failure investigation- dar bagamoyo-paper

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THE ENGINEERING PROPERTIES OF CORALLIMESTONE USED AS SUB BASE IN DAR-

BAGAMOYO ROAD PROJECT

By Gerald Roosevelt Maregesi



1

THE ENGINEERING PROPERTIES OF CORAL LIMESTONE USED AS

SUB BASE IN DAR-BAGAMOYO ROAD PROJECT

By

Gerald Roosevelt Maregesi,UNETEC LTD/Advanced Engineering Solutions

P.0 Box 19074 Dar Es Salaam, Tanzania E-mail: [email protected]

Abstract

This paper presents and discusses the unusually poor engineering properties of corallimestone from Mpiji Borrow area, Bagamoyo District - Tanzania. The discussion is

based on the study and assessment of road subbase made of coral limestone at the project site in the coast of Dar Es Salaam area, which involves about 148,000 squaremetres of pavement from chainage 28+900 to 42+800. The road length is 42.8Kilometres starting at Wazo Hill Junction (Chainage 0+000) ending at Bagamoyotown (Chainage 42+800). The deterioration and damage pavement started while theconstruction was on-going. This resulted into immediate forensic investigation so asto establish the probable cause of pavement failure. The detailed discussion presentedin this paper concentrates on scope of the investigation taken, the investigation results(which included Benkelman Beam data, laboratory testing and soil testing data), the

performance characteristic of the soil and finally a summary and conclusion section.

INTRODUCTION

As a result of the significant formation of potholes in flexible pavement fromKm 28+900 - 42+800 on the Dar Es Salaam – Bagamoyo road, a forensicinvestigation was undertaken. It was established earlier beyond all reasonabledoubts that the cause of failure was the coralline sub base material used.Thus, the study was confined mainly to assess the properties of the corallimestone used as sub base material. The sub base material in question wastaken from Mpiji borrow area which was among the eight borrow areas

indicated in the project Materials Report as a potential source of sub basematerials. Much of the cracking and potholes formation resulted within thefirst rainy season of October – December 2000, about 2-3 months aftercovering the subbase with stone base and priming. This paper discusses thedamage investigation, firstly, the scope of the investigation followed by thepavement subbase characteristic and, then the Benkelman beam test results.With this data the performance of the soil subbase as it affected the pavementis evaluated.

SCOPE OF INVESTIGATION



2

The geotechnical investigation which was carried between 2001-2004consisted of a review of available project documents, a visual inspection ofdamaged area, Benkelman Beam survey as well as laboratory testing.Benkelman beam tests were taken along the alignment at an interval spacingof 50-metres in a staggered pattern. Likewise, bulk soil samples were takenalong the alignment while some samples were taken from Mpiji borrow pitfor laboratory testing. It is to be observed that the subbase material wasblended with natural sand from Mpiji so as to reduce the plasticity indexand/or to improve the bearing capacity prior to laying. The tests carried outon selected soil samples consisted of modified and standard proctor tests, un-soaked and soaked CBR test, Atterberg limits, moisture content tests, in-placesoil density determination, grain size analysis, X-Ray Diffraction tests andCBR swell tests.

PAVEMENT STRUCTURE

The pavement thickness was 550 mm comprising of 150 mm of high qualitycrushed granitic-gneiss stone base (CRR), 200 mm of natural gravel coralsubbase and 200 mm of topping material. The subgrade below the toppingmaterial consisted of sandy clay or silty/clayey sand.

LABORATORY RESULTS

Physical Properties of Sub base Before and After Construction

The physical properties of the coral material were tested prior to placing. Allengineering properties tests specified in Technical Specification were carriedout to determine the borrow-material quality. The compaction specificationfor the sub base soil that was excavated from the designated borrow areaincluded average 98% of AASHTO T180 but no single value should be lessthan 96%. In accordance with project specifications, the coralline sub basematerial was compacted within the moisture range of ±2% of OMC. It wasprojected by the designer that if the sub base can be compacted as specified,then it should meet the long-term performance needs of the highwaycarrigeway.

During construction, care was taken to ensure that the sub base wasconstructed in compliance with the designated specifications and standard.Consequently, laboratory and field density tests were performed on regularbasis to verify that the compaction and other physical properties were met. Asthe sub base laying progressed, localised failure in the form potholes wasobserved during first rainy season. The operation was halted so as to allow adetailed investigation to be carried out.

