2 the Challenges of Problematic Soils for Infrastructure Development - PAPER

8/2/2019 2 the Challenges of Problematic Soils for Infrastructure Development - PAPER

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[Mukabi JN (Ph.D) SPECIAL LECTURE] Page 1

The Challenges and Solutions for Problematic Soils for Infrastructure Developmentin Kenya and the East African Region

John MUKABI1, Kazuyuki GONO

2, Bernard NJOROGE

3, Rikiya HATEKAYAMA

4, Abiy TESFAYE

5, Justus

OTWANI6, George AMOYO

7, Protus MURUNGA

8, John NDEMI

9, Joram OKADO

10, Silvester KOTHEKI

11

1Kenstesu Kaihatsu Consultants Ltd. [email protected]/[email protected],

2Kajima Corporation

[email protected], 3 University of Nairobi [email protected], 4Kajima [email protected],

5European Union [email protected],

6Kensetsu Kaihatsu Ltd.

[email protected],7Kensetsu Kaihatsu Ltd. [email protected],

8Norken Ltd.

[email protected],9Norken Ltd. [email protected],

10Kensetsu Kaihatsu Ltd. [email protected],

11Kensetsu Kaihatsu Ltd. [email protected]

Abstract: The existing technologies that deal with problematic soils are far from preciseand have been found to be grossly inadequate and usually culminate in extremelyexpensive designs and methods of construction. On the other hand, when adopted withoutdue considerations, structures constructed on or within problematic soils have a tendencyof either deteriorating at an alarming rate and/ or failing altogether.

This Paper provides some pragmatic engineering solutions that have been developed onthe basis of long-term research and have been successfully adopted for design andconstruction of various civil engineering structures within this region.With the increased rate of rapid development in Africa, coupled with the wide existence ofproblematic soils in this region, it has become a necessity to develop techniques that cancharacterize such geomaterials as precisely as possible and subsequently innovatetechnologies that can make their optimum use, either fully or partially as constructionmaterials.

1. INTRODUCTION

Tropical problematic soils found in various parts of Africa, commonly classified asexpansive, collapsing, pedogenic, volcanic, saline and industrial wastes, are usuallyproducts of physical and chemical in-situ weathering of igneous, sedimentary andmetamorphic rocks influenced by complex interaction of variables as well as environmentalmatrices and changes. Due to this, their characterization for use for geotechnicalengineering purposes has hitherto been deemed an extremely complex exercise.Furthermore, precise determination of the appropriate extent or level of their suitability forlight structures has hardly been achieved due to the nature of their intrinsic properties andthe difficulty of precisely characterizing these soils. As a consequence, construction withinor above these soils has in most cases proven to be either disastrous and/or unreasonably

expensive.In this study, the results of some long term experimental research are discussed. Some ofthe new methods developed to provide cost-effective solutions aimed at enhancing theengineering properties related to plasticity, shear strength, elastic, elasto-plastic andresistance to deformation are also introduced. These methods, which include the OPMC,Suction-Stress, Moisture-Swell Control Interface and Rerap, were developed bycomprehensively carrying out conventional and innovatively modified laboratory and in-situexperimental testing, under various physical and mechanical conditions, particle matrixinteractions, stress-induced loading factors and environmental changes.Innovative testing methods for comprehensively characterizing the physical andmechanical properties of the expansive geomaterials were developed to effectively study

among other phenomenon:-1) the influence of clay content, 2) nature of shape and structure of the soil minerals inrelation to the surface area in contact, 3) soil particle orientation, 4) micro-aggregate


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cluster formation, 5) interaction of sesquioxides, 6) contribution of the chemicalcomposition of the colloid, and 7) influence of exchangeable ions.Powerful analytical methods either modified or newly developed during this study based onscientific and/or engineering concepts mainly associated with consolidation and stressratio functions, energy mobilization /dissipation, dynamic loading, excitation, stress-induced transversal and shear wave propagation , elastic properties as well as theories for

back analysis of deformation stress history, are also introduced.Some of the detrimental aspects of problematic expansive soils are depicted in Plate No. 1

Plate No. 1 Depiction of Some of Detrimental Aspects of Problematic Expansive Soils

2. TESTING REGIME

The In-situ and laboratory testing regimes which were innovatively designed are described

by Mukabi (2001c) and Gono et al (2003a) as well as in various other Engineering Reportsprepared by Kensetsu Kaihatsu Consultants.


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3. TEST RESULTS AND MATERIALS CHARACTERIZATION

Numerous tests have been undertaken comprehensively for purposes of characterizingproblematic soils as precisely as possible.

