water control in construction

8/8/2019 Water Control in Construction

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CE 4630 Geotechnical Engineering

Water Control in Construction

Construction in soil is an exciting work because of the variables that are inherent in the soil system. Each site has different soil types and

layering, each soil has different strength and deformation behaviors. Construction is performed on a site to make it have the final

characteristics that the owner wants. It is the role of engineering (and the engineer/contractor, however that role is defined) to develop a

construction plan that will provide the most cost effective solution to safely finish the project. (Checkthe Canon of Ethics to determine who

has the greater claim to your expertise, the client or the public.) In order to do the "best possible project at the lowest possible price"

requires optimizing the various functions required for successful completion so there can be economies of scale and procedure.

One of the areas in which this is most significant and obvious is that of ground water control. The presence of water usually has

detrimental effects on a project, even a dam. Increasing the water pressure decreases the effective stress that can cause a reduction in soil

strength. Flowing water in soil can move particles and create recharge zones in areas that the contractor wants to keep dry. And it is very

difficult to run heavy equipment when the engine intake stacks are below the water level.

Good engineering requires knowing what the various techniques of water control are and how they can be incorporated into a project

design. For example, a diaphragm wall may be expensive as a water control technique, but may provide a very inexpensive foundation if

its construction is part of the overall design. Sometimes it may be less expensive to handle the water and soil separately.

Construction dewatering means controlling, and generally lowering, the water so that construction can take place. There are two primary

types of water control.

Predrainage is removal of water from the upper aquifer prior to construction so that the soil behaves like it is essentially dry.Equipment can then work in the soil neglecting that the soil was initially saturated.

Pressure reliefaffects water away from the excavation, especially below it. In some situations, water under pressure, usually in aconfined or artesian aquifer, can cause the soil to deform, to heave and possibly to blow the base of the excavation out. Pressure

relief lowers the water pressure and reduces the chance for catastrophic failure.

Predrainage and pressure relief can be accomplished using two broad groups of procedures.

Exclusion creates a barrier so that water cannot enter the excavation or flow paths are lengthened to lower the hydraulic gradientof the flowing water and reduce the pressure. Various exclusion measures include penetration grout curtains, steel sheeting, slurry

walls, jet grout walls and freezewalls.

Controlled Water Flow Alternatively, sumps, wells and wellpoints can be used to control the flow of water and to maintain safeworking conditions.

In the discussion to follow, we will talk about procedures for water control that are often used for other purposes, typically structural

support for foundations or temporary support for excavations. From my point of view, I see them as water control first and others second.

As is said often on the net, "your mileage may vary." Your expertise is to understand how a system can be successful in multiple ways, soyou can save your client money.

The soil profile below has an upper unconfined or phreatic aquifer that is open to the atmosphere and is separated by a clay layer from the

lower aquifer that is confined. The water levels (shown by the black triangles) in the piezometers on the left indicate that the initial water

pressure in both aquifers is hydrostatic. The water level in the unconfined aquifer is just below the soil surface and the excavation will

need to go below the water level.


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One way to construct like that is to use a dragline, which consists of a cable-rigged crane which has a bucket attached to it. The bucket is

cast out into the excavation just like a side-armed fishing cast, and then spooled back onto the shore. The bucket scrapes up the soil like a

scraper does in common construction. The bucket is lifted out, swung to the side, emptied and cast out again. This system works fine for

gross excavation of a large amount of material as long as it is acceptable to work under the water surface.

http://www.cmte.org.au/

Exclusion Systems

The following figure shows two forms of exclusion, steel sheeting and Freeze wall.

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(The two systems are shown for examples, you wouldn't normally have two different exclusion systems at a site.) Once the sheeting has

been driven or the soil has been frozen to create a barrier, the water in the upper aquifer can be pumped out all the way down to the clay

and all the work can be performed in a "dry" condition. Without some way of relieving the pressure in the lower aquifer, however, the

pressure could be great enough to cause the clay layer to heave and possibly to blow out in a particularly weak area.

Controlled Flow Systems

Barriers are normally not used in controlled flow systems but there are many situations in which one may be the compliment of the other.

(As above, normally you wouldnt mix wellpoints and well systems, but you could.) Either the wellpoints or the wells would pump out of

both aquifers. In the upper aquifer, the water level is pumped down below the bottom of the excavation so that work can proceed in the

dry, i.e., predrainage. In addition, the piezometric surface of the lower aquifer is showing lower values (i.e., pressure relief) because it is

being actively pumped. By relieving the pressure in the lower aquifer, it is much less likely to "blow out."


