File Name: construction dewatering and groundwater control new methods and applications .zip
Dewatering is a term to describe the removal of groundwater or surface water from for example a construction site. In construction the water is pumped from wells or sumps to temporarily lower the groundwater levels, to allow excavation in dry and stable conditions below natural groundwater level. There is a variety of pumps used for it: submersible pumps , self-priming pumps, borehole pumps, etc. The presence of undesired water in a construction site can lead to safety risks , increased costs and delays to your project.
Dewatering in Construction Works. IntroductionThe control of groundwater is one of the most common and complicated problems encountered on a construction site. Construction dewatering can become a costly issue if overlooked during project planning.
In most contracts, dewatering is the responsibility of the contractor. The contractor selects the dewatering method and is responsible for its design and operation.
Many civil engineering construction projects typically require the excavation of soil. Depending on the geographical location of the project and depth of excavation, groundwater may be encountered. Areas that were once deemed unsuitable for construction due to a high water table are now being developed to meet the needs of an increasing population.
As population numbers rise, the need for transportation infrastructure to accommodate traffic volumes has increased. With the decline in land availability to build or expand highway networks above ground, underground facilities, such as mass-transit lines, may represent the future of transportation.
Underground structures naturally require the control of groundwater to complete construction. Dewatering is a process for the temporary removal of subsurface water to enable construction to be completed under dry conditions. Projects are specific in nature; dewatering techniques depend largely on the scope of the project, prevailing soils, hydrological characteristics, degree of difficulty, and associated cost.
Controlling groundwater can be costly, not only during construction, but also from resulting damages to infrastructure or property if not done carefully. Consequently, insurance claims associated with subsurface site conditions are disproportionately high.
Abdul-Rahman et al concluded that dewatering or drainage issues led to 8. Dewatering processes can also hinder the completion of a project if inadequately planned Serag et al. The public transportation construction industry is a multi-billion dollar industry where costly delays can add up quickly. This short review paper explores common dewatering techniques currently in use for transportation engineering projects, common problems that arise, and lessons learned in the industry.
A series of 13 engine driven wells were used to dewater the site constituting the first documented well system approach to controlling groundwater during construction Powers et al.
Subsequently, the treatment of saturated sandy soils by steampowered pumps, French drains, gravel, sheeting, and sandbags dominated the construction industry until the early s Ratay, Wellpoints with electric submersible pumps then gained popularity with successful dewatering of sandy soils up to the mids Powers et al.
At that time, Thomas F. Moore, a designer and manufacturer of excavator equipment used in trenching, introduced the first effective wellpoint design for dewatering in fine-grained soils Ratay, During the s, in the midst of the industrial revolution, the need for reliable infrastructure to transport people and goods led to a surge of groundbreaking civil engineering projects in the United States U.
Many techniques used at the time in the U. Moreover, many early American engineers first trained in Europe before leading the design and construction efforts of these landmark structures Brown, While the design elements appeared to be sound, construction methods were more or less trial-and-error methods with costly results in loss of life.
Up to the s, controlling groundwater was primarily based on field experience, more of an art than a science Ratay, With the integration of soil mechanics and hydrology, a more scientific approach to dewatering changed the construction industry.
Today, pre-designed dewatering models incorporate science and technology and allow more complex underground construction projects to be possible. Although modern theory has replaced much of the art, field experience still plays an important role in identifying unanticipated problems that can never be fully emulated by computer models. Dewatering methods are project specific by nature, and are generally a critical path element in construction.
A thorough understanding of the subsurface rock and soil conditions at the site is paramount in the development and implementation of a successful dewatering operation Ratay, Identifying the soil types, densities, permeability, stratigraphy the branch of geology that studies the arrangement and succession of strata and consolidation through testing is the first step.
Secondly, the hydrological characteristics of the site must be defined to determine the source of the water and expected water level during dewatering Ratay, Other factors to consider are surrounding structures, local climate and time of year, any of which could affect surficial groundwater levels.
Effects of dewatering on adjacent structures such as foundation and structural damage due to differential settlement must also be considered, especially in urban areas and historic districts.
A cost benefit analysis can often be the deciding factor in the dewatering design. All of these factors must be taken into account to determine the most effective dewatering system that will accomplish the primary construction goal. Published literature on case studies involving successful dewatering systems used in the construction industry are few, and even scarcer for transportation related construction projects.
Typically, only catastrophes are highlighted Ren et al. Literature on large-scope transportation efforts often concentrate on engineering elements other than groundwater control measures, which are essential for the success of the project. Kiziltas and Akinci argue for better documentation of case study data to improve construction cost estimates and limit potential problems.
