Deformgfggation and Shear Strength Characteristics Of

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Pertanika J Sci. Techno . 14 1 2): 6 - 74 (2006)ISSN: 0128-7680

Universiti Putra Malaysia Press

Deformation and Shear Strength Characteristics o f o m Tropical Peat and Organic Soi ls

Bujang B Huat

Department Civil EngineeringFaculty Engineering Universiti Putra Malaysia

43400 Serdang Selangor MalaysiaE mail:[email protected]

Received: 26 March 2004

ABSTRAK

Tanah-tanah gambut da n organik lazimnya terdapat sebagai endapan yangtersangat lembut yang merupakan sebahagian daripada sistem tanah berair.Tanah-tanah ini boleh juga terdapat sebagai s tr at um d i bawah endapan tanahyang lain. Tanah-tanah ini lazimnya dikategorikan sebagai tanah bermasalaho le h k er an a ia tersangat boleh mampat ser ta mempunya i kekua tan ricih yangsangat rendah. Di beberapa negara seperti Malaysia, tanah sebegini terdapatdengan banyaknya. Oleh yang demikian penggunaan kawasan-kawasan sebeginisemakin diper lukan dengan berkurangnya kawasan-kawasan lain yang lebihsesuai. Dengan itu parameter k ej ur ut er aa n se rt a k ae da h pembinaan yangbersesuaian perlu dicari. Kita perlu mengembangkan pengatahuan kita mengenaitanah- tanah se begi n i, kh ususnya dari aspek keju ru te ra an sepe rtikebolehmampatan dan kekuatan ricih tanah. Kertas ker ja ini memerihalkankaj ian yang telah d ij alankan di makmal da n juga di lapangan mengenai ubahbentuk da n kekuatan ricih tanah organik d an g am bu t tropika. Sampel-sampeltanah diambil di beberapa lokasi di Malaysia, Johor Perak, Sarawak da nSelangor. Tanah-tanah ini mempunyai kandungan organik yang berjulat diantara 50 hingga 95 . Indeks mampatan t anah d idapat i bertambah denganbertambahnya kandungan organik da n ai r tanah. Manakala kekuatan riciht anah berkurangan dengan ber tambahnya kandungan organik da n ai r tanah.Kekuatan ricih tanah turut didapati dipengaruhi oleh dar jah pereputan tanah.Tanah dengan serat yang tinggi mempunyai kekuatan r ic ih yang t inggi.

ABSTRACT

Peat an d organic soils commonly occur as ext remely soft, unconsolidatedsurficial deposits that a re an integral part of th e wetland systems. They may alsooccur as strata beneath other surficial deposits. These soils are problematic asthey ar e very highly compressible an d ar e of very low shear strength. Incountries like Malaysia, peat and organic soils ar e found in abundance.Utilization of this margina l ground is required in inc reas ing number ofinstances in th e recent years. Hence suitable geotechnical design parametersand cons truct ion techn iques needed to be found for this type of groundc on di ti on . T hi s paper presents results of laboratory an d f ield tests on th edeformation an d shear strength characteristics of tropical organic and peatsoil. The soil samples were collected from several locations in Malaysia, namelyfrom th e states ofJohore Perak, Sarawak an d Selangor. These soils representedtropical peat an d organic soils with organic content ranging f rom 50 to 95 .


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The soil compression index is found to increase with increase in th e organiccontent and natural moisture conten t. While fo r case of undrained strength,t he s he ar s tr en gt h of tropical peat and organic soil is found to decrease withi nc re ase in th e organic content a nd na tu ra l moisture con tent . The shearstrength of th e soil is also d e pe n de n t o n th e degree of humification of th e soil,

with more fibrous soils having higher undrained strength.

Keywords: Defonna t ion , index propert ies, organic soil, p ea t, s he ar s tr en gt h

INTRO U TION

Peat and organic soils commonly occur as extremely soft, wet, unconsolidatedsurficial deposits that ar e integral parts of the wetland systems. They may alsooccur as strata beneath other surficial deposits Garrett 1995. These soils arefound in many countries throughout the wor ld . In the US peat is found in 42sta tes with a total acreage of 30 million hectares. Canada and Russia a re the two

countries with a large area of peat, 170 and 150 million hectares respectively Hartlen an d Wolski 1996 . Fo r th e case of tropical peat, or tropical peat lands,th e total world coverage is about 30 million hectares, two thirds of which ar ein Southeast Asia. Malaysia has some 3 million hectares - about 8 of th e landarea is covered with tropical peat. While in Indonesia peat covers about 26million hectare of the country land area, with almost half of th e peat land totalis f ou nd i n I nd on es ia s Kalimantan.

