Evaluation of Liquefaction Potential of Some Sites in Uyo Metropolis, Akwa Ibom State, Southeastern Nigeria

Liquefaction potential analysis were carried out for some locations within Uyo metropolis, Akwa- Ibom state, Southeastern Nigeria using mostly cone penetration test data. The dominant soil is the Coastal Plain Sands. Unified Soil Classification System places soil within 20 m depth in the clayey sand (SC), silty sand(SM), poorly graded sand(SP) and dual combination of the three, namely SC-SM, SM-SP. Factor of safety(FS) was calculated for potential earth tremors with 7.5 and 4.5 moment magnitude and a peak ground acceleration (PGA) value of 0.16g. Other than the first layer in some the sites in which values of FS is greater than 1.0, the factors of safety for the 7.5 magnitude for all depth up to 20.0 m are less than one. For the 4.5 moment magnitude, the FS is less than 1.0 in some layers and greater than 1.0 in others. Liquefaction potential index (LPI) values for the sites range from 0 to 26.65, this places the level of liquefaction severity for all the sites in the ‘ very low ’ to ‘ very high ’ category based on level of severity classification. Potential settlement of the liquefiable layers estimated at the sites ranges from 8.38 cm to 58.48 cm. Settlement values were used to correlate liquefaction zones. Liquefaction potential index has a coefficient of correlation of 0.54 with settlement values.


Introduction
Although Nigeria is not located within the major seismic zones of the world, over the years, several earth tremors have been experienced in some parts of the country. These tremors with local magnitudes (between 4 and 5) were experienced from the early 1930s, and this led to in depth studies into the seismology of the country (Tsalha et al. 2015). Some studies have The study uses Cone Penetration Tests (CPT), Standard Penetration Test (SPT), and lithologic borehole with Soil Behavior Type (SBT charts- Robertson and Cabal, 2005) to characterize, and profile soil at seven sites in Uyo metropolis, southeastern Nigeria. The liquefaction potentials of the soil were then evaluated by estimating the factor of safety for soil profiles using different criteria for the CPT and SPT data as detailed by relevant previous workers (Boulanger andIdriss, 2014, Youd et al, 2004) on liquefaction. Potential settlements associated with the liquefiable layers were calculated, and implications of the results for the study area were highlighted 2.0 Study objectives 1. Geotechnical characterization of the selected sites using Cone Penetration Tests (CPT) and Standard Penetration Tests (SPT) data 2. Using SPT data but mostly CPT, determine soil resistance developed by the soil in a Magnitude 7.5 event; the cyclic resistance ratio(CRR) 3. Determine shear resistance, due to magnitude 7.5 and 4.5 event. The cyclic shear ratio (CSR).
4. Estimate the factor of safety for each of the event for the different soil layers characterized in '1' above.
5. Evaluate liquefaction potential index for each of the site 6. Evaluate the potential settlement of areas where factor of safety of the magnitude 4.5 event is 1.3 or less

Description of study area and geology
Uyo is the capital of Akwa Ibom State in Nigeria, a State located in the coastal southern part of the country within the oil rich Niger Delta. Generally, it has relatively flat terrain with no undulating hills or valleys, except at the North eastern part of the city which has deep gully erosion sites. Uyo, lies between latitudes 4 0 58' and 5 0 6' North, and longitudes 7 0 48' and 8 0 02' East; and is located within the West African tropical rainforest belt. Its location in the Niger delta region necessarily leads to extensive urbanization that is characterized by infrastructural developments.
According to the Nigerian Geological Survey Agency (2006) (Short and Stauble, 1967). The Coastal Plain Sands commonly called 'Benin Formation' is one of the six major geomorphic units that makes up Niger delta. The grains are sub angular to well rounded, and are believed to have been deposited in a continental fluviatile to deltaic environment.

Ground water level and ground condition
Sand or sandy soil will not liquefy in unsaturated condition. Laboratory test results (Sherif et al., 1977) show liquefaction resistance for soils increases with decreasing degree of saturation. The ground water

CPT versus SPT tool of liquefaction analysis
Among other features such as Quality control, repeatability of test, and detection of variability of soil deposits, the cone penetration test (CPT) has greater repeatability and reliability, and provides a continuous profile compared to SPT. It is the main tool used in this study, and also the fact that within the study area, SPT data are scare.

