bridge approach embankments on rigid inclusions · international conference on geotechnics, 24-26...

International Conference on Geotechnics, 24-26 July, 2018 Yogyakarta, Indonesia

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Bridge Approach Embankments on Rigid Inclusions

M. Rizal Rekakarya Geoteknik, Jakarta, INDONESIA,

[email protected]

K. Yee Regional Synergy Consulting, Kuala Lumpur, MALAYSIA

[email protected]

ABSTRACT

Rapid development in Indonesia calls for a new highway to be constructed in Central Java. Along the highway alignment,

bridges are to be constructed over rivers and existing local roads. Based on the soil conditions, performance requirements,

construction schedule and project budget, ground reinforcement using Controlled Modulus Columns (CMC) was adopted to

support the bridge approach embankments to minimise post construction settlements; and to improve bearing capacity and

slope stability. The CMC system consists of vertical cylindrical grout columns installed in a predetermined grid spacing using

displacement auger. Typically, the CMC column was terminate at stiff layer, which found at depth 15m to 24m. Due to the

huge loading and thick compressible cohesive soil, selection of CMC column spacing and length is importance to ensure CMC

capacity within design. To confirm design termination depth of CMC column a set of drilling instrument were fitted into the

drilling rig. In this paper, 2D numerical modelling were verified by the 3D model and the results are presented. Also presented

in this paper is a brief description of the installation method used in soft ground condition together with a description of the

quality control procedure and acceptance testing. After completion of the CMC works, approach embankments up to 11m were

constructed.

Keywords: ground reinforcement, rigid inclusions, embankment, numerical modeling.

1 GENERAL INFORMATION

1.1 Project Background

In the north-central Java, a new highway of 39 km is

being constructed linking Pemalang and Batang. This

new highway forms part of the Trans Jawa Highway.

The project site is a paddy field of flat terrain. Figure

1 shows the project location and the ground

improvement areas.

It is necessary to ensure smooth transition between

flexible pavement and rigid bridge structure. With a

thick compressible soft cohesive soil deposit,

excessive post construction settlement is a major

concern. Also there is potential instability during

embankment filling works if the bearing capacity is

exceeded. The excessive differential settlement at the

transition area between flexible pavement and rigid

concrete bridge abutment will cause abrupt bump

which will cause discomfort to road users and

endanger lifes. Long term maintenance works is

required which disrupt the smooth operation of the

highway and it is a costly affair.

Structural solution using RC piles and concrete slab is

expensive. Geotechnical solution using ground

improvement is a viable solution. Since the bridge

abutments have already been constructed before

commencement of any ground improvement work, the

choice of ground improvement technique is limited to

techniques that are environmental friendly i.e.

techniques having minimum vibration and minimum

lateral soil movement during construction works to

avoid potential damage to the completed bridge

abutments and the foundations. The solution of

Controlled Modulus Columns (CMC) was selected.

CMC columns were installed at nine different

locations adjacent to the bridge abutments (Figure 1).

Figure 2 shows the location of CMC. A typical CMC

treatment area is 60m by 30m. The base width of the

embankment is about 60m and the treatment covers a

distance of 30m from the bridge abutment. The

embankment height varies from 6m to 11m.

1.2 Ground Conditions

For each treatment area, pre-treatment site

investigation was carried out to ascertain the ground

conditions. Two numbers of deep boreholes with

standard penetration tests (SPT) and one number of

in-situ cone penetration test (CPT) were carried out.

Figure 3 shows the typical SPT N-values and cone

resistance (qc) values.

International Conference on Geotechnics

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Generally, the ground condition can be described as

an upper layer of 6m thick soft alluvium (NSPT 5) of

grey colour low plasticity marine origin overlying

firm clay layer (average NSPT = 7) to depth of 18m.

Below, a layer of hard clay (average NSPT = 20) to

depth of 28m. Following layer is back to firm silt

layer again (average Nspt = 7) to maximum drilling

depth borehole of 40m.

