Chapter Five: essential basic field construction skills

From Ministry of Water DCOM Manual


In this chapter, a number of select and essential filed construction skills needs have been summarized for adherence in the implementation of Water Supply and Sanitation Projects regarding Dam construction, Boreholes, Intakes, Storage tanks, Transmission mains, Water points and Sanitation works.

1.1 Dam Construction

During the construction of the embankment earth fill or rock fill dams, the following are the key issues to be taken into account at each stage of the project implementation. The construction works should be conducted as per design specifications.

(a)Construction Supervision Guidelines

Supervision of the embankment dam is very critical due to the fact that weather is the major constraint which needs care during construction planning stage. The following are the key issues to be observed in order to achieve the intended output of the project:

(i) The supervisor should make sure that the project has been designed according to the defined standards. Therefore, in case of a separate supervisor from the designer of the project, thorough review of the project should be done and feedback submitted to the client at the agreed time.

(ii)The supervisor and the contractor should be familiar with the project at the initial stages of project implementation to avoid setbacks which may be caused by differences in project understanding.

(iii)The project cost and work plan should be well communicated to the client to make sure there is sufficient fund to run the project smoothly. Delay of fund release may result in project delay and damages may result from weather impact (probable seasonal rains).

(iv)Mobilization of equipment should be well communicated to the contractor so as to facilitate timely commencement of the project.

(v)In the construction contract, the delay caused by any irresponsibility of either party should be penalized so as to minimize the possibility of large liquidated damages caused by weather factors and sometime abandonment of the project.

(vi)The supervisor should define all resources related to the project and keep updating the client on time basis so as to avoid any delays related to miscommunication between the parties.

(vii)The client (of the project) should set a reasonable number of technical personnel to be involved in the project implementation team as counterpart staff so as to activate ownership of the project, hence reduce Operation and Maintenance costs of the project.

(viii)In the contract it should be explicitly stated that the contractor does not require to remove any equipment or plant from the site without prior notification to the supervisor and approval writing in communication with the client.

(ix)All technical specifications of the project should be approved by the supervisor of the project.

(b)Site Clearing and Preparation

Base of the dam
All trees and roots, grass, grass roots and top soils must be removed. Once the trees have been removed (usually by hand) the dam bulldozer and excavator can be used to remove about 300 mm of the top soil which can then be left in a position from which it can be later retrieved to dress the completed embankment or other disturbed areas.

Borrow pit areas
Borrow pit areas should have been demarcated and maintained to be used on some projects. The borrow pit areas sampled during soil test analysis should be used for construction of the project. The passed borrow pits should be well documented and their GPS coordinates should be used for identification during construction.
Excavated soils (from the borrow pits) must be frequently monitored to check that its quality and moisture content has not changed and that it is still suitable for emplacement in the embankment. The core and cut-off trench requires good quality clay, the downstream shoulder needs poorer and coarser materials (drainage is important) and the upstream shoulder needs a clay soil of some permeability.

(c)Setting out of the embankment core trench
The Centre-line pegs should be installed at the ends of the embankment and at every change in ground level. For each change in ground level, a ‘mating’ peg should be established by level machine, Differential GPS or theodolite on the opposite side of the valley, but still on the Centre line.

At each peg, on the Centre line of the embankment, the distances of the toe pegs upstream and downstream are calculated and set out at right angles.

For large dam project the consolidation test results should be used to determine the freeboard due to settlement. Using rule of thumb extra 10% of dam height is added to cover the loss in dam height due to settlement.
The toe peg offset distances from the centre-line are calculated using the formula:
Offset distance (m) = S. H + 0.5 Cw…………………………………………………(5.1)
S is the slope value
H is the height of the embankment (m)
including 10% allowance or calculate settlement in dam design software such as Geostudio software (Sigma/W)
Cw is crest width (m)

(d)Constructing the Embankment
The core/cut-off trench
Excavation of the core trench
Excavation of the core trench should be conducted by using an excavator. Due to the size of an excavator arm, the operator should follow the setting out established in the design report. Due to stability reasons of the trench and seepage management reasons, the core trench is excavated in trapezoidal shape. Trimming of the trapezoidal shape is conducted when the excavator is on the side of the trench. The depth of the core trench depends on the design specifications given in the design report and detailed drawings.

