pilot training course in bioengineering

introduction to bioengineering

Nepal is prone to natural as well as human-induced hazards. each year, several hundreds of lives and properties worth of several millions of dollars are lost, and the soil ecosystem is also disturbed. earthquake, landslide, debris flow, glacier lake outburst flood (GLOF), avalanche, and cloud burst take toll of life and property in the mountains and sudden flooding in the Terai. when such hazards occur, bridges, roads, and power intakes located in the mountainous regions are destroyed. at the same time landslides and debris flows not only take the fertile field and houses but also add to the sediment load in the river, which in turn washes away the paddy fields located along the bank of rivers in mountain areas. In the Terai area, many paddy fields are either eroded or submerged with floodwater.

in this context, it is a big challenge for engineers to solve the problem of erosion and slope stability. as it is too costly to construct heavy civil engineering structures and use high technology for solving these problems. the experiences of past several years have shown that such types of problem can be solved by using living plants. the use of livening plants either alone or in conjunction with small-scale civil engineering structures or non-living plant materials for the purpose of reducing the shallow-seated instability and controlling erosion on slopes of any watershed can be named as bioengineering. it is not a new technique for Nepal. the indigenous methods similar to bioengineering are in practice for centuries.

scope of bioengineering
bioengineering can be applied in different fields. for example, slope stabilisation on embankments and cut slopes, erosion control, water course and shoreline protection, wind erosion control, noise reduction, traffic control, mining and reclamation, construction sites, waste disposal and public health, reservoirs and dams, buildings, highways, railways, and the like.

functions of bioengineering system

any structures is constructed to fulfil a concrete function. this means, any structure fulfils its engineering as well as other functions. as described earlier in bioengineering, small-scale civil engineering (inert) and vegetative structures are used. generally, these structures are used for fulfilment of the following six engineering functions.

catch function
loose materials have the tendency of rolling down the slope because of gravity as well as erosion. this tendency can be controlled by constructing any structure, which could catch the rolling-down materials.

armour function
some slopes are very water sensitive. it means, they start moving or are liquefied easily when they intercept water. or, there may be the case of a high rate of infiltration, which later, causes shear failure. therefore, such types of slope should be covered so that the water could be diverted easily. it is called the armouring function.

reinforce function
because of presence of voids, the soil may not be compacted, and it may need bonding of the grains. the structure constructed for this purpose fulfils the reinforce function.

support function
on the slope with the length more that 15 m, the lateral earth pressure causes the lateral and outward movement of the slope material. this tendency can be controlled by constructing any retaining types of structure. they fulfil the support function.

anchor function
if there is the case of failure of overlying layers with respect to stable underlying strata, the upper strata can be pinned up with the underlying ones. this activity fulfils the anchor function.

drain function
water is the main problem leading to instabilities on slopes. it could be the surface water or the groundwater. therefore, the water should be diverted safely from the slopes.

figure: engineering functions of plant

engineering and hydrological functions of plant

engineering functions

effect

1 stems and trunks trap materials that are moving down the slope.

2 roots bind soil particles to the ground surface and reduce their susceptibility to erosion.

3 roots penetrating through the soil cause it to resist deformation.

4 woody roots may open the rock joints due to thickening as they grow.

5 the root cylinder of trees holds up the slope above through buttressing and arching.

6 tap root or near vertical roots penetrate into the firmer stratum below and pin down the overlying materials.

7 vegetation exposed to wind transmits dynamic forces into the slope.

good

good

good

bad

good

good

bad

figure: hydrological functions of plant

hydrological functions

effect

1 leaves intercept raindrops before they hit the ground.

2 water evaporates from the leaf surface.

3 water is stored in the canopy and stems.

4 large or localised water droplets fall from the leaves.

5 surface runoff is checked by stems and grass leaves.

6 stems and roots increase the roughness of the ground surface and the permeability of the soil.

7 roots extract moisture from the soil, which is then released to the atmosphere through transpiration.

good

good

good

bad

good

site dependent

weather
dependent

criteria for the use of bioengineering

criteria	application
1 reducing instability and erosion - 2 increasing the slope's factor of safety - 3 physical flexibility - 4 versatility in application - 5 only solving some problems - 6 cost-effectiveness - 7 environmentally advantageous - 8 socially advantageous -	by observation in the field. by measurement in the field. by observation in the field. by observation of a range of applications in the field. this may be difficult to evaluate. by cost comparison. by observation and comparison of sites in the field. by discussion with road corridor inhabitants and extension groups.

limitations of bioengineering

there are mainly three specific aspects not covered in bioengineering.
1. vegetation in relation to buildings:
     - damage due to water removal on shrinking clay soils,
     - root penetration on foundations and drains, and
     - risk of toppling onto buildings.
2. vegetation in relation to water quality:
     - choking of waterways with plant growth as a result of eutrophication, and
     - the use of reed beds for land treatment of effluents and nutrient harvesting.
3. vegetation growth on structure:
     · accelerates weathering and corrosion, or causes adverse effects
       on the performance of concrete and steel.
4. needs of aftercare:
     - vegetation cannot perform its engineering function in its initial stage, and
     - it demands regular repair and maintenance.