ABSTRACT
The rocks and soils underlying Lambata-Minna and Minna-Bida roads in central Nigeria were mapped with the view to determine their impact on the stability of the roads underlain by them. Vertical electrical sounding was done along the roads to determine the soil profile of the roads and statistics of the roads utilization was also done to infer if the roads are overused by vehicles. The geophysical studies revealed that the soil profiles of the two roads are composed of laterite, sand and clayey soils while the road statistics revealed that the traffic densities in the two roads are within the permissible limits. A total of 5 and 4 rock samples were collected from Lambata-Minna and Minna-Bida road respectively and each subjected to thin section, XRD and XRF analyses. The thin section and XRD were used to determine the mineralogical composition of the rocks while the XRF was used to determine the chemical composition of the rocks. Thirty five (35) and twenty eight (28) water samples respectively were collected from wells along Lambata-Minna and Minna-Bida roads and subjected to hydrochemical tests to determine the variations in the ionic concentration and physical properties of the groundwater with the underlying lithologies. A total of 47 and 60 soil samples respectively were collected along the same roads and each subjected to grain size distribution test using wet sieving to determine the soil group dominating the soil occurring on each lithologic unit. Thirty nine (39) and fifty (50) samples from the above were selected and subjected to Atterberg limit tests to determine their plasticity. Twenty two (22) and nineteen (19) samples from the above were subjected to compaction, permeability and California bearing ratio (CBR) to ascertain which of the soils are suitable for sub-grade, sub-base or base material. The field mapping, thin section and XRD results revealed that Lambata-Minna road is underlain by migmatites, gneisses, granites, marble, granodiorite and schist while Minna-Bida road is underlain by granites, migmatite, schist and sandstone. The chemical compositions of the rocks indicate they are mostly acidic rocks/protholith. The physico-chemical tests revealed that the groundwater occurring within the sandstone terrain of Minna-Bida road has the least ionic concentration and physical properties while that occurring within migmatite/schist terrain along Lambata-Minna road has the highest ionic concentration and physical properties. The grain size distribution test revealed that the soils occurring within the sandstone terrain are composed mostly of sandy soils (SW and SP) while those within the migmatite gneiss and granite terrain along Lambata-Minna road are gravely (GW and GP) soils. Permeability of the soils ranges from 4.73 x 10-4 to 9.78 x 10-3 cm/s. The compaction test revealed that the soils occurring within the sandstone terrain along Minna-Bida road has optimum moisture content (OMC) ranging from 9.4 to 18.0%. The OMC of the migmatite gneiss and granite terrain along Lambata-Minna road ranges from 18 to 27% while those within granite terrain ranges from 15 to 18%. The soaked CBR of soils within the sandstone terrain along Minna-Bida road ranges from 45 to 95% while those within the migmatite gneiss and granite terrain along Lambata-Minna road ranges from 0.9 to 70%. The unsoaked CBR of soils within the sandstone terrain along Minna-Bida road ranges from 70 to 144% while those occurring within the migmatite gneiss and granite terrain along Lambata-Minna road ranges from 5 to 70%. Results of the permeability tests revealed that permeability of the soils is generally low and does not vary with the different underlying lithologies. Grain size distribution, Atterberg limits, compaction and CBR reveal that soils underlying Minna-Bida road are generally more competent than those underlying Lambata-Minna road. The results also show that soils occurring within the sandstone terrain along Minna-Bida road are more stable than other portions of the studied roads and can satisfactorily serve as road sub-grade and sub-grade in their natural state. The consistent failure of the Lambata-Minna road portion underlain by migmatite gneiss and granite is attributed to the fact that the soils occurring within those terrains have poor geotechnical properties to serve as either sub-grade, sub-base or base material in their natural state.
CHAPTER ONE
INTRODUCTION
1.1 Background Information
Engineering geologists and geotechnical engineers are integral parts of design team for virtually all civil engineering projects like roads and buildings that involve site characterization and geotechnical design. To understand the geologic conditions of a site for civil engineering project and their implication(s) in design criteria, a common understanding of the site geologic origin and geotechnical properties of the construction aggregates to be used in the civil engineering project is essential. Generally, the engineering geologist provides basic information for the planning of land-use and for the design, construction and maintenance of civil engineering works. Such information is needed to assess the feasibility of a proposed land-use which will in turn assist in the selection of the most appropriate type and method of construction in order to ensure the stability of the intended structure as well as aid in the performance of necessary maintenance. Engineering geological research and mapping are therefore mainly directed towards understanding the interrelationships between the geological environment and the engineering structure; the nature and the geological relationships of individual geological components; the active geodynamic processes and the possible effect(s) that can result from the changes being made.
