LIQUEFACTION POTENTIAL ASSESMENT OF SOME COASTAL SANDS OF LAGOS, SOUTH-WESTERN NIGERIA, USING GEOPHYSICAL AND GEOTECHNICAL METHODS

This paper presents the results of the investigation conducted in some parts of wetland areas of Lagos, Nigeria, using Multi-channel analysis of surface waves (MASW), Cone penetration test (CPT) and Standard penetration test (SPT). The primary aim was to delineate probable areas that were prone to induced seismicity. The MASW of the seismic method was used to generate the shear wave velocity (Vs) of the near surface soil while the CPT and SPT were employed to infer the penetration resistance and the blow count in the assessment of the stress-based liquefaction potential of these soils respectively. SeisImager and liquefaction assessment software (known as Cliq) were used to process both the MASW and CPT data respectively. The values of shear wave velocity generated for most sands in the study areas range from 120m/s -200m/s. This value fell within potentially liquefiable sands. From the CPT results, 41.67% of Ikoyi data showed a very high risk to liquefaction while 37% of Badore data indicated severe liquefaction potential. The factors of safety (F.S) against liquefaction potential obtained from the plot of cyclic stress ratio (CSR) against s-wave velocity (Vs) were less than 1. This also corroborated the presence of liquefiable sands within the study areas. Background of the Study Liquefaction phenomenon is one of the geological hazards that resulted from sudden ground shaken (Earthquake). It leads to the loss of strength/stiffness of saturated or partially saturated cohesionless soil which makes it to lose their bearing capacity as they behave like liquid. The solid state of soil is been transformed into liquefied state in response to increased pore pressure and reduced effective stress. Loose, partially/saturated soils are then subjected to shearing, which led to re-arrangement in the soil grains to kind of dense packing devoid of much space as a result of forceful ejection of water in the pore spaces [1]. This action led to decrease in effective stress and shear resistance as the stress from the soil skeleton has been transferred to the precipitating pore water. Coastal Plain Sands is the recent stratigraphic formation of the Dahomey Basin. It is characterized with poorly sorted sands with clay lenses. Nigeria lies on the eastern flank of the Atlantic Ocean, generally believed to be quiet when compare to Pacific Ocean margins which are characterized by subduction tectonics and occurrence of devastating earthquakes. Few Seismic activities have been recorded inform of Earth tremors in Nigeria since 1933 with more than twentyeight (28) cases (Table 1). These occurrences has been attributed to various factors ranging from the presence of fault zones along Ifewara Zungeru, uncontrolled mining capable of inducing earth Tremors and others. Dahomey basin is believed to have seated on equatorial fracture zones such as Romanche, Chain and Charcot fracture zones (Figure 1). These fracture zones are reported to have been responsible for shearing and migration process of Earthquakes from South American plate to African plate via Mid-Atlantic Ridge [2,4]. Table 1: Showing Earth Tremors in Nigeria. (Modified after [3]) S/N DATE TOWN 1 1933 WARRI 2 JUNE 1939 LAGOS 3 JULY 1948 IBADAN 4 JULY 1961 OHAFIA 5 DECEMBER 1963 IJEBU-ODE 6 APRIL 1981 KUNDUM 7 OCTOBER 1982 JALINGO 8 JULY 1984 IJEBU-ODE/ SAGAMU 9 DECEMBER 1984 YOLA 10 JUNE 1985 KOMBANI YAYA 11 JULY 1986 OBI 12 JANUARY 1987 GEMBU 13 MARCH 1987 AKKO 14 MAY 1987 KURBA 15 MAY 1988 LAGOS 16 JUNE 1990 IBADAN 17 NOVEMBER 1994 IJEBU ODE 18 JUNE 1997 OKITIPUPA 19 MARCH 2000 BENIN 20 MARCH 2000 IBADAN/ABEOKUTA 21 MAY 2001 LAGOS 22 AUGUST 2002 LAGOS 23 MARCH 2005 YOLA 24 MARCH 2006 LUPMA 25 JUNE 2016 KADUNA 26 JUNE 2016 KADUNA 27 JULY 2016 BAYELSA 28 AUGUST 2016 SAKI Figure 1: Represents the map of Nigeria showing the fracture Zones which acts on