Particle size analysis

For the present study, a CILAS laser diffraction instrument was used (Figure 3.6). The instrument proved to be a good method for analysing the range of grain size found in the Yola floodplain. The CILAS 1180 can characterise particle size distributions between 0.04 and 2,500 µm (Dietmar, 2006). The fine particles are measured by the diffraction pattern by using Fraunhofer or Mie theory (CILAS, 2004). Two hundred and fifty six air-dried sediment samples were analysed. The results obtained from the CILAS were analysed using Gradistat version 8.0, a statistical package develop by Blott (2011).

The sediment samples were weighed to 0.05 g, soaked in 10 ml 10% tetra sodium pyrophosphate, and left over night to deflocculates, before starting measurement. The samples were then added into the CILAS 1180 instrument and analysed using the program Size Expert (Figure 3.6). Care was taken in introducing the amount of sample into the CILAS mixing chamber to avoid high obscuration of sample in the mixing chamber. Optimal obscuration

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occurs when a sufficient number of suspended particles are present in the mixing chamber, which significantly diffract the laser beam, without blocking it. The obscurations for the samples were maintained between 15 to 25% for coarse-grained sediment (following Sperazza et al., 2004). Background measurements and rinsing were performed in between each sample measurement in order to keep the results consistent and reliable. Twenty seconds of ultrasound, twenty seconds of pumping and ten seconds of fast pumping were used for each sample before taken readings. Each sample was run three times for the data consistency and reliability.

Figure 3.6: Schematic diagram showing particle size analyser setup for wet mode’s mimic screen (Modified after CILAS, 2004).

The statistical analysis for the alluvial sediment samples was carried out using the Gradistat software produced by Blott (2011). As suggested by Pye and Blott (2004) statistics can be calculated using the method of moments either arithmetically (based on normal distribution) or geometrically (based on a log normal distribution) that gives a good approximation for well-sorted soils and sediments in the present study. The approach used here was the

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arithmetic method as this gives a normal distribution of the floodplain alluvial sediments. The statistical descriptive method (mean, D10, D50, D60, D90, D90/D10, D90-D10, D75/D25, D75-D25, Cu, Skewness and Kurtosis), for sediment samples were used to describe the particle size distribution of the sedimentary deposits, as these parameters describe key components of a given distribution for use in the interpretation for the grain size deposition.

The particle size distributions were also presented on the surface plot. Surface plots allow a quick overview of the sediment particle size distribution, so providing useful information on the sediment conditions (Beierle et al., 2002).

3.4.3 Loss on ignition

Loss on ignition (LOI) for the sediment samples was determined in the laboratory following the method by Bengtsson and Enell (1986) as shown in Appendix B. The method is fast, inexpensive; it gives good estimates of the sediment LOI and thus is useful for analysis of a large number of sediment samples. The method by Bengtsson and Enell (1986) is widely used by many researchers to estimate LOI of the alluvial sediment, for example Heiri et al. (2001), Veres (2002), Santisteban et al. (2004), Beasy and Ellison (2013), and Ledger et al. (2013).

There was a time gap of 3 months between the determination of the LOI and the sampling of the sediment in the field. The sediment samples were collected in April and May 2011. The LOI for the sediments were determined in August 2012. Even though the samples were stored in a sealed bag, there nevertheless a risk of moisture loss.

Three stages are involved in determining the LOI of the sediment samples and the samples losses were determined by mass with a scale at a precision of two decimals. In the first stage, moisture content of the sediment was determined by heating the sample at 105 ^°C (Figure 3.7). At first the samples were heated at 105 ^°C for about 12 hours period and were further heated at 105 ^°C for more 12 hours period and lastly the samples were further heated at 105 ^°C

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for 12 hours period making in total of 36 hours period until constant weight at room temperature was reached. In the second stage, LOI at 550 ^°C was determined by heating the samples at 550 ^°C for 2 hours period in order to destroy the organic matter and other stuffs in the sediment. In the third stage, the samples and ash were heated at 950 ^°C for 4 hours period to destroy carbonate content in the sediment samples. The LOI by mass were calculated for each sample by taking the average of five readings for the sediment sample in order to obtained representative values. When the LOI is very low, it may be loss of moisture bond within a clayey silt of the sediments.

