre-reinforcement elastic behavior. The mechanical parameters are are modeled and the process is explained in the defined in Table 2. The parameters of support and following paragraph. tunnel invert are determined by homogenization based on steel and shotcrete characteristics In order to avoid boundary effects, the extension and rib’s spacing. A rigid interface is considered of the mesh is equal to 150 m in X and Y directions between the support and the ground. and 70 m in Z direction. The cover depth is about Figure 2: Geological section and instruments

www.plaxis.nl l Spring issue 2013 l Plaxis Bulletin 11 South Toulon Tube: Numerical Back-analysis of In-situ Measurements

The calculation is carried out in drained reinforcement system considered in the numerical 4 Comparisons Between Numerical Simulation conditions. The tunnel excavation is simulated in simulation, based on works realized in the studied and Measurements a first stage by 10 steps, 3 meters long, followed zone. Figure 5a shows the settlements of different by 60 steps with an excavation length of 1.5 m as surface points, placed directly above the tunnel done in situ. In each phase, the tunnel lining is As far as the umbrella pre-support is concerned, axis in the analyzed zone, against their distance installed 1.5 m behind the tunnel face, on which 13 autodrilling pipes (51/33 mm) were taken from the tunnel face. The excavation started to the shotcrete application is simulated as well. The into account and renewed every 9 m. The same influence settlements more or less 30 m ahead tunnel invert is activated 39 m behind the tunnel inclination of 6° has been kept all along the model of the tunnel face. Afterwards, settlements face progress. in order to simplify the meshing. accelerated and finally reach a threshold 50 m behind the tunnel face with a settlement of The pre-reinforcements, i.e. the pipes constituting As for the face bolts, a constant length of 18 m around 20 mm. The progression of settlements the long forepoling and the face bolts, are is considered. Besides, Dias and Kastner [2005] obtained with the numerical calculation fits with simulated using “embedded pile” structures (see proved that the bolting system is characterized the measurements evolution. Similarly, three- Fig. 3). Thanks to in situ pullout tests, a realistic by the global stiffness (E•S). Therefore, the same dimensional modeling seems to well represent the limit skin resistance could be introduced between number of bolts (20) is kept all along the model shape of the transverse settlement trough both in the piles and the soil. The maximum friction in order to simplify the meshing. The real bolting terms of maximum settlement and trough width resistance measured along the soil/bolt interface density, installed in situ, is simulated varying (see Fig. 5b). is equal to 135 kN/m. proportionally the bolts’ modulus and the friction resistance based on the material characteristics The final measurements of Figure 4 shows the main characteristics of the pre- listed in Table 3. movements in the monitoring section plane

Steel bolts Fibreglass bolts

Modulus E (GPa) 210 40

Cross section A (m2) 0.488 10-3 0.8 10-3

Moment of inertia I (m4) 0.0327 10-6 ~ 0

Table 3: Face bolts characteristics

Figure 3: Umbrella forepoling and face bolts modeled with embedded piles

Figure 4: Simulation of pre-reinforcements in the numerical model

Figure 5: Comparison between in situ measurements and 3D simulation - Settlement evolution against tunnel face advance (a) and transverse settlement trough (b)

12 Plaxis Bulletin l Spring issue 2013 l www.plaxis.nl South Toulon Tube: Numerical Back-analysis of In-situ Measurements

