SOIL INVESTIGATION OF FAILED PORTIONS OF CHOBA, EAST-WEST ROAD

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SOIL INVESTIGATION OF FAILED PORTIONS OF CHOBA, EAST-WEST ROAD

ABSTRACT

There is an increasing incidence of road failures in Southwestern Nigeria. Many asphalt roads have failed sections soon after construction and rarely reach their design life span before major repair is required. The aim of this study is to conduct geotechnical properties of sub-grade soil along failed portions of Choba, East-West road in the Northern part of Ondo State, Southwestern Nigeria with a view of determining the possible causes of the failures experienced along the road. Four samples were collected from four different locations and stabilised with varying percentages of rock fines. Laboratory tests conducted on the soil samples were natural moisture content, liquid limit, plastic limit, grain size analysis, linear shrinkage, specific gravity, compaction and California bearing ratio (CBR). The test results showed that the natural moisture content of the soil samples ranges from 9.3 to 26.4%, the liquid limit from 0.0 to 46.4%, plastic limit from 0.0 to 40.0% and plasticity index from 0.0 to 31.61% which did not conform to specifications. The grain size analysis shows that the amount of fines passing 425µm sieve ranges from 33 to 51% which are higher than the standard specifications. The linear shrinkage ranges between 5.8 and 15.3%. The specific gravity ranging

Background of the study

Road networks play a vital role in the economy of any nation especially in developing ones like Nigeria, which require good roads for transportation of goods and services [1]. Roads aid easy and smooth vehicular movement. In developing countries where other means of transportation such as rail, underground tube, air and water transport systems have remained largely under-developed, roads remain indispensable in the transportation of people, goods and services from one point to another. Oguntuase [2] and Igwe et al. [1] broadly classified the road network in Nigeria into three distinct groups namely: Urban roads, inter-state and inter-city highways and the rural roads. Urban roads are wide paved roads found within cities and towns. The inter-state and inter-city highways are those that connect two or more States together. The third category of roads is the rural roads which are mainly earth roads. These roads connect farms and villages with inter-city roads. Choba, East-West road falls within the second category of roads (inter-state and inter-city road). A sub-division of the Nigerian road system was done by Fadaka [3]; Nnama [4] and Akobi [5] considering important details such as ownership, construction and maintenance responsibility. The road serves as inter-links between towns, states and countries and therefore is known as the common means of transportation.

Road failure is generally detected and observed as cracks, depressions and removal of the bituminous pavement, which exposes the subbase. The punctuation in a smooth ride is generally regarded as road failure [6]. Consequently, the pavement will no longer be able to absorb and transmit the wheel loading through the fabric of the road without causing a fairly rapid deterioration of the road pavement. The rate at which roads (highways) fail in Nigeria is quite alarming in recent times and this has been causing concerns among the stakeholders in the maintenance of Nigerian highways. Most of these roads have become death traps in many places across the country with the attendant loss of lives and properties. The failed segments of major highways across the country usually display a variety of failed pattern ranging from pitting, rutting and waviness patterns. A number of contributing factors have been identified, such as mis-use of highways or over-use of highway, lack of maintenance culture, poor construction practices and geotechnical factors [7].

Engineering properties have been identified as the major factors influencing the failure of many roads within and outside the country. Despite the technological improvement in the country, the cause of pavement failures has remained a serious concern in almost all the major highways in Nigeria. A pavement is the durable surface material laid down on an area intended to sustain vehicular or foot traffic, such as road or walkway. The terms base and sub- base are sometimes used interchangeably to refer to the sub-surface layers of a pavement, sub-base is the layer of aggregate material laid on the sub-grade on which the base course layer is located. It may be omitted when there will be only foot traffic on the pavement, but it is necessary for surfaces used by vehicles. The base and sub-base layers at the study area were exposed by the different grades of pitting along the road. Road failures are almost invariably linked directly to the underlying soil (sub-grade soil) which usually bears the load carried by the road. The term “sub-grade” as used in Civil Engineering refers to the in-situ material upon which the pavement structure is placed. It may be the material reworked in-situ or even the imported material from another source such as a cut. Sub-grades are also considered layers in the pavement design, with their thickness, assumed to be infinite and their material characteristics

assumed to be unchanged or unmodified. The sub-grade serves as the foundation for the highway pavement and distributes the load from the upper layers to the soil underneath. A satisfactory sub-grade is able to resist the effect of both traffic and weather. Under heavy traffic loads, sub-grade soils may deform and contribute to distress in the overlying pavement structure manifesting as failure. It is therefore very important to adequately prepare this layer if the road is to last.

