design aspect of small scale civil
engineering structures
small-scale civil engineering structures
civil engineering structures
jute netting
materials and equipment:
- woven jute netting;
- hardwood cutting from shrub or tree, 2 to 5 cm in diameter
and 30 to 40 cm. long;
- tools for cutting wood and jute;
- iron bar for making hole; and
- wooden mallet.
method:
a - trim to an even slope : make sure there are no small protrusions
or depressions which will interfere with the netting, remove protruding
rocks if possible;
b - peg the netting : starting at one end of site, peg the end of one roll
of netting 30 cm above the slope to be covered;
c - slowly unroll the netting down the slope;
d - peg the netting: allowing some slack in the netting, begin to peg
it from the bottom of the slope. hammer hardwood cuttings or pegs through
it at intervals of 50 to 100 cm leaving the cuttings protruding about 8 cm.
while working on the slope never hang on the netting - always stand
on the pegs;
e - cover the whole slope with netting : repeat the process, making sure
that the vertical edges of the net meet, until the whole slope is covered
in netting;
f - butt joint the strips: place a series of pegs down each side of the butt
joint so that the jute is held together as a continuous net;
g - carefully adjust the netting : if necessary adjust the netting in order
to reduce the tension and let it hug the surface closely. if it remains
tight it will not lie right against the slope surface;
h - place additional pegs : add further pegs as necessary to ensure
complete surface contact;
i - trim lower edges : cut the netting strips to the length required.
advantages:
- it provides rapid cover for the slope surface;
- even on the harshest sites the young seedlings are protected from
run-off and drought until they become established;
- jute netting is easily produced locally.
disadvantages:
- it can only be used in limited places because it has a high moisture holding
capacity;
- netting has a short life span unless it is bituminised, and even then it will
last for no more than 3 years.
gabion bolster panels are normally 5 m x 1 m. where larger bolsters are required 5 m x 2 m panels can be woven. they are made on a conventional gabion-weaving frame but with a much smaller mesh than usual. heavy coated 10 swg wire is used for the border and 12 swg for the mesh.
materials and equipment:
- woven gabion panels;
- 12 mm mild steel rod cut into 2 m lengths;
- boulders;
- tools for digging trenches and for working with gabion wire; and
- hammers.
method:
a - trim the slope : first trim the slope to be treated to an even slope with
no small protrusions or depressions, which will interfere with the bolsters.
remove protruding rocks if possible;
b - mark out a contour : starting about 2 metres from the bottom of the slope,
mark out a contour line across the slope with the aid of a spirit level;
c - dig a trench along the line : the trench should be about 30 cm wide
and 30 cm deep;
d - lay a gabion bolster panel lengthways along the trench : make sure
the edge of the panel on the lower side is flush with the edge
of the trench;
e - fill the bolster with stones larger than the mesh size;
f - fold the upper edge of panel over the stones and join it to the lower
panel edge. Leave a 10 cm flap from the upper edge extending over
the lower edge;
g - join abutting bolsters across lope : form the bolsters into a continuous
line across the slope and close the extreme ends with wire;
h - backfill: backfill the material around the bolsters, compact it and clean
away surplus debris;
i - peg with steel bars: drive mild steel bars into the ground at right angles
to the slope every 2 metres along the bolsters. Position them immediately
below and touching the bolsters, and drive them in far enough so that they
cannot be pulled out by hand;
j - cover remaining site: repeat steps 'b' to 'i' 2 m higher up the slope
and repeat again until the area is covered;
k - starting from the top of the slope, clean away surplus debris
and make sure that backfill is complete and firm.
advantages:
- provide very strong and durable surface scour checks;
- stronger and longer-lasting than wattle fences; and
- freely drained and so little moisture is accumulated on the slope.
disadvantages:
- relatively expensive to install; and
- contour bolsters give rise to an increase of infiltration, which can
cause slumping on some slopes.
key features of small check dams
check dams are often poorly constructed and either fail or require remedial work. you should give attention to the following points:
- build sound foundations on a good base;
- key dam well into gully sides;
- include weep holes to drain water from behind wall and reduce
hydrostatic pressure;
- make a notch and slope top of dam towards centre so water
does not scour sides;
- point top of wall with cement mortar; and
- shape to get counterbalance moment.
