the effect of soil improvement on foundation super structure design

International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print),

ISSN 0976 – 6316(Online) Volume 4, Issue 2, March - April (2013), © IAEME

258

THE EFFECT OF SOIL IMPROVEMENT ON FOUNDATION & SUPER

STRUCTURE DESIGN

Thulaseedharan V

1, Narayanan S.P

2

1(Department of Civil Engineering, Government Engineering College,

Thrissur, Kerala, India) 2 (Department of Civil Engineering, University Technology PETRONAS, Malaysia)

ABSTRACT

For a particular arrangement of superstructure, soil strength is the most important factor

affecting the design of its supporting raft and hence the cost of construction. Folding of raft

or folded plate foundations can be used to reduce the material consumption in a raft design.

The beneficial effects to foundation and superstructure design by the selective soil

improvement below a raft or folded plate foundation were studied in this paper with the help

of Winkler and continuum methods of soil modelling.

Key words: Raft foundation, folded plate foundation, Winkler model, coefficient of subgrade

reaction, continuum analysis, Mohr-Coulomb model, soil improvement.

1.0 INTRODUCTION

1.1 Overview

Raft foundations are provided in situations where soil is not that weak to need pile

foundations but isolated footings cannot be recommended due to the possible higher

differential settlements. The design of a raft depends on the stiffness of superstructure (SS),

column spacing, and projection of raft beyond the outer lines of boundary columns (PR), raft

thickness, soil stiffness, strength of concrete and yield strength of steel. Among the various

parameters listed above, strength of concrete is decided considering the site specific needs

and difficulties in achieving the maximum strength of concrete. Column spacing is decided

by the architectural needs. The SS and substructure are provided with sufficient sizes to give

a safe and economic structure. The soil related term is the least controllable among the above.

In the conventional method of raft design, a constant bearing pressure is assumed below the

foundation which is possible only in the case of very soft soils almost in a fluid state. When

analyzed using commercial software, the soil media may be represented by a system of

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ISSN 0976 – 6308 (Print)

ISSN 0976 – 6316(Online)

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259

identical but mutually independent, closely spaced, discrete and linearly elastic springs and

this method is based on Winkler’s hypothesis (1867). The elastic constant of the springs is

obtained from the modulus of subgrade reaction, ks and is defined as

ks = �

� (1)

Where q is the load per unit area (or contact pressure) and is the settlement under the

loaded area. The base pressure below a raft may vary from point to point depending on the

load and moment, rigidity of SS and type of soil. As load is applied, the raft settles and the

contact pressure under the raft is re-distributed depending upon the stiffness of foundation

and SS. The settlement of the raft also depends on factors such as the increase in stress-strain

Modulus (E) with depth of soil below raft and the consolidation of soil. Since ks is a variable

depending on the several factors listed above, its computation is very difficult. The common

methods used for the determination of ks are Plate load test, Triaxial Test, Consolidation Test,

CBR test and Empirical relations (Bowles, 1997). Plate load tests are conducted using small

plates of size varying from 30 cm to 76 cm. The stress increase in the soil due to loading on

the plate is felt over a small area whereas that under a raft influences a large area and hence

Terzhagi (1955) suggested an expression for correction for size effect of foundation. Bowles

(1997) suggested the ranges for ks for sandy soil as given in Table 1. Scott (1981) proposed

empirical expression connecting ks with standard penetration resistance (N) for sandy soils.

The simplifying assumptions of Winkler model itself cause some errors (Terzhagi, 1955).

The springs are neither elastic nor independent. The settlement due to the applied load at one

point in the raft is felt at the adjacent areas and hence a uniformly loaded raft may exhibit a

dish shaped settlement, unlike the uniform settlement predicted by Winkler (Coduto, 2001).

Hence efforts were made to couple the springs so that the effect of vertical load is transmitted

in the lateral direction also (Horvath, 2011). In continuum methods the soil media is

represented by 3D finite elements. However continuum analysis is time consuming and

getting the representative soil properties for assigning to the model is very difficult. In

contrast, the Winkler foundation is very simple and large numbers of software are available

based on this method, capable of analysis and design of rafts meeting different country

specific codes of practices. The difficulty in computing ks led ACI committee 336 (1988) to

recommend that the raft designs be carried out varying the value of ks over a range of one

half to 5 or 10 times the furnished value. The furnished value in a soil report is hereafter

called the designated value of ks.