showed two important phenomena (see Fig. 6). References On one hand, the first few meters of inclinometers • Constantin, B, Durand, J.P., Thone M. (1998), converged towards the tunnel and, on the other Progrès technologiques dans le cadre de hand, a local convergence (“belly”) is observed l’utilisation de la méthode du prédécoupage at the tunnel level. The numerical approaches mécanique à Toulon. In : Journées d’études correctly estimate the two phenomena. Internationales de Chambéry, Association A comparison between the tunnel support Française des Travaux en Souterrain, Paris, pp. deformation obtained with numerical calculation 171–180. and that inferred from in situ measurements is • Dias, D. (1999), Renforcement du front de taille also performed. The 3D model proves to be able des tunnels par boulonnage - Etude numérique to represent the support deformation with an et application à un cas réel en site urbain. Thèse acceptable accuracy. The difference between Doc. Lyon : INSA de Lyon, p. 320. the measurements and the numerical results, • Dias, D, Kastner (2005), Modélisation numérique especially in the bottom part of the rib, is probably de l’apport du renforcement par boulonnage related to the simplifications related to the way of du front de taille des tunnels. Canadian modeling (homogenization of steel and shotcrete Geotechnical Journal, Vol. 42, pp. 1656–1674. characteristics, excavation of the stross carried out • Hejazi Y, Dias D, Kastner R, (2008), Impact of at the same time as that of the tunnel face). constitutive models on the numerical analysis of underground constructions. Acta Geotechnica, 5 Conclusions Vol. 3, n° 4, pp. 251–258. The monitoring section, installed during the • Janin, J.P., Dias, D, Emeriault, F, Kastner, R, Le construction of Toulon tunnel’s south tube, allows Bissonnais, H, Guilloux, A, (2011), Settlement analyzing the evolution of soil deformation during monitoring and tunneling process adaptation the excavation progress. It also permits to create – case of South Toulon Tunnel. The seventh an important database, used later to validate a International Symposium on “Geotechnical numerical simulation. Aspects of Underground Construction in Soft Ground”, tc28 Rome. Rome. A 3D numerical model was performed with PLAXIS • Lunardi, P. (2008), Design and construction of 3D, taking into account the real excavation process tunnels – Analysis of controlled deformation in and the pre-reinforcements installed in situ. The rocks and soils (ADECO-RS). Berlin: Springer. good fitting with the different measurements recorded in situ shows that the three-dimensional numerical modeling, with discretization of the inclusions, is a reliable tool to simulate the complex phenomenon of interaction between the excavation process, the reinforcements and the ground reaction. Finally, PLAXIS 3D seems to be a useful and efficient tool to predict the movements caused by the excavation of a tunnel, realized with ground pre-reinforcements.

Figure 7: Comparison between in situ measurements and 3D simulation – Deformation of the tunnel support Figure 6: Comparison between in situ measurements and 3D simulation - Inclinometer movements in monitoring section plane

www.plaxis.nl l Spring issue 2013 l Plaxis Bulletin 13 Stability Analysis of the Red River Dike: The Past to the Present

Pham Quang Tu, P.H.A.J.M van Gelder, Technical University of Delft, The Netherlands Trinh Minh Thub, Hanoi Water Resources University

The Red River delta is located in Northern Vietnam with an area of around 15,500 km2 and a population of nearly 20 million people. The delta is protected by approximately 3000 km of dikes which are classified into four grades from III, II, I to “special”; and each dike type attains a different level of safety. In the past, most of the dikes, which were not high enough, failed mainly due to overflowing; therefore the area was usually threatened with flooding. Nowadays, as a result of many measures that have been imposed in the delta such as strengthening the dike systems, constructing the dams in the upstream river for multiple purposes, the flood risk in the area is reduced significantly. In this paper, the authors compare the different levels of safety in the past with those of the present situation by using Plaxis code that includes several material models.

The Red River (known as Song Hong in Vietnamese) flows from the mountain areas of Southern China to the Gulf of Tonkin (East Sea) »with a total length of over 1150km (the length in Vietnam’s area is about 510km), as seen in Figure 1. The River is the biggest one in the northern part of Vietnam with the total dike length of nearly 1700km that take up more than a half of the length of the dike systems of the delta. The dikes are divided into different parts due to their specifically protective functions, and each part will follow a different level of safety. In the Hanoi area, the length of dikes of 1 and “special*1” is around 250km in a total of 470km (DDMFC, 2009), (see Figure 2). In this part, four periods of dike development will be discussed to give readers an overview of dike rehabilitation for hundreds of years ago.

The first period of the stability analysis is the empirical period of Vietnam which started in the 11th century, when the first dikes were built, till 1890s. However, the significant progress in flood protection was only made from the beginning of the 19th century in the Nguyen dynasties. At that time, people knew how to measure the height of flood water level and strengthen the dike system after each flood. As a result, the dike crests were increased each year after dike breaches and floods, without any prediction for the next levels of Figure 1: Red river basin (from Khoi [2010])

*1 “special” grade is used in Vietnam dike system as the top grade which is more important than the grade 1(for instance, the dike system in Hanoi area, the capital of Vietnam)

14 Plaxis Bulletin l Spring issue 2013 l www.plaxis.nl safety. In this period the last geometry of dike and the severity of the typical flood in the year 1893 were opted for the stability analysis. There were 38 years with dike failures during this period causing serious damage to the economy and fatalities (GDM, 1995), meanwhile in the 20th century, only 8 years with dike failure were observed.

The second period of the analysis is from 1890s to 1945 when France fully established their ruling in Vietnam. After assisting the Nguyen dynasties in governing the whole country, the French widely exploited the natural resources; as a result, infrastructures were developed for their purposes. At that time, French engineers were requested to transfer strategically their experiences of flood protection in Hanoi, for instance, the 1st and the 2nd dike program, under the supervision of the Dikes Commission ( see GDM, 1995; Phan, 1985). Consequently, there were some noticeable progresses in flood mitigation during this time; for instance, the increase of the dike crest from 10.5m to 13.0m and the construction of Day Dam for flood diversion to lower the water level in Hanoi area, etc.

