Geotechnical investigations for infrastructural development: A case study of Daki Biyu District, Federal Capital Territory, Abuja, Central Nigeria

The geotechnical properties of Daki Biyu district in the Federal Capital Territory, Abuja, Nigeria was investigated to ascertain the suitability of the sub-surface soil to support massive infrastructure such as high rise building and industries that might be carried out in the future. The particle size distribution shows that the soil is predominantly sandy-clay to sandy-gravels. The plasticity indices suggest low to medium compressibility while the co-efficient of volume change (Mv) and the coefficient of consolidation (Cv) are generally low for most of the pressure ranges. The average allowable bearing capacity values of 150 kN/m 2 -240 kN/m 2 and an average of 460 kN/m 2 -700 kN/m 2 for the ultimate bearing capacity agrees with the national building code (1983) for safe bearing capacity for cohesionless soils. The sub-surface soils were found to possess good geotechnical properties that are capable of supporting infrastructural development.


INTRODUCTION
Sequel to the relocation of the Federal Capital Territory (FCT) from Lagos to Abuja on the 12 th day of December, 1991, the city has experienced rapid population growth (Dikedi, 2012).This has resulted to the need for land for infrastructural developments such as high and low rise buildings for offices and residential purposes, water supply pipelines to all parts of city, sewage treatment and purification plants as well as construction of fly-over bridges etc.This has posed serious concern to engineers, engineering geologists and planners alike.A good understanding of the geology and geotechnical characteristics of the rocks and soils in the Capital Territory is essential for planning and construction of all engineering structures because of the heterogeneous nature of rocks and soils which these structures are constructed on.The importance of geotechnical investigation as well as engineering geology has lately been emphasized and encouraged due to reported cases of structural failure and collapse especially in Lagos, Abuja and other parts of the country (Ejembi, 2016).Geotechnical site investigation involves among others the need to assess the general suitability, safety and economical design of foundations and temporary works, knowing the nature of each stratum and engineering properties of the soil and rock as well as foresee and provide solutions against difficulties that may arise during construction due to ground conditions (Clayton et al., 1996).The complexity of a site investigation depends upon the nature of the ground conditions and the type of engineering structure (Bell, 2011).Accordingly, a site investigation should attempt to foresee and provide against difficulties that may arise during construction because of ground and/or local conditions.Such investigations should reveal the strata that would be significantly affected by the structural load, the ground water condition, the degree and extent of weathering, the structural orientation of the rock masses is equally important.Also worthy of note and investigation are areas that were previously used for mine workings as they may pose as potential areas of subsidence (National Building Code, 1983).This information when carefully recorded and interpreted with accompanying geological sections and where possible a 3D picture of the subsurface strata can be of great assistance in the design and subsequent construction of the super structure.
It is in this regard that the sub-surface geotechnical investigation of Daki Biyu and environs in the Federal Capital Territory, Abuja was conducted with the aim of carrying out particle size distribution analyses for engineering soil classifications as well as the determination of the bearing capacity and settlement rates necessary for foundation design of structures.The data generated and analysed will help in ascertaining the suitability of the site for future infrastructural development.

Location and geology of the study area
The study area is located within the Federal Capital Territory (FCT), Abuja and lies between Latitudes 9.058°N and 9.035°N; and Longitudes 7.412°E and 7.434°E (Figure 1).It covers an area of 6.15 km 2 and is defined by two renowned rock formations; the Zuma Rock and Aso Rock.The lowest elevation is found in the extreme south western part of the city where the flood plain of the River Gurara is at an elevation of about 10 m above sea level.The land rises irregularly eastward, northward and north westwards.The highest part of the territory is in the northeast where there are many peaks over 76 m above sea level.In some other areas within the territory, there are many rounded and isolated hills called inselberg.The Gwagwa plains lie between the major hills and are very extensive.The surface geology is composed of predominantly high grade metamorphic and igneous rocks of Precambrian age which trend in NNE-SSW direction.These rocks consist of gneiss, migmatites and granites.Schist belt outcrops exist along the east margin of the area.This belt broadens southwards and attains a maximum development to the south eastern sector where the topography is rugged and the relief is high.In general, the rocks are sheared (Kogbe et al., 1983).The rocks can be divided into the following major groups: Migmatite Complex (migmatite, migmatite gneiss, granite gneiss, porphyroblastic granite gneiss, leucocratic granite gneiss), Metamorphosed Supracrustal rocks (mica schist, marble, amphibolites and amphibole schist, fine-medium grained gneiss); and minor intrusions such as rhyolites, quartz-feldspar porphyry, dolerites and basalts.Others include quartzite, pegmatite and quartz vein (Rahaman, 1988) (Figure 2).

METHODOLOGY
This involves the use of two major equipments for the collection of samples; a number 4 2.5 ton Dutch cone penetrometer which was employed for in-situ bearing capacity determination and a number 4 Percussion Dando 150 motorized shell as well as Auger rigs which employ light cable percussion drilling techniques (Johnson and Degraff, 1988).The rigs were used for the collection of disturbed and undisturbed soil samples; and the execution of Standard Penetration Test.A total of sixty penetrometer tests and thirty boreholes drills were executed.The maximum depths of most of the boreholes were determined at depth to basement though at some instances hard pans were encountered and stopped.The resistance to penetration was recorded at every 0.25 m for each penetrometer test point.Some basic sampling and testing procedures employed in the research also included; collection of disturbed samples at each change of strata and execution of standard penetration test in cohesionless and mixed soils at intervals of 1.50 m with disturbed samples recovered from SPT spoon (Sanglerat, 1972).A split spoon, 50 mm was driven into the soil by a mass of 65 kg hammer dropping from a height of 760 mm.The resistance to penetration was then expressed as the number of blows (N-blows) required to penetrating a distance of 0f 300mm below an initial thickness of 150 mm.Collection of undisturbed 100 mm diameter (U4) samples in cohesive strata and noting depths at which ground water was struck.All samples retrieved from boreholes were carefully logged while representative disturbed samples were subjected to classification test and undisturbed samples were subjected to Oedometer consolidation test which is of immense importance for estimation of settlement.Hence, results obtained were plotted and examined in CorelDraw ® , Surfer ® and ArcGIS ® softwares.

