Poromechanics V © ASCE 2013 1372 - uni- · PDF fileEAB (2012) the recommendations EB 73 “Excavations with circular plan”, ... excavations with rectangular plan in frictional...

Deep Excavations with special ground shape

Christian Moormann1 and Linus Klein2

1Universitiy of Stuttgart, Institute of Geotechnics, Pfaffenwaldring 35, 70569 Stuttgart, Germany; PH (0049) 711 685-62437; FAX (0049) 711 685-62439;

email: [email protected] 2Universitiy of Stuttgart, Institute of Geotechnics, Pfaffenwaldring 35, 70569 Stuttgart, Germany; PH (0049) 711 685-62071; FAX (0049) 711 685-62439;

email: [email protected]

ABSTRACT

The extension of our infrastructure in urban areas and also in rural embossed areas is often linked with the design and construction of complex geotechnical structures. Especially the design of the temporarily required excavation structures with the plurality of construction phases and the varying deformation states and stress situations has to be mentioned. In the most cases the examination of the bearing and deformation behavior in plain strain models is preferred to the detailed three-dimensional investigation of the soil-structure interaction. In reality deep excavations are spatial systems significantly influenced by the geometric and geotechnical boundary conditions [Chiou et. al. (1993)].

The three-dimensional soil-structure interaction and the influence on the bearing and deformation behavior of nearly rectangular excavations in cohesive soils are investigated by the analysis of several field measurements and the execution of a parametric study. The stiffening effect of the corners and its influence on the bearing behavior of the structure is apparent and leads to a more realistic and more economic design. The realistic approach of the acting earth pressure is contrary to the approach formulated in the technical guidelines, e.g. EAB (2012). Further investigations of the bearing and deformation behavior of deep excavations with special ground shape under consideration of the geotechnical and the structural boundary conditions shall lead to more realistic design approaches. CASE STUDY: RIVER POWER PLANT IFFEZHEIM, GERMANY

Owing to the construction of turbine No. 5 the river power plant Iffezheim at the river Rhine evolves to the most efficient facility of its kind in Germany (Figure 1). The three excavations for the construction of the turbine house and the supply structures are located in subsoil dominated of sand and gravel layers with high thickness and partly in back filling of sandy gravel in consequence of the position at the former working panel of the existing power plant.

The main excavation for the hydro turbine with a depth of 34 m has an elliptical plan with dimensions of 54 m x 36 m without bracing or anchors and is

1372Poromechanics V © ASCE 2013

effected by heavy earth pressure and a water pressure of maximal 25 m. The diaphragm wall was executed with a maximal segment length of 3,2 m and special Y-panels for the highly stressed transitions to the adjacent excavations [Raithel et al. (2011)]. The design provides a head board and a pressure ring for the stiffening of the excavation and an anchored underwater concrete slab.

Figure 1. River power plant Iffezheim (Germany). a) Aerial photography of construction site. b) Three-dimensional model.

To predict the soil-structure interaction, the deformation behavior of the

excavation itself and of the adjacent structures a three-dimensional model of the excavations was required (see Figure 1). The ascertainment of the spatial bearing behavior was required in consequence of the complex geometry with asymmetrical load distribution and the reciprocal interaction of the excavations with changing distributions of the subgrade reaction.

An extensive monitoring program was installed to compare the predicted bearing and deformation behavior with the effective interaction of the excavations and the adjacent structures. Figure 2 documents the horizontal deformations of the diaphragm wall in the north-north-eastern elliptical curve with small radius after the complete lowering of inner water table. It is obvious that due to the supporting effect the deformations in direction of the semi-major axis increase. The chart shows the threshold (dashed line), which is defined according to the calculated deformations, the intervention level (dash-dotted line) and the alarm value (solid line). ANALYSIS OF IN-SITU MEASUREMENTS

The analysis of international case studies exhibits that the spatial bearing behavior of (nearly) rectangular excavations is mainly affected by the lowly deformable corners [Moormann (2012)]. The occurrent vertical and horizontal deformations increase from the minimal values at the corners to the maximum values in the middle of the side walls. In case of high ratio of the length of the side wall L to the excavation depth H the horizontal displacements come up to 90 % of their


maximum value in a distance from the corner of x/H = 1,0 to 2,0 (compare Figure 3). The maximum values are less than predicted by a plain strain model. The reduction of the wall deformations and the deformations in the area of influence of the excavation reach up to a distance of two to four times the excavation depth H.