Figure 2: Red river around Hanoi area

Year Flood water level (m) Flood duration (days) Note*

1893 10.5 (13)* 7 9

1915 12 12 10.5

1945 13.5 5 10.5

1971 14.6 5 11.5

1996 12.98 23 11.5

Future*2 14.9 16 11.5

Figure 3: Probablility distribution of maximum water level Table 1: Flood water level

*2 Flood duration is calculated following a threshold level (here, alarm level 3~11,5m at Hanoi)

www.plaxis.nl l Spring issue 2013 l Plaxis Bulletin 15 Stability Analysis of the Red River Dike: The Past to the Present

The third period, the Vietnam war, spread from 1945 till 1975 which resulted in the anomalies of river dikes. At that time, the dikes could be bomb targets or were excavated to slow down enemy assaults. Although much effort spent on the rehabilitation of the whole dike system of the Red River Delta, they were still facing a high flood risk, and even the rehabilitation works had been undertaken for the whole dike system in the Red River Delta. Additionally, the attempts to heightening and strengthening the dike system such as millions of man-days, and cubic meter of , have been done in this period, which made it to the top of amount of dike rehabilitation work (see FPD, 2000). In terms of dike engineering, Figure 4: Typical dike strengthen strategies the anomalies of dike did not only come from its very construction, but also from such social activities.

The last period for the analysis is the Renovation- GL + 7.9m of-Economy time (known as “Doi Moi” in Vietnam) Ground level Ground level Ground level Ground level Ground level from 1986 till now. Economically, private-owned enterprises were established and equally treated as state-owned ones. As a result, new technologies were applied to dike safety in the projects funded by the Asian Development Bank (ADB) or by local governments. The safety level of the flood defence system has been improved significantly due to the application of effective measures such as planting and protecting forests in upper basins, constructing dams and reservoirs to store water, strengthening dikes, and raising awareness of flood protection in the public. In short, the Red River dikes are now able Figure 5: Typical geotechnical profiles to withstand a flood frequency from 1/250 year to 1/500 year comparing to 1/50 year in the third period (Ha, 2010). carried out, such as planting trees in the upstream calculation; the 4th and the 5th types seem to be During the past hundred years, the overflowing basin, constructing reservoirs for many multiple more dangerous for dike embankments, but these and the resulting erosion of inner slope, that purposes, strengthening dike systems, and profiles are vanished from the study area and they came from the limited height of dike system, applying flood diversion. A rising level of around will be discussed in another report (see Figure 5). caused many dike failures. In times from the 14.9m*4, equivalent to a flood frequency of 1/500 Finally, soil properties were selected from the empirical period until now the dike embankment year, will be considered for the near future (for report of Geotechnical investigation, which were compaction ratio, K*3, has varied considerably. In more details, see Table 1). carried out in 1994 for the ADB project (HEC1, addition, soil properties and their distribution also 1994). The dike embankment is represented by changed underneath the dikes. These two aspects Secondly, dike geometry was collected from the different compaction ratios which is assumed play an important role in dike safety assessment, local dike boards and historical documents. In varying from K < 0.80 (in 1890s) to K~ 0.95 particularly for flood wave durations. In this paper, the 19th century-empirical period-there were (currently). The soils of embankment have been dike stability is studied following a deterministic no drawings of dike cross sections due to the remoulded in the laboratory with the mentioned approach taking into account first the historical limitations of both scientific knowledge and poor compaction ratios, then the strength and the data to illustrate different stress states and water archives of works. In the period dominated by compression were determined by levels, and second different soil models. the French, the dike dimensions were formulated and . In fact, the soil parameters based on historical documents and updated (of layers 2b, 7, 8 following HEC1 (1994)) could Input Data with research works (Phan, 1985; Gauthier, not be determined at the construction period, The selected location in this case study is 10km 1930; Monsieur and Dominique, 1991). The data for instance during 1890s, 1915, 1945, etc. It is upstream of Hanoi Old quarter, namely Lien Mac including documents of dikes have been properly assumed that the changes of soil parameters are (in Vietnamese). In 1915, there was a serious dike protected, so all typical sections relating to each disregarded in this case study. breach at this dike section caused by overflowing- dike program are quite clear and available now. the main failure mechanism (see Local Dike Board The strategies of dike rehabilitation are described Strength and stiffness of soils may be influenced No.3, 1971-2010). geometrically in Figure 4. by different stress states that resulted in actual loading progresses. In this case, both Mohr - Firstly, the hydraulic boundary condition is Thirdly, from geotechnical investigations, the Coulomb (MC) and Hardening Soil (HS) models considered with the statistical data of over profiles of soil are illustrated with different typical were applied, and parameters were selected 100 years. After that, the maximum water level combinations. Nguyen (1999) summarized the four following Table 2 and 3. and flood duration are fitted to the theoretical possible cross sections for analysis in the project distributions. The water levels in the calculations of Red River dike rehabilitation. Tran (2012) also Calculation Phases, Results, and Discussion have been chosen following the historical data recommended five types of ground conditions for Following the manual of Plaxis 2D-2011 of the typical flood waves in the years 1893, 1915, the assessment of the Red River dikes. In fact, both (Brinkgreve, 2011) these problems could be 1945 and 1971. It could be seen from Figure 3 that the studies are nearly similar, although the latter analysed by the Plaxis finite element code which the water level was maximum at 14.6m in 1971, described more clearly the influence of the silty includes multiple choices for material models, and equalled a flood return period of 1/50 year. fine sand layer to the dike stability. In this paper, construction stages, and calculation types, etc. Additionally, flood mitigating measures have been the authors take in to account the 1st profile for However, it is hard to model stress states from