Filling of the core trench
As per engineering specifications, the materials obtained from the tested borrow pits are hauled, transported and spread in the core trench at layers not more than 0.3 m thick and compacted at an optimum moisture content to attain maximum dry density of the compacted soil. The degree of compaction is a ratio of compaction carried out at site and that achieved in the soil test laboratory. The degree of compaction is obtained by using different methods and namely sand replacement methods and the nuclear method.

Raising of the Embankment
The embankment is compacted in the same way as that of the core trench and the only difference is to maintain a clay core in case of a zoned embankment where the clay core may be vertical or inclined depending on the design choice made. The designer may select to use a certain alignment of the clay core or concrete cut-off wall depending on the site-specific conditions.
As this is the most important part of any embankment, great care is necessary in the excavation, filling and use of materials.

The minimum depth necessary will depend on the site conditions but in all excavations the cut-off trench must be taken from good quality impermeable materials such as clay or solid rock or to a minimum of three-quarters of the dam’s crest height. If a suitable rock is located and is generally good, it is permissible to fill any cracks or fissures with compacted clay or mortar, provided they can be fully cleaned and traced to ensure seepage paths will not develop later. If an impermeable layer of sufficient thickness has not been reached and the trench depth has attained the required height of 0.75H, the cut-off trench excavation can stop only if the material encountered is not of a coarse or gravels nature (as it often occurs in streambeds). If permeable material is found it is vital that the cut-off is taken through it to a depth sufficient to find more impermeable materials.

Before backfilling, the excavation should be checked to ensure that the conditions above have been complied with. Short cuts taken at this stage can prove costly later and seepage through the embankment can become excessive if the correct depth into the correct material is not achieved. A little extra time and care in the excavation of the core is usually worthwhile.

Other requirements such as construction of a coffer dam, special compaction, dewatering equipment and safety provisions in the trench should be considered before excavation starts, to allow the work to be carried out efficiently. For example, an assessment of the site condition, such as to ascertain groundwater levels, at the design stage would allow such special provisions to be included in the cost estimates. Water bowsers or other water sprinkling equipment may be useful in assisting compaction of the embankment.

Ant heap material or cracking clays are not recommended for core filling but if the former is used it should be chemically treated and in all cases kept as far as possible below the ground level sections of the core (which should remain wet throughout the year).

Once the cut-off has been brought up to ground level, the embankment can be constructed. If necessary, and usually because of time limitations, it may prove prudent to construct the cut-off some time before the rest of the dam (i.e. during the previous dry season ensuring the works are protected from erosion). The removal of the soil from the borrow pit areas can be assisted by ripping or irrigating the area involved (avoid over-watering which could lead to traction problems). The latter is especially desirable for core and upstream sections where the soil, if used wet, may be more readily compacted.

At stages determined by the designer/supervisor, the embankment as constructed should be surveyed to check that the slopes conform to the design specifications. If there is any variation, remedial measures will be necessary. It is better therefore to avoid such problems by careful and frequent monitoring of the structure as it takes shape, especially at the beginning of the work when operators and other staff are more prone to making mistakes.

When the embankment is at the correct height it must be surveyed to check in particular that the crest has been built slightly convex with more soil laid in the centre where the most settlement will occur. The crest should have a slight slope (cross fall) towards the upstream side of the embankment to permit the safe drainage of rainwater to the reservoir rather than the downstream slope. A channel may be necessary to reduce the risk of erosion.

It is very important that good grass cover, preferably of creeping grass type, is established on both the embankment and the spillway before the likelihood of heavy rains. This could mean constructing most of the spillway before work on the embankment itself starts, ideally at the end of the previous rainy season when water for establishing grass is available.

