optimum design for highway embankment … · the highway embankment with stone column modeling with...

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International Journal of Civil Engineering and Technology (IJCIET) Volume 8, Issue 1, January 2017, pp. 863–872 Article ID: IJCIET_08_01_102

Available online at http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=8&IType=1

ISSN Print: 0976-6308 and ISSN Online: 0976-6316

© IAEME Publication Scopus Indexed

OPTIMUM DESIGN FOR HIGHWAY EMBANKMENT

WITH STONE COLUMN

Abdul-Hassan K. Al-Shukur

PhD., Professor, Department of Civil Engineering, College of Engineering,

University of Babylon, Babylon, Iraq

Huda Nihad Al-Khafajy

M.Sc. Postgraduate Student, Department of Civil Engineering, College of Engineering,

University of Babylon, Babylon, Iraq

ABSTRAT

In this paper discusses how to design the highway embankment with an optimum process to

get a minimum area of the highway embankment to reduce the cost of construction, and the

problems of soft clay soil in southern of Iraq when construction highway embankment as low

bearing capacity and excessive settlement and the way to treat it.At beginning a model of the

high way embankment without improving of soft clay soil for height of highway embankments

(H=2m and H=3m) was built to note the problems which will be faced when construction of

highway embankment in the future. When the height of embankment is (H=2m) the excessive

settlement appears but when the height of embankment is (H=3m) the low bearing capacity as

well as the excessive settlement will appear. To avoid these problems, the soft clay soil will be

improved by using stone columns and design the stone columns also with optimum process to

get minimum area of stone columns that can carry the applied load without any problem like

low bearing capacity or excessive settlement with lowest cost. When the stone columns are

used to improve the soft clay soil, it can note reduce in settlement by (99%) for height of

highway embankment (H=2m), and increase in bearing capacity to (15%) for height of

highway embankment (H=3m) for certain diameter as minimum increase can carry the load

applied on foundation. The highway embankment with stone column modeling with ANSYS

software program and this program very useful to help to find optimum design by optimization

tool, and use geo slope program to find slope stability for highway embankment by Bishop’s

method.

Key words: stone columns, highway embankment, soft clay, and diameter.

Cite this Article Abdul-Hassan K. Al-Shukur and Huda Nihad Al-Khafajy, Optimum Design

for Highway Embankment with Stone Column. International Journal of Civil Engineering and

Technology, 8(1), 2017, pp. 863–872.

http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=8&IType=1

Optimum Design for Highway Embankment with Stone Column


1. INTRODUCTION

The purpose of optimization is to find the best possible solution among many potential solutions

satisfying the chosen criteria. Designers often base their designs on the minimum cost as an objective,

taking into account mainly the costs of the structure itself, safety, and serviceability.

An embankment is an artificial barrier that typically is used to hold back water or to support a

roadway, railway, or canal. The material use in embankment can be naturel like (sand, rock….etc.) or

manmade, in general any material can be use but non organic material. The highway embankment

consists of soil that is placed on the embankment foundation in order to raise the level of the roadway

to the correct grade and to provide adequate support for the layers of base and surface courses that

make up the roadway. Since the embankment foundation and embankment provide the basic support

of the entire roadway, it is very important that they be properly constructed in accordance with all

plans and specifications. Grades, slopes, density, and all other controls must be accurately monitored.

The highway embankment founded on soft clay soil. Soft clays represent a well-known category of

problematic soils which are generally encountered under the form of deposited layers in coastal areas.

Several problems are faced when dealing with the study of soft clays from field investigation, soil

characterization, behavior modeling, and stability of geotechnical structures up to ground

improvement solutions. Studying the behavior of soft clays especially requires a thorough

determination of their geotechnical parameters. In Iraq, the problems of this soil taken space from the

attention of geologists and civil engineers, because about 35% of the Iraqi clay soils (especially

southern Iraq) are weak[1,2]. So, necessity to improve soil properties for road building has resulted in

the use of various stabilizers. Ground improvement is normally understood as the modification of the

existing physical properties of the ground beneath a site to sufficient depth to enable effective,

economic, and safe permanent or temporary construction in practical timescales [3]. Typical objectives

would be one or a combination of the following:

1. Improve bearing capacity by increasing in shear strength or density.

2. Minimize total or differential settlements of buildings or structures by reduction in compressibility.

3. Minimize flow of ground water to prevent inundation or water damage by reduction in permeability

4. Prevention of liquefaction or reduction in lateral spreading beneath or near both new and existing

structures during earthquakes, employing densification, replacement with stronger materials, or deep

drainage.