Figure 1 Project location and ground improvement areas where CMC columns are installed

Figure 2 Typical cross-section of CMC treatment area.

To Pemalang

CMC diameter 0.42m, length 15m

to 24m, Spacing 1.9m to 2.2m

Pile length up to 50m

To Batang

Pekalongan City

Project location


3

Figure 3. Typical SPT N-values and cone resistance

(qc) values.

1.3 Performance Specifications

The performance specifications are as follow:

(1) The maximum allowable residual settlement shall

be less than 100mm after the 10 years;

(2) The factor of safety against slope failure shall be

not less than 1.5.

The traffic loading shall be 15 kPa.

2 CHOICE OF GROUND IMPROVEMENT

TECHNIQUES

The choice of suitable ground improvement

techniques is governed by environmental constraints,

a tight construction schedule and the soft ground

conditions. Since the bridge abutments have already

been constructed before commencement of any

ground improvement work, the choice of ground

improvement technique is limited to techniques that

exhibit minimum vibration and minimum lateral soil

movement during works to avoid potential damage to

the completed bridge abutments and the foundations.

Vibro stone columns or any casing driven granular

columns will cause excessive ground vibration during

installation works. Also, due to the low shear strength

of the underlying soft soils, there will be excessive

column bulging and possible column failure during

loading. Vertical drains and surcharging requires

sufficiently long consolidation time and time for stage

loading of the embankment and surcharge fill

construction. With time constraint, this solution is not

feasible even with close spacing of vertical drains and

with the addition of reinforcement geotextile at the

base of the embankment. The rapid loading placed on

the soft soils below will cause excessive lateral

movement of the underlying soft soil deposit during

placement of embankment and surcharge fill which

may cause deflection of the installed piles supporting

the abutments.

Based on all the above constraints, an environmental

friendly solution of ground reinforcement using

Controlled Modulus Columns (CMC) was considered

most suitable. The CMC columns are installed by a

non-vibratory soil displacement augering process. The

columns are cement-grouted columns and hence, have

no column bulging problem and they are having

higher load bearing capacity than any other granular

columns. The columns are 42cm in diameter with a

cement grout compressive strength of 20MPa. The

column spacing varies from 1.9m to 2.2m square grid

subject to the embankment height.

3 CONTROLLED MODULUS COLUMNS

3.1 Concept of CMC system

The components in a CMC system consist of a load

transfer platform (LTP) of 1.0m thick compacted sand

or gravel to facilitate the transfer of fill load on to the

columns uniformly. Two layers of reinforced wire

mesh are placed inside the LTP layer to provide

traction reinforcement. Cylindrical vertical grout

columns (or also known as inclusions) are installed

below the LTP using displacement auger.

The process of load sharing mechanism in CMC is

illustrated in Figure 4. Since the ratio of stiffness

between CMC and the soil is between 1:1,000 to

1:10,000 it is necessary to consider the vertical

deformation separately for the CMC and the soil. The

deformation of a point inside the CMC at a given

initial depth is different from an adjacent point at the

same depth in the soil. In other words, there exists a

different field of deformation between the CMC and

the surrounding soil as explained below:

Stage 1: Due to the transfer of imposed stress

to the soil (soil) through the load distribution

layer (sand blanket), vertical deformation

(settlement) of the soil (soil) occurs due to

consolidation.

Stage 2: As a result of consolidation

settlement, stress is transferred from the

5 4

12 4

8 9

6 7

10 5

8 23

19 21

16 31

25 28

31 13

8 5 5

8 9

7

0

5

10

15

20

25

30

35

40

45

0 10 20 30 40 50 60

Dep

th (

m)

Nspt

0 4 8 12 16 20 24

qc (MPa)

at 0m to 18m

qc<1 MPa


4

surrounding soil to the CMC. The

deformation at the same given depth (except

at neutral plane) in the soil (soil) is different

from the CMC (CMC) due to different

stiffness (ECMC > Esoil) and that soil > CMC,

negative skin friction is developed in the

CMC.