Maintaining the geometry of the embankment
During raising of the embankment, the contractor should maintain the designed geometry of the embankment by trimming of the raised embankment wall slopes using the excavator. Based on the trapezoidal equation the upstream and downstream slopes of the embankment is maintained by proper trimming of the embankment at any reasonable dam height. Computation of the top width is done at each stage of the embankment raise. Note that in case of miscalculating the top width at any stage of embankment raise, will alter the final crest width or slope of the embankment or both.

For large dams spillways are very sensitive structures which need great care during construction. The engineering design of the spillway should be maintained during construction phase of the project. In all cases the movement of machinery over the spillway area should be minimized to avoid disturbing the topographical set up of the spillway proposed area due to erosion which may be caused by moving machine. Any large volume spillway cut should be done at a time when the excavated material (if suitable) can be included with the material being moved to construct the main embankment or reserved to fill in borrow pits.

As the dam settles, the crest should fall close to the horizontal. The monitoring benchmarks or beacons should be used to monitor the horizontal and vertical movement of the embankment.

(g)Plant and Equipment
Consideration of what plant and equipment is available, the conditions of operation and distances materials are to be moved, as well as size and type of dam to be built, are the most important factors in determining the plant and equipment to be used.<br. Bulldozers are not generally recommended as they make it difficult to achieve the levels of compaction and layering essential in any earth embankment. Very small dams made of impermeable materials, up to heights of 2m, can be successfully constructed with bulldozers (calling for settlement allowance of up to 20 percent). In context of the manual, large earth fill embankment dams are highly considered. Heavy earthmoving machines – such as elevating scrapers and push loading scrapers are really necessary for large dams construction.

(h)Compaction Equipment and Techniques
The compaction of soils is essential to increase the shear strength of the materials to achieve high levels of embankment stability. A high degree of compaction will increase soil density by packing together soil particles with the expulsion of air voids. Comparing the shear strength with the moisture content for a given degree of compaction, it is found that the greatest shear strength is generally attained at moisture contents lower than saturation.
If the soil is too wet, the materials become too soft and the shear stresses imposed on the soil during compaction are greater than the soil’s shear strength, so that compaction energy is dissipated largely in shearing without any appreciable increase in density.

If the soil is too dry, materials compacted in this condition will have a higher percentage of air-spaces than a comparable soils compacted wet. It will take up moisture more easily and become more nearly saturated with consequent loss of strength and permeability. A damp soil, properly layered and compacted with a minimum of air voids also reduces the tendency for settlement under steady and repeated loading.

Sheep foot rollers can compact layers of soil up to 350 mm deep gross (i.e. about 300 mm after compaction) and satisfactory densities can normally be obtained with 8-12 passes at a roller speed of 3-6 km/h when the soil moisture content is right. It is important to keep these rollers clean as soil collecting between the feet will reduce compacting ability. Sheep foot rollers are more effective than other rollers in compacting drier clay (but will require more passes) and will churn and blend the soil which is useful in distributing water throughout the construction surface when borrow pit water spraying is not possible. Note the weight of the compaction and vibration energy are key issues to be considered when selecting compaction equipment.

Vibrating rollers are more suited to the compaction of sandy soils and where resulting very high densities are required. In dam construction their usefulness is usually limited to small-scale works such as narrow cut-off compactions and trench works.

Rammers and plates have much the same application and are used where space is a limitation and in specialized works such as trenches, behind concrete and around pipe works. They suitable application of the equipment is on the outlet pipe. On clay soils, smooth-wheeled rollers can form seepage paths between layers of soils laid on the embankment. If a sheep foot roller is not available to compact such soils, the layers of clay should be reduced in gross depth and final surfaces roughened (by harrowing or similar) to permit a good bonding between compacted layers.

1.2 Borehole/Wells

The method of construction of a borehole/well should depend upon the depth of the aquifer tapped, the diameter required, the nature of the geological formation to be penetrated and the amount of data backup available.