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Selection of Systems

Water control systems are selected to coordinate with other project considerations. For example, if the excavation is occurring in a

congested location, it may be appropriate to use steel sheeting to support the excavation and exclude water. If the soil is clayey, this may

be sufficient. If the soil is sandy, the sheeting may still leak enough that additional drainage may be required. Freezewalls are generally

very strong and stiff. It can be used for vertical or lateral support, but then revert back to normal conditions after the work is completed.Again, the concept of multiple usage is very important and can save hundreds or thousands (or Hundreds of thousands) of dollars per

year.

Exclusion Systems

Penetration Grout Walls

Penetration grout walls consist of two types, permeation grouting for soils and intrusion grouting for rock. A row, or typically 3 or 5

rows, of grout holes are lined up along the centerline of a dam, a levee or dike. Penetration grouting using either bottom up or top down

procedures are used to create a vertical column of grout penetrated material. Multiple rows are installed to insure that the vertical

columns overlap and/or intersect so that little or no water can pass between them. It can be a relatively inexpensive alternative because it

can be done with small crews and generally simple and small equipment. This is primarily a grout plant, pump, hoses and pipe. It can

also be very effective in the appropriate procedures.

Steel Sheeting

Steel sheeting is a rolled steel section with special grooves formed along the long edge. The grooves are made so that they interlock. The

rolled sections are made with different

thicknesses and with different cross sectional areas to produce a variety of section moduli depending on the characteristics needed for aparticular job. Interestingly, the narrowest sheets are the best under tension (where they can hold over 8000 pounds per foot of section)

such as a cellular dam whereas the widest sheets (with the deepest sections) are the best for high walls and long straight segments that

must support lateral loads.

Steel sheeting is a very versatile material, and can be used under a wide range of situations. They are installed by driving (MKT, ICE

drivers) the first sheet or sometimes an H pile that has a set of interlocks welded to it so it is certain that the first sheet goes in straight.

Then the second sheet is engaged into the first set of interlocks and driven into place. The photo above shows using a Foster's Vibratory

hammer which acts by vibrating the steel into the soil. This is the same vibrator used on the Terraprobe discussed earlier in deep

compaction.


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Steel sheeting is commonly used to support trench excavation. Trenches may be a few meters to over 30 meters in depth. (Much below

that may be better done by tunneling.) One difficulty with steel sheeting is that it cuts through the profile like a knife, severing utility and

communications lines. An alternative procedure used in urban areas is soldier beam and timber lagging, which is placed as the soil is

excavated so that service lines may be accounted for during the construction rather than before it.


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Two sets of Cellular Cofferdams. Note the different construction style. The upper style represents Circular Cofferdams while the lower

ones are Diaphragm Cofferdams. Circular cofferdams are free standing and generally require fewer sheets per unit length of wall.

Diaphragm walls have uniform stresses between the sheets.

Two significant problems with steel sheeting are noise and vibration of installation and shearing of service lines. In tight and congested

areas, steel sheeting may not be the best alternative and soldier beam and lagging may be used.

Slurry Walls/Diaphragm Walls

Slurry walls make very effective hydraulic barriers. They are essentially deep trenches, excavated with special excavators and backfilled

with a mixture of the natural soil and a relatively thin clay grout slurry or a thicker bentonite/cement mixture. Diaphragm walls are

constructed the same way, but a solid concrete wall is either placed or cast using tremie techniques inside the trench to provide supportand additional cutoff.

There are two basic excavators. One is a backhoe with an extended arm, including some that can excavate to a depth of70-80 feet. The trench is

usually very narrow, on the order of two to four feet, and the bucket has a volume less than one cubic yard. The trench is kept filled with

bentonite slurry to create an impermeable mudcake against the side of the wall and to provide pressure to prevent the trench from

collapsing. As the backhoe digs, it casts the spoil along the side of the trench. When it is time to backfill, a front-end loader or a bulldozer

pushes the waste back into the trench. Because of the viscosity of the slurry, it creates a natural angle of repose of about 15 degrees. It isimportant that the backfill material rolls down this slope. This causes the material to mix in with the slurry and fill in void spaces and

break down "softer" chunks of soil. While this process takes place, it is necessary to maintain the correct depth of slurry in the trench.