Peat actually represents an accumulat ion of disintegrated plant remains,which have been preserved under condition of incomplete aeration an d highwater content. accumulates wherever the condi tions are suitable, that is, inareas with excess rainfall and th e ground is poorly drained, irrespective oflatitude or altitude. one theless , peat deposits tend to be most com mon inthose regions with comparat ive ly cool we t climate. Physico-chemical andbiochemical process cause this organ ic mater ia l to remain in a state ofpreserva tion over a l on g p er io d of time. In other words, waterlogged poorlydrained condition, not only favor th e growth of particular type of vegetationbut also help preserve the plant remains.

These soils ar e generally referred to as problematic soils du e to the ir h ighcompressibility an d low shear strength. Access to these superficial deposits ar eusually very difficult as th e water table will be at, near or above the groundsurface. Undoubtedly, these are the consequences of th e tendency to eitheravoid construction and buildings on these soils, or when this is not possible, tosimply remove, replace or displace them, that in some instances may lead topossibly uneconomical design and construction alternative. However in manycountries including Malaysia, substantial areas ar e covered by this material. Th ethickness of this deposit varies from just about 1 m to more than 20 m thick.Pressures on th e land us e by industry, housing and infrastructure ar e leadingto more frequent utilization of such marginal grounds. is therefore necessaryto expand th e knowledge of their geotechnical propert ies and mechanicalbehavior, in particular those in rela tion to their deformation an d shear strength

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Defonnation an d Shear Strength Characteristic of Some Tropical Peat an d Organic Soils

characteristics, and subsequently devises suitable design parameters andconstruction techniques on these materials.

Review of literature indicates that peat and organic soils ar e very variable intheir properties, both from on e deposit to a no th er a nd from point to point inth e same deposit. Such variations ar e associated with t he origin of these soils,th e type of plant from which they ar e derived, t he m in era l content of thedeposit and the amount of decay or humification that had occurred. All thesefeatures ar e reflected i n t he mec hanica l be ha vior compressibility an d shearstrength with which th e geotechnical engineer is concerned Tresidder 1966 .When a soil is subjected to an increase in compressive stress due to foundationload, the resulting soil compression generally called settlement generallyconsists of elastic compression immediate settlement , primary compression consolidation settlement an d secondary compression. Compared with mineralsoils, peat soils ar e highly organic an d highly compressible. Its compression orsettlement process may take a considerably longer amount of time. Peat

generally posses low undrained strength an d high compressibility. Buildings onpeat are usually suspended on piles, but the ground around it may still settle,creating a scena rio as depicted in ig l below.

Fig. 1: a Typical section a housing estate on peat immediately after completion construction b Several years after completion construction scale exaggerated

The calculation of t he s et tl em en t requires evaluation of soil parametersfrom the compression curves which are usually obtained from laboratoryoedometer tests. The results of incremental loading oedometer tests ar e usually

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presented as th e relationship between void ratio, an d effective vertical stress, v The vertical effective stress may be plotted o n a l inear scale to determine thecoefficient of volume change, v and oedometer modulus, M or on logarithmicscale to d et er mi ne t he compress ion index, e

As with mineral soils silt and clay), th e settlement parameters of peat i.e.consolidation settlement) may also be determined from standard incrementaloedometer o ne d imensional compression) t es ts Edil 1997). The parametersar e interpreted f rom tradit ional e - lo g v plots. There may be differences in th emagnitudes of various quantities measured but th e ge nera l shape of th econsolidation curves appear reasonably similar and the formulat ion developedfo r clay compression can be used to predict t he m ag ni tu de an d rate ofsettlement Edil 1997).