Materials and Method
6.1 Data acquisition.
Cone penetration test data from seven locations were utilized in the study. Their approximate geographical coordinates are listed in Table 2 and are indicated in Figure 2. One of the seven locations has Standard penetration test (SPT) data which is the only SPT data utilized in the study, while one other site has lithology borehole drilled up to 20.0 m in addition to CPT data at the site. The lithologic borehole is used as a check on the soil types indicated by the CPT data. Disturbed soil samples up to 3.0 m depth were taken at other location for soil type identification and classification purposes.
Five of the CPT data were acquired with 2.5 ton Guada Dutch cone penetrometer, while two were acquired with a 10 ton motorized rig CPT. The cone penetration tests acquires cone resistance ( ), sleeve friction ( ). Three of the sites were located within University of Uyo The two steps above were used to established soil profile at each of the CPT test location iii. ,For each identified soil layer, an average cone, and friction resistance is determined by summing up the different resistances that make up the layer and finding their average.
The average cone resistance is then normalized for overburden pressure using the following equations; where, qc1N = equivalent clean sand normalized cone resistance for over-burden (Robertson and Wride 1998); C Q = normalizing factor for cone resistance; qc = cone tip resistance; Pa = atmospheric pressure; σ' vo = vertical effective stress; n = stress exponent defined in equation (4) above iv. The cyclic resistance ratio at 7. 5 quake magnitude, 7.5 is then calculated as given by Boulanger and Idriss, (2014) 7.5 = cyclic resistance ratio for an equivalent magnitude 7.5 event And (q c1N)cs = equivalent clean sand normalized cone penetration resistance; 1 = 1 + ∆ 1 (9) 1 is as defined in equation (6) Where FC = Fine content (%) and given by the expression is a constant that varies with soil layers. The percentage of fines obtained from shallow investigation was used to model this value on each site Vii) Cyclic stress ratio, CSR for a 7.5 magnitude event is calculated using Where , a max = peak horizontal ground acceleration; g = acceleration of gravity; = total vertical overburden stress and / = effective vertical overburden stress, respectively, at a given depth below the ground surface; r d = depth-dependent shear stress reduction factor MSF is the magnitude scaling factor, which is equal to 1 for the 7.5 event = the overburden correction factor computed using equation given by Boulanger and idriss, Where is computed as And depth-dependent shear stress reduction factor computed as ii) The field value of SPT blows is then corrected into standard value, ( 1 ) 60 as follows Where = Field SPT 'N' value Where C N = is a factor to normalize Nm to a common reference effective overburden stress; C E = correction for hammer energy ratio (ER); C B = correction factor for borehole diameter; C R = correction factor for rod length; and C S = correction for samplers with or without liners C S , C B , and C E are assumed to be 1.0, 1.0, and 0.6, respectively. Rod length correction with respect the depth (CR) at the borehole location is corrected using the v) The cyclic stress ratio (CSR) is calculated using the same equation (24) above vi) Factors of safety computed as in equations (20) and (21) above.

6.3
Computation of liquefaction potential index Iwasaki et al. (1978Iwasaki et al. ( , 1982 proposed liquefaction potential index (LPI) as a single value expression to evaluate regional liquefaction potential for soil profile up to 20 m depth, they proposed the following; Where z = is the midpoint of the soil layer dz = is the differential increment of depth F(z) = severity factor, which is calculated using the following expressions ( ) =1-FS for FS< 1.0 ( ) = 0, for FS≥ 1.0.
( ) = weighting factor and is computed as Instead of a single value of factor of safety assigned to depth less than 20 m, Luna and Frost (1998), proposes presents the following procedure which evaluates LPI of each soil layering and then sum them up. Their procedure is as follows = is the is thickness of the discretized soil layers; n = is number soil of layers; = is liquefaction severity for i-th layer; = is the factor of safety for i-th layer; = is the weighting factor =10-0.5 = is the depth of the i-th layer (m).
The Luna and Frost procedure is the one adopted for computation of LPI in this study.

Computations of settlements
Possible settlements in the case of seismicity of Magnitude 4.5 for each location were estimated using the relationships developed by Tatsuoka et al. (1990). Due to the large volume of computations involved, excel worksheet was used in carrying out all the computations in this study.