Figure 2. River Power Plant Iffezheim - Inclinometer profile H4 located in the

north-north-eastern elliptical curve with small radius.

The decisive influence coefficients on the spatial bearing behavior of deep excavations are the ratio of the side wall lengths L1/L2, the excavation depth H and the stiffness of the support system. The kind of retaining system and hereby the flexural stiffness of the retaining system in longitudinal direction has less influence. APPROACHES IN TECHNIACL GUIDELINES

In chapter No. 8 with the title “Excavations with special ground plans” of EAB (2012) the recommendations EB 73 “Excavations with circular plan”, EB 74


“Excavations with oval plan” and EB 75 “Excavations with rectangular plan” are included. With regard to the consideration of the spatial bearing behavior of deep excavations with rectangular plan in frictional soils respectively minimum stiff cohesive soils EB 75 contains two approaches, which are formulated as a function of the ratio of the stiffness of the corners of the excavations to the middle of the sidewalls. The bases for the subsequent exemplified approaches are the theoretical-analytical models illustrated in Figure 4.

Figure 3. Summary of the analysis of the spatial behavior of deep excavations observed by field measurements. a) International case studies from literature.

b) Excavations in stiff Frankfurt Clay.

For excavations with corners likewise flexible as the middle sections of the excavation walls the approach according to Figure 5 a) can construed from the model which assumes shear forces in the flanks of slipping earth wedges. Herein the reduction of the earth pressure is considered in the corner region aL resp. aB in the form of chamfering or in the form of steps in the continuous earth pressure Eh.


Figure 4. a) Model with shear forces in the flanks of slipping earth wedge.

b) Model with three-dimensional failure mass.

Eh designates the earth pressure on a continuous wall from soil self-weight, unbounded distributed load pk ≤ 10 kN/m² and, if applicable, cohesion. If the corners of the retaining wall are less flexible than the middle sections of the excavation walls a reduction of the earth pressure Eh in the middle section corresponding to Figure 5 b) can be applied.

Figure 5. Simplified approach of earth pressure reduction a) Excavations with

corners likewise flexible as the middle sections of the excavation walls. b) Excavations with corners less flexible than the middle sections of the

excavation walls.

The wall lengths aL resp. aB for which a reduction may be applied, can be formulated according to Walz (1994) as a function of the excavation depth H:

aL = �0,35− 0,06∙H

L� ∙ H for the wall with length L (1.1)

aB = �0,35− 0,06∙H

B� ∙ H for the wall with length B (1.2)


The precondition for the reduction of the earth pressure Eh is according to EAB (2012) a retaining structure, which is either not supported, flexible supported or slightly flexible supported and permits hereby the required deformations for the approach of the active earth pressure. In case of a design based on the at-rest earth pressure the consideration of the three-dimensional bearing behavior is not permitted. By applying the increased active earth pressure an interpolation between the at-rest earth pressure and the active earth pressure in the areas with attenuation is allowed. The distribution of the earth pressure across the wall in areas with a reduction of earth pressure can be applied with the well-known pressure diagrams.

A basis for the analysis to which extent the corners of the retaining wall show a more flexible behavior than the middle sections is not defined in EAB (2012) and consequently a project-related decision is required. NUMERICAL ANALYSIS

A parametric study using a three-dimensional numerical model was carried out to investigate systematically the mechanical behaviour and the consequences of spatial effects on the earth pressure, the deformations as well as on the internal forces and stresses of the wall and the support system of deep excavations. For the analysis a spatial finite-element model with an elastoplastic constitutive model was used [Moormann et al. (2002)]. The geometric dimensions of the excavations like the excavation depth H and the lengths of the side walls L1 and L2 as well as the stiffness of the walls, which are assumed to be diaphragm walls or contiguous bored pile walls, are varied within the numerical study. Exemplarily Figure 6 shows the finite element mesh used for the analysis of a 20 m deep excavation with rectangular shape and the surrounding homogenous clay layer overlapped by 4 m quaternary sands.