If the trench must be deeper than about 70 feet, or difficult bedrock is encountered at the bottom of the trench, or a segmented diaphragm

wall is being constructed, a clamshell or a continuous miller/grinder excavator is used.


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Extended Kelly System Cable System

(Click on Pictures to go to Boston's Big Dig)

For water or for contaminant control, usually just the slurry wall is used. If the wall needs to be, or would be convenient to be, structural,

then either panels can be dropped into the slurry and interconnected, or the wall can be built using the rotary mill.

Another technique, called Bio-Polymer Slurry Trenchs (hit Technologys and Bio-Polymer to get to the page), uses the same procedure but a

different mud technology. A slurry trench is constructed in the usual way, but using a bio-reactive polymer. Initially the mud is clay like and

behaves like bentonite. After a period of time that can be controlled, the mud itself breaks down and becomes a liquid and a trace amount

of non-toxic chemicals. You are left with a loosely mixed open wall of soil which is free draining, so it acts like a large deep french drain.

Jet Grout Walls

Jet grouting is a drill rig/borehole technique in which a 150mm hole is drilled to depth and grout is shot out of the pipe at velocities up to

200 m/sec (650 ft/sec). As the pipe is rotated, the grout stream erodes the soil around the pipe and exchanges the void space in the soil

with grout. Diameters of 0.6 to 1.2 m are possible in gravelly soils and larger in loose sands and silts. Withdrawal of the pipe while

operating creates a column of uniformly mixed soil materials. In a common construction sequence, the first column is made, the nextcolumn is placed at distance of 1.5 column diameters. This spacing is continued for the desired length. Then, another set of columns are

developed, starting midway between the first two columns. The column created in this position erodes out the sides of the first two

columns, cutting into them and insuring a good seal. A solid wall is constructed with this technique.


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(Figure from Hayward Baker)

If a larger diameter column is wanted, or a thicker wall, "double rod jetting" or "triple rod jetting" is available that can shot air and/or

water out into the formation also. The three tube procedure separates the air and water from the grout. The initial erosion of the soil isdone by the air and water and a more uniform grout can be installed after the soil is loosened. Hole diameters from 0.9 m to 1.4 m can be

developed using the three tube procedure. Hayward Baker Project

Freezewall

Soil freezingGround FreezingRKK-Soil Freeze

Horizontal Trencher

Wellpoint Systems

A wellpoint is a simple suction well and consists of a screen, usually 18 to 24 inches long, a riser pipe, typically 2" diameter x 16-24 ft.

long and a swing, which connects the wellpoint to the header line. The header carries the water under a vacuum to the wellpoint pump,

which is a combination centrifugal and vaned vacuum pump. The water is brought up to atmospheric pressure and then dischargedthrough a discharge line.


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Sanford Edgar's Project File

Wells

A well consists of 5 major parts. Typically, a hole is first augered so that a well casing (which is solid steel pipe) and a well screen

(which is slotted or vented openings) can be lowered into position so the screen is in the most permeable zone. Then, the pump assembly

is lowered into position. The assembly varies depending on pumping rate. For flows below about 20 gpm, an electrical submersible pump

is typically used. For greater flows, a vertical turbine pump is used with power supplied by an engine or motor at the surface turning a

shaft mounted inside the column. The shaft drives the vertical turbine pump.

The water flows from the aquifer through a filter layer and the screen into the well. It then flows into the intake of the pump bowl

assembly, through the stages of the pump, up through the annulus between the column and the pump shaft, then through the discharge

head assembly. The head assembly provides an outlet for the water and a support for the drive shaft. If the shaft is being motor driven,

the motor has a hollow shaft that the drive shaft passes through. If the pump is engine driven, the discharge head assembly has a right

angle drive that spins the drive shaft.

Wells are convenient to use because they can accommodate flows from several gallons per minute to several thousand gpm. They are

especially useful for pressure relief where the confined aquifers often require several hundred gpm.

Ejector Systems

Ejector pumps are a cross between a well point and a well. An ejector pump has no moving parts under the ground surface, and uses a

process of shooting a jet of water through a nozzle to create a vacuum to draw the groundwater into the pump. It requires two pipes at thesurface, a supply water pipe and a return water pipe. The supply pressure ranges from 60 to 120 psi and the return pipe usually ranges

from 10 to 30 psi.


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These are generally used in situations in which high vacuum and low

flow is needed, for example, in a silt or fine sand. They are expensive to operate and maintain, but sometimes are the only viable options.

water control in construction

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