As part of this forensic investigation, samples were taken in the vicinity of thepotholes and tested for grading, Atterberg limits and CBR. The test results



3

indicated that the coralline sub base is undergoing mechanical breakdownunder repeated traffic loading. After being trafficked for about two years, thegrading changed remarkably in comparison to the original grading. Particlesbreakdown under repeated traffic loading resulted in increase of fines.Consequently, due to degradation of this coralline material, the soaked CBRdropped drastically in comparison to the original un-soaked CBR beforetrafficking. Further, due to generation of ultra-fine particles from smectitebearing coral rock, increase in plasticity index and CBR swell index was alsonoted.

The comparison data before and after trafficking is given in Figures 1-4

(a) (b) Figure 1: Atterberg limits before and after trafficking (a) Liquid Limit (b) Plasticity index

(a) (b) Figure 2: CBR and swell index after and before trafficking (a)CBR (b) Swellindex

(a) (b) Figure 3: Grading before and after trafficking (a) % passing 4.75 mm sieve (b)

Passing 20 mm sieve

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% p

a s s i n g 4 . 7

5 m m

before traffic after traffic

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% p

a s s i n g 2 0 m m

Before traffic after traffic

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110

29 31 33 35 37 39 41 43

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C B R

before trafic After traffic

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29 31 33 35 37 39 41 43

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i d l i m i t %

before traffic after traffic

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13

18

29 31 33 35 37 39 41 43

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P l a s t i c i t y I n d e x




4

(a) (b)

Figure 4: Grading before and after trafficking (a) % passing 0.075 mm sieve (b)% passing 1.18 mm sieve

Clay Mineralogy

X-Ray Diffraction (XRD) test were performed on the bulk sample from 8borrow-pits namely Mpiji, Kimambi, Wazo (Teacher), Wazo (Civilian), Boko(Amir) and Kunduchi quarry. It was established that all borrow areas containscalcite, microline, albite, anorthilite and sanidine. These borrow-pits wereindicated in materials report as potential source of subbase material.However, through X-Ray Diffraction analysis, it was detected that all borrowsamples contain expansive clay mineral smectite. The detailed investigationrevealed that all sources were equally contaminated with smectite exceptKunduchi quarry. The XRD performed on the particles passing 0.063 mmrevealed that the Mpiji borrow material contains significant amount of

smectite ranging from 60%-70%. If this is converted to represent the wholemass, then the percentage of smectite in this coralline subbase material rangedfrom 20%-30%. The summary of the test results are summarised in Table 1.

Table 1: The summary of X-Ray Diffraction test results [7]

Sample No. Smectite Kaolinite Remarks

1 60% 40% The main constituent is likely to beCalcium smectite, its presence wasconfirmed after testing with acid.

2 70% 30%3 60% 40%4 70% 30%

Smectite are important minerals in temperate regions due to its large volumechange potential. This is attributed to the large breadth-to-thickness ratio inexcess of 100 and specific surface area as much as 800 m2/g (Terzhagi et al,1996).(1) Because of their surface areas and adsorptive properties a significantvolume change (shrink/swell) and softening can occur as a result of moisturechange. Smectite clay even in small quantities in soil as low as 2% has beenreported to cause instability to a 5m embankment (6). When dry, the smectitebearing soils are hard and stable but when placed above optimum moisturecontent or exposed to infiltration/ground water inflow, the bonding capacity

of the clay is lost and the soil fill is of very low strength and unstable material.

10

15

20

25

30

29 31 33 35 37 39 41 43

Chainage

% p

a s s i n g 0 . 0

7 5

before traffic After traffic

25

45

65

85

29 31 33 35 37 39 41 43

Chainage

% p

a s s i n g 1 . 1

8




5

Smectite sheet is similar to that of mica and vermiculites with basic buildingblock of alumina-octahedron and silica tetrahedron. (2) Aluminium atoms arepartially replaced by magnesium in octahedral sheet. Each replacementproduces a unit negative charge at the location of the substituted atom, whichis balanced by exchangeable cations, such as Ca+2, in which case it is calledcalcium smectite, or Na+2, in which case it is called sodium smectite. Unlikethe case of sodium smectite, the electrostatic attraction of the Calcium cationsin calcium smectite links the successive sheets together and preventsseparation beyond 1 nm resulting in a smaller volume change compared tothat of sodium smectite yet, the softening and loss of bearing capacity effectwhen exposed to water infiltration is a characteristic properties of both Na +2 and Ca+2 smectite.