*3 Compaction ratio K = gdry/gmax where, gdry is gdry density of soil; gmax is the maximum density of soil, which is found by plotting a density - moisture curve in compaction test in laboratory (ASTM D698 & ASTM D1557). *4 Predicted results of flood frequency of 1/500 year, (Khoi, 2010)

16 Plaxis Bulletin l Spring issue 2013 l www.plaxis.nl Stability Analysis of the Red River Dike: The Past to the Present

a hundred years ago; here we ignored the time The procedures are repeated from phase 1 to 3 for with the level of dike crest of 10.5m, and the safety interval in each calculation phase, and we opted each flood in 1915, 1945, and 1971 with water level factor (Msf) minimizes at the predicted situation for undrained behaviour as the drainage type. and flood duration given in Table 1. . with a flood frequency of 1/500 year. However, in Time dependent conditions in each flooding Figure 6 shows the results of the safety terms of flood management, the dike in the year season are considered by transient analysis. The calculation phases, and the plastic zones enlarge 1893 might be safe at the water level frequency calculation phases are illustrated below. depending on both the dike geometry and the of 1/1 year (see Figure 3). It means that if there is flood water levels. The phreatic levels reach a flood with a higher water level of 10.5m, dikes • Initial: Gravity loading, the initial conditions the critical situation in most cases, and the dike could failed by the overflowing and erosion of the have been calculated and the classical mode is embankments were saturated. The level of dike inner slope, even if its safety factor reaches 1.54. chosen for calculation; crest has increased many times with different In 1971, the dike could be able to withstand in the • Phase 1: The 1st dike stage of construction construction methods (Phan, 1985). In this paper, critical states, e.g., Msf ~ 1. It showed reasonable follows the dike geometry of 1893; the new dike only four different geometry of dikes are selected evidence for dike assessments around Hanoi at is built up and the ground water level is ap- because they were dealing with the typical flood that time with some dike sections at the birth proximately set on the surface level because the waves and the major project of dike rehabilitation. of collapse (see FPD, 2000). At the predicted earthwork had been carried out in dry seasons; flood level of 1/500 year, the water level is not • Phase 2: The high water level in 1893, which is Dike failure was observed twice in the year 1915 raised freely because the Red River discharge set at 10.5m and transient flow is modelled for at the study location, comparing to eight cases is redistributed by the reservoir systems at the the analysis with the flood duration of 7 days; of dike failure during the 20th century (Ha, 2010), upstream rivers. However, the flood duration • Phase 3: The analysis of dike safety has been and where the overflowing was the dominant increases dramatically, from 5 days (in 1893) to used to determine the factor of safety in the mechanism of failure. Table 4 and Figure 7 show 16 days (in the predicted flood of 1/500 year) and flood in 1893. that we have the maximum of dike safety in 1893 dikes are also at the risk of critical situations. If

