INTEGRATION OF SEISMIC REFRACTION AND LABORATORY TEST TECHNIQUES FOR SLOPE STABILITY ANALYSIS, SOUTH-SOUTH, NIGERIA

ABSTRACT


INTRODUCTION
Slope stability has become an essential factor to consider in engineering projects. It involves calculating and assessing the amount of stress a slope can manage before weakening as a result of ground movement (Gordon et al., 2016;Ray et al., 2020). A slope will likely fail whenever the driving forces (gravity) overcome the slope material's shear strength (resisting forces). According to a study, analysis of slope stability is a standard practice in geotechnical engineering, and it involves the calculation of the factor of safety (FOS) (Asteris et al., 2022). Studying slope stability enable the location of risky areas, causes of slope failures and can aid in designing of reliable slope. Through comparability of slope stableness, earth scientist and engineers established a moderate and innovational framework within the province which in the erstwhile considered hazardous for construction.
Moreover, the insight gained by determining slope stability has given subsurface geophysicists, geologists, and civil engineers, an expanded understanding of natural laws and forces, which they can study to improve future projects, as well as progress in the civil engineering and geophysical application globally, sloppy areas subjected to high loading from high-rise buildings will require higher FOS to minimize the probability of slope failure occurrence (Roc, 2005;Xiao et al., 2016). Thus, in a hilly terrain area, accessibility will be difficult for the boring technique which fetches much time, energy, and cost constraints. To lessen this menace, this current research was carried out using an integrated approach of seismic refraction with laboratory test techniques which have several advantages. Seismic refraction methods are non-destructive and more efficient with much aerial coverage data compared to laboratory borings.
Selecting the slope stability analysis method is important; however, the accuracy of the analysis relies heavily on the input data. Ideal input data requires a thorough understanding of the slope's subsurface conditions including soil types, layer thicknesses, soil strength parameters, soil unit weights, location of the water table, and relative elevations along the slope. The absence of any of these parameters decreases the accuracy and reliability of the study. However, the seismic refraction method can boost the conventional geotechnical investigation of the slope area to obtain accurate parameters needed for slope stability analysis in contrast to laboratory testing (Abudeif et al., 2017). The seismic refraction method will give information on slope layers which will be estimated based on wave velocities. That is compression wave velocity (Vp) and shear wave velocity (Vs) toward the depth of the slope area (Uhlemann et al., 2016).
According to a group researcher, analysis of slope stability is a standard practice in geotechnical engineering, and it involves the calculation of the factor of safety (FOS) (Asteris et al., 2022). Studying slope stability enables the location of risky areas and causes of slope failures and can aid in designing a reliable slope. Through comparability of slope stability, earth scientists and engineers established a moderate and innovative framework within the province which the erstwhile considered hazardous for construction. Moreover, the insight gained by determining slope stability has given subsurface geophysicists, geologists, and civil engineers, an expanded understanding of natural laws and forces, which they can study to improve future projects, as well as progress in the civil engineering and geophysical application globally, (Roc. 2005;Asteris et el., 2022).
The stability condition of slopes is a subject of study and research in soil mechanics, geotechnical engineering, and engineering geology. Slope stability assesses the strength of earthwork, burrow, and congenital gradient in rocks (Rasouli and Mahyar, 2011). It is a measure of how resistant a natural or artificial slope is to failure due to collapse or sliding, as well as to prevent the initiation of sloppy movement, slowing it down, or arresting it through extenuation relief (Sharma et al., 2017). On the other hand, soil parameters classified as elastic, bearing, and engineering parameters can be obtained from the seismic refraction method as expressed in Eqns. 1 -7 and summarized in Table 1. Comparison will be made with the same parameters acquired from laboratory testing. In laboratory tests, rock core samples collected are used for strength tests. These include uniaxial rock tests to yield uniaxial compressive strength of soil classification and triaxial strength tests to analyze the mechanical properties of the soil parameters.

LOCATION AND GEOLOGY OF THE STUDY AREA
The study area is Mary Slessor School terrain in Ibiono Ibom Local Government Area of Akwa Ibom State, Nigeria. It is underlain by excavated slopes, soft rock trails in reservoirs, and forests of sedimentary formation of late tertiary and Holocene ages within the tropical rainforest belts of Nigeria (Figures 1 and 2a). It is located within latitudes 5.10 0 N -5.15 0 N and Longitude 7.50 0 E -7.55 0 E (Aka et al., 2020). The terrain was established in 1895 by a Scottish Presbyterian Missionary; Mary Mitchell Slessor to educate Ibibio and Efik people in then Calabar axis of the Southern protectorate of Nigeria (Imbua, 2013). However, becomes a tourism center for landscape, infrastructure, architecture and cultural studies globally, as shown in Figure 2(b). It is enriched with natural resources and hydrocarbon (Akpan and Okwueze, 2006). It lies in the equatorial climatic region which experiences two seasons: wet and dry seasons between June to October and November to March respectively (Udeagha et al., 2013). Also found in the terrain are rivers, streams, and wetlands which support aquatic lives like fishes, periwinkle, shrimps, and oyster (Aka et al., 2020).

