Construction Construct ion of Earth Dam
1
Phases of Construction •
Evaluation of plans, specifications, basic requirements, and features of the site
•
Planning and scheduling of the job
•
Making the site ready
•
Building the structure
•
Clean up
2
Phases of Construction •
The first phase of evaluation consists of a detailed study of the requirements of design and of the site itself, including topography, weather and river flow, and
of
the
problems
of
labour,
materials,
and
equipment supply •
The second phase is most important if the job is to be
done
economically.
The
plant,
labour,
and
materials for each stage of construction process must be available at the correct time; if they are too early there is unnecessary down time, while if too late the remaining schedule suffers
3
Phases of Construction •
Site readying includes developing access roads and the provision of the fixed construction plants such as ware houses, crushers, concrete mixers, offices and housing for the staff and workers
•
The major part of the time and money is spent on the building phase 4
Site Preparartion Clearing: •
It
is
the
removal
of
tress,
boulders,
fences,
buildings, and other structures from the area. This is for the borrowpits, and the area to be occupied by the dam Grubbing: •
It is the removal of roots, stumps and organic matter. Scattered and isolated roots can be left, but stumps
and
matted
roots
should
be
removed.
Grubbing is done only in the actual borrow pits and within the outline of embankments
5
Site Preparation Stripping: •
It is the removal of all topsoil, organic materials and soft pockets which are unsuitable for the borrow in the borrow pit areas or unsuitable for supporting structure within the embankment outlines
Scaling: •
It is the removal of loose materials, weathered rock and weak materials from the abutments. It also includes the shaping of abutments to eliminate overhangs and provide uniform slopes
6
Diversion and Cofferdamming Diversion: • Diversion is the re-routing of the river around the construction area. This is done with a temporary dam or cofferdam which walls off and protects the area, and a dive di vers rsio ion n cha chann nnel el or tu tunne nnell whi which ch ca carri rries es the wa wate ter r around the site Cofferdam fills: • Many types of cofferdams are used depending on the materials available and the river currents which flow past it
7
Diversion and Cofferdamming • The simplest is a fill of rock and earth dumped in place to form a crude dam, Fig. 1 • The heart of this is a triangular mound of the coarsest material obtainable such as boulders and large broken rock
Fig.. 1 Fig
8
Diversion and Cofferdamming • This is dumped, starting from the abutments on both sides and working toward the centre of the cofferdam • If it is constructed in a flowing stream, the rocks placed on the bottom must be coarse enough to resist being carried away by the water velocity • As the centre of the stream is approached, larger rock must be used since the velocity increases as the flow is restricted • The final closure is effected with very large rocks or even pre-cast blocks of concrete which are quickly 9
Diversion and Cofferdamming Combined coffer-dam: • In some dams it is possible to utilize a rock toe of the permanent embankment as the rock core of the cofferdam and thus reduce the overall cost of the cofferdam (Fig. 2) • Alternatively the rock toe can be made wider than normal to compensate for the poor foundation and placed directly on the un stripped ground
Fi g. 2 10
Diversion and Cofferdamming Combined coffer-dam: • In dams with an upstream core it is frequently possible to use the upstream part of the dam itself as a second stage cofferdam to protect the remaining foundation area • A small cofferdam, quickly built after the flood season is over, permits construction of the upstream part of the dam • This becomes the cofferdam for the next flood season (Fig. 3)
Fig.. 3 Fig 11
Diversion and Cofferdamming • When the currents are swift and no coarse rock or boulders are available, cellular cofferdams of steel sheet piling are used, Fig. 4 • Several patterns of cells are used depending on the height. The circular cells are the most stable since each cell is structurally independent of its neighbor
Fig.. 4 Fig
Foundation Preparation Cut-off trench: •
The soil-filled trench is most widely used form of cutoff for earth dams on an earth foundation. It is also used for the earth dams on rock where the upper surface of the rock is weathered or cracked
•
The construction procedure is employed is governed by the nature of the foundation material, and in particular
by
the
excavation slope
ground
water
and
the
stable 13
Foundation Preparation • For greatest economy an open excavation is used. This is dewatered by pumping from sumps when the foundation is made up of rock, clayey soils, or gravels • The sumps, which are merely collecting pits, should be dug just outside the trench line so that their pumping