7.0
Results and discussion

Soil indices and classification
The results of laboratory analyses used to identify and classify the soil types at three of the seven penetration sounding locations are presented in Table 3. The table presents one of these results for shallow depths starting from the ground surface up to 4.0 m and the inferred soil type from soil behaviour type analysis (SBT) up to refusal depth. The table also presents the soil log from SPT and lithologic boreholes at two locations up to 20 m. Both borings provides soil profile at these locations up to 20 m. Table 4 presents a typical worksheet analysis of CPT data for the classroom site location.
The table presents soil index values Ic, unit weights, division of the soil into layers based on both soil index (Ic), and unit weights. The two parameters were reconciled to be able to group and profile the soils into layers. From all the CPT data, I c values ranges from 1.22 to 4.26 indicating soil types ranging from gravelly sand to dense sand to organic soilsclay. By reconciling the soil behavior index values(I c ), and the computed unit weights, the soil profile is developed for each site. Soil profiles at each site indicate highly stratified pattern. . For example the classroom site has seventeen soil strata within 11.75 m range, and the Bank Avenue site has thirty two strata within a 20 m depth range. Table 4 presents a typical profile The soil profile for the study area is made up of sand mixtures which consists of clayey sand and silty sands, interspersed frequently with clays and silt mixtures, and sometimes pure sands.
7.3 Potential for liquefaction due to soil type Figure 1 is a chart developed by Tsuchida (1970), which is a sieve analysis plot that shows envelopes for both potential liquefiable soils and soils that are liquefiable. A plot of sieve analysis results of soil recovered from SPT boring, borehole three(BH3) and from lithologic boring puts all the soil within the envelope for liquefiable soils. This indicates that soils within the study area will undergo liquefaction.

Liquefaction analysis
The liquefaction analysis results can be group into two, those on University of Uyo permanent site campus, and those outside the campus. Tables 5, and 6, presents typical liquefaction analysis results for two sites; namely; the hostel block site, and the classroom block site, all at the University of Uyo permanent site campus, while Table 7 presents SPT analysis for the specialist hospital site. Table 8    The SPT analysis presented in Table 7    though Zhang et al. uses Robertson and Wride (1998), approach in calculating ( 1 ) , whereas this study uses the method by Boulanger and Idriss, (2014).

Implication of findings to the study area
There is at present no provision for seismic consideration in building design codes for the study area. In the light of the findings of this study, provision for such consideration should now be given in the design and construction of buildings within this area. Most of the buildings in the study area are bungalows structures, single or two storey structures, and multistorey structures whose foundations are mostly placed at between 1.20 m to 1.80 m depth representing the second layer in most of the sites except for the multistory which are mostly on deep foundation. All such structures will undergo settlement associated with the soil layer on which they are founded upon if such liquefy. In a study of building that were affected by 1999 Turkey earthquake. Called "Adapazari failures", Gazetas et al (2004)  in the cited case is similar to the one under study, based on the above, and as a first step in limiting potential damaged to buildings within the study area, the ratio of building height to width should be limited to the ratio H/B < 0.8 since from the case cited it ensures such buildings will not undergo tilting and ensure uniform settlement. Although the type of shallow foundation (independent footing or mat) the buildings have was not indicated in the example, it appears it is not too significant to the result.

Conclusion
The soil profile at seven sites within Uyo metropolis were evaluated for liquefaction potential based on assumed 7.5 and 4.5 moment magnitude earth tremors with a possible peak ground acceleration (PGA) of 0.16g.
The soils at the sites based on classification of liquefiable soil types classify as 'most The study shows that some location within Uyo city may be more affected by liquefaction than others as indicated by LPI values that range from 0 to 29.74 This study represents a baseline study for liquefaction potential evaluation for the study area.

Acknowledgement
The authors wish to acknowledge the Directorate of physical planning unit of the University of Uyo, for making available the CPT data for the sites within University of Uyo permanent site campus used in this study. We also acknowledged Nigerpet Strucutres , Ewet Housing Estate Uyo for some other CPT data made available for our use in this study.

Declaration
The authors' wishes to declare that the interpretation of the CPT data provided by the two bodies acknowledged above and as used in this study are the authors interpretation and in no way have influence on the way or decision such data have been used by such bodies. Idriss IM, and Boulanger RW, (2006).Semi-empirical procedures for evaluating liquefaction potential during earthquakes, Soil Dynam. Earthq. Eng., 26, 115-130, Iwasaki T, Tokida K, Tatsuko F, and Yasuda S, (1978). A practical method for assessing soil liquefaction potential based on case studies at various sites in Japan, Proceedings of 2nd International Conference on Microzonation, San Francisco, 885-896,.

Figure 4
Log pattern of the normalized cone resistance and factor of safety values with depth for the hostel block site for 4.5 magnitude event.