Table 1 Values of ks for sandy soils. Table 2 Stress –Strain Modulus (Bowles, 1997)

Thulaseedharan and Narayanan (2013a) studied the impact of varying ks in the design

of raft foundations. The maximum settlement and the maximum values of top and bottom

bending moments (BM) in a raft generally reduce as ks increases. The comparisons of

Type of Soil ks in kN/m2/m

Loose Sand 4800-16000

Medium Dense Sand 9600-80000

Dense Sand 64000-128000

Clayey Medium Dense

Sand

32000-80000

Type of soil E in kN/m2

Sand-Silty 5000 to 20000

Sand-Loose 10000 to 25000

Sand Dense 50000 to 81000



260

Winkler and Continuum methods showed slightly lower maximum top moments and more

maximum bottom moments in the later method. It was also concluded that ks based methods

are sufficient for design purpose of flat rafts. For unsymmetrical rafts the design for a range

of ks varying from half to four times the reported value for site resulted in an increase of 2 to

2.5 % in the total reinforcement required. This was further validated using continuum

method. For symmetrical rafts, the increase in reinforcement was around 1% at bottom with

slight reduction in top reinforcement. Designing the raft for such a range of ks or E values

gives a conservative design at a small additional expense. The raft models incorporating

superstructure showed lower settlement values compared to the models without

superstructure implying that the superstructure stiffness helps to re-distribute base pressure.

PR is a possible variable which may be restricted depending on the site conditions.

Calculations were carried out on rafts with varying PR from zero to 1.5 m. The settlement

pattern changes with PR and a dish like settlement (Central part of raft settling more) was

observed for nearly 1.5 m PR in Winkler method. With continuum modeling using Plaxis 3D,

a dish shaped settlement of raft was observed for low values of PR of nearly 0.25 m. As soil

strength is reduced, the settlement pattern changed with more deflection at the edges and

corners of the raft for the same PR. An increase in PR reduces the maximum top moments in

the raft- which usually occurs in the outer-spans of raft. The effect of top moments in a raft is

felt over a large area and hence requires reinforcement to be given for a greater area.

Generally the design bending moments at bottom at the face of columns are much more than

the top moments. However the bottom moments reduces to minimum value within a small

area around the load transfer area of column and rest of the raft in the bottom portion is given

only nominal reinforcement for satisfying crack width requirements. In general the settlement

computations by Winkler method are not comparable to continuum methods, especially when

the soil is soft. Hassan (2011) studied the variation of raft deflection with ks at various

locations in a raft and the influence of column spacing and raft thickness on settlement of

raft. In the current research, column positions are fixed and raft thickness is to be reduced as

much as possible. Hence these aspects are not given much importance. Gupta (1997)

compared the analysis results of rafts using conventional method and Winkler foundations

and concluded that the former is generally on the conservative side. He also reported that

bending moment in a raft may vary several times depending upon the raft size and soil

properties under the raft. This variation increases further as the deviation from symmetry of

the shape or loading of the raft increases. However a variation from 10 kNm to 70 kNm in

Bending Moment (BM) in a raft is an increase by 7 times which may not require an increase

in member size or reinforcement. We are more interested in finding out the change required

in the size of member or area of steel.

Folded plates are widely used in SS for spanning large areas. Due to its folding,

bending moments are reduced, which reduces the required concrete and reinforcement.

Folding is done in straight lines and form work can be placed very easily. In foundations, if

the folding is done in such a way that steep slopes are not provided, then form work can be

avoided. The construction of folded rafts are easy compared to beam and slab rafts and the

additional space created at the basement level by folding a raft can be used for storage of

water or for using as cable trenches. Hanna and Rahman (1990) investigated on the

geotechnical aspects of triangular strip footings and concluded that there is 40 % increase in

bearing capacity when such structures are used as foundations with consequent reduction in

settlement. Thulaseedharan and Narayanan (2013b), compared raft and folded plate

foundations, giving importance to material savings that could achived by using the later. The