The discussions of these test results and the corresponding materials characterization are

presented in various Engineering Reports prepared by and compiled by KensetsuKaihatsu Consultants and partly reported by Mukabi (2001c) and Gono et al. (2003a).

4. SOME SOLUTIONS AND APPLICATIONS4.1 The Rerap Methods

Determination of appropriate counter-measures

1) Replacement Method - Tables 4.1 and 4.2 as well as Figures 4.1 and 4.2 show the

design and QC criteria developed on the basis of research and adopted for theconstruction of the Addis Ababa ~ Goha Tsion Trunk Road Project. In determining the

necessary thickness tCL to replace the expansive soil, the following equations proposed in

this study were adopted.

SPbpPCL xStTt (4.1)

The total pavement thickness TP is expressed as:

vfbPP txRtT (4.2)

And the coefficient of subgrade structural performance SSP is computed from:

5.0//1 edCBRSP eS (4.3)

On the other hand, the basic pavement thickness tPb from Eq. (3.10) is computed from the

following equation.

P

dPdPP

b

P

DN

CBRCCBRBAt

/log

loglog2

(4.4)

Where the roughness factor 25.02 itif RRRR : Ri is the initial roughness factor and R t is the

terminal roughness factor, tV in Equation 11 is the positive value of the specified tolerance

for pavement thickness, AP=219, BP=211, CP=58 and DP=120. The parameter e in

Equation 12 is defined as:

cneee

MLLVCB

ee eA

(4.5)


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where Ae=0.23, Be=0.54, Ce=0.08 are constants and Ve=Annual Average

Evapotranspiration in m/year (ref. to Mukabi et al. (2003c), LL=liquid Limit in percentage

and Mcn=Natural Moisture Content of the subgrade material expressed in percentage form.

All thickness are calculated in mm. Continuous assessment and evaluation of the

performance of the sections already constructed by adopting this criteria indicates that the

method has so far been quite successful.

Table 4.1 - Determining Required Capping Layer Thickness (cm) for RE 1 Type Phase II

CodingOption

Plasticity and

Swell Condition

Required Thickness

for Different Subgrade

Bearing Capacity

Pla

sticity

In

dex

Swell(%)Sm

CB

R=1

CB

R=2

CB

R=3

CB

R=4

445 Sm>10 140 90 70 60 30

B 35


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4.2 MC Technique

The importance of moisture control has been clearly demonstrated by Mukabi etal. (2007a

and 2007c) through nomographs that depict the influence of moisture~suction variation

on the bearing strength of varying geomaterials and can also be witnessed from plate No.2.

Plate No. 2 Typical Problematic Soils Black Cotton Soil in E. Africa

The variation in the nature of geomaterials is simulating through the application of varying

plasticity characteristics and magnitudes. The inset equations also indicate that the moduli

of deformation or deformation resistance are susceptible to moisture~suction variations, a

fact that has also been discussed by Mukabi (2008a-this conference). The ongoing

research on this subject intends to develop a technique of controlling the moisture contentof a subgrade of an expansive nature by systematically and technically imbedding sand

columns in predetermined areas or zones. Figures 4.3 ~ 4.4 present part of the preliminary

results that have been obtained in the initial stages of testing. Although definite

conclusions cannot be derived from these results yet, the trends exhibited from these

graphs are distinctly clear. In other words, embedment of sand interface layers seams to

be effective in reducing swell and increasing the bearing capacity notwithstanding the

magnitude of the surcharge pressure.

Figure 4.3 - Free swell soaking Figure 4.4 Soaking period for CBR

4.3 Suction Stress Method


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Research for purposes of developing this method is still in the initial stages. The basic idea

is to develop a technique of constructing a subsurface drainage layer underlain by a layer

compacted to a higher degree in order to induce high but varying suction stresses. The

layer is intended to facilitate in directing any excess moisture away from the pavement

structure. Reference can be made to Mukabi (2008a-this conference) which discusses

maintenance and Mukabi (2004a). Figures 4.5 and 4.6 are a representation of how thistechnique was used by incorporating a suction stress column in the design of an OPMC

Stabilized retaining wall and maintenance along the Addis Ababa ~ Debre Markos

International Trunk Road, the all important northern corridor that connects Sudan and

Eritrea.

