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Transportation Curriculum Coordination Council |
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Chapter 7
Shaft Excavation and Cleaning
Contents
- This Chapter contains an overview of the drilled shaft construction from the Inspector's viewpoint and an overview of the Inspector's responsibilities during this phase of drilled shaft construction.
- A short quiz is provided at the end of the Chapter.
Learning Objectives
When you have completed this Chapter, you will be able to:
- Describe, in general, the Inspector's role during the shaft excavation process
- Describe, in general, the Inspector's role during the shaft cleaning process
- Determine shaft tip elevations
Trial Shaft
On most projects, the Contractor will be required to install a "Trial" shaft. In some parts of the country these are also referred to as Technique or test shafts. Regardless of the name, the purpose is the same; to determine if the method and equipment the Contractor proposed in the Drilled Shaft Plan will work. This Trial shaft will help determine critical items such as:
- Can the dry shaft construction method be used
- Can the hole be stabilized with casing
- Can the hole be stabilized with the proposed slurry
- These are just a few of the reasons for performing the trial shaft.
The Inspector's role during shaft excavation is essentially the same as for production shafts, except that typically the trial shaft will be located on the project plans a certain distance from the production shafts and the Inspector needs to verify that the shaft is performed at the specified location. Upon successful completion of the shaft, the Inspector must verify it is "finished" per the plans.
In the event, the Contractor fails to install a successful trial shaft, they must revise the Drilled Shaft Plan and attempt another trial shaft until successfully installing one that the Engineer accepts.
Shaft Excavation
The Inspector has a variety of functions to perform during the shaft excavation process. From verifying the shaft is located in the proper place to verifying the shaft meets the cleanliness requirements upon completion of excavation, the Inspector needs to document construction events.
Shaft Location and Alignment
- Is the shaft being located at the correct plan location indicated on the plans?
Typically there will be a plan tolerance which the Contractor must achieve.- Is the kelly bar plumb? This is critical as there are tolerances for axial alignment that the Contractor must achieve.
This photograph shows the Inspector checking and verifying that the shaft is in the correct plan location.
This photograph shows the Inspector and Contractor checking and verifying, on the project plans, the shaft location(s).
This photograph shows the Inspector checking the vertical alignment of the kelly.
Excavation
If the Drilled Shaft Plan specified the use of casing and or slurry, the Inspector must verify and document its use.
On many projects, a "surface casing" will be temporarily installed to stabilize the surface soils during the construction process. The constant in and out of the hole with drilling tools can quickly degrade the surface soils conditions if not protected.
The Inspector needs to be concerned with, in general, the following.
- Documenting the type of drilling tool and its diameter, and condition. Also remember to record its length, as the Inspector needs this to add to the kelly bar to determine depths.
- Documenting the length, diameter and type of any casing used.
- If slurry is used, verifying and documenting that the required sampling and testing is performed.
- Maintaining, in the required format, a log of the material excavated. Typically, there will be forms for Rock Coring, Soil/Rock Excavation and possibly others.
Document, Document, Document.
Job site photographs are a very valuable form of documentation.
Photograph of a section of steel casing being prepared for installation by vibratory hammer. Notice the casing is "marked" in feet.
Pictured here is the Inspector examining the material on the auger that is coming out of the hole. It is important to accurately identify and document the material being excavated.
Pictured here is the Inspector checking his documentation of the shaft excavation. Notice the right-hand page is a sketch of the material identified, versus depth.
Slurry Testing
Slurry needs to be maintained properly, as discussed earlier in, if it is to be effective.
Typically, the specifications for a project will specify the type and number of tests to be performed on the slurry.
The most common tests are:
- Viscosity- also know as Marsh Funnel Test, is the test used to measure the flow rate or consistency of slurry.
- Mud Balance Test- also known as the Mud Density Test, is used to measure the density of the slurry.
- pH Test- used to determine the alkalinity and acidity of the slurry.
- Sand Content Test- used to determine the sand content of the slurry. Generally the specifications have a maximum allowable percentage of sand permitted.
Pictured here is the Inspector pouring slurry mud into the cup of the Mud Balance Test apparatus. When filled and sealed, the knife (graduated bar) is placed on the fulcrum and the sliding weight moved until the cup and arm are balanced. The density of the mud is then read from the bar.
Pictured here is a typical "slurry sampler".
The lower cap is lower to the desired depth and the tube then lower on the cable to that depth. The top cap is allowed to slide down thereby trapping slurry at the sample depth.
Shown here is the sand washed and collected from slurry at the final step of the Sand Content Test. The percentage of sand is read from the graduated glass vial.
Shaft Cleaning
Depth Verification and Cleaning
During shaft excavation, the Inspector estimates the bottom of shaft depth by noting the depth marks on the kelly and adding the length of the particular tool to it, the sum of which provides the total depth. Upon achieving the desired shaft tip elevation and following cleaning of the shaft bottom, the Inspector needs to verify the depth and cleanliness.
Generally, cleanliness requirements will be specified and are typically based upon the amount, or thickness, of sediment permitted on the bottom of the shaft.
In making this determination, the Inspector uses a weight tape and takes "soundings" at numerous locations (normally 5) around and in the center of the shaft. These are recorded on the specified form. This should be done as soon as possible, as the longer the hole is open, the greater the potential for problems.
Pictured here is the Inspector "sounding" the shaft for depth and cleanliness.
Pictured here is a typical "weighted" tape used to measure shaft depth.
Illustrated below is a typical 5 location sounding pattern
to check for depth and cleanliness.
Determining Tip Elevation
The Designer has designed the drilled shaft foundations based upon a variety of factors and their design is based upon a certain shaft diameter and depth of penetration below existing ground surface. Where the bottom of the shaft is to be located is referred to as the "shaft tip elevation".
This elevation is determined from a fixed point elevation provided by the Contractor. Typically, this is the top of casing or some other fixed reference. Using this elevation and the depth measured on the kelly or weighted tape, the Inspector calculates the shaft tip elevation, to verify the Contractor is at the specified elevation.
To determine the shaft tip elevation merely subtract the depth (in feet or meters) of the shaft below the reference elevation from the reference elevation.
Remember to watch for + and - elevations.
EXAMPLE:
1. Reference Elevation = + 135.75 feet
Depth to bottom of
Shaft = 55.0 feet+135.75' - 55.0' = Shaft Tip Elev. of + 80.75 feet
2. Reference Elevation = + 25.75 feet
Depth to bottom of
Shaft = 55.0 feet+25.75' - 55.0' = Shaft Tip Elev. of - 29.25 feet
I have completed Chapter 7 and am ready to take the Quiz
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