Identification Unit Dikes1880 Clay1 Clay2 Sand Dikes1945 Dikes 2000

Drainage type Undrained (A) Undrained (A) Undrained (A) Drained Undrained (A) Undrained (A)

gunsat kN/m3 17 18.5 19.5 19 18 18.7

gsat kN/m3 17.5 19 20 19 18.7 19.3

E kN/m2 3000 4500 7000 12000 3800 4500

n(nu) 0.33 0.3 0.28 0.33 0.32 0.27

G kN/m2 1128 1731 2734 4511 1439 1772

Eoed kN/m2 4445 6058 8949 17780 5438 5623

2 cref kN/m 9 12 20 0.1 12 18

f (phi) deg 7 9 12 28 10 12

Flow model Van Genuchten Van Genuchten Van Genuchten Van Genuchten Van Genuchten Van Genuchten

Soil type Silty clay Medium fine Fine Sand Silty clay loam Silty clay loam

kx m/day 0.05 0.014 0.0003 1.27 0.04 0.03

ky m/day 0.05 0.014 0.0003 1.27 0.04 0.03

Table 2: Soil parameters for Mohr - Coulomb model

Identification Unit Dikes1880 Clay1 Clay2 Sand Dikes1945 Dikes 2000

Drainage type Undrained (A) Undrained (A) Undrained (A) Drained Undrained Undrained (A)

gunsat kN/m3 17 18.5 19.5 19 18 18.7

gsat kN/m3 17.5 19 20 19 18.7 19.3

ref 2 E 50 kN/m 3000 3150 5500 7500 4400 5500

ref 2 E oed kN/m 3400 5500 7500 7800 4300 6000

ref 2 E ur kN/m 10000 16500 23500 20000 13000 17000

power (m) 0.5 0.5 0.5 0.5 0.5 0.5

cref kN/m2 9 12 20 0.1 12 15

f(phi) deg 7 9 12 28 10 12

Flow model Van Genuchten Van Genuchten Van Genuchten Van Genuchten Van Genuchten Van Genuchten

kx m/day 0.05 0.014 0.0003 1.27 0.04 0.03

ky m/day 0.05 0.014 0.0003 1.27 0.04 0.03

Table 3: Soil parameters for Hardening Soil model

Notes: The data used in tables 2 & 3 are adapted from HEC1 [1994]

www.plaxis.nl l Spring issue 2013 l Plaxis Bulletin 17 Stability Analysis of the Red River Dike: The Past to the Present

Figure 6: Stability of dikes vs construction stages

Figure 7: Msf vs Material Models and flood frequency Figure 8: Msf vs steps calculation of MC model

Msf Flood wave Year Return period MC HS Frequency (%) (1/year)

1893 1.538 1.549 100 1.0

1915 1.282 1.226 91 1.1

1945 1.32 1.279 20 5.0

1971 1.032 0.987 2.2 45.5

1/500 1.031 0.989 0.2 500.0

Table 4: Summary of calculation results

18 Plaxis Bulletin l Spring issue 2013 l www.plaxis.nl Stability Analysis of the Red River Dike: The Past to the Present

other failure mechanisms presented, the dike References could have failed. • Hanoi DDMFC. Report on current condition of river dike systems in Hanoi area before flood Comparing the material models of MC and HS, season 2009. Technical report, 2009. we can see from Figure 7 that the former has • General Department of Meteorology GDM. His- a higher value of Msf than that of the latter. A tory of Vietnam Meteorology, Vol 1, Ministry of possible explanation is that the HS model mobilize Natural Resources and Environment of Vietnam. soils strength lower than that of the MC model Hanoi Meteorology publishing, 1995. (Brinkgreve, 2009); in this case, HS model is • Phan, K. (1985). A brief history of water manage- preferable to application. ment in Vietnam (in Vietnamese). Hanoi: Hanoi Social Science Publishing. Conclusions • Ha Van Khoi. Study the scientific background An adequate assessment of dike safety is a for un-flooded some lower area in the red river complex issue that should include not only basin and Hoanglong river basin. Technical the instability but also piping and other failure report, 2010. mechanisms. In stability analysis, PLAXIS 2D 2011 • Hanoi Local Dike Board No3. General back- demonstrated successfully the dike safety factor ground of red river dike systems in Hanoi area, in flood waves with different construction stages. board no3. Technical report, 1971. A dike location in Red River in Hanoi with historical • J Gauthier. Digues du Tonkin (Hanoi, Imprimerie data of dike failure is selected and the calculation D’extreme-orient). M6409 – National library of results demonstrated reasonably comparing to Vietnam, 1930. the past observations, and dike’s stability has • M.D Hemery. Histoire du euve rouge, gestion been significantly improved from the past until et amenagement d’un systeme hydraulique now. The dike’s stability was calculated in critical au tonkin des annees 1890 jusqu’a la seconde states of both the flood in 1971 (with return period guerre mondiale, tome 1 – History of the Red of 1/50 year) and the flood frequency of 1/500 River, management and Layouts of a system of year and was found to be unsafe. In contrast, it hydraulic tonkin 1890s until the Second World was estimated to be safe in the flood with a return War, Volume 1. Universite de Paris 7, 1991. period of 1/1 year in 1893, with regard to stability • Nguyen, C. M. Some geotechnical engineering only. Further research should be carried out to aspects of dike in red river delta, calculation re- investigate the influences of the heterogeneity vitalization. In Proceeding of Geology engineer- of dike embankment and subsoil data, and ing and geo-environment in Vietnam, 1999. other related issues such partially saturated soil • Tran, V. T. Study on the geological conditions behaviour and advanced soil models. of the red river dikes in Hanoi area. Technical report, 2012. Acknowledgements • HEC1. Geology investigation reports, Hanoi This paper is funded by the project of Upgrading dikes improvement project. Technical report, the Training Capacity in Coastal Engineering 1994. of the Water Resources University of Vietnam • R.B.J Brinkgreve. Plaxis 2D-2011 manual, vols from the Royal Netherlands Embassy in Hanoi of 1:5. Plaxis BV, 2011. Vietnam, and CICAT and Hydraulic Engineering • Hanoi. FPD. Hanoi - 50 years with natural hazard Department of TU Delft; the authors wish to thank protection work, Flood Protection Department for their support. of Hanoi. Hanoi publishing, 2000. • R.B.J Brinkgreve. Material models for rock and soil - Lecture note CT4360. Technical University of Delft, 2009.