The X-Ray Diffraction detects the total amount of smectite in the sample.However, XRD cannot determine the percentage of each when both sodiumand calcium smectite are present in the sample. It was noted that the presenceof lime/calcium in the Mpiji borrow material is very apparent. The presenceof lime was indicated by the effervescence of the soil when Mpiji coralmaterial was tested with acid. This gave an indication of the presence of Ca+ smectite.

Compaction Properties

A total of 3 standard proctors (AASHTO T99) and 3 modified (AASHTOT180) maximum dry density curves were developed for samples taken fromMpiji borrow area and as well as from the road under consideration. Bothstandard and modified compaction results are given in Table 2.

Table 2: Summary of Compaction Data for Mpiji Borrow material

S a m p l e

N o .

L i q u i d

l i m i t ( L L )

P l a s t i c i t y

I n d e x P I

T e s t

D r y

d e n s i t y

( k / m 3 )

O p t i m u m

m o i s t u r e

c o n t e n t

R e m a r k s

1 31.3 19.8 MOD 2095 6.6% Sample from B/Pit1 31.3 19.8 STD 2002 9.1% Sample from B/pit2 30.1 17.6 MOD 2102 6.5% Sample from B/pit2 30.1 17.6 STD 2012 8.7% Sample from B/pit3 31.2 16.5 MOD 2110 6.5% Sample from B/pit3 31.2 16.5 STD 2015 8.8% Sample from B/pit1* 21.4 10.1 MOD 2132 6.8% Sample from the road1* 21.4 10.1 STD 2027 8.7% Sample from the road2* 22.8 8.0 MOD 2175 6.4% Sample from the road2* 22.8 8.0 STD 2055 8.2% Sample from the road3* 22.3 11.0 MOD 2150 6.6% Sample from the road3* 22.3 11.0 STD 2040 8.5% Sample from the road



7

0

50

100

150

0 2 4 6 8 10 12

water content

C B R ( % )

Soaked CBR Unsoaked CBR

0

0.1

0.2

0.3

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0 2 4 6 8 10 12

Moisture content (%)

S w e l l i n d e x ( %

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80

0 2 4 6 8 10 12

water content

C B R ( % )


0

0.20.4

0.60.8

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0 2 4 6 8 10 12


S w e l l i n d e x ( % )

020406080

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0 2 4 6 8 10 12

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C B R ( % )


0

0.2

0.4

0.6

0.8

1

0 2 4 6 8 10 12


S w

e l l i n d e x ( % )

wet of optimum (i.e OMC + 2%) at 95%-101% of modified compaction(AASHTO T180). This is demonstrated in all samples tested across a range ofmoisture content from dry to wet. Figure s5-8 and Figure 11 provides thesummary of the test results.

(a) (b) Figure 5: soaked and unsoaked CBR versus moulding moisture content compacted at 95% of AASHTO T180 (Modified) and the associated swell(a)CBR (b) CBR Swell index

(a) (b) Figure 6: Soaked and un-soaked CBR versus moulding moisture content compacted at 97% of AASHTO T180 (modified) and the associated swell (a)CBR (b) CBR swell index

(a) (b)



8

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200

0 2 4 6 8 10 12

water content

C B R ( % )


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0.1

0.2

0.3

0.4

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Figure 7: Soaked and un-soaked CBR versus initial moisture content compacted at 99% of AASHTO T180 and associated swell (a) CBR (b)CBRSwell index

(a) (b) Figure 8: soaked and unsoaked CBR versus initial moisture content compacted at 101% of AASHTO T180 and associated swell (a)CBR (b)CBRswell index