There ar e however a certain class of peat, typically peat with high organicand fibre content with low degree of humif icat ion, tha t do not conform to theconcept of conventional clay compression because of their different solid phase

properties and microstructures. The analysis o f compression of such materialspresent certain difficulties when convent ional methods are applied because thecurves obtained from th e conventional oedometer tests an d behaviour exhibitedby them may show lit tle similarity to th e clay behaviour Edil 1997; Den H aan1997).

When conduct ing laboratory test, great care s ho ul d b e t ak en in determiningth e deformat ion parameters of these soils fo r t he s et tl em en t calculations.Usually the laboratory consolidation tests ar e time consuming, rather expensiveand requi re great care in handling and in te rpret ing the results. Consideringth e above-mentioned factors, an effort has been made to co rre lat e th ecompression index of these soils to some other easily determined soil indexproperties. There ar e a number of correlations fo r clays, but very few fo rorganic soils. This is th e focus of this paper.

Shear s trength is another important parameter in soil mechanics. Shearstrength always playa vital ro le w he n engineering decision comes across withany soils including peat. Shear s t rength is a concern both during constructionfo r supporting construction equipment as well as at the end of construction insupporting th e structure. Low shear s tr en gt h a n d high compressibility of thepeat soils however confined them in the problematic category. Accuracy indetermining the shear strength of these soils is associated with several variablesnamely; origin of soil, water content, organic content an d degree of humification.Fo r th e case of peat, the presence of fibers modifies our concepts of strengthbehavior in several ways. can provide effective stress where there is none an dit induces anisotropy.

Early research on peat strength indicates some confusion as to whether peatshould be treated as a frictional material like sand or cohesive like clay.Commonly, surficial peats ar e encountered as submerged surficial deposits.Because of their low unit weight and submergence, such deposits develop verylow vertical effective stresses fo r consolidation and th e associated peat exhibithigh porosities and hydraulic conductivities comparable to those of fine sand

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Deformation an d Shear St rength Characteristic of Some Tropical Peat an d Organic Soils

or silty sand Dhowian and Edil 1980) . Such a mater ia l can be expected tobehave drained like sand when subjected to shear load ing. However, withconsolidation, porosity decreases rapidly an d hydraulic conductivity becomescomparable to that of clay. There is a rapid transition immediately from a welldrained mater ia l to a n u nd ra in ed material Edil et 1994 .

Determination of shear s t rength parameters for organic soils, as with othersoils, is important and somehow a difficult job in geotechnical engineering. Fororganic soils, several methods have been used to d et er mi ne t he undrainedshear strength in the laboratory namely Swedish fall-eone test, triaxial test,shear bo x test an d vane shear test. For the case of field tests, f ield vane andDutch Cone Penet ra tion tests a re o ft en used.

T ST PROGR MS

A series of laboratory an d field test have been carried out to study th edeformation and shear strength characteristics of tropical peat and organicsoils.

For the study on deformation, samples were collected from nine differentlocat ions in the state of johore, Perak, Sarawak and Selangor, generally atdepth between 0.5 m - 1.0 m below th e ground surface. The organic contentof th e samples r an ge fr om 50 to 95 , with natural water content in th erange of 200 - 800 , and liquid limit of 150 to 400 . In terms of VanPos t scale Landva and Pheeney 1980), th e samples were H to H9 that ishemic to sap ric peat. The consolidation characteristics of th e samples weredetermined from standard incremental oedometer tests, with sample size of 75mm diameter an d 20 mm high, and applied normal stress of 5 kPa to 160 kPa.The consolidation parameters were interpreted from the t radi tional log J vplots.

For th e study on shear strength, both laboratory an d field tests were carriedout. Several p ea t a nd organic soil samples were collected an d tes ted for theirundrained shear strength using laboratory shear box test with sample size of 60mm x 60 mm by 25mm thick. Th e samples ar e generally obtained at depth of0.5m below th e ground level. In situ tests were also performed on th e same siteusing a small 50 mm diameter field vane shear at depth of about 0.5 m belowground surface. The samples were coll ec ted f rom five different locations inSelangor an d Negeri Sembilan. The organic content of th e sample range f rom79 - 98 , with liquid limit of 160 - 377 , an d water content of 200

- 800 .In

order to e xa mi ne t he effect of degree of humification on th e soilshear strength, tests were done on samples with Van Post scale Landva an dPheeney 1980) ranging from to lO that is f rom fibric to hemic to sapricpeat.