Stage 3: At greater depth, the point

deformation CMC > soil resulting in a stress

transfer from the CMC back to the competent

soil. This induces positive skin friction and

base resistance.

Stage 4: Overall, an equilibrium state of load

distribution is achieved where the tip

resistance, friction resistance and soil

resistance is equals to the total load.

Figure 5 shows the locations of the neutral plane

where point deformation of CMC and soil is the same.

At this location, the CMC column carries the

maximum stress.

Figure 4. Design concept for CMC

Figure 5. Graphs of vertical displacement, shear

stresses and vertical stresses

3.2 CMC Design

Numerical analysis using Plaxis 2D was carried out to

estimate the deformation and slope stability. The 2D

analysis was checked against 3D model. The results

show minimum difference.

Axisymmetric 2D model with long-term stiffness

material is used to determine maximum vertical

settlement and 2D plane strain model with short-term

stiffness is used to excess slope stability.

The analysis is carried out using three different

models that is, a) drained and undrained axisymmetric

b) single plane strain d) full plane strain.

Results of SPT and CPT tests were used for the soil

properties. They are compared with the laboratory test

results.

3.2.1 Long-term settlement

For long term settlement, axisymmetry model was

used.

CMC was modeled as a soil volume and inside the

CMC a “dummy” plate was assigned. In doing so, the

result of axial and shear forces on the CMC can be

accessed.

In the field, CMCs were installed in square grid

pattern while in the axisymmetry model, it is circular.

Hence, necessary correction is made based on area

ratio. The result of stress inside the CMC and stress at

CMC head were extracted and compared with plane

strain model. To be conservative strength increase due

to installation effect to the adjacent soil is not taken

into the design. Maximum stress inside the CMC

occurs at the neutral plane, and this value was used to

determine the compressive strength of the cement

grout. Typical example of settlement obtained from an

axisymmetric model is shown in Figure 6.


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Figure 6. Vertical displacement result CMC axisymmetric

model

3.2.2 Short term for stability

Step 1: Model Single plane strain

CMC was modeled as embedded beam row (EBR).

EBR is not fixed or attached to the soil which allows

the soil to flow through. EBR can carry axial force

and bending moment value unlike soil volume. The

top and bottom of the CMC was set to be free to

move. The shaft resistance value was pre-defined

based on initial soil stress.

𝜏𝑠 = 2𝜋𝑟𝑡𝑚𝑎𝑥(𝑧) (1)

where s is shaft resistance, r is the column radius, tmax

is shear resistance

𝑡𝑚𝑎𝑥 = 𝑅𝑖𝑛𝑡𝑒𝑟 [𝑐′+𝜎′(𝑧) tan 𝜃′] (2)

Since the EBR is a single line it cannot capture load

transfer to the column surface to account for arching.

To simulate this, a fictitious CMC head was modeled

by extending the top of the CMC (Figure 7). At the

top tskin was pre-defined based on value obtained from

previous axisymmetric model. In EBR CMC spacing,

axial skin friction was modelled in an elastic-plastic

behaviour. The stress inside the CMC was then

compared with the axisymmetric model as shown in

Figure 8.

Figure 7. Single unit plane stain with fictive extension at

CMC head

Figure 8. Comparison stress to CMC in Axisymmetric and

Plain strain model

In this model the position of the neutral plane was

located at elevation -6.0mRL.

Step 2: Full plane strain model

After calibrating the single plane strain model with the

axisymmetric model, a full plain strain model can be

made with actual embankment shape. In this model

steel wire mesh and CMC with steel bar were

included. For the steel bar inside the CMC, a limit

plastic moment, Mp of 33 KN.m was assigned based

on steel bar size and the numbers of bars used. For

CMC without steel bar, a very low value of Mp was

used. Figure 9 belows a typical full plane strain

model.