ANALYSIS OF TEST RESULTS

Volume Change Characteristic

In some of the references and based on the plasticity index values, the projectcoralline subbase soil would be classified to have low to medium swellpotential (Nelson and Miller, 1992). (3) This classification could be deceivingand the impact on swell and softening could be far greater than anticipateddepending on the type of structure considered and construction phase (i.e.,before, during or after construction). A soil may undergo little free swell butmay exert great swelling pressure under confined condition. Further,significant damages can still occur from developed strain during volumechange without mobilising large swelling pressure. (11)

Because of the high smectite content in the coralline subbase soil, significantswell resulted from soaking. Swell was measured on soaked CBR testsperformed with modified effort (AASHTO T193). Based on the data collected,the swells at optimum, 95%, 97%, 99% and 101% compaction and at the peakCBR value can be interpolated. From Figure 6-7, it can be seen that if thesubbase material is compacted above 98% which was a specified minimumcompaction, the CBR swell below the specified value of 0.5% is achieved.However, if the moisture content is reduced below the optimum moisturecontent, the CBR swell tends to increase exponentially. This swell data are

summarised in Figure 5-8. As expected, the greatest CBR-swell indices are



9

obtained on the dry side of optimum. Based on samples compacted withmodified effort, swell index as high as 1.48% can occur at 95% compaction(Figure 5-6).

Without specifically testing the swell and softening effect, the swell andsoftening potential of the coralline subbase soil from Mpiji could remainunsuspected because of low range of plasticity characteristics. Note that theplasticity index of the subbase material after blending with Mpiji sand wasless than 12%. The smectite soils have the potential to swell when themoisture content is increased ; they conversely shrink if the moisture isreduced. It is to be observed that no shrinkage tests were performed.

The swelling for all tested samples appeared to stabilise within the soakingperiod of 4 days. However, it should be noted CBR swell index should not beconsidered as a free swell potential of the soil at any dry density- moisture butan indication of the swell. Restrained soil swell resulted due to restrictions ofthe soil to heave caused by the compaction mould and the compaction energyapplied. In order to confirm this hypothesis, a site performance trial wascarried out to simulate the wet condition at Km 30+150-30+175 and 30+100-30+125 using subbase material blended with 40% of the screened natural riversand from Mpiji. The trial section was brought to OMC (5.9-6.1%) prior rollingand compacted to relative compaction of 99.7% and 99.5% of MDD (AASHTOT180) and proof rolled using water bowser. Thereafter, the sub base materialwas saturated using water bowser and the density retested. It was found thatthe relative compaction drastically decreased to 95.6% (4.1% drop) and 95.0%(4.5% drop) while the moisture content averaged to 10.1%. During proof-rolling, severe pumping was observed signifying that the strength of thecoralline subbase material was substantially reduced to a level that it wasimpossible to support the loading imposed on it. Reduction in density istypical characteristic of the Smectite bearing swelling soil, when soaked thesoil tends to absorb large amount of water leading to increase in gross volumeof material, thus density and bearing capacity of soil dramatically reduced.Using dry density data before swelling (at OMC) and after swelling thepercentage of swelling can be computed which were found to be 4.3% (for thefirst section) and 4.7% (for the second section) as opposed to CBR swell indexvalue of less than 0.5% measured from CBR-swell test. This is implying that,the CBR swell index test does not realistically depict the swell in field sincethe sample in CBR is restrained from swelling by compaction mould. The truein-situ swell was found to about 9 times the value measured using CBR swellindex test due to the fact that the subbase layer was not restrained fromswelling, thus actual free swell was achieved. The percentage of swellingafter saturation was computed using the expression given in equation 1,where pd is the dry density before swelling and pswell is the dry density afterswelling.



10

0

20

40

60

80

95 97 99 101 103 105

Compaction

C B R

CBR at OMC CBR at -2% OMC

0

0.5

1

1.5

95 97 99 101 103 105

Compaction

S w e l l i n d e x %

Swell at OMC Swell at -2% OMC

0

20

40

60

94 96 98 100 102

compaction %

C B R ( % )

3% MC 5% MC 7% Mc 9% Mc

0

0.5

1

1.5

94 96 98 100 102

compaction (%)


3% MC 5% MC 7% MC 9% MC

=

1 + %………(1)

The trial section carried out to simulate the wet condition indicated that thematerial used as subbase material is expansive as evidenced by reduction in

in-situ density associated with reduction in soil bearing capacity. This studysuggests that even blended with sand; the activity of the smectite-montmorillonite clay cannot be substantially reduced as demonstrated withsevere swelling during on-site performance trial.

