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ENGINEERING PROPERTIES OFSUBGRADE SOILS IN IMO STATE

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Foreword-

This report covers a phase of the Institute's research programme in which the main subgradesoils of Nigeria are investigated in an ordered manner to establish their engineering classification andproperties. The information and data generated will be found invaluable to hiqhwav planners, desig-ners and contractors who can obtain from these data a fairly accurate assessment of the subgrade soilproperties even before they engage in any field work.

NBRRI Report No. 13 describes the investigation of subgrade soils in Imo State of Nigeria. Inview of the high erosion potential of this area, it is hoped that highway engineers VIi II find the resultscontained in this publication very useful in their preliminary design and construction programmes.

A. O. MADEDORDirector

December 19QO

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The Nigerian Building and RoadResearch Institute embarked on a.programme of studying the enginee-.ring characteristics of subgrade sol Isin .N~geria starting with problemsoils. The overall long term objec-tive was to develop a comprehen-sive road soils map and the relevantclassification and basic engineeringdata with regard to economicdesign and construction of roads.The informa tion and data generatedwill constitute a data base and aninvaluable aid to road planners,designers, highway engineers, con-tractors etc., not only in road routelocation and land resource evalua-tion but also assist in the prel imi-nary preparation for detailed fieldwork in the area of interest.

published. Investigations - ...-grade soi Is of the FederalTerritory and Cross Rive :12'"=,ha~e been concluded, andreports are being published.

1. BACKGROUND INFORMATION

1.1 Introduction

1.3 Present Work

The present project focuses 0'"study of the engineering charac-teristics of subgrade soils of lrroState. The scope of the work isthree-pronged and involves:

(i) conDucting detailed field 'n-vestigation of Imo State 'sub-grade soils

(i i) conducting a comprehensivelaboratory testing programmeon field samp les, and

(iii) analysing test data to charac-terise Imo State soils

1.2 Past Work 1.4 Geology

Under th is programme, subgradesoils in Nigeria were investigatedin an ordered manner starting withthe problem soils. AI ready theblack cotton soil of Bauchi, Bornoand Gongola States as \tW1las thesubgrade soils of Rivers State havebeen investigated and the reports

1.4. 1 The general topography of ImoState is characterised by gulliedhillslopes underlain by unconsol i-dated sedimentary rocks wh ichdate back to the Upper Cretaceous.The general stratigraph ic lithologyin which Imo State lies is shownin Table 1.

T.able 1. Stratigraphic Succession of Sedillientary Rocksin lmo State (After Kogbe 1976, Ola 1985)

Age Formation Lithology.Quaternary Miocene Alternation of coarse

-Pleistocene Benin - sands with clays andshales

Oligocene-Miocene Ogwashi - Asaba Clays with lignite

TertiaryEocene Ameki Grey clayey sandstone

and sandy claystone

Paleocene Imo Shale Lami na ted clayey shales

Upper Nsukka Shales with coal andCretaceous Maestrichjan Ajali sandstone beds

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1.4.2 The Nsukka Formation consists ofalternating sequences of sandstone,dark shale with thin coal seams andlies unconformably over the Ajalisandstone (Reyment, 1965).

2. FIELD INVESTIGATION

A three-week field investigation wasconducted to cover the state. Thescope covered visual observation andsampling.

1.4.3 The Imo formation consists of thick•

grey, fine-textured clayey shale withoccassional admixture of clay iron-stone and sandstone bands. The typearea is along the Imo River betweenUmuahia and Okigwe where it attainsa th ickness of about 500 meters andrests conformably on the NsukkaFormation, (Azeez, L.O., 1976).

2.1 Visual Observation

The general conditions of the pave-ment, drainage, soil and topographyetc. were observed and some of theseare discussed.

2.1.1 Highway Pavement

1.4.4 The Ameki Formation overlies theImo Formation and consists offossi Iiferous green-grey clayey sand-stone and sandy claystone (Kogbe etai, 1976). This is overlain by theOgwashi - Asaba Formation consis-ting of an alternation of lignite seams,clays and sandstone. Above this is theBenin Formation made up of yellowand white sands and sandstone.

There are few existing old roadscompared with new/modern roads.Some of the existing old roads includeAba - Ikot Ekpene Road, Umuahia --Afikpo Road, Okpuala - MberichiRoad, Omoku - Oguta Road etc.These roads generally have poorgeometric standards and narrow singlecarriageways. They are characterisedby snarp vertical and horizontal curves(Photos 1a and 1b), and they exh ibitsimi lar pavement distresses characte-rised by excessive potholes, pavementdrop-offs (Photos 2a and 2b), etc.

1.4.5 The development of the relief isclosely related to the lithologicarrangement. In general, argillaceousbeds underlie the valleys with sandsforming ridges and escarpments. Theargillaceous rocks are composed ofshales, clays and' mudstone wh ich areoften affected by the process oflaterisation, (Grove A.T., 1951). How-ever, the arenaceous rocks are com-posed of unconsolidated sands andsandstones which are characteristicallycross-bedded.

On the other hand, the newlyconstructed roads such as the PortHarcourt - Enugu Expressway, Aba -Owerri Road, Orlu - Okigwe -Umuahia Road etc. have acceptablegeometric specification with the

•• •carnaqewavs In some cases rangingbetween 7.0 and 7.4m in width whilethe shoulders are 1.0 to 2.5m wide.

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1.5 Gully ErosionSome of these new roads exhibitpremature pavement distress alongsome sections but to a lesser degree.

Some of the sandy deposits are veryfine, loose, unconsolidated and friable.Where these become exposed to highrainfall intensity, they are prone toexcessive erosion which often leads tothe formation of erosion gullies thatare prevalent in the area. The develop-ment of erosion gullies have tremen-dous impact on the land use in thearea. Indeed there are many instanceswhere erosion gull ies have threatenedto undermine or undercut existingroads.

2.1.2 Pavement Shou Ider

In some cases, the pavement distressesalong the old roads are so much thatmotorists resort to using the pavementshoulders. A typical example is theAfikpo - Umuahia Road, Photos 2a,2b. The distresses also extend to theshoulders but to a lesser degree. Mostof the shou Iders are overgrown withweeds, Photo 1b.

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