When all the requirements needed are at hand, fishpond construction starts as planned. The system of farm construction will be based on the prepared program and schedule of development. In cases where there is a limited capital, partial construction can be practiced, that is by developing a portion of the area and start the production and the income will then be used for the development of the remaining area.
a. Underbrushing. In underbrushing, vegetation, including nipa trees and shrubs, of less than 10 cm in diameter are cut with the use of the bolo. This operation must be done systematically in blocks and be completed immediately before the perimeter dike and the main gate are completed. This is to prepare the pond for the next process. Underbrushing is done by manual labor and the work should commence when the foundations of the main dike and the main gate have been established.
b. Withering (optional). Withering is to kill back the trees by filling up the pond with water. It has been found that mangrove trees, especially the group of Rhizophora, usually fade when their trunks are constantly soaked with water at a depth of more than 0.5 meter for a period of 4–6 months. They will eventually die if inundated continuously for a period of about one year.
c. Falling and burning. Falling is simply cutting down big trees left after underbrushing. The falling operation should commence when the tree bark begins to peel, but before the leaves and branches fall. The prerequisite of falling is to dry and harden the ground, which can be done by keeping the water table of the entire pond area at 0.3–0.5 meter below the surface for a period of 1–3 months depending on the weather conditions. Either manual or mechanical method, or a combination of both, can be employed for falling. A chain saw is effective in falling big trees and cutting logs. It is a fast method and economical to use.
Initial burning can usually be carried out when the dead trees have become dry. Pruning and reburning operations should start as soon as the site is cooled sufficiently to allow access to the unburned portions.
d. Uprooting of Stumps. Uprooting means the complete removal of tree stumps and root system embedded in the soil. Manual labor is usually practiced in fishponds, although uprooting by the use of machines can also be done. Manual labor is effective and economical in smaller areas having small stumps and roots, but for big stumps and larger areas, it is more economical to use a mechanical puller.
The mechanical puller consists of a winch and its associated equipment. The winch is powered by 8 to 16 HP gasoline engine for lightness and portability. The winch and the engine are set on a common steel base provided with an anchor, holdfast or a log-deadman (see Fig. 30). The steel cable, provided with a quick-detach shackle, is then tied securely to the stump. If all are properly set, the engine is started and the winch will pull out the stump. For extra large trees, roots opposite the winch may have to be cut if the winch cannot pull the tree out entirely.
The mechanical puller is moved from place to place, until all the stumps have been up-rooted. The pond is then flooded wide tide water, the uprooted stumps will partially float and can thereby be removed from the pond.
Figure 30. Mechanical Puller and Types of Anchor
a. Main Concrete Gate. The main gate location depends upon the layout plan of the fishpond project. Usually, however, it is constructed at the central side of the proposed fishpond facing the source of water.
The main gate is constructed at the same time the perimeter dike is constructed to allow the curing of concrete while the main dike is completed. The best time for construction is during neap tides or “aya-ay”, preferably during the months of February and March. Construction should be completed before the spring tide comes. All the materials needed should be in the area before the construction of the gate is started.
Prior to construction, conduct tidal observation in the place where the main gate is to be constructed. To calculate the elevation of the ground surface based on the height of the tide observed, set a tide pole and determine the highest mark the tide has reached for the day. If no tide pole is available, any permanent structure such as a tree can be used. Locate in the tide table or tide calendar the time and date the high tide occurred and note its height, and apply correction to the measurement as discussed previously in the text. This height of tide is reckoned from the MLLW. From this data, you can approximate the elevation of the existing ground surface by adding the difference in elevation (g) to the elevation of the tide observed (T), (see Fig. 31).
Figure 31. Calculation of Ground Level from Tide Observations
The elevation of the ground surface could also be determined based on pre-established Bench Mark, if it is available.