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TEST RESULTS ND DIS USSION

Deformation Characteristics

i) Compression Index c, an d Liquid Limit WI effort is made to correlate compression index with liquid limit, wI voidratio, and ratio of c / 1 e ).

~ plot of o m p r ~ s s i o ni ~ d e x versus th e liquid limit of the soil showsthat c, increases with th e l iquid limit w L) of the soil as shown in Fig. 2. Farrellet a 1994) conside red the empirical relationship between the compressionindex an d the l iquid limit suggested by Skempton an d Petley 1970) for organicsoils equation 1) as to give a reasonable approximation of this parameter.Hobbs 1986) estimated the compression index fo r fen peat using equation 2,which gave a slightly lower value of Ct Values of c, of tropical peat samples testedhowever were apparently a little higher than th e above two relationships Fig.2).

C, = 0.009 w L - 10)

c, = 0.007 w L -10

1)

2)

Fig. 2: Compression index c) versus liquid limit wL)

The Cc values of the tropical peat studied range from 1 to 3, m uc h h ig he rthan sedimentary soil such as clay whose c, is only 0.2 - 0.8. is of interest tonote that th e c, of th e Irish peat range f rom 1 - 4, as shown in Fig. 3, which isquite close to th e tropical peat.

Azzouz et al. 1976) reported th e following relationship for organic soil an dpeat

C = 0.0115 W, 3)

Where w is soil natural water content in percent. Note t ha t th e natural watercontent of the peat s tu di ed ran ge f rom 200 - 800 . Using the aboveequation this would give c, of 2 - 9, which is higher than the measured values.

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Deformation an d Shear Strength Characteristic of Some Tropical Peat an d Organic Soils

4 . - - - - - - - - - - - - - - - - - -- - - - -

1 - 1 . M i l l I

_ _ lIL NT :.a C4

i

Fig. 3: Compression index c - liquid limit w/ Farrel et al. 1994

ii Void Ratio with Liquid Limit and atmal Water Content ig 4 shows a plot of th e initial void ratios versus liquid limits of th e peat andorganic soils from several sites in Malaysia together with th e normally consolidatedpeat found by Miyakawa 1960 , an d Skempton and Petley 1970 . The figureshows an increasing trend in void ratio with th e increase of th e soil liquid limits.Void rat io of th e tropical peat studied is found to r ange f rom 1.5 - 6 that is

J

Fig. 4: Relationship between the void ratio and liquid limit

for the case of amorphous peat For the case of fibrous peat it can be as highas 25. Such h igh void ratios gives rise to phenomenally high water contents Fo rcomparison Malaysian marine clay for ins tance, has an in itial void ratio in therange of 1.5 to 2.5. The natural void ratios of th e peat indicate t he ir h ighercompressibility.

ig 5 shows the graph of void ratio e an d natural water content w . Th ebest-fit line in th e above figure is expressed by:

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30.65 w o +0.88 116 -30e =o 2

Bujang B. K H u a t

4

As fo r th e case with liquid limit, void ratio increases with increase in naturalwater content. A similar trend of behavior is observed by De n Haan 1997 .

: .: - 4 J _ iJ ... 1 ...... 1 f ..

Ji u

1

: Dft:i.u isJ.n .. J .M :3 ~ J 2 } 1 I t u . ti : : t ~ Ja ~ l r . l * n . . . . . . . .

It, C - W l r l . S ~ h f h lV , r..,

- D.. lj ..... .,.wi

Fig. 5: Initial water content-void ratio

iii Compression Index Ratio, c/ 1+e , and Liquid Limit

Hobbs 1986 found that despite th e large variations, which occur within peatan d organic soils, th e variation in th e ratio of c / 1 +e is relatively small. Th evalues of this ratio determined f ro m t he l ab or at or y test on tropical peat soilsamples ar e plotted in Fig. 6. In agreement with Hobbs observation, th e trendobtained is similar to the other researches. However th e compression indexratios, c / 1 +e , of th e tropical peat ar e generally slightly higher than theothers. This is likely due to their h ig he r in s itu void rat io e . The value of depends on the in situ vertical stress; hence m us t b e that appropriate to th every low effective stress conditions.