5. Conversely, an improvement in deep drainage in order to assist preloading or surcharge techniques.

6. Controlled displacement of the ground in order to dispel previous differential settlements or ground

2. FORMULATION OF OPTIMIZATION PROCESS

2.1. The Objective Function

The objective function for embankment is based on minimum cost for this purpose the area of

embankment should be minimize. It can be formulated as follow:

Min. A = H*(b + H/m) + ��

Where:

H: embankment height (m)

b : top width of embankment (m)

m:slope of embankment

D: diameter of stone column (m)

Abdul-Hassan K. Al-Shukur and Huda Nihad Al-Khafajy


2.2. The Constraints

The objective function is minimized subject to a set of constraints. Some of these constraints can be

explicitly defined in terms of the design variables (i.e., side constraints), such as dimensions (m: must

be ≤ ½) [4],(0.5m ≤ D ≤ 1.2m) [5]), while others are implicitly related to design variables.

2.2.1. Bearing Capacity

Qu = σv∗��∗�

.�…… (Qu˃Qall)(1)

σv = σrLKpcol(2)

σrL= pp= γ*kp*z + 2*cu* �� (3)

Where :( σv) limiting axil stress in the column when it approaches shear failure due to bulging

,(σrL) limiting radial stress,(Kpcol)passive earth pressure ( Kp= tan2(45+��), (pp ) passive pressure,(γ)

effective unit weight of soil within the in influence zone,(z) average bulge depth (2 time the column

diameter ),(cu )cohesion of soft clay,(kp )passive pressure coefficient of soft

clay(kp=tan2(45+�/2),where � angle of friction of soft clay) and (1.5) factor of safety [5].

2.2.2. Slope Stability

F=

∑��∑[{�� + tan�′} �&'�

()*+, )*+-.

/

] (4)

Where :(F) Factor of slope stability (for embankment must be ≥ 1.5), (W)weight of each slice, (cꞌ)

cohesion of soil, (b) width of each slice, and (�ꞌ) angle of friction [6].

2.2.3. Settlement

After construction the stone column and at the center of building the settlement for each layer i is

expressed as follows:

S=12∗34

56∗7�89({:;56)∗7<869} (5)

Where: (ai) the replacement ratio for layer i (ai===>

,A: total area within the unit cell ,Ac: area of

stone column),(Ecol)young modulus of stone column, (Esoil) young modulus of soft clay, (σt)average of

vertical stress exerted by building and (hi) layer thickness.

The total settlement (∑S) should remain below the values set by the operation condition.

2.2.4. Maximum Allowable Stress for Stone Column

It is need to determine the vertical stress-rupture point (qr) for an isolated column at first to calculate

the maximum allowable stress, based on characteristic of column and the soil after treatment and

according to possible mode of failure[7].

Bulging failure

qre = σrL*kpcol (6)

• (σrL) calculate as equation (3) above

Punching effect shear failure

The vertical shear within the column is most intense at top and decreases as it moves down.

qrp = 9 ∗ @A + B>:2 ∗DEF�− H>) (7)

where :

γc: unit weight of stone column material



Lc:length of column

Rc: average radius of the column

• Vertical failure stress (qr) within the column equal to :

qr = min(qre , qrp , 1.6 Mpa).At the service limit state (SLS), the allowable vertical stress (qaSLS) within

the column is obtained by applying safety factor of 2 to the vertical failure stress (qr) :

• Value of stress within column at the layer i (σci) can be expressed as :

σci=7�89∗34

56∗7�89(�:;56�∗7<869% (8)

• The stresses should remain below maximum allowable values (σci˂ qaSLS)

3. PLOT MODEL BY ANSYS

For design optimization model in ANSYS should be built the model to get optimum shape and

dimensions for embankment, this built include drawing shape define material, element used with

meshing for finite element process, boundary condition and load applied.

