Geophysical and Geotechnical Investigation of Building Failure in A Typical Basement Complex Environment – A Case Study in Ile-Ife, Nigeria

A building located within the Basement Complex of the ancient town of Ile – Ife, Osun State, Southwestern Nigeria was observed to have failed due to excessive total and differential settlement. The failure was investigated using the electrical resistivity and geotechnical methods The electrical resistivity method involved the 2-D electrical resistivity imaging (ERI) technique using the dipole-dipole array along four traverses of 30 – 60 m in lengths. The geotechnical method involved the cone penetration test (CPT) using the 2.5-ton static penetrometer machine. Quantitative and qualitative analysis of the ERI data were made using the DIPROfWIN software for the pseudo-inversion while the CPT data were interpreted for lithology using standard chart. The results show that the topsoil, about 1.0 m thick, is composed of sandy clay/clay that is characterized by cone resistance (q c ) of 0.2 – 2.0 MPa and resistivity of 75 - 200 Ω mm. The underlying clayey weathered layer, which constitute the shallow foundation soil is characterized by thickness of 4 - >10 m, q c of 0.2 – 1.0 MPa, resistivity of 25 - 75 Ω mm and estimated consolidation settlement of 200 – 500 mm. The basal layer is the saprock/fresh bedrock characterized by q c of > 8.0 MPa and resistivity of 100 - 1000 Ωmm. The subsoil is thus characterized by variably thick incompetent clayey weathered layer within which the shallow foundation was placed; hence the excessive total and differential settlements.


Introduction
The incessant incidence of foundation failure of structures is becoming alarming in Nigeria. This failure has been attributed to a number of factors such as inadequate information about the soil and the subsurface geological material, salinity, poor foundation design and poor building materials (Fatoba et al., 2010). Building failure in the form of excessive total settlement and intolerable differential settlement or both are attributed to highly compressible foundation soils. While these types of building failure are common in sedimentary terrain underlain by recent transported soils in the form of soft clays and loose sands; they are uncommon in the Basement Complex terrain where residual soil and partially weathered rocks constitute the foundation soils.
Conventional pre and post-construction investigations of foundation soils, which usually involve shell and auger exploratory borings along with cone penetration tests, are generally effective in area underlain  Figure 1).

GEOLOGY OF STUDY AREA
The study area is part of the Ife-Ilesa schist belt which consists of three major units: the Ilesa amphibolite Complex which occurs as lenticular bodies, the Ilesa metaclastics made up of sheared biotite schists, and the Effon quartzitic sequence which occurs as massive quartzite, schistose quartzite and quartz schist.
The study area is underlain by pegmatized schist (Figure 2) which has been severely weathered. The pegmatized schist is generally dark grey in colour with texture ranging from medium to coarse grain. Its points where the machine anchor legs were lifted or pulled out of the subsurface. The procedure described above was then repeated for subsequent location tests.
The CPT data were processed by plotting the cone resistances against depth at each location point using the Microsoft Excel software. The layer sequences were interpreted from the variation of the values of the cone resistance with depth. On the basis of the expected resistance contrast between the various layers, in ection points of the generated penetrometer curves were interpreted as the interface between the different lithologies.

Results And Discussion
Electrical Resistivity Survey Traverse 4: The resistivity structure (Fig. 3d) shows that the subsoil sequence along the southwestern ank of the site is characterized by low resistivity varying from 6.0-138 Ωm. The topsoil in green/yellow colour, occurring to 1.0 -2.0m depth, has resistivity values that generally vary from 48 -138 Ωm, hence interpreted as residual soils composed mainly of clay and sandy clay. The underlying layer in blue/green colour, occurring to about 4.0 m depth, with resistivity values of 21 -62 Ωm is interpreted as clayey weathered layer. The underlying layer, in predominantly blue colour, characterized by low resistivity values of 6.0 -43 Ωm is considered to be soft clayey material. The fresh bedrock was not delineated beneath this traverse.

Geotechnical Test
Results of the cone penetration tests are presented as plots of cone resistance (q c ) against depth ( Figure   4). For the purpose of lithological interpretation, the q c values were correlated with Schertmann chart (1978). The friction ratio, which could not be measured with 2.5 ton penetrometer machine, is taken to vary from 2-5 based on classi cation of Look (2007).

SYNTHESIS OF RESULTS
The composite resistivity structure of the site was generated by integration of the 2-D along Traverses 1 -4, with the aid of Surfer 11 software, and was used to determine the generalized resistivity of the subsoils beneath the failed building at different depths.
The results (Figs. 5 -8) show that, beneath the investigated building, from the ground surface to 1.0 m depth, the subsoil is chracterized by resistivity values varying from 75 -200 Ωmm and q c varying from 0.196 to 1.96 MPa. The layer is thus regarded as dry portion of clays and sandy/silty clays delineated by the CPT. The underlying subsoil to depth of 4 m is chracterized by resistivity values varying from 25 -75 Ωm, with resistivity values decreasing with depth. This horizon is considered to be clayey weathered layer occurring as mottled zone of saprolite. The horizon corresponds to the soft clay and stiff silty clay delineated by the CPT to depth of 2.5 m. At depth of 4 to 7 m, the subsoil is characterized predominantly by resistivity values of 100 to 500 Ωm, which is typical of pallid zone of saprolite and corresponds to the sandy and silty clays delineated by the CPT. The basal layer at depth of 7 to 10 m characterized mainly by resistivity values of 100 -1000 Ωm portrays occurrence of saprock and fresh bedrock. The southwestern portion is however characterized by subsoil with low resistivity values to depth beyond 10 m. This suggests occurrence of buried river channel in that part of the study area.
The clayey subsoil constitutes the horizon for shallow foundation. Undrained shear strength (Cu) parameters of the horizon were estimated from q c values (kPa) based on Look (2007) relationship: The results shows that the horizon is generally characterized by low undrained shear of 11 kPa except in the northwestern portion of the site that has undrained shear of 54 kPa.
In addition, the horizon has thickness varying from 4 m in the NE to >10 m in the SW portiom of the site. Consolidation settlement (S c ) of the horizon was estimated from constrained modulus ( ), which is inverse of coe cient of volume change (Mv); hence: was estimated from q c based on relationship (Sanglerat, 1972). The lower value of = 3.0 was adopted for this study due to the low q c value of 0.2 MPa. Also, the failed building has two oors (ground oor and rst oor), hence loading intensity of 30 kPa was adopted. The estimated consolidation settlement varies from 200 mm in the NE section to 510 mm in the SE section of the building.
The subsoil is thus characterized by variably thick incompetent clayey horizon within which the shallow foundation was placed; hence the excessive total and differential settlement.