261

design was arrived after conducting analysis for a wide range of soil properties in order to get

a conservative design of both raft and folded plate foundation. Nearly 40% savings in

concrete, 30 % savings in steel and overall 35% savings in cost of construction could be

achieved by folding the raft. Between folded and flat rafts, folded raft structure requires

slightly lower reinforcement for columns and beams. In continuum method, the settlements

under corner and edge columns were more in folded rafts compared to flat rafts, especially

when the raft projections were low. This is found to be due to the heavy lateral loads at those

supports and due to which folded raft was deflecting in the fold direction. This increases the

reinforcement in the outer spans of folded raft and in the corner and edge columns at the

ground floor of the building. BM in the outer beams of ground floor also showed some

increase compared to flat raft. As E of soil increases, the BM in raft, column reinforcement

and BM in beams reduces. The increase in PR brings significant changes to top moment

values in the raft. Column and beam reinforcement were slightly reduced as PR increases. It

also reduces reinforcement required and settlement of both flat and folded rafts. In general

Winkler method is sufficient for the design of flat rafts and in the case of folded plates, lateral

stiffness of soil need to be considered in the analysis. The column designs obtained by giving

fixed supports to the columns and sometimes even that obtained from Winkler method may

not give a conservative design pointing the need for elaborate soil structure interaction

studies including continuum analysis for important structures. The designs of folded plates

are affected by the central rise or fall of fold.(In foundations, the folded part is going below

ground level and hence the term fall is used). With increase in the fall, reinforcement required

can be further reduced. As the fall of the folded raft increases, settlement reduces due to the

increase in stiffness of foundation. Reinforcement required in folded raft also reduces.

Similarly column and beam reinforcement in the superstructure was slightly lower compared

to one with less fold height.

1.2 Objectives of study The present paper compares the performance of folded plate and raft foundation under

identical SS stiffness and loading when soil strength parameters are varied below it at

different locations. The positions of maximum settlement in a raft were identified using

Winkler and continuum methods and soil strength was then increased at those places.

Numerically, this strengthening was accounted with increase in ks and E values. Winkler

method is very popular and easy to use for soil modeling and its inherent disadvantages were

taken care in this study with the help of continuum methods. There is uncertainty in the

determination of ks and E values and hence the impact of its variation over a range in

foundation and SS design were also studied.

1.3 Importance of the study The increased soil stiffness reduces settlement of raft and with several other benefits

to substructure and superstructure. The localized improvement in soil strength (LISS) can

reduce differential settlements along with considerable reduction in material requirements of

steel and concrete. Folded plate foundations are provided to reduce material consumption and

localized strengthening of soil may further improve the advantages.



262

2.0 MODELING AND STRUCTURAL IDEALIZATION

2.1 Winkler and continuum modeling: StaadPro (2008) and SAFE (2009) were used to model the Winkler Foundation. The

ks of the proposed site is given as 25000 kN/m2/m and this value is hereafter referred to as

designated ks. The analysis was carried out with and without the SS. The SS was modeled

using line and plate elements and the substructure with quadrilateral plates. Several analyses

were carried out taking typical situations. All designs were carried out using BS 8110 and

BS 8007. Continuum modeling was carried out using Abaqus (2011), Plaxis 3D (2004) and

StaadPro (2008) . Solid elements were used to model the soil mass. In Abaqus and Plaxis,

interface option is possible, where as in StaadPro, rough contact is assumed. In StaadPro

modelling, only E is varied. Mohr- Coulomb model was used for the study purpose in Plaxis

and Abaqus as the present comparison is made for a site containing sandy strata. Mohr-

Coulomb model in Plaxis requires 5 parameters as input namely the Cohesion (C), the angle

of Internal Friction ( θ), the Modulus of Elasticity (E), the dilatancy and the Poisson’s ratio.

Here the exact evaluation of E is very difficult and hence a range of values are taken

consistent with the known soil properties. In this study the E values were varied from 15000

to 60000 kN/m2 and from 30 to 43 degrees. A very small value of cohesion is given to aid

computation as recommended in Plaxis 3D foundation user manual (2004). The

recommended range of variation of E values is given in Table 2 (Bowles, 1997). Dilatancy

value is given as zero and the Poisson’s ratio was varied from 0.35 to 0.4. Abaqus was used

only for comparing the results obtained from the structural software Staad for the folded

plate.The comparison of designs were done between flat slab raft and folded raft. The rafts

were analyzed for 43 service load combinations and 53 ultimate load combinations. M40

concrete and Steel of grade 460 was used. The comparison was made on a raft consisting of 4

equal spans of 8 m in the X direction and 7m in the Y direction. Two projections of raft in X

direction, 0.3m and 1m were considered in the present studies. Raft projection in the Y

direction is kept at 0.3 m. A 3D view of the folded raft is given in Fig. 1. Folding is

introduced in the X direction in such a way that the inclination of the surface is less than 32

degrees. The building is seven storied with column sizes of 600x600 mm. Seismic forces are

generated as per UBC (1997) for zone 2A and soil classification Sc as per the site

requirements.