Figure 4.5 - Use of a Suction Column for OPMC Stabilized Retaining Wall.

tABC=15cm

tAf=12.5cm

tBf=20cmNatural Gravel Boulders

Filter Course

Crushed AggregateFilter Course 0~40 only

Crushed Aggregate Base

Course M.S @ 3:20.5 : 0~40

Asphalt Concrete

Cons tructed to Specifications

Carriageway

Shoulder

Stepped & Compacted to higher degree

to achieve h igh suction stresses

Subgrade

10cm 25cm 75cm

tAS=7.5cm

Crushed Aggregate Base

Course M.S @ 3:20.5 : 0~40

tAf=7.5cm

tAf2=7.5cm

Figure 4.6 Suction-stress method

4.4 Long Term Consolidation

Mitchell (1976), Leroueil and Vaughan (1990), Mukabi (1995a), Mukabi and Tatsuoka

(1999c) and Mukabi et al. (2003d) have shown that long term consolidation enhances the

strength and deformation resistance of geomaterials subjected to various conditionsincluding disturbance and swelling. Anderson and Stokoe (1978) performed resonant

column tests under constant isotropic confining pressure 0, and showed that the value of


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low amplitude shear modulus increases with time and expressed this empirically as

depicted in the following formula.

G0 (t) = G0 (t-tp) [1+NGlog (t/tp)] (4.6)

On the other hand, Mukabi (1995), generated formulae related to the enhancement of

strength, deformation resistance (maximum elastic modulus) and linear elastic limit as

graphically represented in Figs 4.8 and 4.9. Figure 4.7 presents the fundamental concept

of stage construction.

Note:

Shortcomingsrequires

Long Term Planning Long Term

Construction Close Monitoring Proper Construction

& Maintenance of

Drainage Facilities

Construct Separating orImpermeable OPMS Layer with

Good Drainage Facilities

DetermineLinear Elastic

Range &deformationProperties

Apply LongTerm

consolidationConcept

Construct Gravel Layer

Continue StageConstruction Until

EnhancedEngineering

Properties are

Achieved Accordingto Design

Requirements

Implement StrainControlled Preloading

Determine Range ofLinear Elasticity

Is Linear ElasticRange Expanding?

Prepare & Compact Expansive SoilSubgrade In Accordance with

Specifications

YES

NO

Figure 4.7 - Fundamental Concept of Stage Construction Applying The Concept of Elastic Limit Strain

y = 3E+30x3E+30

5000

5200

5400

5600

5800

6000

6200

6400

6600

6800

7000

0 200 400 600 800 1000

ElasticM

odulus,E

max

(kgf/cm2)

Consolidation time (hrs)

Pre-swelling (D79-3)

Post-swelling (D79-3)

Undisturbed (D79-5)

OAP Clay

a = 0.01%/min

y = 3E+30x3E+30

0

0.001

0.002

0.003

0.004

0.005

0.006

0.007

0.008

0 200 400 600 800 1000

Elastic

limitstrain

,a

(%)

Consolidation time (hrs)

Least disturbed

Pre-swelling stage (D79-3)

Post-swelling (D79-3)

Linear (Post-swelling (D79-3))

Fig. 4.8 Effects of Long Term Consolidation on Deformation Resistance


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Figure 4.9 Consolidation time effect on elastic modulus and deviator stress

(1) Effect of Construction Equipment, Vehicular Compaction and Surcharge Pressure

Computation of total and initial settlement resulting from construction and surcharge of

upper layers is considered vital since this influences the characteristics of the roadbed

soils and the magnitude of their engineering parameters. In computing the total settlement,the generalized Eq. (4.7) below was adopted.

ij

KiC

ijo

jii

iC

iijT

P

PP

e

CHS

01,1

10log1

(4.7)

where, Hi = Thickness of each layer in cm. Back Calculation of induced stresses and

strains due to these effects were derived from equations 4.8 and 4.9.

0010 /log PPPeC ici

(4.8)

Rewriting Equation (7) we obtain, )110(0

i

k

scij PP

(4.9)

Where,i

cii

Cei 1

It is assumed that the stress is induced uniformly and that the magnitude of induced stressreduces proportionally with depth. However, the quantitative reduction is average over the

depth of each layer as a logarithmic function of the summed reduction in voids ratio (e)

and compression Index (CC). The stress induced is computed as a resulting value of the

post-construction surcharge. This effect is depicted in Figures. 4.10 and 4.11 for expansive

tropical soils. Figure 4.12 and the insert equation after Mukabi and Gono (2003a) were

used for the design of containing swell for pavement structures constructed within areas of

expansive tropical soils.


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0

0.4

0.8

1.2

1.6

L1 L2 L3

CC

(Co

mpressionIn

dex

)

Layers

BC (Before Consolidation)

AC (After Consolidation

Reduction in

Compression

Properties

0

0.1

0.2

0.3

0.4

L1 L2 L3

Swell

,%

Layers

AC (After Consolidation)

BC (Before Consolidation)Reductionin SwellingProperties

Fi

gure 4.10 Reduction in compression Figure 4.11 Reduction in swell

(2) Dynamic Loading Effects

Subsequent to longterm static loading, the trial sections described by Gono and Mukabi

(2003a) were subjected to dynamic loading. As can be noted from Figs. 4.13, the three

trial sections were initially subjected to around 61 passes of dynamic loading by use of aloaded dump truck of 1.2 axle configuration and front and rear axle loads of 4.5 and 9.5

tons respectively. This vehicle was chosen since it represents the most common type of

traffic along the project road (Addis Ababa ~ Goha Tsion).