www.plaxis.nl l Spring issue 2013 l Plaxis Bulletin 19 3D Modelling of Frozen Ground and Associated Volume Expansion

Joseph Sopko, Services, Moretrench

Plaxis was contracted by Moretrench to provide assistance in setting-up 3D finite element models for the analysis of how ground freezing process and associated volume expansion would impact existing panels. Thanks to PLAXIS Expert Services, valuable results have been obtained in terms of displacement and integrity assessment for the existing slurry walls.

What is ground freezing? heat from the surrounding soil. When the soil ground and propagates thermally, rather than The ground freezing process converts pore temperature reaches 32°F (0°C), ice begins to form by displacement. The ground remains largely water into ice by the continuous circulation of a around the pipes in the shape of vertical, elliptical undisturbed during freeze pipe installation and the cryogenic fluid within a system of small diameter, cylinders. As the cylinders gradually enlarge with freezing process and, in most instances, reverts closed-end pipes installed in a pattern consistent time, they interconnect to form a continuous wall. to its original state once freezing is discontinued. with the shape of the area to be treated. The Once the frozen wall reaches design thickness, the Ground freezing provides both excavation support frozen pore water acts as a bonding agent, fusing freeze plant is typically operated at a reduced rate and groundwater control. While the use of ground together particles of soil or rock to significantly to maintain the condition. freezing for the construction of deep shafts increase compressive strength and impart through water-bearing soils is the most common impermeability. application, the technique is also appropriate for mining of cross passages and connector tunnels, “PLAXIS professionals assisted us tremendously in moving forward our stabilization of mixed-face tunnels, frozen-arch canopies, and containment of contamination. analytical tools and ability to understand the mechanics of our project... “ Finite element modelling Freeze pipes are typically installed vertically in the Unlike other groundwater -off and excavation The dimension of the FE model is 100m*50m*60m. soil and connected in series-parallel. The coolant support techniques, ground freezing is a Only half of the geometry has been modelled is pumped down a drop tube to the bottom of the minimally invasive technique that requires due to symmetry conditions. The model contains freeze pipe and flows up the annulus, withdrawing only limited physical penetration into the a total of 64,000 elements and 90,000 nodes (i.e

FE mesh Calculated displacement after ground frozen

20 Plaxis Bulletin l Spring issue 2013 l www.plaxis.nl 270,000 dof). Each finite element has 10 nodes. The model consists of the following construction The slurry wall panels have been defined around a sequence: 11m diameter shaft. The frozen pipes have been Vacancy: Advisor PLAXIS Expert Services placed along a series of concentrated circles. • Initial stress definition and initial water level • Slurry wall construction Plaxis bv is currently looking for an advisor to In PLAXIS the frozen process is modelled in • Excavation strengthen our expert services team. sequence by applying an isotropic volumetric • Ground freezing part I at diameter D1 prescribed strain of 1.5 % (so 0.5% in X-direction, • Ground freezing part II at diameter D2>D1 The advisor is expected to plan, execute 0.5% in Y-direction, 0.5% in Z-direction ) followed • Ground freezing part III at diameter D3>D2 and report projects and results on his/her by replacing the properties of the swollen clusters • Ground freezing part IV at diameter D4>D3 own. He/she also acts as project leader and by those of frozen soil after each expansion phase. maintains contacts with clients, universities and other external parties.