Dewatering TechniquesDewatering systems used today vary in type and complexity. Yet many systems are similar, although greatly enhanced, to those employed two centuries ago.
Transportation facilities, such as roads, bridges, tunnels, and canals are essential to society worldwide. For a project to be completed safely and successfully, the degree of dewatering required often corresponds with scale of construction.
Open Pumping and Trench MethodsThe simplest dewatering technique involves the use of a sump pump placed in a swallow ditch to remove surficial groundwater Ratay, This method is useful for small excavations, and is often used in the dewatering of roadside ditches for short-term construction. Water can effectively be conveyed outside the area of excavation to a holding pond, enclosed drainage system, or a lower point downstream, until construction is complete.
Removing water during construction by pumping has been a practical method for over two-hundred years. A variety of pumps are available today with a wide range of flow capacities.
For example, a portable gasoline-powered wet priming pump with suction and discharge sizes of 5 cm to 7. Generally, a pre-designed dewatering plan is not required for this method. However, the amount of water to be removed and duration of pumping time hours or days dictate the type of pump and pumping rates required to meet the needs of the project. Sumps pumps are also used to augment larger dewatering systems to collect surface water from rain or groundwater seepage, and remain an important tool in construction Ratay, Surface water in an excavation must be removed quickly to prevent work delays.
However, the use of sumps is not limited to small scale construction. At one time, in New York, sumps were used in most excavations below the water table referred to as "peel the onion", where soil was dewatered and removed in layers to the desired depth Ratay, Perforated drainage pipe encased in gravel, or French drains, can be used at the toe of slope to collect surface water and convey it to a sump.
A similar system can be utilized in surface roads to control groundwater intrusion into the roadway base and sub base materials post construction. Over time, cracks in flexible pavement can occur due to increased traffic and heavier loads, allowing water seepage into the underlying infrastructure foundation.
In areas where fairly high water tables exist and cost or other circumstances prohibit the construction of elevated or built-up roadway sections, subsurface drainage systems can prevent infrastructure failure resulting from subsurface saturation.
Cyr and Chiasson examined the use of perforated drain pipes at varying locations and depths below the curb and gutter for at-grade roadways in Canada. The perforated drain pipe system proved effective in shortening the drainage period of subsurface water at or near the roadway base layer, thus preventing damage.
However, a high percentage of saturation remained present in the lower gravel base during heavy daily rains Cyr and Chiasson, A concern for transportation officials and engineers, saturated roadway foundations can, at a minimum, lead to roadway failure, and in some parts of the U.
S, extreme undermining may produce sinkholes. Well SystemThe wellpoint consists of a slotted or perforated pipe which is covered with a screen mesh. At the foot of this pipe is an orifice which permits jetting of the pipe into the ground during installation.
A well-point dewatering system consists of a series of closely placed small diameter wells installed to shallow depths. These wells are connected to a pipe or header that surrounds the excavation and is attached to a vacuum pump. The construction steps in the wellpoint system are Figure 1 shows small pipes, up to 2. Figure 1 -Wellpoint systemWellpoints typically have capacities ranging from a fraction of a gallon a minute to gallons per minute and they may be used in single stages or in multiple stages to accomplish deep dewatering.
Wellpoint Pumps, Header and Discharge Piping Once the total flow, Q, the required vacuum, the distance to the point of discharge, and the discharge elevation has been determined, a suitable mechanical system should be selected for those conditions. When selecting wellpoint pumps, they must have adequate water and air capacity at the necessary vacuum and must be capable of developing the required total dynamic head in order to be able to deliver the water to the discharge point.
Single pump or multiple pumps may be used, depending on availability of equipment and job conditions. If multiple pumps are used, they should be spaced along the header, or they can be grouped into one single pump station.
In a single pump station only one discharge line is used, but in this case large header pipes are used to bring the water to the central pump station without excessive friction. Redundant or standby pumps must be provided, installed and be ready to operate in case of malfunction of the regular pumps or during maintenance or repair of those pumps. For high volume pumping a suction manifold may be used, which reduces the water velocity in the header line at the pump.
The manifold also causes smooth flow in the critical approach to 8 of 2 3 the pump entrance, reducing cavitations, which will increase the capacity of the pump. Header lines are sized to keep friction at acceptable levels. Valves are used to facilitate installation, trouble shouting, repair and removal. The discharge lines should be braced and strapped, where pressure is moderate to high.
The arrangement and location of header lines, pumps and discharge should be selected in accordance with the plan and schedule of construction sequence and activities so that it is convenient during excavation, but also will not interfere with construction and backfill operations, in order to prevent relocation and modifications during the course of the project. Figure 2 shows a typical dewatering system using wellpoints. However, these did not proceed without challenges.
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