Shear Strengths of Tropical Peat and Organic Peat Soil

i Shear Box TestThe results obtained f rom t he shear box tests ar e shown in Table The resultsshow that th e s he ar strengths of th e peat and organic soils ar e very low. Fromt he s he ar b ox test, it was found that th e soils have low cohesion, c , with valuesin th e range of 6 to 17 kPa. The angle of internal friction, tP , r ~ n sfrom 3to 20. The u values ar e generally lower with increasing degree of humification.

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D ef on na ti on a nd S he ar Strength Characteristic of Some Tropical Peat an d Organic Soils

ig 6: Relationship between c/ i+e o and the liquid limit

Similar trend is also observed fo r cohesion. A typical normal and shear stressesplot of t he s he ar box test is shown in Fig

1li

: .

r

k - --

10i '

u u II I Sir

ig 7: Typical relationship between shear stress and normal stress for sample withhigh fibrous content H i sample location: Banting, Selangor, Malaysia

ii Field Vane Test

From the vane shear test, th e undrained strength of th e soils were found tor an ge f ro m 3 kPa -15 kPa Table 2 .

The plot of moisture content against vane shear strength portrays a decreasing

behaviour of shear s t rength with increasing moisture content as shown in Fig8 At th e same moisture conten t the h igh fibrous Hi-H3 peat gives higherstrength compared with th e medium and low fibrous H4--HiO peat. general th e high fibrous or fibric H i - H3 peat has higher shear strengththan th e medium fibrous or hemic H4 - H6 peat an d th e low fibrous or sapric H 7 - 0 peat. The behaviour is more or less th e same when vane shearstrength is plotted against organic content Fig 9 According to Mitchell 1993 ,th e effect of organic matter an d stiffness of soils depends largely on whether

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TABLE 1Basic soil properties and labora tory shear box test results

of some tropical peat and organic soils

Location Moisture Organic Liquid Van Post Cohesion Angle ofcontent content limit scale kPa internal

friction deg

Banting, 211 85 294 HI 9 11 9-20Selangor 219 94 316 H6 11-12 9-12

802 83 362 HIO 6-10 12-20

Banting, 195 79 219 H2 6 11 9-16Selangor 832 84 361 H5 8-10 7-10

225 85 166 H8 8-12 6 11

Kg Jawa, 215 78 180 H3 10-12 6-14Klang 209 89 325 H6 12-14 7 25

786 85 368 H8 7 11 8-13

Kg Jawa, 680 85 298 H3 11-12 10-15Klang 747 93 352 H5 10-12 5-10

720 83 282 H7 7 9 9-12

Dengkil, 246 98 305 H2 13-17 3-12N. Sembilan 301 98 335 H5 11 13-15

786 83 377 H8 8 9 12-20

th e organic matter is decomposed or consist of fibres which can act asreinforcement.

According to Van Post scale Landva and Pheeney 1980 , t he degree ofhumification is graded on scale from 1 to 10 and designated from to HlO

ig 1 shows the decreasing trend between th e degree of humification an d th evane shear strength. The shear s t rength obtained from vane shear strength testdecreases gradually with high degree of th e humifications.

ig 11 shows th e field vane shear data, measured with the larger 100 mmdiameter vane, from a new mosque pro ject site at Putrajaya, Malaysia. There

is only a slighttendency fo r an

increasein strength

withdepth. The

low bulkdensity of peat together with high water table implies low effective stresses withdepth. Because of this there may not be a discernible increase of strength withdepth within th e peat layer 0.5 m - 4.0 m .

The field vane shear strength seems to relate well with results obtainedus in g t he small vane shear fo r soil with a similar degree of humification ass hown in ig 1 above.