Soaked CBR Results

After soaking, the CBR testing showed that there is egregious reduction instrength in the soil as a result of swelling and softening. This can be seen bycomparing un-soaked and soaked CBR value for the same coral material. As

can be seen from Figure 5-11, soaking causes the strength to be reduced to lessthan 10% of the compacted un-soaked strength on the dry side of theoptimum at 95%-101% compaction. Even when compacted dry of optimum to101% of maximum dry density the subbase soil is extremely sensitive tosoaking and/or moisture content variation with strength reduction to 13% ofthe un-soaked value. It is evident that, the coralline soil can performsatisfactorily as sub base material when compacted at about OMC±1% only.Beyond and above that range, the soil experiences catastrophic drop in CBRvalue after soaking.

(a) (b)

Figure 9: Moulding moisture content, and soaked CBR relationship and theassociated CBR-swell index compacted at various compactive efforts (a)soaked CBR (b)CBR swell index



11

(a) (b) Figure 10: CBR at various compactions level (AASHTO T180), compacted at OMC and OMC-2% and associated swell (a)CBR (b)CBR swell index

It should be noted that the Na+ smectite behaves in different manner incomparison to Ca+ smectite. While Na+ smectite is known as high swellingclay, the Ca+ smectite is known as low swelling clay, but both Na+ and Ca+ smectite are characterized by low strength when soaked. They tend to softenthus reducing the bearing capacity of the soil. During construction thestrength reduction will be even more severe than observed in the laboratory,as the swelling will not be restricted by a compaction mould. For instance ifthe subbase were compacted dry of optimum and the subbase absorbs waterfrom rain, it would swell resulting into an increase in moisture contentaccompanied by decrease in density.

(a) (b) Figure 11: Soaked and un-soaked CBR compacted at various compactionlevels (a) Soaked CBR (b) Un-soaked CBR

In Figure 5-11, soaked and un-soaked CBR values are summarised at variouslevel of compaction. The minimum CBR can be on either side of optimum. Asit can be seen, the resulting soaked CBR values are extremely moisturedependent and are typical of all the other samples tested and that the CBRvalues decrease dramatically after soaking on both dry and wet side of theoptimum moisture content.

The coralline soil characteristic described herein shows that the corallinematerial is extremely sensitive to moisture content. The acceptable CBR wasyielded only when the soil is compacted in excess of 98% at OMC. But, if the

soil is compacted in normally specified range of moisture (OMC±2%), theCBR value less than 20% is recorded with maximum CBR recorded at the oneither side of OMC (OMC±0.5).

Plasticity Data

0

20

40

60

2 4 6 8 10

moisture content

s o a k e d C B R

95% comp 97% comp

99% comp 101% comp

050

100150200250

2 4 6 8 10

Moisture content

U n s o a k e d C B R

95% Comp 97% comp

99% Comp 101% Comp



12

Several samples were taken from the borrow pit and tested for the plasticityindex (PI) in “as dug state” and thereafter blended with 20-25% of naturalMpiji river sand. The Atterberg limit tests on this coralline material yieldedliquid limit ranging from 23-43% before blending and after blending theliquid limit ranged from 23-28%. The plasticity index varied from 12-17% forunblended material and after blending the plasticity index varied from 7-12%.The soil with plasticity index of less than 15% is normally classified as lowswelling potential soil. (3) However, due to the mineralogical nature of thiscoralline sub base material which contains smectite-montmorillonite clay, asignificant swelling on the dry side of optimum was recorded as shown inFigures 5-10.

In-Situ Moisture Content

The in-situ moisture content of the failed subbase sections was determined.The test results indicated that the subbase in most cases was saturated withmoisture content above the optimum moisture content suggesting that thecoralline subbase material has absorbed water from rainfall and/or the watercontent increased due to change of evapotranspiration regime after priming.

The predicted Equilibrium moisture content (PEMC) in the subbase wascomputed and tabulated using two formulae developed by J.F Haupt (12) forcomparison with the in-situ moisture content. The formulae are:

PEMC (LL) = 0.42(LL)0.7(%P)0.3-3.9 ………….(2)

PEMC (LS) = 0.053(LS)(%P)0.7+5.1 ……………(3)

Where LL is the liquid limit, LS is the Bar linear shrinkage, and %P is thepercentage of material passing 0.425 mm sieve.