The causes of roads failure in Nigeria have been attributed to several factors such as poor construction materials, poor design and specification, road usage, poor drainage, geological and geotechnical factors [8,9]. Geological factors are usually not considered as precipitators of road failure even though the highway pavement is founded on the geology. The proper design of highway requires adequate knowledge of subsurface conditions beneath the highway route. Previous studies have revealed that the integrity of the highway pavement can be undermined by the existence of geological features and/or engineering characteristics of the underlying geologic/geoelectric sequences [10,11]. Field observations and laboratory experiments carried out by Ajayi [9] showed that road failures can arise from inadequate knowledge of the geotechnical characteristics and behaviour of residual soils on which the roads are built and non-recognition of the influence of geology and geomorphology during the design and construction phases.It is crucial to develop a sub-grade with a California Bearing Ratio (CBR) value of at least 10 [12]. Islam [13] reported that if a sub-grade has a CBR value less than 10, the sub-base material will deflect under traffic loadings in the same manner as the sub-grade and cause pavement deterioration. The stability and durability of pavement depends on the traffic load or intensity and the strength of pavement layers [14]. Pavement failure is certain to occur if the pavement is not perfectly designed with the present traffic condition being considered. If it is designed without considering incremental traffic in the near future, the pavement life will be successively reduced. A flexible pavement surface reflects the entire behaviour of the sub grade layer, it thereby, emphasises more attention on making the soil sub-grade of superior soil properties. In previous research, road failure occurred due to the following reasons: Inadequacies in pavement structural design [15,16], poor sub-grade soil properties [17], insufficient pavement drainage [18] and effect of seasonal fluctuations of temperature over base course aggregates [19] and poor visco-elastic properties of asphalt binder [20]. Adewoye et al. [21] observed that the flexible highway failures along the OyoOgbomoso road, which manifested as waviness/corrugation rutting and potholes were due to environmental factors such as poor drainage, lack of maintenance and misuse of the highway pavement. It was also noted that runoff due to precipitation, largely found its way into the pavement structure damaging them in the process. A road is usually divided into different parts namely: pavement, base, sub-base and sub-grade. Sub-grade is the soil foundation which directly receives the traffic loads from the pavement and a satisfactory subgrade resists the effects of both traffic load and weather.

The aim of this research is to examine the effect of rock fines on the geotechnical properties of sub-grade soil materials along failed portions of Choba, East-West road, South-western Nigeria in order to reassess their direct productive application as soil materials or the need for incorporating the stabilisation method to enhance their properties for flexible road construction.

CONCLUSION

From the field observations and various laboratory tests carried out on the soil samples collected from the failed sections of the road, the following conclusions were made:

i) Lack of provision of the drainage system on the highway leads to the reduction in the strength character of the soil as a result of ingress of water. The values of natural moisture contents of this failed road indicating that the load bearing capacity of the soils increase rapidly as the moisture content values are lower than the plastic limit values. The higher liquid limit values obtained for most of the soils may have contributed to the failure of those sections as liquid limit correlates to the compressibility of soils.

ii) The linear shrinkage of most of the soils are greater than 8%, this indicates that there will likely be shrinkage problem; most of the samples have linear shrinkage values greater than 10%, hence will pose field compaction problem which might have also contributed to the failure of the sections of the road.

iii) The maximum dry density values of the soil samples classify them as poor highway foundation materials. This poor compaction character can also result in the failure of the sections of the road pavement.

iv) Despite the addition of the stabilisation material in different percentages of 0, 2, 4, 6, 8 and 10 to the soil samples, the CBR values obtained still fall below standard. This is likely to be caused by the poor laterisation of the soil used

v) Sequel to the above findings and conclusion, it is recommended that all the materials used for road construction should conform to specification. State and Local Government should have a maintenance agency to embark on routine, periodic and emergency maintenance; this will also prevent the road from deteriorating before their design life.

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