Factores influyentes en el EPBT de sistemas BIPV

This section presents the criteria for assessing the suitability and sustainability of hand drilling techniques for abstracting the shallow floodplain alluvial aquifers. The following parameters would be used to define criteria for assessing the suitability of hand drilling techniques in the present alluvial sediment and of similar alluvial floodplains.

Estimating the suitability for hand drilling methods can be based on the following criteria:

- The geological suitability

- The suitability based on sediment permeability

37 - The suitability according to the water depth - The geomorphological suitability

- The suitability according to the shear strength forces

2.4.1 The geological suitability

Geological suitability is related to the hardness of the layers of rock formations (Kane et al., 2013). Hand drilling techniques are only suitable for unconsolidated sediments, but do not allow drilling in hard formations. Sediment formations, which are suitable for application using hand drilling, are soft sand, silt and clay (Weight et al., 2013). However, there is currently no available information on geological suitability in the present study despite hand drilling being commonly used. Characterising of the alluvial floodplain sediment in the present research will be used to assess and understand the geological suitability of hand drilling methods. According to the review above, the geological suitability for hand drilling method is based on the soft unconsolidated alluvial floodplain formations.

2.4.2 The suitability based on sediment permeability

The permeability characteristic suitable for hand-drilled wells is about the possibility to manually drill a small shallow borehole in permeable ground that can yield a significant flow rate (Kane et al., 2013). It is important to understand whether the types of sediment formation been drilled are permeable or impermeable. Permeability is an ability of sediment formations to transmit water through it (RWSN, 2010). Different types of sediment formation encountered during drilling include sand and silt, mixed formations and clay. Table 2.2 shows classification of the particle sizes in an alluvial sediment samples.

- Sand and silt formation: It allows easily flows of water through the open space between the sediment particle sizes and thus, very permeable. When drilled through

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this type of formation, water fills the borehole easily, which can be abstracted using hand drilling method for irrigation activities.

- Mixed formations: consists of a mixture of sand, silt and clay. Water flows slowly through the mixed formations and they are described as semi permeable. When drilled through this type of formation, it allows a slow flow of water into the well.

- Clay: clay particles are very sticky and water does not easily flow through its formation and thus, are impermeable. When drilled through this type of formation, it does not allow flow of water into the well and the well will be empty.

Table 2.2: Classification of the particle sizes in a sediment sample (RWSN, 2010)

S/No Particle name Particle size (mm)

1 Clay <0.004

2 silt 0.004 to 0.06

3 Sand 0.06 to 2

Among the criteria discussed above, sand and silt, and mixed formations are suitable for application for hand drilling techniques, because the formations allow the flow of water to recharge a well, which can be easily abstracted for irrigation. Sandy silt formations are very permeable, they allow the flow of the groundwater easily through the open space and are therefore suitable layers for hand drilling methods.

2.4.3 The suitability according to the water depth

Suitability according to water depth is related to the depth where exploitable water level can be found and reached by hand drilling. Study by Kane et al. (2013) reported that the groundwater depth suitability consists of identifying areas where exploitable flowing groundwater at a depth compatible with hand drilling techniques could be found. Generally, hand drilling is suitable when exploiting water not deeper than 40 m (RWSN, 2010; Fussi,

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2011; Kane et al. 2013); although in specific areas, it can be applied up to 100 m or more (Forsyth et al., 2010). Therefore, the depth range between 20 and 40 m in the alluvial floodplain groundwater are suitable for abstraction using hand-drilling techniques. Our literature review has shown that the shallow aquifer on the alluvial floodplain formation at Yola does not exceed 40 m (Obiefuna and Orazulike, 2010).

2.4.4 The geomorphological suitability

Geomorphological suitability refers to the existence of a surface morphology that facilitates the accumulation of unconsolidated materials, the presence of thick weathered layers and the limited depth of water. These zones correspond with bottom of the valley and sometimes with flat area with limited slope (Fussi, 2011; Kane et al., 2013).

In general, hand drilling is considered feasible in unconsolidated alluvial formations (sand, silt and clay) with interspersed layers of soft sedimentary rock or laterite (PRACTICA, 2010). General mapping of hand drilling may be based on analysis of existing data such as local hydrogeological information, interviews and discussion with the population. Detailed data regarding existing water points both from existing information as well as direct field observation, topographical maps, well logs, field surveys, geological maps, satellite images and shuttle radar topography information can provide an indication of the potential for hand drilling in a region (PRACTICA, 2010; Carter et al., 2010).

2.4.5 Suitability according to the shear strength forces

This refers to the forces applied on the alluvial floodplain sediment during drilling process (Eijkelkamp, 2009; Chung et al., 2012). Hand drilling methods are undertaken using human muscle power; therefore, it is useful to estimate the forces applied to the alluvial floodplain sediment. Field Shear Vane Tester gives an idea on the shear strength forces on the floodplain

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sediment. Understanding shear strength forces on the floodplain sediment is one of the criteria for assessing hand-drilling techniques.

There are no clear correlations between shear strength forces in relation to hand drilling in literature. However, some studies reported values of shear forces in relation to sediment formations. Recent study by Hubbell (2003) reported shear strength forces ranging between 12 and 95.8 kPa obtained using Field Shear Vane, for very soft to stiff sediment formations and between 102 to 179 kPa for very stiff to hard sediment formations. In earlier study, Schjϕnning (1986) reported shear strength forces ranging between 38 and 102 kPa on coarse sand formations, and shear forces ranging between 96 and 172 kPa on fine loam sediment formations. Schjϕnning (1986) used a Field Shear Vane together with power drilling to obtain shear strength. However, no mention of shear forces was made only comparing shear forces was reported. Similarly, Eijkelkamp (2009) measured borehole shear forces between 32 to 95 kPa using a Shear Vane. Therefore, based on the above information it can be assumed that the shear strength forces in the range between 12 and 95 kPa are suitable for drilling with human power, and forces greater than 100 kPa are above hand drilling with human power.

In brief, although hand drilling is commonly used in the area of study, we do not know if it is the optimal method, or if it is used near the limit of the (capacity) applicability.