After conducting tidal observation, gate construction can begin. The following is the procedure in constructing a concrete main gate:
Measure the area of the proposed concrete gate. Give a space allowance of about 1–2 meters around for movement as the markers are placed;
Enclose the markers with a temporary dike. Make the temporary dike high and big enough to withstand the pressure of the incoming tide. Pump or bail out the water inside the temporary dike;
Excavate the enclosed portion to a negative 0.6 m elevation. Remove the stumps, roots, and soft soil if there is any. Excavation should include the portion where the toes of the gate will be constructed;
Drive three-meter long bamboo puno or wooden posts into the ground over the entire area of the gate at one-meter intervals. Leave 5 cm of the piles exposed. Continue removing the soft mud displaced as a result of the driving. Drive shorter, (one to two-meter) piling at 30 cm intervals. The driven bamboo puno should be level;
Insert boulders about 20 cm thick between the piles to form a floor. Spread gravel to level and cover the boulders, then compact the fill materials. Leave 5 cm of the bamboo piles exposed. These piles should be embedded in the concrete mixture for a stronger foundation;
Construct the form for the flooring including the toes. Place the reinforcement bar as per plan and tie with no. 16 wire. Check the elevation of the flooring to be sure it is still within the desired elevation;
Install vertical and horizontal reinforcement bars for side walls, wings and collars. Place the reinforcement bars for bridges and tie them one by one. While installing the reinforcement bars, prepare forms for the walls, wings, etc.;
Set the forms for the walls, wings, collars and bridges. Properly reinforce all the forms with wood so that it can withstand the pressure of the concrete mixture. Fix the reinforcement bars at the centers of the forms. Pump the water out if there is any. Continue pumping if necessary. Check if everything is in place and properly fixed;
Prepare a Class A concrete mixture (1:2:4) and pour it in the form continuously until completed. While pouring the mixture, somebody should compact the concrete with the use of a long stick to prevent a hollow or void forming, and at the same time move large stones to the middle of the form in order to insure a smooth finish when the forms are removed. Prevent salt water from coming in contact with the concrete mixture while it is still wet;
Allow concrete to harden for about 2–4 days, then remove the forms. Apply finishing touches, especially at any portion of the gate not properly compacted;
Sprinkle fresh water on the curing concrete every day for two weeks; and
Remove the temporary dike after 30 days. Condition the gate by letting tidal water in and out of the pond.
b. Wooden Gates. Secondary and tertiary gates are usually made of wood to reduce construction cost. Like the main gate, secondary and tertiary gates are constructed simultaneously with the construction of partition dikes. Because it is easier to construct and install a wooden gate, it may not be necessary to have a temporary dike enclosure. Keep the pond dry by closing the main gate. The following is the procedure in constructing and installing a wooden gate:
Buy selected lumber and dry it under a shed for at least 15 days;
While shed drying the lumber, set a temporary BM near the site of the gate to be constructed. Determine the elevation of the BM either by the transit and stadia method or by running a water hose from a known elevation;
Smooth the sides and edges of the lumber by the use of a plane. Cut the lumber according to specifications given in the design of a wooden gate;
Assemble the pillars, and braces. With bronze nails, nail the floor and the walls to the pillars and braces. Nail other parts;
Mix coal tar with a small amount of cement. Paint the wooden gate with the mixture, at least two coatings, and let it dry under the heat of the sun;
Excavate the site to the desired elevation. Check the elevation from a temporary BM previously set for such purpose;
Install the gate at the excavated foundation. Check if the gate is properly installed and in the proper direction;
Fill with mud blocks layer by layer at the sides of the wooden gate. Allow the soil to dry and harden;
Condition the gate by allowing tidal water in and out of the pond.
a. Dike Construction. Dike construction begins by clearing the path of vegetation, stumps and debris. The clearing should be about 2–4 meters wider than the base of the dike. For a main dike, a puddle trench (mitsa) of about 50 cm wide by 50 cm deep should be dug in the middle of the cleared path. For secondary and tertiary dikes, it may not be necessary to dig a puddle trench, but it is advisable in the path of the dike where the soil substratum is loose and root-ridden. The portion of the dike passing through rivers, creeks or low places, if there are any, should be constructed first. If possible, blocks of mud which will be used for diking should be dug near the dike for ease and lower costs. Sandy clay soil makes the strongest diking material. Sand and root-ridden soils are inferior, thus, they should be avoided. When filling with mud, the dike should not be raised at once to its desired height. It should be done in layers allowing each layer to settle before another layer is laid. Diking may be done either manually or by machine or both. It is very important to have uniform dike height. A ½ inch diameter hose, 25 meters long, can be used to measure accurately the uniformity of dike height. The hose is first filled with water, one end should be held at the first station and the other end should be held at the next station. The levels of water at both ends of the hose should be the same. Then mark the height of the next station. Station to station should have a distance of 20 meters. The process is repeated until the last station has been marked.
b. Canal Construction. Canal construction is done at the same time as construction of adjacent dikes. Markers should be staked before construction starts. These markers will serve as guide during the excavation of canals. The canal slope will be the same as the dike. The canal should have a grade sloping towards the main gate and the bottom should be flat so that the flow of water will be uniform.