Figure 6. Three-dimensional finite element mesh of a quadrant of a deep

excavation with rectangular shape.

The calculated levels of equal values of displacements resp. earth pressure are illustrated in Figure 7. The distribution of the horizontal wall displacements shows that the maximum displacements occur in the middle of the side walls whereas the displacements at the corners are very small. This is in consistency to the results of the


analysis of field measurements. For each side wall the both side walls, which are

perpendicular to the treated wall, act as a nearly undeformable support at the corners

and affect a significant reduction of displacements. Behind the wall the horizontal

earth pressures concentrate at the stiff corners whereas the earth pressure in the

middle of the wall is obviously smaller.

Figure 7. Front-elevation of the retaining walls (fold up). a) Levels of equal horizontal displacements [cm]. b) Levels of equal earth pressure [kN/m²].

Wall stiffness. The influence of the flexural stiffness of the retaining wall is shown in

Figure 8 by illustration of the maximum horizontal displacements calculated for the

walls with different thickness. Independent of the thickness of the wall the

displacements at the corner are minimal and decrease with the distance from the

corner. Also in the middle of the side walls the displacements of the excavation with rectangular shape are less than predicted by a plain strain model.

Figure 8. Influence of flexural stiffness of the retaining wall on spatial

deformation behavior. Geometric configuration. In the frame of the study the lengths of the side walls are

varied between 8 m and 100 m keeping all other parameters like the excavation depth

and the flexural stiffness of the wall constant. In the following the wall, of which the

displacements are regarded is called primary wall LA, whereas the side walls staying


perpendicular to the primary wall are called complementary walls LB. For the interpretation of the calculation results a spatial factor κ is defined as: ��uh� = uh

3D uh2D⁄ (1.3)

The spatial factor � is the ratio of the horizontal wall displacement uh

3D of a rectangular shaped excavation to the horizontal wall displacements uh

2D calculated with a plain strain model for an excavation with same width. In Figure 9 the results of the parametric study are summarized by displaying the distribution of ��uh� along the side walls for constant lengths LA of primary wall and varying lengths LB of complementary walls.

Figure 9. Numerical parametric study of the influence of geometric

configuration. The numerical analysis leads to the following conclusions: � With a plain strain model the displacements of a rectangular shaped excavation

with practically relevant dimensions are overestimated. Exemplarily the displacements of a rectangular excavation with dimensions of 60 m x 60 m are by 22 % smaller than predicted by a plain strain model.


� The displacement behavior at the corners is quite independent from the geometric configuration.

� The value of ��uh� in the middle of the side walls depends on the lengths of the side walls.

� With increasing length of the primary wall LA the displacements in the middle of the primary wall converge to the plain strain value, that means, ��uh� converges to 1.0.

� The length LB of the complementary walls influences the displacements of the primary wall. By constant length of LA the spatial factor ��uh� for LA increase to nearly 1.0 by decreasing length LB of the complementary wall.

Bracing system. The strut loads, calculated with the spatial model, show a high dependency on the position in the plan of the excavation. They increase with the distance to the corners of the retaining walls. An increasing stiffness of the struts leads to a minor reduction of the deformations and the earth pressure in consequence of the spatial behaviour of the excavation. Excavation depth H (ratio L/H). The executed numerical study leads to the conclusion that the excavation depth H has, in comparison to the ratio LA/LB, only a marginal effect on the distribution of the deformations and the acting earth pressure. INVESTIGATION SCOPE Corner stiffness. The analysis of international case studies and the presented numerical analysis shows the significant effect of the lowly deformable corners of the excavation on the spatial bearing behavior of (nearly) rectangular excavations. The influence of the assumed stiffness on the reduction of the acting earth pressure and the deformations of the retaining wall shall be identified by variation of the support conditions and the examination of the in-situ behavior of excavation corners. Jet-grouted soil. Ng et. al. (2012) report on a 14,5 m deep multi-propped excavation in soft clays at a greenfield site in Shanghai. The cast-in-place diaphragm wall was supported by a concrete prop at ground level, three steel props and a 5 m thick ´grouted prop´ in a depth of 13,5 m up to 18,5 m. To reach the final depth of the excavation the removal of 1 m of this ´grouted prop´ was required. During the period of five days for the construction of the basement slab the horizontal deformations increased about 22 % up to 45 mm. The presented deformations show the decisive influence of soil improved with jet-grouting on the deformation behavior of retaining walls of deep excavations. Frequently deep excavations in quaternary resp. tertiary sand and gravel layers are executed with bottom sealing by jet grouting. The approach of the characteristic compression strength and especially of the modulus of elasticity for the evaluation of lifting deformations of the construction pit floor resp. the strains in the sealing layer is often based on experienced data [Borchert et al. (2011)]. The influence of jet-grouted soil layers subject to the material behavior on the deformations of retaining structures and on the earth pressure approach shall be investigated.