Factors to be considered during the construction of Boreholes/Wells

(a) Borehole construction should be based on the recommendation of the Hydrogeological and Geophysical Survey Report,
(b) Proximity to the planned service area,
(c) The site should be easily accessible by drilling rigs and other equipment during the drilling, construction and maintenance phases,
(d) It should not be within 100m of the cattle watering pools, latrines and other health hazards, and preferably be upstream of those. Any pit-waste (solid waste) should be placed downstream of the well to avoid the water well-being contaminated by leachate,
(e) It should be safeguarded against flooding. Especially near rivers, the location has to be chosen so that the well is not threatened by any meandering action of the river. Furthermore, the danger of flooding of low-lying areas should be taken into account,
(f) The sub-soil should not render the construction of a well impossible. It is difficult to make a hand dug wells in rocky materials, etc.
(g) Proximity to existing electric power lines. Avoid sites close to existing High voltage electric power lines, otherwise exercise maximum safety precautions.

1.2.1 Drilling Methods

(a) There are several different types of rigs available for drilling water boreholes. They vary in size, capacity and capability depending on the type of formation expected and the depth required. There are rigs which do not perform well in hard rock formations and there are those that are multipurpose.

(b) Percussion and rotary-percussion drilling methods are generally the most applicable techniques for drilling in consolidated formation (igneous and metamorphic rocks). If a significant thickness of granular or other overburden materials is present, a combination of methods can be effective, although not very practical.

(c) Cable-tool, hydraulic-rotary percussion and air-rotary percussion (down-the-hole air hammer) and foam drilling modifications are the most common types of equipment in use today for igneous and metamorphic rocks. (Referred to Web: https// manual)

(d) In unconsolidated loose, unstable, collapsing formations, rotary with appropriate drilling stabilizer should be used. In such a case the drilling diameters will be telescopic starting with diameter large enough to lower temporary casing in upper collapsing formations and continue drilling depend on the final minimum diameter. If other chemical fluids or solids are used to arrest collapsing of formations, the Contractor has to use proper borehole development and cleaning methods to make the use of borehole water safe for drinking purposes. The Contractor will use such fluids or solids with the agreement of the Client.

1.2.2 Borehole Depth

The depth of the borehole should be determined from the lithological log. A borehole should be completed to just below the bottom of the lowest aquifer to be exploited for the following reasons:

(a) More of the aquifer can be utilized as the intake portion of the well, resulting in higher specific capacity,

(b) Sufficient water is available to maintain the yield even during periods of severe drought or re-pumping,

(c) To provide room at the borehole bottom for casing to keep away any loose materials between the casing and the borehole wall.

1.2.3 Casing Materials

The selection of casing materials should be based on the water quality, well depth, cost, borehole diameter, required yield and drilling procedure. The common types of casings used in borehole construction are steel, thermoplastic, fibre glass and concrete. The pipe quality should be approved by Tanzania Bureau standards (TBS).

1.2.4 Gravel Packing and Grouting

The annular space between the casing and borehole wall should be filled with filter packing materials in the screen intervals and materials. The gravel packing mixture to be used depends on the sieve analysis results and the slot size of the screen. The contractor will do the sieve analysis and then determine the gravel pack materials. Gravel packing material will be stored so as to avoid contamination or rain-washing finer materials. Iron and Calcareous grains will not be included in the gravel pack materials. Where those occur in a formation it is best to use blank casing sections. The uppermost section of the annulus is normally sealed with bentonite clay and cement grout to ensure that no water or contamination can enter the annulus from the surface. Where gravel packs are considered necessary the D60/D10 particle size (size passing sieve 60% and 10% respectively) is a guide to selection.

  • Sufficient gravel pack should be placed against the screens i.e. from below the lowermost screen to above the uppermost screen. The gravel pack should extend to approximately 2 - 3 m or more above the uppermost screen to allow for settling during well development.
  • The gravel pack should be capped with a clay seal (pure clay) to prevent contamination via the annular space.