3.1. Drawing Model

It suggested symmetrical model, is drawn after defining the initial values of variable which draw the

initial shape of embankment and stone column and then hole model configuration as shown in

figure(1).

Figure 1 Model of Highway Embankment with Stone Column

3.2. Define Properties of Material

The properties of material used in optimum design of highway embankment with stone column as

shown in table below:

Table 1 Shown Properties of Material Used in Design

material Yong modulus(E)

(Mpa)

Poisson’s

ratio(ϒ)

Density (γ)

(KN/m2)

Cohesion (cu)

(KN/m2) �

Embankment 80 0.3 20 5 30

Soft clay 2.4 0.4 17.7 15.8 11

Sand clay 50 0.3 18.3 5 25

subbase 400 0.35 22 ------ ---

asphalt 5000 0.35 23.5 ------ ---

Stone column 60 0.3 19 0 44



The properties of soft clay material were taken nearby Korek company Building Al-Hilla City.

Figure 2 Shown Korek Company Building Al-HillaCity[8]

3.3. Chose Element

PLANE 82 is used for 2-D modeling of solid structures.PLANE82 is a higher order version of the 2-D,

four-node element (PLANE42). It provides more accurate results for mixed (quadrilateral-triangular)

automatic meshes and can tolerate irregular shapes without as much loss of accuracy. The 8-node

elements have compatible displacement shapes and are well suited to model curved boundaries. The 8-

node element is defined by eight nodes having two degrees of freedom at each node: translations in the

nodal x and y directions. The element may be used as a plane element or as an ax symmetric element.

The element has plasticity, creep, swelling, stress stiffening, large deflection, and large strain

capabilities. Various printout options are also available.

Figure 3 Plane 82 Geometry

3.4. Boundary Condition

The foundation of embankment model is fixed at bottom and sides in which no translation permissible

in X and y directions as shown in figure (4).

Figure 4 Shown the Mesh and Boundary Condition



3.5. Applied Load

The surcharge load (traffic load) applied at the top of embankment model as pressure (uniform

distribution pressure) as shown in figure (5).

Figure 5 The Distribution of Load

4. RESULTS AND DISCUSSION

4.1. Verification of Optimization Process

In this section it will be made verification between optimum design process and ordinary design of

highway embankment and notes the different between two methods of design by shown the reduction

in variable design when use optimum process which lead to minimize the cost of constriction.

The following case study in Hampton, Virginia in interstate route 664 with interstate route 64

involved numerous highway embankment which designed by ordinary method and it will be designed

one of them by optimum method using ANSYS program.

Figure 6 General Location Plan



The table below shown properties of material and information about design of case study:

Table 2 Properties of Material

Input parameter Sample Unit Value

height H m 4

Surcharge qu kpa 11.5

Unit weight of highway embankment γe Kn/m3 20

Friction angle of highway embankment Φe …… 30

Unit weight of soft clay soil γc Kn/m3 13.3

Cohesive of soft clay soil Cu kpa 18

Friction angle of soft clay soil Φc …… 0

Young modulus of soft clay soil Eu kpa 2700

Unit weight of stone column γs Kn/m3 19.5

Friction angle of stone column Φs ……. 45

Spicing between columns S m 2.1

Young modulus of stone column Eu kpa 60*103

Length of stone column L m 7

For optimum purpose it will be get out limitation to get minimum area. When design the case

study above with optimum process by ANSYS program and comparison with ordinary design as

shown in the table below it can be noted the decreasing in diameter of stone column and increasing in

slope of highway embankment.

Table 3 Comparison Results Between Ordinary and Optimum Methods

Variable design Ordinary method Optimum method

m ½(V:H) 1/1.75(V:H)

D(m) 1.2 0.81

Area of embankment(m2) 76 72

Volume of stone column(m3) 8 3.6

The percentage of decreasing in diameter of stone column is (32.5%) and the percentage of

increasing in slope of highway embankment is (14.2%).From above equation it can noted the optimum

method is an effective method leads to optimum values of cost and safety factors compared with

Ordinary design.