will
not
interfere
with
the
surface
preparation for the trench • In sil silty ty soi soils ls and sand sands, s, sump pump pumping ing ofte often n crea creates tes an unstable condition including seepage erosion, sand boils, and quick sand. To avoid this water table must be lowered ahead of the excavation work so that there is never an upward flow of water in to 14the
Foundation Preparation • This is best done by well points, Fig. 5, if the water table is to be lowered by 50ft or less
Fig.. 5 Fig 15
Foundation Preparation Impervious contact: • The preparation of the contact between the cut-off (or core when no cut-off is used) and the impervious foundation requires careful hand work • All loose materials and porous materials are removed. Pits and crevices are cleaned out as deep as possible • Thin horns and over hanging projections are removed because they will interface with compaction and may break off under the action of heavy equipment
16
Foundation Preparation • If foundation is rock, it can be cleaned cheaply by a combination of water-jetting and compressed air • The small pits and cracks in rock must be filled with an impervious material that resists washing so that a zone of seepage will not develop along the contact • Cement-sand mortar is best because it penetrates the cracks easily and then sets up as a solid • A clay paste can be used for the same purpose in the smallest cracks
17
Foundation Preparation Shell foundation: • The foundation preparation beneath the shell is less critical. Seepage is not a factor and no crack sealing or pit filling is required • All soft zones are removed and the embankment placed in layers on the virgin surface • Scarifying and rolling the foundation is sometimes advocated, but it is an unnecessary expense when contact is not a problem
18
Foundation Preparation • If the found foundati ation on is of a loos loose, e, cohesi cohesionle onle
a relative density of less than 70%, dens desirable
• Certainly a blanket of dense material 2
thick is desirable to minimize the effec
structural readjustments in a loose founda
Grouting • Grout is a fluid mixture of Portland cement, water and various admixtures such as sand, clay and rock dust, which hardens to form a semi-solid or solid mass that is impervious and which has some degree of rigidity and strength • Clay grout is a viscous fluid mixture of clay and water, plus sand and other filters, which tends to stiffen slightly by the squeezing of water from the mix
20
Grouting • Chemical
grouts
are
fluid
mixtures
of
various
chemicals which solidify by chemical reaction after their injection • The process of injecting any of these materials in to the ground is grouting • Grouting is a complex process, an art in which the experience and judgment of the operator is the most important guide 21
Grouting Objectives: a) Make the formation impervious b) Improve its structural capabilities (strength, compressibility) • This is done by filling cracks, fissures, cavities and even the voids between the grains with the grouting material • In most earth or rock fill dams the stress concentrations are not sufficiently severe to require structural improvement; in masonry dams, however this is a major factor
22
Grouting Consolidation grouting: • Consolidation grouting is the injection of a larger area of the foundation to shallow depth to reduce its compressibility, increase its strength, and particularly to
minimize
seepage
in
slightly
fractured
or
weathered zone which is so often present in the uppermost few feet of rock • In earth dams it is limited to the contact zone with the core or cut-off or to a limited portion of the contact if it is wide 23
Grouting Curtain grouting: • It is the establishment of a cut-off wall by grouting • The
depth
of
the
curtain
is
determined
from
a
consideration of the seepage characteristics of the foundation • When the fissure pattern is so erratic that no rational analysis
is
possible
the
depth
in
rock
is
often
established by an empirical procedure such as: Depth = 0.5∆h to 1.5∆h or 0.33∆h + c where,
∆h
is the head on the foundation, c is a constant
equal to 25 ft in sound rock and 75 ft in fissured rock 24
Borrow Pit Excavation • Excavation of the embankment materials, processing them, and hauling them to the job are important from considerations of both cost and the quality of the materials delivered into the embankment Excavation method: • The selection of excavation procedure depends on the
type
of
available
materials
and
on
the
configuration of borrow pits • Five principal methods for handling the materials hand, power shovel, dragline, scraper and elevating grader
25
Borrow Pit Excavation Hand excavation • It has been used throughout the history and is still import imp ortant ant in in areas areas whe where re labo labour ur is cheap cheap • Most types of soils can be excavated; but coarse gravels, boulders, and even soft rock cannot be handled so efficiently