For more information please visit: “The implementation of PLAXIS to applications of artificial ground freezing www.plaxis.nl/jobs has significantly advanced the analysis of complex problems... “

PLAXIS Expert Services added value: The PLAXIS consultants eagerly accepted the challenge, fully understood our concerns and • Quick start on the job rapidly and efficiently assisted us in generating • Set-up of fully optimized and ready to run 3D the models that accurately simulated the complex FE models issues with stress and deformation on existing • Regular model review structures. Their assistance has enabled us to • Next business-day advanced technical make PLAXIS a readily available tool to analyse assistance a wide arrange of ground freezing challenges. Their knowledge, accuracy and efficiency are Customer quotes unsurpassed in this industry. The implementation of PLAXIS to applications of artificial ground freezing has significantly About Moretrench advanced the analysis of complex problems, Moretrench is geotechnical contractor delivering particularly the mechanics associated with ground geotechnical solutions to the underground, expansion as well as the decrease in frozen soil industrial and environmental remediation strength with time under load. industries. Moretrench offers a wide range of specialized services with skilled and experienced Applying PLAXIS to this problems was a engineering teams in the field of dewatering major undertaking for our engineers. PLAXIS and groundwater control, ground freezing, professionals assisted us tremendously in earth retention and anchors, deep foundations, moving forward our analytical tools and ability to underpinning, grouting and ground improvement, understand the mechanics of our project as well as environmental remediation. being able to present the results to our client with incredible output graphics.

Stress development in the slurry panel

www.plaxis.nl l Spring issue 2013 l Plaxis Bulletin 21 Recent Activities

Seminar Offshore Foundations Houston material modelling, special attention was given to The seminar was concluded with a live On January 28, 2013 Plaxis organised a short the total stress approach, anisotropic undrained demonstration of building and analysing an seminar in Houston on the use of PLAXIS 3D for , and the NGI-ADP model in PLAXIS offshore foundation in PLAXIS 3D, this clearly offshore foundations in geotechnical engineering. 2D and PLAXIS 3D. illustrated the ease and speed of work in PLAXIS 3D. A description of this case study on a combined PLAXIS 3D is rapidly gaining popularity in Also discussed were the importance of load vs. loading of a suction anchor can be found in our modelling subsea behaviour of soil and structures displacement controlled analysis, side-swipe online Knowledge Base. in the US and around the globe. This free event technique, and the use of interface elements in was well attended by professionals coming from PLAXIS for modelling skin friction, gapping and A similar seminar was also organised in Singapore major oil companies, engineering consulting firms, slipping. Additionally consideration was given end of 2012, and due to the popularity of both and universities. to computational efficiency by optimising the events plans for others around the world are calculation mesh, utilising a multicore solver, already underway, with the first coming up in A selection of offshore relevant features of PLAXIS and using the new option called ‘Gradual Error Australia later this year. So keep an eye out for this 3D where presented. On the subject of soil Reduction’ in PLAXIS 3D 2012. and more upcoming events.

22 Plaxis Bulletin l Spring issue 2013 l www.plaxis.nl New Head Office in Delft Product update: PLAXIS 2D Furthermore the new software has many new and The construction of our new head office is now PLAXIS 2D 2012 is now available! The new improved features such as; Structural forces in finalised, so as of the 2nd of January we have software is equipped with, amongst others, the volumes (When soil is used to model structures it moved to our new location. new embedded pile row facility. With the success is now possible to directly visualize the structural of the embedded pile element in PLAXIS 3D, forces in the Output program), Inverse analysis Our new visiting address will be: users started to ask for a similar facility in PLAXIS of material parameters (parameter optimisation), Computerlaan 14 2D. However, the stress state and deformation and Implementation of free field boundaries to 2628 XK Delft pattern around piles is fully three-dimensional, so enhance the domain of the dynamics calculations The Netherlands at first sight it seemed very difficult to develop an (Dynamics module, available upon request for element in 2D that can realistically model piles. PLAXIS VIP). Further contact details remain unchanged. Nevertheless, we have succeeded to do so.

Our new office offers many new facilities, including space for hosting PLAXIS Academy trainings. For 2013 we have planned to host a series of workshops. Each month a different subject will be addressed for either PLAXIS 2D or PLAXIS 3D, for instance modelling tunnels, dynamics or groundwater.

