Prediction of Field Response of Soil-support Systems in Deep Excavations

8/2/2019 Prediction of Field Response of Soil-support Systems in Deep Excavations.

1/13

PREDICTION OF FIELD RESPONSE OF SOIL-

SUPPORT SYSTEMS IN DEEP EXCAVATIONS.

By

Ashraf Mohamed Hefny1, Tamer Mohamed Sorour2, and Mohamed Ezzat Ezzat

3

1- Associate Professor of Geotechnical Eng., Department of Civil Engineering, Ain Shams University.2- AssistantProfessor of Geotechnical Eng., Department of Civil Engineering, Ain Shams University.3- Lecturer ofGeotechnical Eng.,, Department of Civil Engineering, Al-Shrouk Academy.

.

.

Abstract.

This paper highlights, a parametric study was performed to evaluate the

methodology of predicting soil-support system response. Analytical models are

prepared using finite element program 2D PLAXIS and finite difference program

GEODELFT MSHEET to be calibrated with field data monitored for realistic case

studies, with the results of analytical models and by Changing of support systems for

the same soil profile and collecting results again then compare them, we can insure the

reality of the prediction technique suggested in this thesis.

It concluded from the results of this paper that using Hardening soil model which

depends on soil stiffness (E) can lead us to reach an accurate prediction of soil-support

system response . As stiffness of soil could be accurately determined using laboratory

tests like Triaxial test and Odometer test (Eoed) for field soil samples bored out from

deep excavation site.

Keywords.

Deep excavations, Soil-Support systems, Finite Element Method .

1- Introduction.

The task of predicting the performance of deep excavations is challenging, because

many factors influence the performance of deep excavations. Soil conditions,

groundwater conditions, and the stiffness of the support system are three that are always

important.


2/13

Hardening soil model depends on stiffness of soil which can be accurately

determined using laboratory tests as TRIXIAL test and OEDMETER test. The main aim

of this paper is analyzing two finite element models to compare them with two real case

studies have a completely monitoring data to verify the suggested procedure of

predication soil-support system performance.

3- Hardening Soil Model Methodology :

The Hardening-Soil model is an advanced model for simulating the behavior of

different types of soil, both soft soils and stiff soils,. When subjected to primary

deviatory loading, soil shows a decreasing stiffness and simultaneously irreversible

plastic strains develop. In the special case of a drained triaxial test, the observed

relationship between the axial strain and the deviatory stress can be well approximated

by a hyperbola.

However, supersedes the hyperbolic model by far. Firstly by using the theory of

plasticity rather than the theory of elasticity. Secondly by including soil dilatancy and

thirdly by introducing a yield cap. Some basic characteristics of the model are:

A basic feature of the present Hardening-Soil model is the stress dependency of soil

stiffness. For oedometer conditions of stress and strain, the model implies the

relationship

In the special case of soft soils it is realistic to use m = 1. In such Eoed situations

there is also a simple relationship between the modified compression index * and the

oedometer loading modulus.

- Stress dependent stiffness according to

a power law.

Input parameterm

- Plastic straining due to primary

deviatoric loading.

Input parameterEref 50

- Plastic straining due to primary

compression.Input parameterEref oed

- Elastic unloading / reloading.Input parametersEref

ur, nur

- Failure according to the Mohr-

Coulomb model.Parameters c, j and y


3/13

wherepref

is a reference pressure. Here we consider a tangent oedometer modulus at a

particular reference pressure pref

. Hence, the primary loading stiffness relates to the

modified compression index *

. Similarly, the unloading-reloading modulus relates to the modified swelling index k*.

there is the approximate relationship:

Again, this relationship applies in combination with the input value

m = 1.

More than (50) trial were analyzed in this study using finite element program

2D-PLAXIS to achieve the perfect match with the results of monitored field data within

soil modeling verifications. So , we advise to use this following soil parameters and its

relations to reach the closest prediction of soil-support system field response in deep

excavation.

3- Case Studies Verification:

To be satisfied of the methodology of using Hardening soil model to represent

the soil support system performance we select two different case studies which included

complete mentoring data of soil support system performance, mentored during the

different stages of construction as shown in figure (1).

This case studies were for constructed previous projects, The first was for Flame

Towers in Baku and the second was for Towers in Japan as shown in figure(2). These


4/13

different cases with different soil profiles shown in figure (3) and table (1) and different

support systems were modeled using Hardening soil models with 2D-PLAXIS with the

same soil parameters that depends on soil stiffness parameters which needed in

Hardening soil model shown in figure(4).

The analysis procedure, boundary conditions, initial stress, ground water

condition, soil parameters, support system parameters, stages of constructions and the

results of case study (A) and (B) were accurately presented in finite element model

according to case studies, To verify the methodology of modeling sequence and

technics with the monitored data from the both case studies.