CONCLUSION The stiffening effect of the corners of a nearly rectangular excavation influence distinctively the spatial deformation and bearing behavior of deep excavations. If the corners are less deformable as the middle of the side wall, a large horizontal stress arch is mobilized in the soil continuum behind the side walls using the corners as support and causing a reduction of the earth pressure load in the middle part of the side walls. A plain strain calculation without adaption of the acting earth pressure distribution overestimates the deformations in the influence area of the excavation respectively of the retaining system itself and of the stress resultants in the structural system. The geometric configuration of the excavation, especially the lengths of the side walls, determine mainly the three-dimensional soil-structure interaction. The consideration of the spatial performance of deep excavations leads to a more realistic and more economic design of excavations. For this reason further research activities are necessary to provide simplified recommendations for application of this aspect in design practice. REFERENCES Borchert, K.-M., Mittag, J., Römer, M., Savidis, A. (2011). „Bemessung von

Düsenstrahlsohlen unter Berücksichtigung von Sohlhebungen.“ Ver-öffentlichungen des Grundbauinstituts der TU Berlin, Heft Nr. 58, 199-215.

Chiou, D.C., Ou, C.Y. (1993). “Three-dimensional finite element analysis of deep excavation.” Geotech. Res. Rep. No. GT93001. Dept. Of Constr. Engrg., Taiwan Inst. Of Technol., Taipei.

EAB (2012). “Empfehlungen des Arbeitskreises Baugruben der Deutschen Gesell-schaft für Geotechnik.“ 5th ed., DGGT (Hrsg.), Berlin, Ernst & Sohn.

Moormann, C. (2002). „Trag- und Verformungsverhalten tiefer Baugruben in bindigen Böden unter besonderer Berücksichtigung der Baugrund-Tragwerk- und der Baugrund-Grundwasser-Interaktion.“ Dissertation. Mitteilungen des Institutes und der Versuchsanstalt für Geotechnik der Technischen Universität Darmstadt, Heft 59, 562 S.

Moormann, C., Katzenbach, R. (2002). “Three-dimensional finite element analysis of corner effects on deep excavations behaviour.” Proc. 5th European Conference on Numerical Methods in Geotechn. Engrg., NUMGE 2002, 4-6 September 2002, Paris, Presses de l´ENPC/LCPC, P. Mestat (ed.), 633-640

Ng, C.W.W., Hong, Y., Liu, G.B., Liu, T. (2012), “Ground deformations and soil-structure interaction of a multi-propped excavation in Shanghai soft clays.” Géotechnique 62, No. 10, 907-921.

Raithel, M., Kirchner, A., Rathgeb, R., Kamuf, I. (2011). „Erweiterung des Rheinkraftwerks Iffezheim - Baugruben zum Zubau der Maschine 5.“ 7. Stuttgarter Geotechnik Symposium., Mitteilungen des Instituts für Geotechnik der Universität Stuttgart, Heft 65.

Walz, B. (1994). “Erddruckabminderung an einspringenden Baugrubenecken.” Bautechnik 71, 90-95.


Poromechanics V © ASCE 2013 1372 - uni- · PDF fileEAB (2012) the recommendations EB 73 “Excavations with circular plan”, ... excavations with rectangular plan in frictional...

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