Yogeswaran 1995 reported th e average field vane s he ar s tr en gt h fortropical peat found in Sarawak to be only 10 kPa while th e sensitivity ranges

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Deformation a nd S he ar Strength Characteristic of Some Tropical Peat an d Organic Soils

TABLE 2Basic properties an d vane shear strength parameters

of some tropical peat an d organic soils

Location Moisture Organic Liquid limit Van P ost Field vane

content content scale shear strength kPa)

Banting, 211 85 294 HI 10 - 12Selangor 219 94 316 H6 7 9

802 83 362 HlO 6

Banting, 195 79 219 H2 11Selangor 832 84 361 H5 10

225 85 166 H8 4

Kg. Jawa, 214 79 180 H3 11

Klang 225 84 325 H6 8618 88 368 H8 5

Kg. Jawa, 680 85 298 H3 10 - 15Klang 747 93 352 H5 5 - 10

720 83 282 H7 9 - 12

Dengkil, 246 98 305 H2 9 - 13N. SembiIan 301 98 335 H5 6 - 10

786 83 377 H8 3 - 6

150030010000

High fi r content H1.H3)

* Medium fi r content H4.H6) Low fi r content H7H9)

7000000

r22 20= 18 16 14 12 10

8642O - - - - - . . - - - - . - - - r - - - r - - - . - - - . . . - - - - - i

100

Moisture Content o)

ig 8: Plot vane shear strength versus moisture content

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th e degree of humification of th e soils, with more fibrous soils having higherundrained strength.

R F R N S

Azzouz, A.S., KRIZEK RJ. a nd R B. COROTIS. 1976. Regression analysis of soil compressibility.Soils an d Foundation 16(2): 19-29.

DEN lIAAN E. J. 1997. An overview of th e mechanical behaviour of peats an d organicsoils an d some appropriate construction t echniques . In Proceedings of onference onRecent Advances in Soft Soil Engineering ed. Huat an d Bahia, p. 17-45. Kuching,Sarawak, Malaysia.

DHOWIAN A.W. an d T.B. EDlL. 1980. Consolidation behavior of peats. ST GeotechnicalTesting Journal 3(3): 105-114.

EDlL T.B, P Fox an d L.T. LAN. 1991. End-of-primary consolidation of peat. TenthECSMFE Florence I: 65-68.

EDlL T.B. and X. WANG. 2000. Shear strength an d Ko of peats an d organic soils. InGeotechnics of High Water Content Materials ed . Edil an d Fox, p. 209-225. ASTM STP1374 American Society for Tes ting and Materials.

FARRELL E. R 1997. Some experience in th e design and performances of roads an d roadembankment on organic soils an d peats. In Proceedings of Conference on centAdvances in Soft Soil Engineering ed. Huat an d Bahia, p. 66 - 84. Kuching , Sarawak,Malaysia.

FARRELL E.R C. O NEILL an d A MORRIS. 1994. C ha ng es in th e mechanical properties ofsoils with variation in organic content. In Advances in Understanding an d Modeling theMechanical Behaviour of Peat p. 19-25. Balkema Rotterdam.

HARTLEN J. an d J. WOLSKI. 1996. Embankments on Organic Soils Elsevier.

HOBBS N. B. 1986. Morphology an d the propert ies and behaviour of some British andforeign peats. Quaterly Journa l of Engineering Geology 19: 7-80.

JARRETT E.P.M. 1995. Geoguide 6. Site investigation for organic soils and peats. JabatanKerja Raya (PWD), Malaysia.

LAN OV OA. and P.E. PHEENEY. 1980. Peat fabric an d structure. Canadian GotechnicalJournals 17: 416-435.

MICHELL J.K. 1993. Fundamentals of Soil Behavior. J. Wiley Sons Inc.

MIY K W J. 1960. Soils engineering research on peats alluvia. Reports 1-3. Hokkaido

Development Bureau Bulletin 20. Civil Engineering Research Institute.

SKEMPTON AW . and DJ . PETLEV. 1970. Ignition loss an d other properties of peats andclays from Avonmouth King's Lynn Cranberry Moss. Geotechniques 20(4): 4 ~ 5 6

TRESlDOER J. O. 1966. An overview of existing methods of road constructions over peat.Road Research Technical Paper No. 40.

YOGESW R N M. 1995. Geological considerations in th e development of Kuching areadialogue session. Geological and Geotechnical Considerations in Civil Works,Geological Survey of Malaysia. Kuala Lumpur.

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Deformgfggation and Shear Strength Characteristics Of