Underwater foundations
Contents |
[edit] Introduction
Foundations can often be required for the construction of harbour walls, sea walls, breakwaters, and so on. Underwater foundations, also known as subaqueous foundations, may be used in situations where the use of a cofferdam or caisson is prohibitively expensive or unfeasible.
Typically, dredgers or grabs working from pontoons will excavate the ground and load the spoil into barges. If rock needs to be removed, drill and blast methods can be used, or drilling and cutting by jackhammer. Softer rock formations can be dredged or broken up by air tools and then grabbed. Gravel and sands can be excavated using a suction dredger or ejector tube.
Divers can then place temporary steel formwork, using sandbags to anchor them in place and prevent concrete leakage from under the toe of the form. If the foundation is relatively short and in shallow water, bagged concrete or cement placed in layers may be used as formwork and left in position. Foundations in deeper water may incorporate permanent formwork in the form of precast concrete blocks which are tied in to the foundation.
The concrete mix must have a high degree of workability to maintain a good flow and prevent too much cement being washed out of the mix. This ensures that the leading edge of the concrete is kept moving forward under the pressure of additional material. Extra cement should be added to offset any loss through the water, although too much cement will result in excessive laitance forming on the concrete surface.
[edit] Concrete placing
Concrete is generally placed by one of three methods:
[edit] Placing by tremie
A tremie is a steel tube suspended in the water from a crane. A hopper is fixed to the top end to receive the concrete. The tremie is positioned vertically over the area to be concreted with the lower end resting on the bottom. After the pipe has been filled with concrete it is raised off the bottom to allow it to flow. This should allow the flow to continue to feed the interior of the initial mass.
The flow rate of concrete is controlled by raising and lowering the tremie, but care must be taken not to lift the tremie out of the concrete mass, otherwise the seal will be broken and the concrete will be weakened by water as it rushes out of the pipe. This may result in a damaged surface to the mass of concrete which will need to be removed before work can recommence. One tremie will serve an area of approximately 30 sq. m, and where all the concrete cannot be placed from one position, more than one tremie can be used for simultaneous placing.
For more information, see Tremie.
[edit] Placing by skip
Bottom-opening skips can be used, which are fitted with a top cover held in position by water pressure. The skip is filled with concrete before being gently lowered into the water. On reaching the bottom, the skip is emptied.
If it is lowered too rapidly, the covers may be disturbed and the concrete inside damaged. This is a slower method than a tremie pipe but can be more practical for thin beds, whereas tremies are more appropriate for large pours.
[edit] Pumping
Concrete pumping is a method used when very large concrete pours are required.
Pipe lines typically have a diameter of 110-150 mm and are generally capable of pumping for a distance of 300 m horizontally and 30 m vertically. Pumps may be mounted on a pontoon with an extendable steel boom. The pump works hydraulically with a diesel-driven piston that forces concrete along the pipes.
[edit] Related articles on Designing Buildings Wiki
Featured articles and news
Boiler Upgrade Scheme and certifications consultation
Summary of government consultation which closes 11 June 2025.
Deputy editor of AT, Tim Fraser, discusses the newly formed society with its current chair, Chris Halligan MCIAT.
Barratt Lo-E passivhaus standard homes planned enmasse
With an initial 728 Lo-E homes across two sites and many more planned for the future.
Government urged to uphold Warm Homes commitment
ECA and industry bodies write to Government concerning its 13.2 billion Warm Homes manifesto commitment.
Places of Worship in Britain and Ireland, 1929-1990. Book review.
The emancipation of women in art.
CIOB Construction Manager of the Year 2025
Just one of the winners at the CIOB Awards 2025.
Call for independent National Grenfell oversight mechanism
MHCLG share findings of Building Safety Inquiry in letter to Secretary of State and Minister for Building Safety.
The Architectural Technology Awards
AT Awards now open for this the sixth decade of CIAT.
50th Golden anniversary ECA Edmundson awards
Deadline for submissions Friday 30 May 2025.
The benefits of precast, off-site foundation systems
Top ten benefits of this notable innovation.
Encouraging individuals to take action saving water at home, work, and in their communities.
Takes a community to support mental health and wellbeing
The why of becoming a Mental Health Instructor explained.
Mental health awareness week 13-18 May
The theme is communities, they can provide a sense of belonging, safety, support in hard times, and a sense purpose.
Mental health support on the rise but workers still struggling
CIOB Understanding Mental Health in the Built Environment 2025 shows.
Design and construction material libraries
Material, sample, product or detail libraries a key component of any architectural design practice.
Construction Products Reform Green Paper and Consultation
Still time to respond as consultation closes on 21 May 2025.
Resilient façade systems for smog reduction in Shanghai
A technical approach using computer simulation and analysis of solar radiation, wind patterns, and ventilation.