Fi

gure 4.12 - Surcharge Pressure on Swell Figure 4.13 Deformation characteristics

Deformation during the static loading stages was measured by use of imbedded pegs (ref.

to Gono and Mukabi, 2003a), while steel plates were adopted during the dynamic loading

stage. In order to analyze the seasonal effects, the sets of both insitu and laboratory

tests were carried out in two stages during the wet and dry seasons under static loadingconditions. Dynamic loading was carried out for 20 days subsequent to which the ground

response was monitored for 3 days under static loading conditions. Dynamic reloading

was then effected for another 4 days after which insitu measurement of deformation,

extrusion of least disturbed samples and material sampling for laboratory testing was

undertaken. It can be noted from the results in Fig. 4.14 that longterm consolation and

primary dynamic loading tend to enhance the strength and deformation properties of

expansive soils.


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Figure 4.14 Static/dynamic loading Figure 4.15Coupling design

Notes:

1: Primary Static Loading, 2: Secondary Static Loading, D: Dynamic Loading

4.5 Coupling Method (Drainage Layer and Suction Stress)Figure 4.15 shows an example of the coupling method that was used in maintenance of

some defect areas along the Addis Ababa ~ Goha Tsion Trunk Road in an area that had

very high precipitation levels Ethiopia. This method proved quite effective in enhancing the

strength, stability, bearing capacity and deformation resistance of the pavement structure

under the stated conditions.

4.6 Moisture Control and Interface Technique

This method, depicted in Figure 4.16, was also effectively applied in maintaining varioussections of road in Ethiopia.

tAf=12.5cm

tBf=20cm20cm Boulde rs (NaturalGravel) Filter Course

Crushed AggregateFilter Course 0~40 only

Crushed Aggregate BaseCourse M.S @ 3:20.5 : 0~40

Asphalt Concrete

Constructed to Specifications

Carriageway

Shoulder

M.S Drainage Layer

Subgrade

10cm 25cm 75cm

tAS=7.5cm

Fig. 4.16 Depiction of Moisture Control and Interface Technique

CONCLUSIONS

The techniques proposed in this study have been successfully applied to enhance thegeotechnical engineering properties and overall performance of the problematic soilsinvestigated, particularly Black Cotton Soils.

The following conclusions are derived from this study:


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1. It is demonstrated that, for foundation, geotechnical and highway pavement

structures, these techniques can be effectively utilized in; deriving the consolidation

and shear stress-strain and deformation history, facilitating for the acquisition of

post-construction history data and determining appropriate engineering

countermeasures.

2. The results presented in this paper show that the recently developed methods wereeffective in enhancing physical, mechanical and other integral geomaterial

properties of the expansive soils studied, such as shear strength, bearing capacity,

intrinsic elastic properties, and deformation resistance.

3. Case Study Analysis for some structures already constructed in the East African

region by applying these techniques also introduced in this study indicate that

performance of the geotechnical engineering structures was not only commendable

but also cost-effective due to the effective enhancement of the engineering

properties of the problematic soils.

ACKNOWLEDGEMENTS

The author is highly indebted to the contributions of Professor Fumio Tatsuoka and theUniversity of Tokyo. Sincere appreciation is also expressed to the Japan InternationalCooperation Agency (JICA), Japan Bank of International Cooperation (JBIC), ConstructionProject Consultants Inc., Kajima Corporation and Kajima Foundation for funding thesubsequent part of the study conducted in Africa. The authors wish to express theirsincere appreciation to the Japan International Cooperation Agency (JICA), Japan Bank ofInternational Cooperation (JBIC), Construction Project Consultants Inc., KajimaCorporation and Kajima Foundation for funding most of the study. The paper wouldcertainly not have been completed without the crucial support of Ms. Piera Cesaroni, andthe input of Kenneth Wambugu, Ms. Zekal Ketsella, Joram Okado, Paul Kinyanjui, BryanOtieno, Walter Okello, and Anthony Ngigi. It is also important to mention the cooperationand assistance extended by the Ethiopian Roads Authority as well as the Ministry ofRoads, Public Works and Housing, Kenya.

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2 the Challenges of Problematic Soils for Infrastructure Development - PAPER