-16.0

-14.0

-12.0

-10.0

-8.0

-6.0

-4.0

-2.0

0.0

2.0

4.0

- 2,000 4,000 6,000

Ele

vat

ion (

mR

L)

Stress Inside CMC

Axisymmetry Model

Plain Strain Model

neutral plane

CMC

CMC as plate

CMC as

Soil

Volume

Neutral plane

Fictious

CMC

CMC model

as embedded

beam row


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Figure 9. Full plane strain model

Due to the lesser load below embankment slope CMC

length is shorter than the centre. Due to the higher

tensile force at the embankment edge, steel

reinforcement bar was placed inside CMC at a certain

depth.

The stress distribution below the embankment is

gradually reduced with depth, taking an example at

10m depth, the stress reduces 30% and at 25m depth,

stress is reduced by more than half (Figure 10). Due to

this, CMC length at the edge is shorter and wider.

Figure 10. Stress distribution below embankment without

CMC

The results of a full plain strain analysis are shown in

Figure 11 to Figure 16

The results show maximum vertical and horizontal

settlement of 8.5cm and 3.0cm respectively (Figure

11)

Figure 11. Results of vertical deformation

Figure 12. Results of axial force inside CMC

Figure 13. Results of bending moment inside CMC

Fill

15 KPa

Two layer steel

wire mesh on LTP

CMC with steel bar inside CMC without steel bar inside

FH

𝑅𝑡;𝑇


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Figure 14. Axial force of the steel wire mesh

Figure 15. Slope displacement pattern

Figure 16. Factor of safety for slope stability

3.2.3 Steel wire mesh

Figure 9 also shows the load transfer platform layer

reinforced with steel wire mesh. The wire mesh

consists of transversal and longitudinal steel bars. The

longitudinal steel bars enable to “absorb” the lateral

forces caused by the active earth pressure of the

embankment. The longitudinal bars also serve to limit

lateral soil displacements and thus any lateral

displacement of the CMC. The transversal bars enable

to mobilize friction at the soil-wire mesh interface.

Wire mesh also enhances the efficiency of load

transfer distribution to the CMC thus will minimize

the arching effect. The wire mesh design shall meet

the requirement of the allowable wire mesh strength

greater than the lateral active force, (RtT > FH).

𝐹𝐻 =𝐾𝑎𝛾𝐻2

2 (3)

where Fh is the lateral active force, ka is the coefficient

of active earth pressure of the embankment, is the

unit weight of the embankment fill and H is the height

of the embankment at the crest.

And

𝑅𝑡;𝑇 = 𝑓𝑒

𝛽. 𝑛.

𝜋𝑑𝑦2

4 (4)

where fe is the steel yield stress, β is the reduction

factor for the wire mesh soil interaction (= 1.25), n is

the number of steel bars per meter width of mesh, d is

the diameter of steel bar in the longitudinal direction.

3.2.4 CMC with steel reinforcement bar

CMC without steel reinforcement is good in

compression but not in tension. When subject to

embankment loading CMC will experience both axial

and bending moment at the same time. Thus, it is

necessary to check the capacity of the CMC column in

resisting lateral load. In cases where the lateral force

is large, tensile reinforcement is needed in the CMC

columns. Steel reinforcement is incorporated into the

CMC columns. Excessive lateral force in CMC

columns is normally found near the toe of a high

embankment slope. Calculation for the resistance

capabilities of CMC on lateral loads can be done

according to BS EN 1992-1-1.2004 12.

a) Calculate the axial design load and design

moment.

𝑁𝑒𝑑 = 𝑁𝑝𝑙𝑥𝑠𝛾𝐺 (5)

𝑀𝑒𝑑 = 𝑀𝑝𝑙𝑥𝑠𝛾𝐺 (6)

where Ned and Med is the axial and bending moment

acting on the CMC respectively. Nplx and Mplx is the

axial and bending moment obtained from Plaxis

calculation results. S is the column spacing and G is

the factor of safety between 1.2 to 1.4.

b) Eccentricity e (Figure 17), with the following

equation

𝑒 =𝑁𝑒𝑑

𝑀𝑒𝑑 (7)