3.0 ANALYSIS AND DESIGN RESULTS

The flat and folded raft models were analyzed using Winkler soil model with 3 values

of ks ( 12500, 25000 50000 kN/m2/m). The continuum analysis was carried out in two steps.

In one step, E is the only variable with values of 15000, 30000 and 60000 kN/ m2. In the

second case, E and were varied for analysis with Mohr-Coulomb model. To study the

effect of soil improvement, ks or E values are varied at select locations below the raft; the

increase was 4 times in ks or E values compared to the adjacent area. The impact of the same

on structural design was worked out. Analysis and design results are given in eight sub-parts.

Variation of settlement below raft, base pressure, BM in rafts, Shear and Impact of raft

projections are covered in the first five parts. Part 6, contains the design of raft and folded

plates. Part 7 deals with the impact of soil stiffness on SS design and the eighth part is a

comparison of material and cost savings. In each section, a comparison on the performance of

flat and folded plate foundations are given for both Winkler and continuum modeling.



263

Fig.1 View of the raft folded in X- direction.

3.1 Comparison of settlement The maximum settlement of the foundation was reduced as ks or E increased in both

Winkler and continuum methods. When PR is considerable, the raft may settle in a dish

shape with more deflection at centre. With LISS, the settlement pattern is changing in

addition to reduction in settlement. Without any increase in PR, the location of maximum

settlement of raft shifts to centre from the edges.



264

Fig.2 Variation of maximum settlement below flat and folded rafts in continuum method with

change in E for LISS

In Fig 2, the maximum settlement is more below folded raft in continuum method compared

to flat raft. This is due to the very high lateral forces applied and consequent lateral

displacement at the supports of corner and edge columns in X direction (Fig.1). The LISS

changes the settlement pattern of the folded raft. Even for low PR values, dish like settlement

pattern is obtained. The maximum settlement is reduced as shown in fig.2.

3.2. Variation of base pressure below the raft and the folded plates Base pressure below the raft was more under the column load transfer area. For

smaller PR values, the maximum base pressure occurred under corner and edge columns. As

PR increases, the area of raft increases and maximum base pressure below the raft is reduced

significantly. At interior columns, the base pressure under column load transfer point and

adjacent areas were almost uniform for low values of ks. As ks increases, the difference

between maximum and minimum base pressure adjacent to the load transfer area also

increased. The increase in ks leads to load transfer through a small concentrated area below

load point. The same trend was observed below the folded rafts with much less variation

between the maximum and minimum base pressure anywhere. This may be due to the higher

stiffness of folded raft re-distributing base pressure. Continuum modeling also gave similar

results for both flat and folded rafts. LISS results in very high base pressure at the soil

improved area and the values go on reducing as the ks or E in the central area increases.

3.3 Variation of maximum design moments

The maximum value of bottom moments at the face of columns in the raft is taken as

the design bottom moment in a raft. As ks or E increases, the bottom and top moments are

reduced (Fig. 3a and 3b). With LISS, for the flat raft, considerable reduction in maximum

top moment occurs along with increase in bottom moments for low E values. For folded rafts

with LISS at the boundary area, maximum top moments were slightly increased in the outer

most fold portion compared to the case without LISS as shown in fig.3 (a). Continuum

methods showed a reduction of 67% for the maximum top moment in X-direction compared

to flat raft. Bottom moment (Mx) was reduced by 50%.