26
Borrow Pit Excavation Power shovel • It is well adapted to a wide variety of materials from soft soil to boulders and layered rock • It is also suitable for hard rock that has been shattered by explosives • Stratified materials are easily mixed since the shovel makes a vertical cut or slice
Borrow Pit Excavation Drag line • It is adapted to most soils except soft sticky clays and very hard or cemented materials • Stratified materials can be mixed or they can be segregated, depending on the mode of operation
Borrow Pit Excavation Scraper • It is most efficient in slightly clayey sands, silts, nonsticky clays and it can even function in some soft rocks • Hard materials such as cemented soils and layered rock can frequently be broken up with a rooter (a giant hooked plough drawn by a tractor) and then excavated with the scraper
29
Borrow Pit Excavation Elevated grader • The working of an elevated grader is grader is similar to that of a scraper so far as the handling of various types of materials is concerned • It has, however, some tendency to provide more pulverizing than the scraper • A long level borrow pit is required for best operation
Borrow Pit Excavation Moisture control: • Moisture control is necessary in the borrow pit to permit
efficient
operation
of
the
excavation
equipment and to condition the soil for its future compaction • Dr Drai aina nage ge is nec neces essa sary ry for for pits pits bel below ow grou ground nd wate water. r. This should be done far enough in advance of the construction so that the soil is not water logged • Otherwise the soil must be stockpiled on higher ground to drain and then re-excavated for use in the 31
Borrow Pit Excavation • All pits should be so excavated that the surface water drains away from them and does not collect in them Material control: • The operation of the borrow pit must be carefully controlled for better quality • A testing laboratory is set up at the borrow pit where the moisture content, gradation, and even plasticity can be determined in minimum time
32
Borrow Pit Excavation Modification: • In some cases modification or processing is necessary before the materials can be used • The materials necessary for filters rarely occur in the correct gradation • Even though natural sand-gravel mixtures are found which approximate the requirements, they are variable and should be screened to provide more exact control 33
Fill Placement • The placement of materials in the fill depends on the method of hauling, the processing which will be done in-place before compaction, and the size of the area to be filled • The materials hauled by hand are dumped in small piles to form a reasonably level surface • The materials hauled by trucks and wagons are dumped in uniformly spaced piles • Certain side dump wagons can also form long narrow piles or wind-rows 34
Fill Placement In-place processing: • All objectionable materials are removed during and immediately after the spreading operation • These include roots, clumps of grass and the other organic
matter,
and
large
stones
which
would
interfere with compaction • In some cases it is necessary to mix two materials. An examples in core construction when a limited amount of highly impervious clay can reduce the permeability of more abundant but less water-tight 35
Fill Placement • If the materials are hauled by trucks or wagons they are placed in alternate piles or in parallel adjacent wind-rows Moisture control: • moisture control is necessary when placing the soil, even though it may leave the borrow pit at the proper water content • The soil can be dried by spreading it in layers and turning and aerating it in the sunshine
36
Fill Placement • The pulverizing and mixing machines can help the drying because they beat air through the soil during the process • Moisture
addition
is
done
by
sprinkling
water
followed by mixing with ploughs or the pulverizing mixers • If possible, moisture addition should be done several hours before compaction, so that the soil will have time for absorbing water 37
Soil Compaction • Compaction is the process by which the required density is obtained in an embankment • Two different phenomena are involved in this process: 1. Movement of the particles into new positions 2. Distortion of the particles, particularly the flaky ones with their adsorbed layers, so that the voids between them become smaller
38
Soil Compaction Compaction methods: • The oldest method of soil compaction is tamping tamping.. This provides pressure at the moment of impact and some vibration. Because of this dual action, tamping can be used on all types of soil, but best in clayey sands, sandy
clays
and
similar
soils
of
low
(but
not
negligible) cohesion • The hard tamper , a block of iron, stone, or wood weighing 6 to 10 lb., is the simplest. The compaction effort is small and so the soil must be compacted in thin layers, from 1 to 2 in.