4.2. Embankment without Improving Soil

At begging it was being shown the model of embankment without improving soil to know at any

factor of safety was failed as table (5.1).

Table 4 Initial and Infeasible Set for Model without Improving

* Initial Set

H

m

M

…….

Qu

Kn/m2

Qall

Kn/m2

Se

mm

Sp

mm

Ssec

mm

St

mm

Streq

mm

Fs

…..

A

m2

2 0.50 60 50.246 63.328 176.72

9 249.048˃ 50 4.475 30

3 0.50 60< 69.094 54.572

210.85 9 274.422˃ 50 2.33 51



In the table above which showed the initial set (initial value of design variable) and which factor

failed (cell in blue color), It can be notes in initial set the bearing capacity is less than the required

value and the total settlement is more than the required value (excessive settlement). In Infeasible set

the same problem because the soil is wake and cannot be carried the load without failed. At height of

embankment (H=2m) the bearing capacity of soft soil can carry the applied load but the problem still

with the excessive settlement, the settlement within the soil is very high. The reduction in settlementis

(80%)

That’s mean it needs (80%) reduction in settlement to reach for required settlement.

At height of embankment (H=3m) in addition to the problem of excessive settlement the average

of bearing capacity of soft clay is less than the allowable stress from embankment applied on soil as

shown in the table above that’s mean the soil cannot be able to carry the load applied on it. The

reduction in settlementis (81.8%), and the Increase in bearing capacity is (15.15%).That’s mean it

needs (15.15%) increasing in bearing capacity of soft clay to carry the applied load.

In this case its need to improve the soft clay to increase bearing capacity and reduce the excessive

settlement. In this thesis the stone columns will be used for this purpose as mentioned before.

4.3. Embankment with Improving Soil

Using stone columns to improve weak soil by increasing bearing capacity and decreasing the

excessive settlement, figure (7) shows the different in settlement between soil with and without

improvement for the same properties of embankment in table (5).

Figure 7 The Settlement With and Without Improving Soil

In figure above it can be noted the obvious decreasing in the settlement with improving soil of

certain diameter from stone columns for each height of embankment. The percentage in decreasing of

the settlement is (98.66%) (For height of embankment (H=3m))

The decreasing reaches (99%) for certain diameter, this decreasing in the settlement by stone

columns made soft clay within the acceptable limit.

without improving with improving

d=0.5 h=2 249.048 7.7469

d=0.55 h=3 274.422 7.9735

0

50

100

150

200

250

300

sett

lem

en

t(m

m)

Infeasible set

H

m

M

…….

Qu

Kn/m2

Qall

Kn/m2

Se

mm

Sp

mm

Ssec

mm

St

mm

Streq

mm

Fs

…..

A

m2

*2 0.43 60 50.246 63.615 177.05 9 249.665˃ 50 4.475 31.16

*3 0.43 60< 69.094 54.730 211.25 9

274.98 ˃ 50 2.53 53.6



The improvement in bearing capacity with stone columns shown in figure (8) below, this figure

shows the different in bearing capacity between the soil with and without improvement for the same

properties of embankment in table (5).

Figure (8) the Bearing Capacity With and Without Improving Soil

In figure above it can be noted the improvement in bearing capacity when using stone columns, the

percentage of increasing in bearing capacity depends on diameter of stone column; with increasing the

diameter of stone columns, the improvement in bearing capacity increases for the same height of

embankment. For minimum area of embankment and stone columns using steeper slope for

embankment and minimum diameter of stone columns which achieve the recommendation of design.

The percentage of increasing in bearing capacity after improving soil for height of embankment

(H=2m) with minimum diameter which archive the recommendation of design is (3.14%).In table (5)

it can be noted that the height of embankment (H=2m) doesn’t need to improving in bearing capacity

as refer before but need to reduce the excessive settlement then the minimum diameter of stone

columns (D=0.5m) use to improve soft clay to reduce settlement within acceptable limit. The

percentage of increasing in bearing capacity after improving soft clay soil for height of embankment

(H=3m) is (18.04%). From table (5) and for height of embankment (H=3m) its need to improve soft

clay soil to increase bearing capacity and reduce excessive settlement as refer before, then (D=0.55m)

it’s the minimum diameter of stone columns use for improving soft clay to archive the

recommendation of design.

