CONSOLIDATION ANALYSIS OF EMBANKMENT IN STAGED CONSTRUCTION ON SUBGRADE SOFT SOIL

CONSOLIDATION ANALYSIS OF EMBANKMENT IN STAGED CONSTRUCTION ON SUBGRADE SOFT SOIL

O B A Sompie , A.L.E Rumayar , J.I Kindangen & Budi I Setiawan

1

Department of Civil Engineering, Engineering Faculty, Sam Ratulangi University, Manado, Indonesia

2 Department of Architecture, Engineering Faculty, Sam Ratulangi University, Manado, Indonesia

3 Department of Civil and Enviromental Engineering, Bogor Agricultural University (IPB), Bogor, Indonesia

 Communicating author, email : bsompie@yahoo.com

CONSOLIDATION ANALYSIS OF EMBANKMENT IN STAGED

CONSTRUCTION ON SUBGRADE SOFT SOIL

ABSTRACT

Earthfill dam requires relatively good soil bed as bearer, where the soil consolidation can subside due to excessive loading Two problems are usually faced in the consolidation process, i.e., magnitude of the displacement and time interval required to reach maximum displacement The staged construction is considered a very effective technique of construction embankment on most soft subsoils The paper is devoted to study the behaviour and stability of an embankment in staged construction using the finite element method Stress-strain behaviour and pore-water pressure are investigated for each stage construction of embankment Failure mechanism and factor of safety are also evaluated their progress and values The numerical method is highly appropriate for solving the environment engineering of geotechnical problems

Keywords: consolidation, embankment, staged construction, soft soil

INTRODUCTION

Construction of embankment dams has an economical advantage; i.e., the dam project can be planned in the outskirts of city area because of the merit mentioned above, and construction materials are principally to be supplied near the dam site Embankment dams on soft subsoil are often constructed in stages in order to pre-settle the soft subsoil, and dissipate the excess pore pressure during the construction This thus increases the shear strength of the dams due to consolidation of the subsoil and the dam itself (Hartlen and Wolski, 1996) Therefore, the behaviour and stability inclusion secondary compression of the staged construction on earthfill dams are necessary to be analysed to ensure that the dams are stabilised during and after the construction

The finite element method (FEM) is advantageous over other methods that it can simulate nearly the working conditions of the structures In particularly, the staged construction can be handled

in FEM to evaluate the stress-strain behaviour, consolidation, and the progressive failure up to including overall shear failure (Brinkgreve and Vermeer, et al 2002) The FEM performed by PLAXIS program is used to analyse consolidation settlement and safety factor an earthfill dam in staged construction

MOHR-COULOMB (MC) MODEL

Shear strength tests on compacted fill materials reveal that the strength envelop consists of a combination of two straight lines, as shown in Fig.3.2 This is due to the fact that compacted soils more or less can have high skeleton strength composed by suction effects and the strength becomes greater than that of normally consolidated state under a confining pressure lower than

Compacted Fill Materials:

 Compacted soils show higher strength AB than that in normally consolidated sate AB under lower confining pressure than the pre-compression stress pc

 Pre-compression effects vanish due to wetting of soil, so that c and φ to be used in the design are determined by AB for practical safe-side design

Fig.3.2 Pre-compression Effect of Compacted Fill Materials

The elastic-plastic Mohr-Coulomb model involves five input parameters i.e and for soil

represent a ‘first-order’ approximation of soil or rock behavior It is recommended to use this model for a first analysis of the problem considered

For each layer one estimate a constant average stifness Due to this constant stiffness, computation tend to be relatively fast and one obtain a first impression of deformations Besides the five model parameters mentioned above, initial soil conditions play an essential role in most soil deformation problems Initial horizontal soil stresses have to be generated by selecting

MATERIAL MODELLING OF AN EMBANKMENT DAM

For the simplification of the analysis purpose, an earthfill dam in the model is assumed as homogeneous and built of sandy clay on the subsoil of soft clay and fine sand The dam is 10 m high (H), with the dam crest of 3 m and the slope inclination for both sides of 1V: 3H (Figure 1) The construction of the embankment is divided into 5 stages with each stage of five-month consolidation

Figure 1 Embankment dam Model and five staged construction (not scale)

