Electrical Resistivity Survey
The 2-D resistivity structures along the four traverses depict the subsurface resistivity distributions (Figure 3).
Traverse 1: The resistivity structure (Fig. 3a) shows subsoil sequence along the Northwestern flank of the site as having resistivity values varying from 34 Ω m to 330 Ωm. The topsoil with predominantly yellow colour, occurring to about 1.0 m depth, has resistivity values that generally vary from 102 – 167 Ωm, with minor portion in green colour having low resistivity values of 35 – 67 Ωm. This portrays heterogeneous soils composed mainly of sandy clay and minor clay. The underlying layer in predominantly green colour, occurring from about 1.0 to 4.0 m depth, is characterized by resistivity values ranging from 48 to 95 Ωm, which is typical of sandy clayey weathered layer. The basal layer in reddish colour, characterized by resistivity values of 110 – 330 Ωm, is regarded as the saprock/fresh bedrock. Depth to the fresh bedrock varies from about 2 m at the origin in the NW about at the terminal 4 m in the SE.
Traverse 2: The resistivity structure (Fig. 3b) shows that the subsoil sequence along the Northeastern flank of the site has resistivity values varying from 28 Ωm to 986 Ωm. The topsoil with bluish green colour, occurring to about 1.0 m depth, has resistivity values that generally vary from 51 – 107 Ωm, which is regarded as sandy clayey material. The underlying layer in blue/green colour, occurring from about 1.0 to 3.0 m depth, is predominantly chracterized by resistivity values of 28 – 55 Ωm, hence regarded as clayey weathered layer. The third layer, ocurring from 3.0 – 4.5 m in yellow and green colours with resistivity values of 80 – 120 Ωm is regarded as clayey sand weathered layer The basal layer in red/purple colour, characterized by resistivity values of 122 - 986 Ωm, is regarded as the saprock/fresh bedrock. Depth to the fresh bedrock varies from about 4 m at origin in the NE to about 15 m at the terminal in the SW.
Traverse 3: The resistivity structure (Fig. 3c) depicts the subsoil sequence along the Southeastern flank of the site as having resistivity values of 27 Ωm to 271 Ωm. The topsoil in yellow/green colour, occurring to about 1.0 m depth, has resistivity values that generally vary from 42 – 115 Ωm, thus regarded as residual soils composed mainly of clay and sandy clay. The underlying layer in blue/green colour, occurring from about 1.0 to 5.0 m depth, with resistivity values of 22 – 75 Ωm is interpreted as clayey weathered layer. The basal layer in red/purple colour, characterized by resistivity values of 90 – 271 Ωm, is regarded as the saprock/fresh bedrock. Depth to the fresh bedrock varies from about 6.0 m in the NE origin to over 10 m in the SW end.
Traverse 4: The resistivity structure (Fig. 3d) shows that the subsoil sequence along the southwestern flank 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 (qc) against depth (Figure 4). For the purpose of lithological interpretation, the qc 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 classification of Look (2007).
Analysis of the results show that from ground surface to depth of 2.5 m, CPT 1 and CPT 3 are generally characterized by low qc of 0.196 MPa (2.0 kgf/cm2) indicating incompetent subsoil composed of soft clays. Corresponding section beneath CPT 2 to 2.0 m is characterized by relatively higher qc of 0.196 – 1.96 MPa (2.0 to 20 kgf/cm2), which indicates fairly competent subsoil composed of sandy/silty clays or stiff clay.
From 2.25/2.75 m to 3.5 m, the qc values for the test points generally varies from 1.47 -7.10 MPa (15 to 72 kgf/cm2), which is regarded as moderately competent subsoil composed of sandy and silty clays. The last layer penetrated by the cone, from 3.5-4.0 m has qc that generally varies from 8.82 -10.5 MPa (90 - 107 kgf/cm2), which is regarded as competent subsoil saprock composed of coarse-grained clayey sand.
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 qc 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 qc 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 (Sc) of the horizon was estimated from constrained modulus (), which is inverse of coefficient of volume change (Mv); hence:
was estimated from qc based on relationship (Sanglerat, 1972). The lower value of = 3.0 was adopted for this study due to the low qc value of 0.2 MPa. Also, the failed building has two floors (ground floor and first floor), 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.