To ensure the reality of the procedure anther (12) finite element model were

analyzed for the same case studies .But, with different support systems and compared

with the original support system used in the case studies.4- The Results:

The result of finite element analysis conducted through the program 2D-

PLAXIS as shown in figure (5) were compared with the field measurements in the form

of displacement (m), depth(m) within stages of construction.

This results were calculated at several nodes representing the ground surface ,the

excavation start ,stages of construction and excavation finish according to case studies.

The results were recorded during the excavation and constructing the support system.

Its clear from these results that theres a perfect agreement between the measured

values and monitored ones as shown in figure (6).

Anther (12) finite element model results ensure the reality of the procedure

suggested in this paper as shown in figure(7) and figure(8).

5- Conclusion.

The following conclusions are drawn from the work describe in the dissertation:

1. Finite element analysis and instrumentation monitoring of deep excavations arenaturally complimentary tools for studying deep excavation.

2. Using of Hardening soil model depends on accurate laboratory test results canlead us to an accurate prediction of support system response.

3. Stiffness of soil and support system is the major factor which control the soilsupport system performance.


5/13

Figure 1. Inclinometer readings for case studies (A) and (B)

Figure 2. Case studies (A) and (B)

Case (A)

rsBAKU towe

Case (B)

JAPAN towers

Case (A)BAKU towers

Case (B)JAPAN towers


6/13

Figure 3. Soil profile for case studies (A) and (B)

Case (A)BAKU towers

Case (B)

JAPAN towers


7/13


8/13

Figure 4. Finite element model for case studies (A) and (B)

Case (A) BAKU tower

Case B JAPAN towers


9/13

Figure 5. Results of finite element model for case studies (A) and (B)

Case (A) BAKU tower

Case (B) JAPAN towers


10/13

COMPARISON BETWEEN MONITORING READINGS, FINITE DIFFERENCE

RESULTS AND, FINITE ELEMENT RESULTS FOR CASE STUDY (A).

COMPARISON BETWEEN MONITORING READINGS, FINITE DIFFERENCE

RESULTS AND, FINITE ELEMENT RESULTS FOR CASE STUDY (B).

Figure 6. Verification of finite element model for case studies (A) and (B)


11/13


12/13

Relation between horizontal displacement and excavated depth for soil model.8Figure

r case study (B).supported by different support systems fo


13/13

6- References.

1) Athanasiu, C.M., Simonsen, A.S., and Ronning, S. (1991). "Back-calculation ofcase records to calibrate soil-structure interaction analysis by finite element

method of deep excavation in soft clays", Proceedings of the Tenth European

Conference on Soil Mechanics and Foundation Engineering, 10(1), 297300.

2) Bachus, R.C., Clough, G.W., Sitar, N.C., Shafir-Rad, N., Crosby, N. and Kaboli,P. (1982). "Behavior of weakly cemented soil slopes under static and seismic

loading", Engineering Report, Vol. II, .ohn A. Blume Earthquake Engineering

Center, Stanford University, California.

3) Balasubramaniam, A.S., Bergado, D.T., Chai, ..C., and Sutabutr, T.(1994)."Deformation analysis of deep excavations in Bangkok subsoils",

Proceedings of the Thirteenth International Conference on Soil Mechanics

and Foundation Engineering, 13(2), 909-914.

4) Benham-Holway Power Group (1995). "Deformation Analyses," engineeringreport for the Arkansas Electric Cooperative Dam Number 2 HydropowerProject, FERC Project 3033-006, Tulsa, Oklahoma.

5) Biot, M.A. (1941). "General Theory of Three-Dimensional Consolidation,"Journal of Applied Ph!sics, 12, Feb., 155-164.

6) Bolton, M.D. and Powrie, W. (1988). "Behaviour of diaphragm walls in clayprior to collapse", Geotechnique, 38(2), 167-189.

7) Bolton, M.D. and Powrie, W. (1987). "The collapse of diaphragm wallsretaining clay", Geotechnique, 37(3), 335-353.

8) Bolton, M.D. and Stewart, D.I. (1994). "The effect on propped diaphragmwalls of rising groundwater in stiff clay", Geotechnique, 44(1), 111-127.

9) Bono, N.A., riu, T.K., and Soydemir, C. (1992). "Performance of an internallybraced slurry-diaphragm wall for excavation support," Slurry Walls: Design,

Construction, and Quality Control, ASTM Special Topic Publication 1129, 347-

360.

10)H. A. Afatoglu1 ENAR Case study on a deep excavation in Baku: Flame Towersproject earth retaining system Engineers Architects & Consultants, Istanbul,

Turkey.

11)M.Mitew Numerical analysis of displacements of a diaphragm wall WarsawUniversity of Technology, Warsaw, Poland

12)Zienkiewicz, O.C. and Taylor, R.r. (1991). The Finite Element MethodVolume 2: Solid and Fluid Mechanics D!namics and Non-linearit!, 4th edition,

McGraw-Hill Book Company, rondon, U.K.

Documents

Prediction of Field Response of Soil-support Systems in Deep Excavations