In Nigerian highway construction, attention is mostly paid to the sampling and testing of the aggregates to be used as the base/sub-base and wearing surface of the pavements as well as those serving as sub-grade. However, irrespective of billions of Naira spent by Nigerian government on the design and construction of these roads, most of the constructed roads do not render satisfactory service. The cause of these road failures can be attributed to the neglect of the role of geologists/engineering geologists in highway design by Nigerian contractors/civil engineers. For example, in choosing the route for the highway, one of the important factors to be considered is geology (lithostratigraphy and hydrogeological conditions) of the intended route. Such information can only be confidently ascertained by a geologist/engineering geologist.
Research has shown that the stability of pavements constructed in tropical regions is controlled mostly by the geological/hydrogeological, soil, climatic and drainage conditions of the terrain as well as the design technique, type of aggregates used, construction procedure and age of pavement (Clare and Beaven, 1962; Tanner, 1963; Gidigasu, 1974, 1975, 1983; Okagbue and Uma, 1988). Works by Weinert (1968), Farquhar (1980), Okagbue and Uma (1988) have shown that geological conditions along a highway route are important factor to the proper performance of the highway. Since roads are built on and with geologic materials (rocks and soils), a good knowledge and understanding of these materials is therefore vital for the successful construction of roads.
1.2 DESCRIPTION OF STUDY AREA
1.2.1 Location and Accessibility
This
study was done in North-central Nigeria in the area lying between longitudes 6o01’E
and 7o00′ E and latitudes 9o05I
N and 9o 36ˈN (Fig. 1.1). The study was centered along Lambata – Minna
Road (LM-R) and Minna – Bida Road (MB-R). The Lambata – Minna Road (LM-R) is a
trunk A road constructed/maintained by the Federal government that links most
of the North western state to the Federal Capital Territory. The Minna – Bida
Road (MB-R) is a trunk B road constructed/maintained by the Niger State
Government. The Lambata – Minna Road (LM-R) covers a linear distance of 76 km
and the Minna – Bida Road (MB-R) covers a linear distance of 84 km. Some
important towns along the road traversed are Bida, Mina, Kataeregi, Mingida,
Guto, Pita, Lambata and Gawu.
1.2.2 History of the Roads
Construction work on the Minna-Bida Road started in 1983 and was completed in 1985 by two different companies. The road portion from Minna to Kataeregi (approximately 45 km) was handled by Albishiri Nigeria Limited while the portion from Kataeregi to Bida (approximately 39 km) was handled by Public Works Nigeria Company Limited. Laterite was used as sub-grade and sub-base course while bitumen surface dressing was used as wearing course. Despite series of maintenance, the road has always been in bad shape. Routine remedial work was done before 1999. In the year 2003, Triacta Construction Company re-surfaced the entire road with asphalt. A good portion of the road is presently in a deplorable condition. Examples are shown as Figure 1.2a, and 1.2b.
Lambata-Minna Road was also constructed and upgraded between 1988 and 1994 by two different companies. Minna – Kakaki portion (approximately 34 km) was constructed by Julius Berger (Nigeria) Limited while Kakaki – Lambata portion (approximately 44 km) was handled by Setraco Construction Company simultaneously. In the case of Lambata-Minna Road, laterite was used as the sub-base, crushed stone as base and asphaltic concrete as wearing course. The two portions of the road failed soon after construction and no major rehabilitation has been done since then. Example of the failed area is shown in Figure 1.2c. The portions observed in the most deplorable state are shown in Figure 1.2d.
1.2.3 Relief and Drainage of the Area
The study area is made up of undulating hilly terrains that range from 300m-800m high above mean sea level. The area is drained by Rivers Chanchaga and Wuya which are tributaries of the Kaduna River. According to Abdullahi, (2010), most of the drainages, which are mostly trellis pattern, cut narrow valleys as water flows from the hills to the valleys during raining season. Schists are mostly exposed along river channels.
1.2.4 Climate, Vegetation and Land Use
The study area which is within the middle belt of Nigeria has a mean annual rainfall of 1,100mm ( figure 1.3). Rainfall starts usually in April, peaks by August/September and ends in October. The temperature is low between the months of July and September, at an average of 24oC. High temperature is usually recorded during the months of January to March, at an average of 350C. The harmattan wind is experienced between December and February. Vegetation of the area is mostly grasses with sparsely populated trees.