Whereas computational geotechnics courses focus on the background and application of the Finite Element Method in geotechnical engineering, PLAXIS trainings focus directly on learning and using the features of the PLAXIS software. Hence, the main goal of these workshops is to teach the use and basic concepts of different features in the PLAXIS software and to train the participants in using the software themselves. Besides that, the workshops will, to an extent, be touching upon the new functionality and improved workflow of PLAXIS software.

Furthermore, the participants will get acquainted with the proper use of PLAXIS 2D and/or PLAXIS 3D by doing hands-on exercises. During these exercises experienced Plaxis tutors will provide individual assistance. Scematic representation of embedded pile

Visit our site at www.plaxis.nl/events for a full list of the workshops and topics included.

We wish you all a prosperous 2013, and would like to welcome you to our new office!

www.plaxis.nl l Spring issue 2013 l Plaxis Bulletin 23 Title

Activities 2013

March 3 – 6, 2013 May 1 - 4, 2013 September 2 - 6, 2013 Geo-Congress 2013 Seventh International Conference on Case International Conference on Soil Mechanics and San Diego CA, U.S.A. Histories in Geotechnical Engineering Geotechnical Engineering Chicago IL, U.S.A. Paris, France March 4, 2013 Workshop on the use of PLAXIS 2D for May 15, 2013 September 2 - 4, 2013 Geotechnical Analysis French Plaxis Users Meeting Standard Course on Computational Geotechnics Milan, Italy Paris, France Gdansk, Poland

March 7, 2013 May 16, 2013 September 4, 2013 Grundläggningsdagen 2013 Workshop: Utilisation de PLAXIS 2D dans Workshop on Modelling Tunnels in PLAXIS 3D Stockholm, Sweden l’ingénierie géotechnique sismique Delft, The Netherlands Paris, France March 11 – 14, 2013 September 10 – 13, 2013 Advanced Course on Computational Geotechnics May 21 – 24, 2013 Advanced Course on Computational Geotechnics Schiphol, The Netherlands Advanced Course on Computational Geotechnics Manchester, United Kingdom Berkeley CA, U.S.A. March 18, 2013 September 12, 2013 Belgium Plaxis Users Meeting June 4, 2013 Workshop: Modélisation de l’eau dans les sols sous Brussels, Belgium Workshop on Modelling Dynamics in PLAXIS Paris, France Delft, The Netherlands March 26 - 27, 2013 September 18 – 20, 2013 Training on PLAXIS 2D for Earthquake June 17 – 20, 2013 Advanced Course on Computational Geotechnics Geotechnical Analysis Standard Course on Computational Geotechnics Chennai, India Istanbul, Turkey Manchester, United Kingdom September 29 – October 03, 2013 March 27 - 28, 2013 June 23 - 26, 2013 GeoMontréal 2013 Training on the use of PLAXIS 2D Rapid Excavation and Tunneling Conference 2013 Montréal, Canada Kuala Lumpur, Malaysia Washington DC, U.S.A. October 2, 2013 April 3, 2013 June 25 - 26, 2013 Workshop: Introduction to PLAXIS 3D Workshop: Introduction to PLAXIS 3D Russian Plaxis Users Meeting Delft, The Netherlands Delft, The Netherlands St. Petersburg, Russia November 6, 2013 April 15, 2013 July 3, 2013 Workshop on Soil Parameter Optimisation and Workshop: Utilisation de PLAXIS 3D en Workshop: Introduction to PLAXIS 2D Sensitivity in PLAXIS géotechnique Delft, The Netherlands Delft, The Netherlands Paris, France July 18, 2013 November 6 - 8, 2013 April 17 - 19,2013 UK Plaxis Users Meeting European Plaxis Users Meeting EURO:TUN 2013 London, United Kingdom Karlsruhe, Germany Bochum, Germany August 12 – 15, 2013 December 04, 2013 April 23, 2013 Standard Course on Computational Geotechnics Workshop on Modelling Groundwater in PLAXIS 3D Workshop on Soil Parameter Optimisation and Gothenburg, Sweden Delft, The Netherlands Sensitivity in PLAXIS Delft, The Netherlands

Plaxis bv P.O. Box 572 www.plaxis.nl Plaxis Asia 16 Jalan Kilang Timor Computerlaan 14 2600 AN Delft Tel +31 (0)15 2517 720 Singapore #05-08 Redhill Forum 2628 XK Delft The Netherlands Fax +31 (0)15 2573 107 Tel +65 6325 4191 159308 Singapore