Analysis of the results shows that 57% of the in-situ moisture content isoutside on wet side of range value predicted by the formulae: suggesting thatconditions in the field were slightly wetter than the theoretical work wouldforecast. This is confirming the water affinity behaviour of the smectite-montmorillonite bearing soil. The comparison of in-situ moisture content andthe predicted equilibrium moisture content is given in Table 3.

Table 3: Comparison of In-situ Moisture Content and Predicted Equilibrium Moisture Cotent

Location %Passing0.425mm

LiquidLimit(LL)

LinearShrinkage(LS)

PEMC*(LL)

PEMC*(LS)

In-situMC

OMC

38.166 46.3 21.6 5.4 7.5 9.3 7.5 6.4



13

Location %Passing0.425mm

LiquidLimit(LL)

LinearShrinkage(LS)

PEMC*(LL)

PEMC*(LS)

In-situMC

OMC

39.109 46.6 19.1 3.9 6.6 8.1 9.7 6.839.250 46.5 19.6 4.3 6.8 8.4 9.9 7.0

39.217 48.8 22.3 5.7 7.9 9.7 9.2 6.4

39.128 42.9 19.2 3.9 6.4 8.0 9.0 8.2

39.153 37.7 21.2 5.0 6.7 8.5 8.8 6.6

40.619 43.8 23.4 6.1 8.0 9.7 9.2 6.6

40.647 45.1 22.1 5.7 7.6 9.4 8.7 6.8

*Predicted Equilibrium Moisture Content

Distress Mechanism

Montmorrilonite is a three layer clay mineral that possesses structuralconfiguration and chemical makeup which permits absorption of largerquantities of water in interlayer and peripheral position on the clay structure.The swelling and shrinkage of the smectite bearing soil tends to manifestwhen there is change and/or increase and/or decrease in moisture contentimplying that in order for the potentially expansive soil to actually swell, theymust initially be in a water deficient condition. The construction recordindicates that the subbase material was in most cases compacted within the

moisture range of OMC-2% to OMC or in other words the compaction wascarried out on the dry leg of optimum moisture content in line with therequirements of Technical Specifications. Therefore in most cases the corallinesubbase was compacted in water deficient conditions. Due to the fact that thesubbase was compacted in state of water deficient, after placing, the materialimbibed water either through rainfall or during the stone base placing andmore importantly, water become available after priming due to change inevapotranspiration regime brought about by priming. The availability ofwater led the coralline subbase to soften creating a weak and overly softtransition layer at the interface of subbase and crushed stone base ultimately

leading to loss of support. The softening as well as loss of support is bestillustrated by moisture data given in Table 3 and Figure 12 which do verifythat after placing the moisture content of the subbase increased from about4% (OMC-2%) to between 7-10% (range of moisture content which wasrecorded during this forensic investigation). The data shown in Figure 9 ifread in conjunction with data shown in Figure 5-11 suggests that during thistransient stage of coralline material to equilibrate, the CBR decreased fromabout 100% to below 20% leading to loss of support during loading,eventually forming cracks and/or potholes. The degree of failure or potholesformation varied from place to place due to spatial moisture distribution

within the pavement.



14

Figure 12: Water content increase model immediately after placing and associated moisture content as measured in the field after failure

Structural deterioration under traffic takes place in every road pavementsalthough in a well designed and well constructed one; its development is veryslow and seasonal. Under load the stress condition in the base are analogousto those of loaded beam. Due to bending, the base is subjected to compression

at the top and tension at the bottom. The cohesion-less base material thatmake-up base have little tensile strength and generally depend onsubbase/subgrade to provide a lateral restraint. Due to development ofoverly soft transitional subbase layer, very little restraint was providedleading to loss of support and integrity with subsequent cracking andformation of potholes which were mainly confined at the pavement edge dueto stress concentration immediately under the surface. The distressmechanism and potholing formation model is shown in Figure 13.