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Figure 17. Illustration of eccentrical load distance

c) Calculate resistance area (Aref) with following

equation (Figure 18)

𝐴𝑟𝑒𝑓 = 𝑅2(2𝜃 − 𝑠𝑖𝑛2𝜃) (8)

𝜃 = arccos (𝑒

𝑅)

where R is the radius of CMC

Figure 18. Illustration of resistance area (Aref)

d) Calculate axial resistance (Nrd) with the following

equation

𝑁𝑟𝑑 = 𝐴𝑟𝑒𝑓𝑓𝑐𝑑 (9)

where Aref is the area resistance and fcd is the grout

strength

e) Check if reinforcement is required: if Nrd>Ned,

reinforcement is not required. If Nrd<Ned,

reinforcement is required.

If reinforcement is required, then the CMC

reinforcement need to be designed using Med.

4 CONSTRUCTION OF CMC

CMC columns are constructed by soil displacement

using a displacement auger. During auger penetration

drilling, the lower screw section which has a conical

screw-bit shape with variable auger flight pitches

(Figure 19) will cut and loosen the soil and transport

the soil to the displacement body section. The

displacement body is a cylindrical shape with the

same diameter as the lower screw section which

prevents soil from passing through and thus, pushing

(or displacing) the soil towards the borehole wall. The

counter screw section above the displacement body

has opposite direction flight. Soil collapsed from the

above during drilling is brought downward to the

displacement body and pushed towards the borehole

wall. With this technique, there is minimum spoilt at

the ground surface.

Figure 19. Full displacement auger

When the drilling auger reaches the design depth,

grout is pumped through a flexible rubber hose and

through the hollow steam attached to the displacement

auger. The grout pumping pressure is monitored and

auger-lifting speed is controlled by the CMC rig

operator.

Electronic and mechanical sensors are fitted to the

CMC drilling rig to ensure a good column installation.

Figure 20 shows the on-board computer monitoring

system fitted with various sensors. A monitor display

is installed inside the cabin to display real-time

monitoring installation parameters. These parameters

include:

a) Depth of installation (m)

Counter

screw section

Displacement

body

Lower screw

section

End cap


9

b) Installation and extraction time (sec, min)

c) Penetration rate (m/hr)

d) Rotational torque (Bar)

e) Injection grouting pressure (Bar)

f) Auger lifting speed (m/hr)

g) Grout volume (m3)

h) Computed CMC column profile

Figure 20. Onboard computer monitoring system fitted to

the CMC rig

For a 12m length CMC column, it takes about 11

minutes that is, about 5 mins for drilling and about 6

mins for grouting. Shorter columns will take shorter

construction time. The average production is about

600 to 1,400 linear meters per 10-hour working day

per rig. The supply of cement grout and the condition

of the working platform greatly influence the

production rate.

Figure 21 shows a typical installation record. The

drilling rig was operated with torque 0 to 35 bar at

depth 3m and at constant maximum torque of 40bar to

15m. The penetration rate was constant at 400m/hr

from beginning till 13m and gradually drops to 7m/hr

at 15m depth. At 13m below, it was substantial

drilling resistance cause auger rotation decrease

substantially and reducing penetration rate.

This is consistence with the borehole result which

indicates at the first 3m is soft soil (NSPT 4), and

continue with firm soil do the depth 12m. At the depth

13m below, borehole indicate a layer of very dense

sand which make auger difficult to penetrate.

The CMC length is pre-determine from Nspt, it is

likely during installation the termination of CMC is

deeper or shallower than initial design. CMC auger

drilling is work by mean of penetration so it can give

verification to the in situ test. The actual CMC depth

is determine by live display drilling record by using

penetration rate and torque.

Before commencement of full production work, trial

installation of CMC columns is carried out to calibrate

the operation parameters. The optimum grouting

pressure and the ideal speed of auger retraction during

grout pumping are determined from the trial

installation. Fast auger retraction causes necking of

columns and slow auger retraction causes grout

blockage in the rubber hose. Adequate grouting

pressure recorded indicates the lateral resistance from

the surrounding soil and that the grout has filled the

entire augering and drilling volume.