NB: The amount of gravel (in 50 kg bags) used on each borehole should be carefully recorded by the Supervisor. Back-filling the Borehole

The annular space above the clay seal should be back-filled with inert drill cuttings. The top 3m of annular space should be left for sealing the borehole with cement slurry.

1.2.5 Well Development

The main objective of well development is to remove finer materials like native silts, clays, sand, drilling fluid residues deposited on the borehole walls during the drilling process from the borehole and immediate surroundings (gravel pack and the aquifer). The pack and the aquifer are cleaned and opened up so that water can flow into the well more easily. The well should be developed before the borehole is back-filled up to ground level. The reason for this operation is that the gravel pack around the screens will settle and become compact during development, and therefore more gravel has to be added up to the design level, before any other back-fill is put into the borehole.

Development can be done by either of the following methods:

  • Continuous airlift until water is free from sediment.
  • Intermittent airlift development. The cycles to be determined depending on the rate at which water is clearing. Typical cycles are 10 minutes airlifting followed by 5 minutes’ recovery. Intermittent airlifting should be carried out until water runs clear to the satisfaction of the Supervisor.

The Supervisor should always accurately record date and duration, in hours, for well developing. After a well development, the plant can be rigged down.

1.2.6 Instructions at the end of drilling Sealing the Borehole

The upper 3 m of the borehole annulus should be grouted with cement slurry to provide an effective seal against entry of contaminants. Capping the Borehole

The borehole should always be capped after well development. A borehole reference number should be marked on the borehole casing above the ground surface. Clearing the Drilling Site

On completion of the construction of the borehole the site should be left clean and free from all debris, hydrocarbons and all sorts of waste. All dug pits should be filled with soil or murrum free of hydrocarbons.

1.2.7 Pumping Test supervision

For every successfully drilled borehole it is important to carry out test pumping. Test pumping is performed to determine the optimum yield (quantity of water that can be drawn out of a borehole in a given time - Q) of a borehole and the depth at which the pump needs to be installed. An advice on the pump (hand pump or a certain type of motorised pump) to be installed can be given based on interpretation of the data, leading to an advised yield and an estimated dynamic water level (DWL).

The test pumping procedures and details about test pumping are in the guidelines for test pumping (Vol. I). During a Constant Discharge rate, a sample of water (1–2 litres) should be collected and taken to the laboratory for analysis of physico–chemical properties in order to determine portability and acceptability. It should be stressed that there are agreed water quality as well as quantity limits below which no installation of hand pumps is permitted. The water quality guidelines are found in the technical specifications for borehole drilling.

1.3 Intakes

Stream or river intake is to be located at the eroding part of a river curvature rather than the side where silt is deposited. It is usual to site a weir on hard rocks that is often supported by some dowel bars that are drilled into the rocks. During construction, it is advised to temporarily divert the river until the intake weir construction is completed and the concrete has fully set.

1.4 Transmission Mains

When constructing big pipelines with diameter of 300mm or more, ensure washout valves and air release valves are located on valleys and hills, respectively. When connecting or welding pipe joints do not put soil cover to the joints before testing for leakages and do not use edible oil in fixing the rubbers. Upon testing the pipes remove air locks. Starting from the intake gradually move across the pipe length until water reaches the storage tanks or water point.

1.5 Storage Tanks

Reinforced block wall tanks can be either fully buried underground, partially buried, on the ground or on a raiser of between 6 m and 12 m. Soil investigations have to be done at any site for construction of the raiser in order to decide how to reinforce the foundation of raiser. Use Waterproofing additives to the cement used for construction of concrete elements of the tank including the plaster to the walls. It is useful to be aware that drawings of all standard capacities of storage tanks ranging from 10,000 litres to 500,000 litres are available in the Ministry of Water (MoW) Website. The drawings contain bills of quantities as well as the bar-bending schedules.