2

3

4

5

6

7

8

9

10

11

M o i s t u r e C o n t e n t %

EMC - low

EMC - high

In situ MC

increase in moisture content

after placing, sub base

softening and swelling

leading to loss of support

subbase placed at 4%moisture (OMC-2%)

Crushed stonebase

Softened coralline subbase

Coralline subbase

Subgrade

Load

Loss of su ort

Crack/deformation



15

Figure 13: Potholes forming model due to loss of support

PAVEMENT RESIDUAL LIFE PREDICTION

BENKELMAN BEAM TEST RESULTS

The Benkelman beam test was conducted in December 2001 about one yearafter observing the first sign of distress so as to assess the integrity ofpavement structure after carrying out the potholes repair. AnotherBenkelman beam deflection survey was carried in September 2004, about fouryears after the first sign of pavement distress. The Benkelman beam wascarried out in staggered pattern at an interval of 50 metres. From these data,the residual life of the pavement was predicted at in-situ moisture contents inaccordance with the procedures given in Tanzania Ministry of WorksPavement and Material Design Manual.(13) The 2001 Benkelman beam test

results indicated that the pavement can satisfactorily carry T2 traffic category(0.3-0.7 million esa) for which the pavement was designed for. The estimatedresidual pavement life in December 2001 survey was found to be range of 1.5-6.3 MESA with an average of 4.8 MESA while the 2004 Benkelman test resultsindicates the estimated pavement residual life is ranging from 2.9-5.5 MESAwith an average pavement life of 4.0 MESA. The summary of the test results isgiven in Figure 14.

Chainage

32 34 36 38 40 42

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r e d i c t e d r e s i d u a l l i f e ,

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8

10

0.3 MESA 0.3 MESA0.7 MESA 0.7 MESA

LHS (Dec, 2001)

RHS (Dec, 2001)

LHS&RHS (Sept, 2004)

Figure 14: Predicted total pavement life using Benkelman Beam test results(December 2001 and September 2004)

This Benkelman beam deflection survey test results suggests that the coralline

material regained its strength after drying. In most cases, the unbound



16

material tends to generate very high pore water pressure when the degree ofsaturation is in excess of 80%. However the stability improves significantlywhen the pavement dries to about 50-70% of degree of saturation. This isparticularly true for the moisture sensitive and expansive material like thecoralline gravel used as subbase material for Dar - Bagamoyo road project.Therefore, it can be stated that as far as there is no moisture fluctuation withinthe pavement, which facilitates the swelling and softening to manifest, thepavement is likely to perform as per design. Additionally, seasonalmovements due to wetting drying have a tendency to reach a point ofstabilization after several cycles such that the dry density reaches a criticalvalue at which the swelling and shrinkage equalises thus making the soil tobe in state of equilibrium. Once equilibrated, the moisture content tends tostabilize thus the manifestation of swelling and shrinkage is impeded.

CONCLUSIONS

• The project soil discussed in this paper is coral limestone from Mpiji alongthe coast of Dar Es Salaam - Tanzania. The coral was used as subbasematerial from chainage 28+900 – 42+800 designed to support a trafficcategory of T2 in accordance with Overseas Road Note 31 (0.3-0.7 Millionesa).

• However, when the above soils were tested across the typicalrecommended and specified compaction and moisture ranges ofOMC±2%, this coralline subbase gravel was found to have extremelydetrimental characteristic for pavement support and construction. Fromsoaked CBR testing it was found that this soil had egregious sensitivity tomoisture content. In other words, if this coralline soil compacted dry ofoptimum at high densities will appear sufficiently stable to pass the proofrolling, but become egregiously unstable after imbibing water. It is notedthat the deviation of moisture content from optimum moisture content hasa very devastating effect on the bearing strength of the coralline soil, andresults in drastic drop in CBR value. The moisture sensitivity and very

poor engineering properties of this coralline gravel can be attributed to thesignificant amount of smectite present in the soil. The existence ofexpansive clay was confirmed through XRD tests as well as during the on-site performance trial, From the X-ray diffraction tests the coral was foundto have more than 60% of smectite.

• The failure of the pavement was a combination of the traffic associateddistress and non-traffic associated distress caused by shrinkage, expansionand softening of sub base material which is a characteristic property ofproject coral material which contains significant amount of expansive

smectite clay used as a subbase material.