To ensure flowability of the fresh grout during auger

retraction and grout pumping, high slump grout is

needed. To prevent blockage during auger retraction

each truck delivery is tested for slump value. In this

project slump value between 23± 2cm is adopted.

For the reinforced CMC columns, the reinforcement

steel bars were placed into the columns shortly after

completion of the grouting works.

Figure 21. Typical CMC drilling record


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5 PLATE LOADING TEST

Plate load tests (PLT) were carried to maximum load

of 75 ton or 110% of maximum stress induced in the

CMC column. The tests were carried out after 28

days. A total of nine PLT tests was carried out; one

for each area of treatment. The objectives of the

carrying out the tests were to obtain the load bearing

capacity of the columns; compare the PLT results with

those obtained from Plaxis analysis; and back

calculation of design values to refine further the

numerical analysis.

A typical PLT results is shown in Figure 22. PLT was

carried out for a 42cm diameter column and column

length of 20m. The column was installed through a

layer of soft to firm silt layer and terminated at hard

clay with Nspt = 20. Since the test column was not

tested to failure, Chin Method was used to determine

the ultimate load capacity. An ultimate load capacity

of 88 tons was obtained. The PLT test was modelled

using Plaxis with a soil stiffness modulus Ey equals to

750 Nspt and soil-CMC interface factor of 0.8. The

numerical analysis indicated an ultimate load capacity

of 83 tons. The difference between the two methods is

deemed acceptable.

Figure 22. Load -Settlement result from PLT

The bearing capacity is also compared alpha-cu

method and the result is summarised in Table 1.The

alpha -cu give very close to PLT value.

Table 1. Bearing capacity comparison

Plate

Load

test

FEM

Plaxis

alpha -

cu

Ultimate bearing capacity

(ton) 87.7 83.3 81.0

6 CONCLUSION

Presented numerical modelling gives the possibility to

correctly design 3D problem using 2D plane strain

model. The result of the back-calculation PLT using

FEM gives a gain in confidence to the predict CMC

bearing capacity. Even though no settlement

instrument installed, observation shows no indication

of the differential settlement, soil heaving near the

slope toe and instability when embankment rises

rapidly. This project has demonstrated the successful

application of CMC column to treat thick layer

compressible soil and high fill embankment close to

the newly constructed bridge abutment within given

performance specification. The project also

successfully completed within given time frame and

budget.

7 BIBLIOGRAPHY

ASIRI. (2011). Soil improvement with rigid

inclusions. Paris: IREX.

Chin, F. (1970). Estimation of the Ultimate Load of

Piles from Tests not Carried to Failure.

Southeast Asian Geotechnical Conference.

Chin, F. (1983). Bilateral plate bearing tests.

Proceedings of the International Symposium

on in situ, (pp. 29-33). Paris.

Combarieu, O. (1988). Amélioration des sols par

inclusions rigides verticales application à

l'édification de remblais sur sols médiocres.

Revue Française de Géotechnique, (pp. 44,

57-79).

Plomteux, C. a. (2000). Embankment construction on

extremely soft soils using. Proceedings of the

16th Southeast Asian Geotechnical

Conference. Kuala Lumpur.

Yee, K. (2012). Controlled Modulus Columns (CMC):

A New Trend in Ground Improvement and

Potential Applications to Indonesian Soils.

ISSMGE Technical Committee TC 211

International Symposium on Ground

Improvement. Brussels, Belgium: ISSMGE.

Zhao, R. F. (1982). Estimation par les paramètres

pressiométriques de l'enfoncement sous

charge axiale de pieux forés dans des sols

fins. Bulletin Laison Laboratoire Central des

Ponts et Chaussees, (pp. 119, 17-24).

0

5

10

15

20

25

30

35

0 20 40 60 80 100

hea

d d

isp

lace

men

t (m

m)

Load (tons)

FEM

PLT

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