1.6 Distribution Mains

During construction for laying water pipes, the trenches have to be located on the edge of the road reserves and if there is enough space to be outside the road reserve. The depth of the trenches should not be less than 60cm and again the joints should be buried only after testing the pipes for leakage. Air entrainment has to be minimized through optimal opening of the valves when commissioning the pipe network for the first time. The smallest diameter of the these pipes is 12.7 mm (1/2).

1.7 Water points

Location of water points is often done in collaboration with Community or CBWSOs if this already exists. Regardless of the type of water point constructed, one tap or 2 taps, the water points have to be drained away from the water connection point into a soak away pit filled with gravel or stones. Alternatively, the spillage water can be led to a garden located in the neighborhood. It is important to install a very good quality bib cork and often times these are the first to be damaged. Water points

1.8 Sanitation Works

1.8.1 Site location for sanitation facilities

The site of faecal sludge/wastewater treatment systems need to be carefully selected not to cause too much nuisance to neighboring communities and for faecal sludge not to induce too high transportation costs of the sludge from the points of generation to the point of treatment. Good communication between Local Government Authorities (LGA), town planners and sanitation engineers/designers is important. The LGAs need to allocate land necessary for the faecal sludge/wastewater treatment facilities involving the communities. Minutes of the agreements with communities must be properly documented for future references.

Once land has been allocated the LGA must ensure that it is properly secured with title deed and fencing to avoid encroachment. The fencing should ensure the minimum distances from settlements. Distances of 550-5000 meters are suitable because smaller distance from these treatment installations from main settlements would lead to transmission of odor and pollution to city.

On the other hand, long distance involves huge costs for constructing infrastructure and transportation. Topography of the area should be determined. Whenever possible use gravitational flow to the treatment site in order to reduce operational costs. The layout of the site should also allow gravitational flow from stage to stage except where there is a need of pumping to elevated tanks or units. Use the head generated by the elevated unit to use gravitational flow to the next stages.

1.8.2 Conveyance Systems

Conveyance systems in sanitation may be open channels, sewerage lines and piping of treatment equipment and units. The open channels should be protected against throwing of trash by people and falling accidents. The laying of the sewer for wastewater transport to treatment plants should follow the same principles as any water pipe or open channel.

Sludge transfer stations are part of the conveyance system. Location of transfer stations must involve the communities. Here the LGA involvement is highly necessary and minutes of the meetings and agreement reached must be properly documented for future references. The transfer stations must be paced in a way that they do not cause nuisance to people. They must be hygienic and fenced. Mobile transfer stations may be considered. Feacal sludge from pit latrines in slum areas or unplanned areas may be collected and transported by manually operated transportation.

1.8.3 Treatment System

The site for the treatment facilities must be fenced and protected from un-authorized entry. If biogas is being released in enclosed areas intentionally or unintentionally fire hazard warnings should be clearly installed. Firefighting equipment must be provided including sand buckets. Construction of treatment plant should be accompanied with training of operators of the plant on operational issues and health and safety. Layout of the facilities must ensure smooth flow of wastewater from one unit to the other. In case of waste stabilization pond NEVER place inlet and outlet of a unit linearly aligned. These two must be placed on diagonally opposite sides of the unit to avoid short circuiting.

1.8.4 Storage and distribution of treated wastewater for re-use

Wastewater and feacal sludge treatment should necessarily aim at reuse. The concept of treatment for reuse must be encouraged at all levels. The LGAs and utility companies are responsible for encouraging and regulating the treatment approach. The key for this approach is to ensure that the feacal sludge or wastewater is adequately treated for the intended reuse. For sludge secondary treatment is very important. There may be a need of polishing stages and treated water storage. The storage may be a pond, an underground tank in combination with an overhead tank for distribution to the needed area. Distribution can be achieved in pipeline laid following principles of pipeline design and layout. Reuse guidelines for treated sludge and wastewater can be prepared.

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