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• The pavement distress was due to increase in moisture content broughtabout by rainfall as well due to change in evapotranspiration regimebrought about by priming. The increase in moisture content led thesmectite-bearing soil to expand, soften and decrease in CBR valueultimately leading to loss of integrity and support, thus, the formation ofpotholes was due to inexorable intrinsic and extrinsic nature of thecoralline material used as subbase material. Swelling is normallyassociated with mobilization of swelling pressure and swelling strainwhich contributed to the formation of cracks and subsequently formationof potholes. The pavement is intolerant of this expansion and/orshrinkage ultimately the movement produced cracks which developedinto potholes. The degree of failure varied from place to place due tospatial moisture distribution within the pavement.

• The estimation of swelling potential based on the plasticity index as wellas CBR swell index can be deceiving due to the fact in the compactionmould the soil is restrained while the field the material is not restrainedthus a free swell is achieved. The test results presented herein indicatesthat the field swell can be as much as nine times more than the CBR swellindex measured in laboratory.

RECOMMENDATIONS

The pavement is normally designed using empirical and/or mechanisticproperties like resilient modulus and/or CBR which has to be fulfilled inorder for the pavement to perform in accordance with the design. However, itis a custom practice to use the dry density as the only means of ensuring thatthe pavement will perform in accordance with the design. This studyindicates that without testing the soil within the full range of moisture contentspecified, the density is likely to give misleading results which may lead topremature failure of the pavement. Based on this study the followingrecommendations are made:

• To test the soil across the typically recommended or specified moisture

and compaction ranges to ascertain the critical behaviour of the soil priorapproval of any proposed borrow area. From soaked CBR testing it wasfound that some soil has egregious sensitivity to moulding/placementmoisture content. Testing through typical specified range may allowrelaxation or more strict control of moisture content in the field.

• To review the CML 1.10 and 1.11 standards for testing CBR so as to insertclause explicitly to address the effect of moulding/placement moisturecontent in the strength of soil. Similarly, article 3606(c) of StandardSpecification for Road Works 2000 should be reviewed to allowcompaction above optimum moisture since some soils are likely to show

considerable instability when compacted at moisture content below



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optimum. Further, researches are indicating that swelling clays should becompacted wet of optimum in order to minimise swelling potential.

• It is imperative for the specifying agencies to address that the pavementlayers must not only be compacted to the proper density, but compactionat proper moisture content. This study has clearly demonstrated that soilproperties and therefore performance are dependant on the placementmoisture content during compaction but not on achieved dry density.

• The use of coralline which is soft especially the one which containssignificant amount of smectite clay should be discouraged for use assubbase materials for road constriction.

References

1. Terzhagi, K., Peck, R., and Mesri, G. (1996). Soil Mechanics in

Engineering Practice, John Wiley & sons, Inc., 549.2. Grim, R.E. (1968). Clay Mineralogy, 2nd ed., McGraw-Hill Book Co., New

York.3. Nelson, J.D., and Miller, D.J., Expansive soils, John Wiley & Sons, Inc,

259.4. Dempsey, B.J., and Elzeftawy, A. (1976). Moisture movement and

moisture equilibria in pavement systems. 161, University of Illinois,Urbana, IL

5. Thompson, M.R. (1996). Subgrade stability, TRB, TransportationResearch Record, 705, 32-41.

6. Mahar, James W, (2000) Contract Geotechnical Practice for the New Millennium

7. Mruma, J (2000) Internal communication to IFF , University of Dar esSalaam

8. Maregesi, G.R (2003), The Compaction Specifications for Highways Fill: Placement Moisture Content Range Fixation, Construction Business, Vol. 6No. 4/5,

9. CML-Tanlab (2001), Benkelman Beam Test Results , internalcommunication to IFF

10. CML-TanLab (2004), Benkelman Beam test Results, Internalcommunication to IFF.

11. Marino, G.G and Abdel-Maksoud (2000), Road Subgrade Properties of Loessal soil in Memphis Area, Unpublished report.

12. Haupt, J.F, (1982) Prediction of Equilibrium moisture content, Journal ofSouth Africa Engineer.

13. Ministry Of Works (1999), Pavement and Materials Design Manual,Ministry of Works Tanzania.

failure investigation- dar bagamoyo-paper

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