0

2

4

6

8

10

12

14

16

18

20

0 10000 20000 30000 40000 50000 60000 70000

Se

ttle

me

nt

in m

m

Young's Modulus E in kN/m2

Flat Raft

Folded Raft

Flat Raft with Soil

improvement

Folded Raft with

Soil improvement



265

Fig.3 a and 3b Variation of top and bottom maximum BM in folded and flat rafts using

continuum methods

The top bending moment Mx in the fold portion was reduced by 80% and goes on

reducing with increase in fall of the folded portion. BM in y direction is concentrated in a

narrow width of folded raft supporting the columns and for the rest of the area BM was

reduced by 90%.The increase in fall of the folded portion increases the stiffness and reduces

settlement and which in turn reduces BM and hence the reinforcement needed in a raft.

3.4 Influence of shear The thickness of the folded raft is increased for a small cross-section supporting the

columns as shown in Fig. 1. No other special care was required in comparison to the flat rafts.

3.5. Impact of projections of the raft (PR) beyond the outer line of boundary columns By providing projections to the raft, some other advantages were also observed. There

was a reduction in the total and differential settlements with change in deflection pattern and

reduction in reinforcement in the substructure and SS. The studies on folded rafts with

continuum model also gave similar results. If PR could be increased, then LISS is not

0

100

200

300

400

500

600

700

800

900

0 10000 20000 30000 40000 50000 60000 70000

To

p m

om

en

t in

kN

m


Flat Raft

Folded Raft

Flat Raft with Soil

improvement

Folded Raft with Soil

improvement

0

200

400

600

800

1000

1200

1400

0 10000 20000 30000 40000 50000 60000 70000

Bo

tto

m m

om

en

t in

kN

m


Flat Raft

Folded Raft

Flat Raft with

Soil

improvementFolded Raft

with Soil

improvement



266

required at the boundary area. However if the central area of the raft is subjected to more

settlement, then LISS at that area is beneficial.

3.6 Design of Flat and folded Rafts Minimum reinforcement was decided considering the crack width limitations. For 900

mm raft the value of this moment is 375 KNm for Service load in main direction and 325

kNm for secondary moment for a reinforcement of 20mm@ 200 mm c/c, for a crack width of

0.3mm. Similarly the other ranges are worked out for different diameters of extra bars to be

provided like 20@200 mm c/c, 25@ 200 mm c/c etc. After finding out ranges of BM for

different combinations of reinforcements, the raft BM at different locations were grouped in

to these ranges and reinforcement was provided accordingly. Then the raft was investigated

for one way shear and punching shear. At few locations, the bottom reinforcement is

increased to give additional shear capacity for avoiding shear links. After completing the

reinforcement design, the raft was further analyzed changing the ks values to 12500

KN/sqm/m and locations where reinforcement requires modification were identified. Then

analysis was repeated after varying the ks to 50000 Kn/m 2/m and design was reviewed and

reinforcement detailing done incorporating all the cases of maximum moments. After

working out the total quantity the design was checked for continuum modeling. There was no

increase in reinforcement required for variation in ks, may be due to the symmetry of the

structure. Checking using Continuum method resulted in an increase of 1% reinforcement in

the bottom area. There was a decrease in reinforcement in the top area and was neglected.

For folded rafts, the same design procedure was followed, though the reinforcement required

and ranges of moments were different. The thickness of folded portion is 350mm and it is

more at column supports. The folded portion was subjected to much less moments and hence

much less thickness and reinforcement were needed. The impact of varying ks values from

half to two times the designated value was found to be insignificant as far as structural design

was concerned. However continuum modeling required more reinforcement upto 9%. This

showed the need for modifying the Winkler Method giving stiffness to soil in the lateral

directions also. With LISS, there was a small increase in reinforcement required at the bottom

and reduction in reinforcement required for the top in the case of flat raft, the later being

much more. In the case of folded raft, the top reinforcement needed increased in the outer

spans of fold area and reduced at several other places at the bottom. The net effect was that

there was no significant savings in reinforcement for the folded raft but other advantages

were there in superstructure design along with reduction in raft settlement.

3.7 Impact on SS design.

For Winkler model, it is observed that the column reinforcement decreases with

increasing ks values for both flat and folded rafts. Continuum analysis of flat rafts also

showed similar results. With soil improvement there is further reduction in column

reinforcements as shown in Fig. 4a. Buildings supported on folded raft required more

reinforcement for edge and corner columns at Ground floor (Fig.4b). The figures 4 and 5 also

contain comparisons with cases involving fixed end condition to columns. For the interior

columns, reinforcement required was slightly lower compared to building with flat raft

foundation, for all floors. LISS at the boundary reduces column reinforcements as shown in

Fig. 4a and 4b for both flat and folded plate foundations. In general beam bending moments

are also reduced with soil improvement with occasional local variations as shown in Fig 5a

and 5b.