Hydrogeology of the study area
Substantial settlement may occur when water table rises up to or beyond the footing level of shallow foundations (Shahriar et al., 2013).This settlement can threaten the integrity of a structure and may subsequently lead to the collapse of the entire or part of the structure.The hydro stratigraphic units consist of the top soil which is mostly made up of dark grey sandy-silty-clay which ranges in depth from 0 -0.25m.This is mostly followed by reddish-brown firm sandy-silty-clay with an average depth of 1.30m from the surface.Above this layer is an intercalation of soft sand and clay which ranges in depth from 1.50 -4.50m.The aquifer consists of reddishbrown hard sandy-silty-clay followed by dark grey hard weathered rock whose thickness ranges from 5.50 -9.0m.The groundwater levels ranges between 4.50 -5.25m below the ground surface in almost all the boreholes of the study area and these are however subject to seasonal fluctuations.

RESULTS PRESENTATION
Figure 3 presents the sampled points from the study area while Table 1 shows the particle size distribution and plasticity characteristics of some tested soils.Some of the tested soils were analysed graphically and categorized into various grain sizes (Figures 4 and 5).Again, some of the samples were henceforth tested for their liquid limit competencies.Table 2 shows the consolidation parameters obtained from the compressibility and consolidation analyses of soil samples within the area.Allowable bearing pressure and ultimate bearing capacity of different samples as well as their undisturbed graphical plots are presented in Table 3.The drill details are thus presented in Appendix A.

Grain size distribution analysis
The particle size distribution analysed from the disturbed soil samples obtained from boreholes differs significantly.The soil samples are mostly well graded ranging from clay, silt, sand and gravels and in some locations, a combination of two or more of the above named soils are observed.Dense sands and gravels are important foundation soils because they can carry weight in excess of 600 kN/m 2 with minimal settlement (Terzaghi et al., 1967).Therefore, areas with dense sands and gravels can accommodate structures whose weight exceeds 600 kN/m 2 whereas places with particularly loose sand, soft clays and silts should not be loaded above 150 kN/m 2 , this to ensure stability and integrity of the structure.

Plasticity characteristics
Plasticity parameters also called Atterberg limits are important indices used alone or with other

Consolidation test
The main objective of consolidation test on soil samples is to obtain necessary information about the compressibility of soils for use in determining the magnitude and rate of settlement of structures, Terzaghi, et al. (1996).Carter (1983) gave 0.05×10 -3 kN/m 2 as the permissible limit of coefficient of volume change (Mv) for heavy overconsolidated clays, stiff weathered rocks and hard clays.Also, Terzaghi et al. (1967), investigated the coefficient of consolidation (Cv) of some geotechnical materials with granular soils including rock fills having a coefficient of consolidation of 0.02+ 0.01, shale and mudstone 0.03+ 0.01, inorganic clays and silt 0.04+0.01 and 0.05 +0.01 for organic clays and silts.
In view of the values of coefficient of volume change or compressibility (Mv) and coefficient of consolidation (Cv) as presented in Table 2 compared with the standard values given above, implies that the soil is typical of nonproblematic lateritic soils.The low to medium value of Mv implies that any structure founded on the soils will not suffer excessive settlement exceeding the maximum permissible limit.Most importantly is the low values of Mv at a constant depth under a given pressure range.

Borehole logs and standard penetration test
The significance of geotechnical investigation specifically for borehole logging and standard penetration test is to know the stratigraphy and strength characteristics respectively of each layer.Most of the borehole logs in the study area show dark grey hard to loose sandy-clay at the upper most layer and occasionally hard lateritic soils commonly called hard pan which ranges from 0 -1.50 m in depth.This is followed by reddish brown soft sandy clay to reddish brown hard sandy silty clay which range from 1.50 m -4.50 m in depth.The cone penetration resistance and bearing capacity indicate that the results are good for structures to be sited on them as they would not suffer excessive settlement due to the weight of the superstructure.

Conclusion
The result of the subsoil geotechnical investigations within the study area has helped in arriving at the conclusions that the top soil is generally thin and ranges in thickness from 0 -1 m in most of the boreholes.The depth to competent rock is more than 5 m in some of the boreholes while most of the soils possess good

Figure 1 .
Figure 1.Location of the study area.

Figure 3 .
Figure 3. Borehole and penetrometer test sample points of the study area.

Figure 4 .
Figure 4. Casagrande plasticity chart plots for the study area.

Figure 5 .
Figure 5.General particle size distribution curves for borehole samples.

Table 1 .
Summary of particle size distribution and plasticity characteristics of some tested soil samples from the study area.

Table 2 .
Consolidation parameters of coefficient of volume (Mv) compressibilty and coefficent of consolidation (Cv) of some soil samples.

Table 3 .
Allowable bearing pressure and ultimate bearing capacity at test point.
Appendix A.