design aspects of vegetative
engineering structures
| system |
functions |
method of operation |
applications and site requirements |
time to maturity |
limitations |
| horizontal line grass planting |
catches, reinforces, supports |
dense line retards surface water flow |
dry, slope <45°, erodible, cut slope |
2 seasons |
thin line easily broken |
| diagonal line grass planting |
catches, reinforces, some support |
dense line guides water along the line |
wet, permeable, fine, cut slopes |
2 seasons |
rills break through |
| grass seeding |
catches, reinforces, supports |
dense grass, mat, rooting system |
consolidated debris slopes <45° |
3 seasons |
can cause liquefaction, young plants get washed away or dried |
| palisades |
catches, reinforces, supports |
dense line above and below the ground retards surface and shallow water flow |
slope <30°, dry, erodible and consolidated debris |
2 seasons |
causes small slumps, requires many cuttings, high mortality |
| brush layering |
catches, reinforces, supports |
dense line, strong buried branches retard surface and shallow ground water flow |
slope <45°, dry, erodible and consolidated debris |
one season if planted early and watered |
destructive to slopes during the excavation, requires many cuttings |
| fascines |
catches, supports, drains |
woody bundle, dense stems, porous, can drain soil if laid down slope |
consolidated debris slopes, <45° |
3 seasons |
destructive to slopes, requires many cuttings, slow to develop, high mortality |
| shrub planting |
transpires, catches, armours, reinforces, anchors, supports |
bunchy leaves, multiple stems, lateral roots, root cylinder, tap roots |
any slopes < 45°. |
at least 4 seasons |
|
| tree planting |
transpires, armours, reinforces, anchors, supports |
lateral and near vertical rooting systems, root cylinder |
any debris slopes <45°, gully side slopes |
at least 5 seasons |
top heavy on steep slopes, leaf drip, canopy shades smaller plants |
| bamboo planting |
transpires, catches, armours, reinforces, supports |
dense poles, massive rooting systems, dense leaves, grows all year |
slope <30°, base of slope, erodible slopes, preferably wet places |
at least 5 seasons |
source plant damage, delicate, requires nursery space, heavy to transport |
interaction between plants and civil
engineering structures
in slope stabilisation we may have a choice whether to use:
- civil engineering on its own;
- vegetative engineering alone;
- a combination of the two.
as soil conservation officers, we need to understand the principles underlying the relationship between vegetative engineering systems and civil engineering systems.
1 relative strength of structures over time.
the strength of a structure at different stages of its life can be related to its maximum strength. this can be described as a percentage of the maximum strength.
a. life span of small civil engineering structures
b. life span of vegetative structures
c. combined life spans
as the relative strength of engineering structures decreases, the relative strength of plant structures increases. note that these graphs relate to the performance of each type of structure separately. they do not compare the actual strength of the civil engineering structures compared with the strength of the vegetative engineering structures.
jute net and grass can both be used to perform a catching function. in the beginning the fine soil retaining capacity of the jute net is very high and each small square behaves as mini check dam. with time the jute decays which weakens the net and consequently its soil retaining capacity decreases.ultimately the net will fail to carry out any retaining function. the grass slips grow up with time and start to retain soil on the slope due to the development of root and shoot systems. When grass is fully grown, it stays at 100% relative strength. as the relative strength of the jute net declines the relative strength of the grass increases. the soil retaining function of the jute net is handed over to the grass.
2 physical relationships between civil and vegetative engineering structures
various relationships may exist between the functions of civil and vegetative engineering structures, e.g.:
- toe wall below bamboo - structure protects plant;
- plants around end of toe wall - plant protects structure;
- trees above toe wall - plant improves performance of structure;
- fence with young plants below - plant replaces structure.
these are the four ways in which civil and vegetative engineering structures can be used together.
3 compatibility of engineering structures
in the last example the function of the civil engineering, structure is handed over to the plants. if this is to happen the engineering functions of the two structures must be the same.
ithis exercise will be carried out on the three sites you used for the examination of civil engineering systems. this means that you should be concentrating on the vegetative structures and the interaction between the civil and vegetative engineering systems.
at each site you will you will spend 25 minutes examining the existing systems and evaluating them. You will find that drawing a sketch map of the main features of the site helps your evaluation. then you will discuss your conclusions with the other group working on the site.
your discussion should be based on:
- what systems are present;
- what engineering functions they perform;
- how they perform these functions;
- how the civil and vegetative systems are integrated;
- how the two systems interact;
- how well the systems are suited to the site;
- what alternative systems could be used on this site;
- how well the systems are working. if not, why they are not working.
if they are not yet ready to function when they are going to take over the job;
- any weaknesses in the way in which they have been implemented; and
- any unavoidable weaknesses in the systems.
keep a note of the information that you collect and the points that come up during the discussion. these will form your record of this page.
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