Construction on land presents enough challenges however, construction in water presents even many unique challenges. Engineering is about finding creative ways of using the best suitable materials that are at our disposal for ground-breaking infrastructure and facilities to better human life in every aspect! Every piece of Engineering, regardless of how small or mundane it has become to humans, presents complex techniques blended with relevant materials to create tranquil infrastructure that may be huge and breath-taking or small and subtle but invoking admiration.
Along rocky areas where the ocean meets the land, forms small pockets of water that naturally occur called rock pools. These rock pools have become an inspiration for the rapid construction of man-made tidal pools just at the shoreline of beaches. The South African coastline makes it possible for these pools to be built such that water replenishment can occur during every high-water during both neap and spring tide periods. They were the first pools to be constructed before freshwater pools in Southern Africa and were essentially constructed as safe zones due to the alarming number of shark attacks in the 1950s. They have become amazing recreational facilities for holidaymakers at the beach.
Construction requires a dry working surface which is why it can be difficult to carry out in the water. Although there is an existing lack of design guidelines for tidal pools, their construction, therefore, makes use of a special underwater construction technique called cofferdam. Compared to other techniques whose utility can be quite limited, many situations call for the use of cofferdams as they can also be created from a variety of materials such as rock, concrete, and piles. Cofferdams are one type of temporary structure designed to keep water and/or soil out for the execution of construction at a site so that the permanent facility/structure can be constructed in water.
The two types of tidal pools that exist are those that are partly enclosed by walls and those that are fully enclosed by walls. The landward side of the former consists of sandy beaches with relatively flat slopes while the latter is attached to the shore or can become detached from the shore during high water tides. Pool floors consist of either sand, rock, both, and concrete. The concrete floor pools are provided with drainpipes at the lowest position in the pool to allow drainage during low water spring tides making it possible for maintenance to take place.
When it comes to cofferdams several types exist and can be mainly characterized by the depth at which they need to be constructed. The construction of tidal pools makes use of this theory in that the walls of the pools can be founded on rocks or can be founded on steel sheet piles driven on the waterside into the water. Depending on the depth of the water and the materials cofferdams can be classified as earthen, rock-fill, single-walled, double-walled, braced, and cellular cofferdams.
Earthen cofferdams
These are constructed at locations where the water depth is less than 3m and the current velocity is low. They are built using locally available materials such as clay, fine sand, and soil. The slope is usually 1:1 or 1:2 and the height of the dam is kept 1m higher than that of maximum water level. Rubble stones are pitched on the waterside to protect the embankment.
Rock-fill Cofferdams
These are like earthen cofferdams but are constructed from rock that is available on site instead of sand or soil. The dam height is up to 3m, and the slopes are at 1:1.5 to 1:1.25. Rubble stones are also pitched on the waterside to protect the dam from wave action. They can be quite permeable and must be filled with a soil membrane to make them watertight.
Single-Walled Cofferdams
These are commonly used at bridge construction sites and can prevent water intrusion for depths more than 6 meters. Wooden or timber sheets are driven into the riverbed along the perimeter of the area of construction. On the inside, steel or iron sheets are driven into the riverbed and are placed at equal distances through the aid of wales which are bolted to both sheets for either side.
Double-Walled Cofferdams
These are the same as single-walled cofferdams, except for the construction of a double wall in place of a single wall. They are typically used when the area to be enclosed is large and the water depth is too high. The use of single-walled cofferdams becomes uneconomical due to increasing supports. The difference between single wall and double wall cofferdams is that two walls provide extra stability and can hold water as high as 12m.
Braced Cofferdams
These are used when driving piles into the bed under the water body is difficult. Two piles are driven into the bed and are laterally supported with the help of wooden cribs installed in alternate courses to form pockets. The empty pockets are filled with stone and earth. The wooden framework of the cofferdam is prepared on the ground and then floated to the construction site. The layers of loose material overlying the impervious bedrock are dredged first. A crib is then sunk into position to fit the variation of the surface of the bedrock. The cofferdam is removed after the concrete structure is completed above the water level.
Cellular Cofferdams
These are used where the water depth is more than 20 m and are the common type of cofferdam used during construction of dams, locks, weirs, etc. They are constructed by driving web steel sheet piles arranged to form a series of interconnected cells that are filled with clay, sand, or gravel to provide stability against lateral forces. The cells are constructed in various shapes and styles to suit the requirements of the site.
Ironically, the process of constructing in water is to avoid water. Building underwater is more about finding creative ways to work around the water and making structures that can withstand the water pressure after the construction is complete. For dam construction two cofferdams are usually built, one upstream and one downstream of the proposed dam, after an alternative diversion channel has been provided for the river flow to bypass the dam foundation. Depending on the geography of the dam site, a “U”-shaped cofferdam is used in the construction of one-half of a dam.
Cofferdams also make it possible to repair oil platforms, bridge piers, and other structures built within or over water as well as to repair ships when it is not practical to put the ship in a dry dock for repair or alteration. Further to that, they can be used in ensuring protection to shorelines from eroding as well as other catastrophic water occurrences. Tidal pools also assist in this purpose, as they ensure protection against wave action to a certain extent, drowning, and can generally be diverse marine ecosystems offering protection to marine life.