The dam is modelled to analyse before the impoundment so the water level is assumed on the ground The input soil properties of the dam and subsoil are described in Table 1

Table 1 Material properties of embankment modelling

Mohr-Coulomb

1 Sandy Clay (Dam)

2 Soft Clay

3 Fine Sand

RESULTS OF ANALYSIS

1 Stress-strain behaviour

The total stress distribution in principal (major and minor) directions at the end of the last construction is drawn in Figure 2 The magnitude can be provided at any desired stress node plotted in the finite element mesh It can be achieved both effective stress and pore pressures

at each construction stage Besides, deformation and settlement can also be determined at any time of construction

Fig 2 Total stress distribution in major and minor directions at last phase

The time-displacement curve at the point A and B is plotted in Figure 3 After five-staged consolidation of 25 months, the displacement of point A and B ( see figure 1) are about 0.038 m and 0.120 m respectively (Fig 2) The total displacement of the soft subsoil and dam is 0.187m (Fig 6) and the degree of consolidation reaches approximately more than 95% Pre-settlement of the deposit is very important factor that the dam can be stable in its construction and long-term operation In some cases, additional treatments such as geotextiles, vertical drain or others might be applied to increase the consolidation process if necessary

Displacement [m]

Time [day]

0.00

0.04

0.08

400 0.12

2 Pore pressures

The excess pore pressures during construction highly develop and take dominantly place

at the dam body as predicted in Figure 4 This partially explains for the reason of the reduction of effective stress, and of that the factor of safety decreases in accordance with increasing the dam height When high excess pore pressures occur in the dam body, it might result in a crack or collapse of the dam So to reduce the development of excess pore pressure in the dam body is very important factor in construction embankment dams

Figure 4 Excess pore pressure after last filling in principal directions

(uextreme= -0.048 kN/m2, , pressure=negative)

Figure 5 Development of excess pore pressure during staged constructions (at point B)

As soon as each filling layer is placed, excess pore pressure immediately increases and after that gradually dissipates for the time being of consolidation as seen in Figure 5 With the adequate time of consolidating the subsoil and the filled embankment, the excess pore pressures dissipate and become negligible It is then stable to be placed the sequent filling of 20 months wherein the excess pore pressures will be disipatted Therefore, construction stage and its period of time to consolidate the soft subsoil and dam usually need to be determined in the design step

0.04

0.00

-0.4

400

-0.8

Time [day]

3 Failure mechanism and safety factor, SF

The failure mechanism of the dam at the end of construction is indicated as the vectors of displacements in Figure 6 The potential slip failure tends to occur downwards to the soft subsoil Fig 6, For stability of the two slopes, the safety factor (FS) consecutively decreases from the first staged construction the final stage as computed: FS first

term construction stage (SF= 1.26 ), wherein the type of land slide of long term construction stage in Figure 7

Although the displacements are not relevant, they indicate whether or not failure mechanism has developed The long-term stability suitable the modelling of earthfill dam (see Fig 1), the factor of safety changes based on a presence of water table in the upstream side, therefore FS increases in the upstream water table decreases and type of failure (land slide) accur two side of dam body

Figure 6 Failure mechanism – vectors of total displacements (max = 0.187m)

Figure 7 Type of land slide

Figure 8 Safety Factor

CONCLUSIONS

The staged construction is a very useful technique to build earthfill dams on soft subsoil The simulation and calculation of staged earthfill dam can be performed by finite element method The numerical modelling describing nearly the field conditions can be computably solved and predictably evaluated the behaviour and stability of the structures The main purposes of FE-analyses are the determination of stresses including pore pressures, of stress/strain distribution, and the prediction of deformation and settlement Safey factor in each construction phase can be defined by phi-c-reduction assumption The widespread use of the numerical method should be considered as an advanced approach for agricultural engineering of hydraulic structure analyses

REFERENCES

J Hartlen, W Wolski (1996): Embankments on Organic Soils ELSEVIER 424p

ISBN 0-444-88273-1

R B J Brinkgreve, P A Vermeer, et al (2002): PLAXIS 2D- Finite Element Code

for Soil and Rock Analyses Versi 8 Delft University & Plaxis b.v., A.A BALKEMA/

ROTT/ BROOKFIELD The Netherlands