At height of embankment (H=3m) it can notes the dominate factor in design it is bearing capacity

for this reason its need to increasing diameter of stone columns from (0.5m for (H=2m) to 0.55m for

(H=3m)) to improve bearing capacity until the foundation can carry the load applied on it.

5. CONCLUSIONS

• The ANSYS APDL is efficient tool to simulate highway embankment foundation interaction problem

and optimization process.

• The individual-accumulative optimization technique used in this researcher gives reasonable procedure

to carry out the factor of safety individually.

• The soft clay soil in southern of Iraq cannot carry the applied load and it is failed by bearing capacity

and excessive settlement for that it is necessary to improve it by suitable method before construction of

highway embankment to avoid failed it by bearing capacity and excessive settlement, in this thesis

improve soft clay soil by stone columns.

• The percentages of saving cost and minimize the diameter of stone column between ordinary and

optimum method is (32.5%) it is mean preferably used the optimum process.

without improving with improving

d=0.5 h=2 60 61.884

d=0.55 h=3 60 70.826

54

56

58

60

62

64

66

68

70

72

be

ari

ng

ca

pa

city

(kn

/m^

2)



• At height of highway embankment (H=2m) without improving soft clay soil the foundation failed by

excessive settlement and at height of highway embankment (H=3m) the foundation of soft clay failed

by bearing capacity addition to excessive settlement.

• For height of highway embankment (H=2m) without improving soft clay soil the excessive settlement

reach about (80%) above acceptable limit and For height of highway embankment (H=3m) without

improving soft clay soil the average bearing capacity of soft clay less than about (15%) from allowable

stress applied on foundation by highway embankment and traffic load.

• For height of highway embankment (H=2m) and after improving soft clay soil gat redaction in

settlement about (99%) for certain area replacement ratio (35%).The minimum diameter of stone

columns it need to improve bearing capacity for height of highway embankment (H=3m) is (D=0.55m)

and this diameter of stone columns it is gave about (18%) increase in bearing capacity of foundation.

REFERENCES

[1] Dr. Hussein H. Karim, Dr. Mohammad M. Mahmood & Raida G. Renka, “Soft Clay Soil

Improvement Using Stone Columns and Dynamic Compaction Techniques”, Eng. & Tech. Journal

,Vol.27, No.14,2009

[2] Shaker K.,E. 2013-2014, “Strength Properties of Soft Clays Stabilized With Saw Dust Ashes”,

Submitted to the Building & Construction Engineering Department University of Technology

[3] Klaus Kirsch and Alan Bell 2013, “Ground Improvement“ Third Edition CRC Press Taylor &

Francis Group

[4] Manual Geotechnical Design Standard–Minimum Requirements, State of Queensland (Department

of Transport and Main Roads, February 2015)

[5] Indian Standard design and construction for ground improvement guidelines (part 1 stone columns),

Soil and Foundation Engineering Sectional Committee, January 2003

[6] ShyamalK., B. B. Sc. (CE) M. Eng. (WRE) August 2009, “Highway Embankment Design in

Bangladesh”, School of Civil Engineering College of Engineering and Physical Science, the

University of Birmingham

[7] G. Billoet & J. R. Gauthey, “Recommendations for the design, calculation, construction and quality

control of stone columns under buildings and sensitive structures”, USG, the French geotechnical

union association Version No. 2 - March 16, 2011

[8] Awad A. 2016,“Geotechnical Maps for the Soil of Babylon Governorate Using Geographic

Information” the College of Engineering the University of Babylon

[9] Asst. Prof. Dr. Qassun S. Mohammed Shafiqu and Fadhl Abbas Ahmed Al-Assady. Analysis of

Embankment Supported by Stone Columns Encased with Geosynthetic Material. International

Journal of Civil Engineering and Technology, 6 (10), 2015, pp. 97-107.

[10] Hussein H. Karim, Zeena W. Samueel and Huda K. Karim, Iraqi Gypseous Soil Stabilized by

Ordinary and Encased Stone Columns. International Journal of Civil Engineering and Technology

(IJCIET), 7(6), 2016, pp.179–192.

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