267

(a) Winkler Method

(b) Continuum Method

Fig. 4. Variation in corner column reinforcement with soil strength

(a) Winkler Method

1500

2000

2500

3000

3500

4000

4500

0 20000 40000 60000Are

a o

f S

tee

l in

mm

2

Modulus of Subgrade reaction in kN/m2/m

Flat Raft without

soil improvement

Flat Raft with soil

improvement

Fixed Support

1500

2000

2500

3000

3500

4000

4500

0 20000 40000 60000 80000Are

a o

f S

tee

l in

mm

2

Young's Modulus in kN/m2

Flat Raft without Soil

improvement

Folded Raft without soil

improvement

Flat Raft with Soil

improvement

Folded Raft with soil

improvement

540550560570580590

0 20000 40000 60000Ma

x.

Mo

me

nt

in

be

am

in k

Nm

Modulus of subgrade reaction in kN/m2/m

Flat Raft without soil

improvementFlat Raft with soil

improvementFixed Support



268

(b) Continuum Method

Fig.5. the variations in a GF beam BM in Winkler and continuum method.

3.8. Comparison of material consumption and cost of execution. For a flat raft, LISS reduces the maximum top moment and hence reinforcement

considerably and it more than compensates the small increase in bottom reinforcement. There is a

small reduction in BM of the beams for all floors of the building with LISS and the variation was

very small and may not affect design. For folded rafts, the top BM in the outer spans were

slightly increased in fold direction. In superstructure design, there was an overall reduction in

beam moments and column reinforcements. If the cost of LISS is not substantial, considering the

overall advantages- especially in settlement reduction- it is a better option.

4.0 CONCLUSIONS

A raft and folded plate foundation were designed for varying ks values from half to

two times the designated values. Soil improvement was done at the boundary of the raft

below columns. For flat rafts, the bottom maximum BM increased and the top maximum BM

reduced in the case of LISS compared to the case without soil improvement. Continuum

methods showed similarity in the BM values obtained with those computed using ks with

slightly higher bottom moments and lower top moments. The top reinforcement required

reduces and the decrease is much more than the increase in bottom reinforcement. Compared

to the design made for the superstructure of flat raft without soil improvement below it,

column reinforcement required reduces with LISS. Similarly the bending moments are

reduced for the beams throughout the building due to LISS, the variation being very small.

LISS also helps to reduce the total and differential settlements. Between folded and flat rafts,

folded raft structure requires slightly lower reinforcement for columns and beams. With

LISS, the reinforcement required further reduces in both cases. In continuum method, the

settlements under corner and edge columns were more in folded rafts compared to flat rafts,

especially when the raft projections were low. This is found to be due to the heavy lateral

loads at those supports and due to which folded raft was deflecting in the fold direction. This

increases the reinforcement in the outer spans of folded raft and in the corner and edge

columns of ground floor. Hence Winkler methods may not give conservative results as far as

column designs are concerned. The effect of LISS reduces such local high values of

settlement and the increase in column reinforcement. In folded rafts, slight increase in top and

bottom reinforcement was needed with LISS at certain areas and there was a reduction in

some other areas. Top reinforcement increases for the outer spans and bottom reinforcement

525

530

535

540

545

550

555

560

0 20000 40000 60000 80000Ma

x.

Mo

me

nt

in b

ea

m i

n

kN

m

Young's Modulus in kN/m2

Flat Raft without soil

improvement

Folded Raft without soil

improvement

Flat Raft with soil

improvement

Folded Raft with soil

improvement

Fixed Support



269

in the central area. Overall there was not much change in the reinforcement required. BM in the

outer beams of ground floor also showed some increase in BM compared to that with the flat raft.

As E of soil increases, the BM in raft, column reinforcement and BM in beams reduces. At

locations where settlement of the raft is computed to be more, with LISS, there are several

advantages in the overall performance of sub-structure and superstructure. However, the cost

savings depends on the cost of LISS and may not be substantial.

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