The weathered products from the rocks in the area give rise to agriculturally rich soils. Farming is therefore the main economic activity of people living in the area. They produce crops like guinea corn, maize, melon, groundnuts, rice and yam. Those that live along the river banks are mostly fishermen. Some few women are engaged in pottery works using the weathered clayed products from the granitic rocks. A few people are also involved in illegal mining of gold, gravel and sand especially around the Chanchaga River.
1.3 STATEMENT OF THE PROBLEM
The Nigerian government spends billions of Naira on road construction/maintenance and yet most of these roads are not found in stable and good conditions most of the time. Most often, the maintenance of these roads involves the replacement of the wearing surface which also does not stand the test of time. Works like Gidigasu, 1974, 1975, 1983; Okogbue and Uma, 1987 have shown that the stability of pavements in tropical regions depends on a number of factors which include the type(s) of aggregates used, design technique, construction procedure, age of pavement as well as the geologic, soil, climatic and drainage conditions. Often, the geology of the area and the geologic materials used in construction play the most significant roles on the stability of the roads.
1.4 AIM AND OBJECTIVES OF THE RESEARCH
The aim of the research was to evaluate the geotechnical characteristics of the sub-grade, base and sub-base of Lambata-Minna and Bida-Minna roads. The specific objectives include:
- To produce an up-to date and accurate geological map of the area that will show the rock types along the roads.
- To carry out geophysical survey in order to determine the various depths to bedrock along the stretch of the roads.
- To investigate the geotechnical properties of the in-situ soils so as to determine their suitability as road construction and road foundation materials.
- To produce an engineering geological map of the area
- To establish the impact of geology on the performance of the roads
1.5 PREVIOUS WORKS
Some previous works that relate to geology and highway studies are presented below under the following aspects:
1.5.1 Engineering geologic mapping
Malomo et al (1983) carried out the engineering geological mapping of Abuja, the Federal capital of Nigeria. Their investigation involved field mapping comprising the determination of lithology, structures, weathering character and laboratory tests such as particle size distribution, Atterberg limits and compaction. Particular emphasis was placed on housing sites, heavy building structures, highways, tunnels and underground structures. Their work contributed to the infrastructural development of Abuja, the Federal capital city of Nigeria.
1.5.2 Lateritic soils
Lateritic soils occur in most parts of the tropical world (latitude 300 North and South of the equator) and have found wide application both as foundation and aggregates for construction of structures like highways, houses, dams etc (Gidigasu, 1972). Gidigasu (1978) showed that progress in the field of identification and evaluation of laterite for engineering purposes depends on the simultaneous consideration of all the major factors which affect the behavior of rocks and their derived soils (e.g. rock type, weathering condition, degree of weathering, geological origin, their chemical and mineralogical composition). According to him, such a pedological approach is bound to make predictions and assessments of the engineering behavior of laterite soils more accurate. Adeyemi and Abolurin (2000) investigated a granite-gneiss derived lateritic soil taken from around km 4 along the Ile –Ife/Sekona road in South Western Nigeria. Their study was centered on the determination of the unconfined compressive strengths of samples stabilized with cement, lime and mixtures of both. Okogbue (1986) studied some laterite gravels from southeastern Nigeria, which mostly formed from sandstones and shale, in order to evaluate their physical characteristics that affect their strength. His work showed among others that the lateritic gravels commonly used for highway construction in eastern Nigeria performed satisfactorily because of their high strength, low water absorption and high specific gravity. Those that performed badly are those that are weakly indurated.
1.5.3 Pavement failures
Failure of flexible highway pavement is a common phenomenon in most parts of the tropics. Some workers like Ajayi (1982) attributed some of these failures to misuse or poor construction as the pavement he studied was founded on saprolite rather than on strong lateritic horizons. Adeyemi (1992) investigated some geotechnical properties of the residual lateritic soil in the Ajabe and I’slare area adjacent to some sections of the Lagos – Ibadan expressway, South Western Nigeria. He found out that the degree of stability of the flexible road increased with the kaolinite content, California Bearing Ratio (CBR) and unconfined compressive strength (UCS) of the subgrade soil. Ayangade (1992) observed that there was a positive correlation between the strength characteristics of the foundation soils and the stability of the pavement along the Osogbo – Gbongan road, south western Nigeria.
