Introduction

Figure

The nature and scope of bio-engineering

USING THE SITE HANDBOOK

This handbook provides the information needed to design, plan, implement and maintain roadside bio-engineering works. It also covers the establishment and maintenance of bio-engineering nurseries. It is intended that the handbook cover all subjects that an engineer would need on site. (The companion Reference Manual provides background and supporting information and is intended for office use.)

Each subject is covered in a separate section and sections are marked with a vertical coloured bar for easier reference.

Words that appear in the glossary have been highlighted in orange.

THE NATURE AND SCOPE OF ROADSIDE BIO-ENGINEERING

What is this handbook for?

This Site Handbook is to inform engineers and overseers on the use of bio-engineering in Nepal. It is written specifically for use on roadside slopes. It covers all of the practical aspects of designing, planning, implementing and maintaining bio-engineering site works. The companion Reference Manual provides all the supporting information required.

What is bio-engineering?

Bio-engineering is the use of living plants for engineering purposes. Vegetation is carefully selected for the functions it can serve in stabilising roadside slopes and for its suitability to the site. It is usually used in combination with civil engineering structures. Bio-engineering offers the engineer a new set of tools, but does not normally replace the use of civil engineering structures. Incorporating the use of bio-engineering measures usually offers a more effective solution to the problem. The materials and skills are all available in rural areas, however remote.

Placing jute netting on a steep cut slope

What does bio-engineering do?

· Bio-engineering can be used to protect almost all slopes against erosion¹.
· Bio-engineering reduces the instances of shallow planar sliding².
· Bio-engineering can be used to improve surface drainage and reduce slumping³.

¹ Erosion is the gradual wearing away of soil (or other material) and its loss, particle by particle.

² Planar sliding is a mass slope failure on a slip plane parallel to the surface (i.e. not rotational). It is the most common type of landslide and is usually shallow (less than 1.5 metres deep). It is also called a debris slide or a translational landslide.

³ Slumping is a form of saturated flow of soil or debris. It occurs mostly in weak, poorly drained materials, when a point of liquefaction is reached following heavy rain. It is usually shallow (less than 500 mm deep).

Bio-engineering systems work in the same way as civil engineering systems and have the same functions. They are effective at depths of up to 500 mm below the surface. They are not effective for deep-seated landslides or failures.

Designs that incorporate bio-engineering are usually the most effective and the most economic solutions for the shallow-seated problems listed above. Although bio-engineering costs more in the short term than the 'do nothing' approach, in the long term there should be additional benefits from reduced maintenance costs.

How does bio-engineering work?

Bio-engineering structures can provide a range of engineering functions (see below). The civil engineering systems given in the table for comparison are the nearest equivalent, but are not always appropriate for slope stabilisation in Nepal.

Where should bio-engineering be used?

Bio-engineering techniques for stabilising slopes should be used on:

· all areas of bare soil on embankment and cut-face slopes;

· wherever there is a risk of gullying;

· all slopes where there is a risk of shallow slumps or planar slips of less than 500 mm depth;

· any slope segment in which civil engineering structures are planned or have been built, and the surface remains bare;

· any area that has failed and needs to be restored, other than rock slopes;

· any area, such as tipping and quarry sites, or camp compounds, that requires rehabilitation.

As with all engineering works, it is most important that the techniques selected are correct for the site to be treated, and that the work is carried out with all due care and attention.

How is bio-engineering done?

In the Department of Roads, executive authority and responsibility for bio-engineering lies with the Division Chief or Project Manager, with assistance from the Regional Office or from the Geo-Environmental Unit. A Supervisor is usually responsible for each nursery (as the naike), and others for specialised bio-engineering labour gangs. Engineers and Overseers are now expected to understand bio-engineering enough to organise programmes under their chief's direction.

Some bio-engineering contractors operate nationally, but most are local, 'D class' contractors.

When is bio-engineering done?

Bio-engineering works are planned in the same way as other works, following the annual pattern of planning, budget estimation and submission, detailed site assessment, estimation and implementation. However, some differences exist: the need to establish and maintain nurseries, for example, and the fact that timing is controlled by seasons.

The main engineering functions of structures, with examples of civil and bio-engineering structures

FUNCTION*	CIVIL ENGINEERING TECHNIQUE	BIO-ENGINEERING TECHNIQUE	COMBINATION OF BOTH
Catch	Catch walls	Contour grass lines	Catch wall with bamboos above
Armour	Revetments	Grass carpet	Vegetated stone pitching
Reinforce	Reinforced earth	Densely rooting grasses	Jute netting with planted grass
Anchor	Soil anchors	Deeply rooting trees	Combination of anchors and trees
Support	Retaining walls	Large trees and large bamboos	Retaining wall with bamboos above
Drain	Surface or sub-surface drains	Downslope vegetation lines	French drains and angled grass lines

* The six main engineering functions are defined in Section 1.2 on page 15 and are elaborated in the Reference Manual.