c. Pond Bottom Leveling. Mechanical leveling is a cheaper and faster means of leveling a pond bottom than manual leveling if the soil condition will support the equipment used. Farm tractors, or tillers, with a back blade may be used. Manual leveling is done by slicing the soil into cubes using digging blades. The cubes are loaded onto a “sliding board”, a bamboo raft or flat boat. The pond bottom should slope gently towards the gate when leveling is finished. The steps in leveling pond bottoms are as follows:
Bring the water down to the desired pond bottom elevation and place a wooden mark. Beside the mark, place another stake about 2 m long calibrated from 0 to 100 cm to serve as a leveling guide for cut and fill operation as well as indicate the depth of water inside the pond. Set the stake so that the zero mark is level with the wooden mark. The zero mark is the required elevation of the pond bottom;
Mark area of the pond above and below zero elevation marks, then increase the depth of water inside the pond to 40 cm for the flatboat to float;
Excavate a portion of the pond marked above zero elevation. Transport the excavated soil with a flatboat and unload it at portion of the pond marked below zero elevation. One to two workers should be at the receiving end to supervise the dumping of soil in the deep portions. The receiver is equipped with a meter stick and from time to time check the elevation of the pond bottom by the sounding method. If the desired elevation is attained the receiver transfers to another low area;
When all marked portions have been excavated, drain the water to the zero mark. The remaining soil on the sides can be leveled using the spade;
The process is repeated until all lower portions are filled; and
Excess soil materials are transported to other compartments having low areas or are utilized as berm for the dike or to increase its height.
Creeping grasses should be planted on the top and slope of the dike for protection from erosion as soon as the construction of dike is completed. Grass can be maintained by fertilizing and minimizing traffic. “Dampalit” is the most common grass that grows in brackishwater ponds and it also can be planted over the dike. Carabao grass could also be grown on the dikes.
Mangrove vegetation is the best protection to main dikes so salvage or buffer zones are required along the river or bay. Vegetation left undisturbed in a wide belt counteracts the destructive action of the waves. The salvage zone also serves as an ecological balance in the area. Erosion to dikes inside a pond is controlled by placing twigs along the side of the dike. The twigs also serve as protection for natural fish food. Some fishpond operators construct broken dikes inside the pond to break and minimize the wave action. Construction of berm along the foot of the dike also protects the dike from wave action.
Dikes which are exposed to open water are at times subject to the destructive action of waves. Protective measures to safeguard such dikes are:
a. Riprap. Riprap is stone, concrete block or concrete slabs hand-placed compactly or irregularly on the dike slope in order to prevent the scouring action of water. The provision for a banquette on the seaside slope of the dike is an added protection, as riprap placed on the banquette is prevented from slipping. The banquette area is covered with coarse crushed stones to a thickness of from 0.10 m to 0.20 m before the riprap is placed. The crushed stone fill forms a bedding layer that prevents the erosion of the soil beneath the riprap.
Stone used for riprap should be dense and hard enough to withstand exposure to air and water. Rough angular stone is preferred to smooth rounded stone as rounded stone does not interlock or resist movement.
b. External Support. Failure due to rotational slipping is quite common in dikes. This is due to loose shearing resistance of the soil when wet or saturated, as well as its corresponding increase in weight. The dike slope is stabilized by providing external pile support at the toe or by the application of heavy loading on the toe. The use of bamboo culms as piles serves the purpose very well and the use of boulders provide the external load.
Frequent and scheduled inspection of the dike must be established as this provides a means of early detection of erosion, and will allow time for repair before major damage occurs. The dike must be patrolled periodically especially after storms. Leakages and seepages must be thoroughly inspected as they are the possible points of failure.
Burrowing aquatic animals such as crabs can cause serious damage to a dike. Traps may be installed to eliminate them or placing plastic sheets over the dike slopes may be used to prevent them from boring holes.
Harvesting of oysters, if they grow on the boulders placed at the dike toe, must be prevented as the harvesting process causes the rocks to be moved or disturbed.
Trees and brush should not be allowed to grow on the dike as the roots loosen the dike soil.
Depressions on the top of the dike should be filled with a suitable earth material. Surface drainage must be effected immediately so that no water will percolate into the dike soil.
While many of the fundamentals of safety engineering apply to all kinds of construction, an effective safety program must be developed to fit the particular operational conditions in pond development.
The following guidelines will be effective in reducing the accident rate during construction:
The management should give full support to an effective safety program. This program should be placed under the direction of a capable person.
Each employee should know that there is such a program and must be told how he will be benefited by it. When an accident does occur, let the employee know how it happened and how it could have been prevented.
Prior to construction, the safety director should analyze the operations of each job and together with his superintendent determine what hazards will exist. Together, they develop a safety program for the project.
All new employees should be subjected to medical examination prior to employment and he should be informed of the hazards of his work and how he can reduce the danger of accident to himself and to other workers.
Employees who persist in causing accidents should be replaced. It is too dangerous to keep them.
The foreman should hold short safety sessions with his gang as often as condition warrants. This is done during the beginning of work, and all employees should be given the opportunity to participate in all discussions.
First-aid facilities should be maintained at the job site and all members of the group should know how to provide the treatment required.