Theory
The seismic refraction practice employs the concept of Snell's law to operate seismic energy which propagates through the subsurface to the top through refracted beam pathway. The seismic energy exists as Primary (P) and Secondary (S) body waves and is measured as seismic wave velocities of VP and VS. (Aka et al., 2020). These measured velocities link to the calculation of elastic and bearing properties as shown in Table 1 and Eqns. 1-7.
Modulus of Elasticity measures the rigidity of the soil materials, which relates the shear modulus "µ" and Poisson ratio "σ".
Young Modulus or Modulus of Elasticity (E) = 2µ (1+σ) Compressibility (β) measures the reciprocal of the bulk modulus (equation 6), which is expressed as volumetric strain to volumetric stress of a material.
Compressibility (β) = 1 (6) Compliance(C) measures the inverse of modulus of elasticity expressed as the tendency of an object to resist deformation of a material and is expressed in equation 7.

Data Acquisition
In seismic refraction, the survey was carried out at 16 different locations for three-layer formations, using 24 channels ABEM 3000S enhancement seismograph. A sledgehammer of 75kg and metal plates was used as a source. S and P wave geophones of 48Hz frequency were used as detectors as shown in Figure 3(a). 120 m profile length at an inter-geophone spacing of 5 m, shot to 1 st geophones separation was 5 m and total geophones of 24 per profile were used as shown in Figure 3(b). The traveling waves refracted based on the differences in soil layer before reaching the geophones. The source of elastic P and S waves was generated by blowing the sledgehammer towards the metal plates that are placed 4 mm on the ground. In P waves, 2 shots were recorded at each location with 2 stacks per shot location.
In S waves, 4 shots were recorded at each location with 2 stacks per shot location. During the blowing session of the sledgehammer to detect P and S waves, any small movement should be avoided to minimize error as geophones trigged any small walking motion. However, for geophones located close to the hammer source, the P and S waves signal will travel directly to the geophones whereas, for geophones located far away from the source, the waves will be refracted before reaching the geophones. This step has been repeated throughout the entire survey to avoid large errors in the survey results. On the other hand, other conditions that hindered the placement of geophones within the terrain include the availability of vegetation, landslides, and soil erosion.

Data Processing and Interpretation
Seismic refraction data acquired were processed and interpreted using Pickwin and IXrefrax software collection. Three phases were considered, the first phase elaborates on the exact picking of the first break from the seismic wave by using Pickwin software to obtain the time (T) and distance (X) of both P and S trajectories. This was grounded at outline length, geophones spacing, and its first arrival times. The second phase elaborates on the computation of P and S wave velocities from the inverted slope of T-X trajectories. The correlation of the S velocity rises with lessens in travel times owing to evolution and immersion of moisture in pores formation. However, lessening in P velocity rises as regards travel time on account of lessening in bulk modulus and soil solidity. On the other hand, the P and S velocities scheme with other parameters indicates a lineal correction allying the parameters. The third phase elaborates on the modeling of 2D near-surface parameters as shown in Figure 2. 2D modeling was preferred over 3D, to maintain the model creation complexity, simulation time, and lower costs of software purchase.

Laboratory Test Analysis
The test was performed to characterize the rock properties. The rock core samples were collected in two categories: that is, disturbed and undisturbed samples. The samples were analyzed to define the required soil parameters for slope stability analysis. The tests carried out were uniaxial rock tests to yield uniaxial compressive strength, which is a typical rock property used to correlate with the seismic velocities obtained from seismic refraction investigations. Triaxial strength tests were also performed to analyze the mechanical properties and soil classifications.
The experimental index was presented as shown in Tables: 2-4. Schematically, in seismic refraction analysis, the scheme covers data acquisition, processing, and interpretation while the laboratory analyzes covers the disturbed and undisturbed samples for resistivity parameters accompanied by the geological interpretation.

RESULT AND DISCUSSION
The seismic refraction survey with a laboratory test technique was carried out within the study area. The result from the analysis showed that the study area primarily is hidden by shale, silt and sandstone of sedimentary rock which is due to the long-term effect of weathering and deposition of mineral or organic particles of the earth's surface as shown in Table 2. Three traverses were covered, each 60 m long and inter-geophones separation of 5 m apart for 13 intervals of 12 (P and S)-wave Geophones. The shot point adopted was a linear array arrangement of geophone at three-point for each three-layer formation. Fig. 4 shows the travel time versus distance plot of two-shot point of SPI and SP3. Three portions for each of the shot point were anticipated for every point; longitude, latitude along with elevation were taken and contoured ( Figure 5).  ; 2270, 1202, 310, 123, 40.5 and 1.2 kN/m 2 respectively. The general classification of slope stability was established based on the soil properties obtained from seismic refraction and laboratory test methods. Thus, the comparison was made to justify their similarities as shown in Table 4. The results showed that the six soil parameters obtained from both methods were similar with percentage error ranging from 5.3 % to 16.6 % as illustrated in the histogram as shown in Figure 6.
The deformation observed in the soil parameters is due to the non-linear behavior of most soil and rock masses which depends on the occurring frequency. The frequencies in seismic waves are normally not the same as frequencies used in dynamic testing. On the other hand, shear stress and strain in seismic waves are smaller compared to stress and strain in the laboratory tests. Finally, the slope stability was delineated in terms of slope FOS which was analyzed based on the data captured from laboratory tests. These were aimed to differentiate between safe slope and slope with the elevated prospect of failure occurrence. In view of this, a safe slope was observed to have FOS of at least 1 whereas the slope with failure prospect was having FOS of less than 1. Hence, a general classification of slope stability is made on the evaluation by virtue of soil parameters and clearly slope factor of safety.