Adeyemi and Oyeyemi (2000) carried out an investigation along Lagos – Ibadan expressway with an attempt to identify the geotechnical and geological factors that are likely to have the greatest influence on the stability of this highway. The result of the investigation of the subgrade revealed that the soils below the stable section have higher maximum dry density (MDD), unsoaked California Bearing Ratio (CBR), uncured unconfined compressive strength than those below the unstable section. The soils below the stable portions have a lower proportion of fines and clay sized fraction and a lower optimum moisture content and linear shrinkage than the material below the unstable section. They surprisingly noted that soils in the portion below the unstable pavement had a lower plasticity index and higher soaked CBRs than those below the stable pavements. Their investigation showed that significant difference need not exist between the geotechnical properties of soils below stable portions and unstable portions and that such parameters (geotechnical properties) can serve as bases for predicting the stability of flexible highway pavements in the tropics.
The incidence of highway pavement failures and the maintenance operations for Nigerian highway systems has been investigated by Jegede and Oguniyi (2004). The study revealed that the pot holes, cracks, pavement incision, corrugation and rutting common on the studied roads was as a result of compacted edges of the pavements and non-provision of drainage facility along the roads, low California Bearing values among others. Mineralogical and geotechnical characteristics of some subgrade soils in a section of the Ibadan – Agowoye expressway were investigated by Adeyemi et al (2003). Kaolinite was found to be the most abundant mineral in the soil sample while other minerals include quartz, illite, chlorite and vermiclulite. They found out that although the soil falls into the group A5 of the American Association of State Highways and Transportation Officials (AASHTO) Classification System, which indicates poor to fair subgrade soil, their plasticity and strength characteristics (unconfined compressive strength) are typical of good subgrade soils. They also established strong correlation coefficients of 0.955 and 0.844 between the amounts of kaolinite and unconfined compressive strength respectively. They thus attributed the observed remarkable stability of pavement in the study area to the preponderance of kaolinite the high strength of subgrade. Some laterite soils from different terrains along Lagos–Ibadan expressway in South Western Nigeria have been studied by Adewoye and Adeyemi (2004) with the aim of establishing the geotechnical basis for the stability of the flexible highway pavement in parts of the expressway. Their studies showed that the degree of the stability of the flexible highway pavement can be said to depend to a large extent on the geotechnical characteristics of the subgrade soil. Adebisi and Oloruntola (2006) carried out a geophysical and geochemical evaluation of foundation condition of a site in Ago-Iwoye area, South western Nigeria where they emphasized the usefulness of geophysical method in complimenting geotechnical studies in establishing variation in lithology accompanied by variation in allowable bearing pressure of foundation soils. Some other works like those of Weinet (1968), Gidigasu (1976), Farquhar (1980), Mesida (1987), Ajayi (1987), Ayangade (1992) and Adeyemi (1992) have shown that the majority of highway failures can be attributed to geological, geotechnical and hygrogeological factors.
In terms of design and construction, Gidigasu (1983) attributed the causes of road failure to one or more of the following reasons: deterioration of the base and sub-base materials; inadequacy in pavement thickness; weakness in the shoulders and inadequate drainage allowing water to enter the pavement structure. Base and sub-base materials deteriorate if water comes into contact with them and areas that are not properly drained do not meet adequate specifications on allowable plasticity and content of fines. Road failure could also result when inferior base and sub-base materials are used and when specifications for the thickness of the pavement are not followed during construction. Gidigasu (1983) have shown that, an average thickness of 6 inches was used in 40% of the failed roads out of 2150 surveyed roads in West Africa instead of 11 inches specified thickness for the pavements. Inadequate width of the shoulder which provides lateral support to the pavement would result to road failure especially when the shoulders are made of poorly compacted cohesive soils. Rain water penetration during the wet season weakens the material in the road pavement and evaporation of soil water from the clayey shoulder material during the dry season causes soil moisture suction under the road (Gidigasu, 1983). Deterioration of the pavement tends to increase in both conditions. Clare and Beaven (1962), and Okogbue and Uma (1988) have observed that the patterns of pavement performance in West Africa are considerably controlled by geology, topography, soil and drainage conditions. Russam and Croney (1961), Gidigasu (1983) and Okagbue and Uma (1988), have noted that depth to water table appears to be the most dominant of the climatic, topographic and drainage factors that affect pavement performance. According to them, when the water table is at the depth of less than 1m, the chances of failure seem to be highly independent of the climate and other environmental conditions.