Figure 3.7: Crucibles with samples heated in a muffle furnace for the determination of loss on ignition for the sediment samples (The photograph taken by the author on 2^nd September 2011).

3.4.4 Magnetic susceptibility

Sub-samples of air-dried sediments were packed into standard plastic vials. The magnetic susceptibility (MS) measurements were taken on a Bartington MS2 instrument, which was housed within the sedimentology laboratory of the Institute for the Environment, Brunel

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University London, UK. A total of two hundred and fifty six different sediment samples were analysed in order to determine the sources for the sediment and the process of their formation in the floodplain.

Figure 3.8 shows the set up for MS instrument. MS was measured five times on each sample and an average value was taken for each measurement. All MS measurements were determined as soon as possible after the sediment was placed into the vial to avoid magnetic diagenesis when exposed to air.

Figure 3.8: Taking reading for magnetic susceptibility in the laboratory with the Bartington MS2 instrument (The photograph taken by Nik Nik on 10^th August 2011).

3.4.5 Field Shear Vane Tester

In order to assess the suitability of the hand drilling method, shear strength forces on the alluvial floodplain sediment is one of the key parameter. Field Shear Vane Tester (FSVT) is

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used in this study to understand shear strength forces on the floodplain alluvial sediments for the suitability of the hand drilling method.

The FSVT was applied at twelve different drilling locations on the floodplain during my fieldwork survey in April and May 2011 in order to determine the shear strength forces on the sediment. The maximum depth of the FSVT is 3 m that is the limitation for the instrument.

The FSVT was carried out according to the British Standard guideline (BS EN 1997 – 2, 2007).

Figure 3.9: Taking readings with the Field Shear Vane Tester on the floodplain at borehole location 2 transect 1, for location (see Figure 2.28) (The photograph taken by Mohammed Abana Girei on 9^th May 2011).

The operation principle involves pushing the FSVT into the ground to the required depth, then gradually turning the handle in a clock-wise direction until the sediment/soil fails. Then readings for the shear strength forces on sediments were obtained on a graduated scale (Figure 3.9). After taking the reading, the graduated scale is turned back to zero position for the next

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reading. This process was repeated for the rest of the locations. At each location, shear strength forces were determined for 15, 30, 50, 75, 100, 130, 170, 210, 250 and 300 cm depth.

The maximum depth that could be reached by the available FSVT was 3 metres.

The distributions of stresses around the shear vane failure were computed from the following equations:

where Tv is torque employed in creating a vertical failure surface; D is Vane diameter; H is Vane height and Su is Undrained Shear Strength.

where Th = torque on two horizontal failure surfaces at the top and the bottom of the material.

From equations 3.1 and 3.2 above, the shear strength (Su) at the shear vane failure surface can be computed as follows:

[ ]

where T is the total torque applied on the shear vane (Griffiths and Lane, 1990; Foguet et al., 1998; International Organization for Standardization, 2009).

The shear strength was corrected to account for the effect of time and strength anisotropy as proposed by Chung et al. (2007). The corrected vane strength is defined as:

( ) 3.5

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where is the correction factor varied from 0.225 to 0.245 with the lower value used for sand sediment an high value for silt and clay sediment, is shear strength.

The most common errors that occurred from the FSVT include incorrect computation of the spring factor and if the sediment contains organic materials (e.g. decayed wood). Care was taken for the spring factor during computation for the shear forces. The error, which may occur as plasticity in clayey silt of the floodplain, were corrected using empirical correction factors as proposed by Aas et al. (1989).