Chapter Five: essential basic field construction skills
1 CHAPTER FIVE:ESSENTIAL BASIC FIELD CONSTRUCTION SKILLS
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)
Where:
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).
Embankment
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.
(e)Spillways
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.
(f)Settlement
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.
(i)Rollers
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.
== 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.