39
Soil Compaction • Pneumatic tampers , with the tamping part weighing about 6lb., are faster than the hand tampers but are only
slightly
more
effective
in
producing
high
compaction • Jumping
tampers weighing
from
160
to
500lb.,
powered by gasoline, are more effective. These jump from 6 to 18 in. in to the air and then drop back by gravity to produce substantial impact pressure • They are capable of compacting soils in layers of 6 to 12 in. thickness to densities of 100% of standard proctor maximum at optimum moisture
40
Soil Compaction • The sheep foot roller consists of a steel drum with projecting lugs or feet. It applies high static pressures over a relatively small area: 1 to 9 sq. in. or an equivalent diameter of 3 to 3.5 in. • Ro Roll llers ers wi with th foo foott pres pressu sure res s as lo low w as 150 ps psii an and d as high hi gh as as 1,20 1,200 0 psi psi are av avai aila labl ble e • Because of high pressure and the small width of application the sheep foot roller is the best adapted to the highly cohesive soils like clays
41
Soil Compaction • Low plasticity soils can be compacted with the sheep foot rollers but lower pressures must be employed to avoid a bearing capacity failure • A modified sheep foot roller with wider feet, 6 to 8 in. across acr oss,, is far far more more effec effective tive in silty silty soi soils ls
42
Soil Compaction • The heavy pneumatic-tired roller consists roller consists of four large pneumatic
tires,
mounted
side
by
side
on
a
suspension system which permits them to move up and down independently • These support a ballast box which exerts loads of 50 to 200 tons depending on the types of tires, the box sizes and the earth or rock ballast • The pneumatic tired roller develops pressures of 80 to 150 psi psi ove overr an area area of of 250 to 700 700 sq.in. sq.in. or or equiva equivalen lentt diameters of 18 to 30 in
43
Soil Compaction • It is very well adapted to cohesive soils of low plasticity, such as clayey sands and silts, and to cohesion less soils such as sands and gravels where the bearing capacity depends on width of the area of application
44
Soil Compaction • Various forms of vibrating rollers are available. One is a two-wheeled rubber-tired roller with a gasolinepowered vibrator on the axis • Another is smooth steel drum , ballasted with water, and with a vibrating unit in the supporting frame • Both the types are effective
in compacting cohesion
less sands and gravels • Layers from 12 to 18 in, thick can be compacted to 100% of the standard proctor maximum, often in one or two passes of the machine
45
Soil Compaction
• Ordinary Ordinary pavement pavement rollers with rollers with steel drum wheels can be used for soil compaction. They exert a moderate degree of pressure across a wide drum • They can be used in most soils provided the layers are very thin. Their main disadvantages are ¾
The tendency of the drum to bridge over small soft pockets
46
Soil Compaction • Flooding
of
sands
and
puddling
of
clays
are
sometimes mentioned as compaction methods. While these are better than no compaction at all, the degree of compaction produced seldom exceeds 85% of the standard proctor maximum
47
Soil Compaction Fill operation: • The selection of the proper methods of compaction, the layer thickness, pressure of the equipment, and the best moisture is the joint responsibility of the designer and the constructor • The most important factor is the pressure distribution in the layer being compacted. Based on Boussinesq equation,
the
pressure
beneath
the
compaction
device decreases with depth 48
Soil Compaction • A high pressure applied over a small area decreases rapidly with increasing depth (as in a sheep foot roller); Fig. 6 • The tendency is to produce better compaction on the surface of the compacted layer than at the bottom, if the layer is very thick
Fi g . 6
49
Soil Compaction • The wide compaction device with a smaller surface pressure
produces
a
more
uniform
pressure
throughout the layer. This is the characteristic of rubber-tired roller • The average pressure in a layer being compacted can be increased by increasing the surface pressure or by decreasing the layer thickness • For earth dam cores, where uniformity is needed, the moisture should be kept as high as possible. For the shell where strength is most important, the moisture 50
Soil Compaction • The surface pressure, however, is limited by the soil bearing capacity. Once that is exceeded, the soil compaction is less • Before any part of the embankment is compacted a test
strip
should
be
s et
up
where
the
best
equipment, moisture, layer thickness, and number of passes are determined by experiment • A little money spent on this may well save much time, money and argument later
51
Soil Compaction Stage compaction: • The maximum pressure which can be exerted by the compaction equipment is limited by the soil bearing cap apa acity.
The Th
uncompacte un ted d
soil
with wi
its its
loose
structure has very low bearing • If heavy equipment is used in an attempt to compact this soil, a bearing capacity failure is likely. If, however, the soil is first partially compacted with the light
equipment,
the
bearing
strength
will
be
increased sufficiently to permit the use of heavy equipment
52
Soil Compaction Stage compaction: • Such stage compaction can be very effective in higly cohesive soils which require high pressures for compaction, and it is useful for all soils when very high degree of compaction are required
53
