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Transportation Curriculum Coordination Council |
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Knowing "When To Stop" driving is one of the most important responsibilities the Inspector has. Depending upon the situation, this decision has numerous ramifications, such as extra Contractor pay, damaging the pile, etc., so it is imperative the Inspector know and understand the When To Stop Driving or Pile Acceptance decision process.
First, let's review some of the terms and definitions the Inspector needs to understand.
Ground Surface Elevation- The elevation of the existing ground surface.
Cutoff Elevation- This is the specified finish elevation for the top of the pile.
Pile Length Driven- The length of pile between the Cutoff Elevation and the Tip Elevation.
Tip (Tip Elevation)- The elevation of the bottom (tip) of the pile.
Penetration- The length of pile below the lowest of three elevations; Ground Surface: Bottom of Excavation; Scour Elevation.
Scour Elevation- a specified elevation representing the depth of potential scour, which is the action of soil being removed by water movement.
Reference Elevation- This is a fixed point and elevation on the template, string line or other stationary object, used to observe the increment marks on the pile relative to the reference point (reference elevation).
Bottom of Excavation- the elevation of the bottom of an excavation (i.e. footing).
There are several points at which the Inspector needs to make this decision of when to stop driving, those being:
The Inspector has the responsibility to tell the Contractor when to stop driving due to achieving one of the above conditions. Naturally, the Inspector may also ask the Contractor to stop driving when they suspect the Contractor's equipment or operations are outside the specifications.
The Engineer may accept a driven pile when the pile has achieved minimum penetration, the blow count is generally increasing and the minimum required bearing capacity obtained for 24 inches [600 mm] of consecutive driving. At the Engineer's discretion, a driven pile may be accepted when the minimum penetration is achieved and driving has reached practical refusal in firm material.
When a Minimum Tip Elevation is specified in the Contract Documents or Driving Criteria Letter, this is the elevation or depth that the tip pile must be driven to as a minimum. Based upon results of the Test Pile program, the designer determined that the piles must achieve this amount of penetration to obtain the design capacity.
More specifically, Penetration Requirements ...".When the Contract Documents show a minimum pile tip elevation or a minimum depth of penetration, drive the tip of the pile to this minimum elevation or this minimum penetration depth. In all such cases, the Engineer will accept the bearing capacity of a pile only if the Contractor achieves the required bearing value when the tip of the pile is at or below the specified minimum tip elevation or depth of penetration..."
Next we will review how to calculate "Tip Elevation" for plumb and battered piles.
Tip Elevation = Reference Elev. - Length Below Reference
Using the illustration below, we know the following:
For battered piles the calculation is essentially the same except you must use a Correction Factor to compensate for the batter.
Tip Elev. = Reference Elev. - [Length below Ref. Elev. X Correction Factor]
Using the illustration below and the information on the next page we know:
Batter Ratio = V/H or V:H
| Batter Ratio (V:H) | Correction Factor (R) |
|---|---|
| 12:1 | 0.997 |
| 10:1 | 0.995 |
| 12:2 (6:1) | 0.986 |
| 10:2 (5:1) | 0.981 |
| 12:3 (4:1) | 0.971 |
| 10:3 | 0.958 |
| 12:4 (3:1) | 0.949 |
| 10:4 (5:2) | 0.928 |
| 12:5 | 0.923 |
Definitions:
L = Pile Length Below Reference Point (ft)
a = Reference Point Elevation (ft)
V = Corrected Pile Depth (ft)
C = Pile Tip Elevation (ft)
Formulas:
V = L*R
c = a - W
Therefore:
Don't forget to use the + & - signs for elevations. Also always do the work in the brackets () or [] before doing the calculation
Tip El. = TE
Ref. El. = +7.00'
Length Below Ref. = 37.5'
Correction Factor = .981 (from table right)
TE = + 7.00' - [37.5' X .981] (Do the calculation in the [] prior to subtracting from +7.00')
TE = +7/00' - 36.78'
TE = -29.78'
The Specifications set forth the minimum penetration requirements should the contract documents, plans or Driving Criteria Letter not specify a Minimum Tip Elevation or a Minimum Penetration.
"When the plans do not show a minimum depth of penetration, scour elevation, or minimum tip elevation, ensure that the required penetration is at least 10 feet into firm bearing material or at least 20 feet into soft material, unless otherwise permitted by the Engineer. If a scour elevation is shown in the plans, achieve these penetrations below the scour elevation..."
First, the Inspector must understand what the term "penetration" means.
Penetration- To obtain Penetration, "Measure the penetration of piles from the elevation of natural ground, scour elevation shown in the plans, or the bottom of excavation, whichever is lower..."
Next we will review how to calculate "Penetration for plumb and battered piles".
Has the pile achieved minimum penetration?
Penetration (P) = Lowest of 3 elevations. - Tip Elev.
Just as with Tip Elevations, the Penetration calculation for battered piles is essentially the same as vertical piles except that a Correction Factor is applied to compensate for the batter.
P = [Lowest of 3 Elevs (Grd. Surface, Scour, or Excavation).-Tip Elev.]/Corr. Factor
Using the illustration below and the information on the next page we know:
[See correction factor table, figure, definitions and formula from previous battered pile example]
Therefore:
P = [+1.5' - (-20.00')]/Correction Factor
P = +21.5'/.981
P = 21.91'
The Driving Criteria, Blow Counts, set by the Geotechnical Engineer and presented in the Driving Criteria Letter, is what we are hoping to achieve in driving the pile.
Remember in the Test Pile Program section we discussed how the Geotechnical Engineer, based upon the driving of the Test Pile, developed these Blow Count Criteria.
More specifically, Blow Count Criteria: The Engineer will determine the number of blows required to provide the required bearing according to the methods described herein. Determine the pile bearing by computing the penetration per blow with less than 1/4 inch [6 mm] rebound averaged through 12 inches [300 mm] each of penetration considered necessary by the Engineer. The Contractor may determine the average penetration per blow by averaging the penetration per blow through the last 10 to 20 blows of the hammer.
So has the pile driving met the Driving Criteria for 2 consecutive feet?
Practical refusal is the point at which there is a great risk of damaging the pile should driving continue. This is generally caused by encountering dense material and more specifically:
Practical Refusal: Practical refusal is defined as 20 blows per inch [20 blows per 25 mm] with the hammer operating at the highest fuel setting or at a setting determined by the Engineer and less than 1/4 inch [6 mm] rebound per blow. Stop driving as soon as the Engineer determines that the pile has reached practical refusal. The Engineer will generally make this determination within 2 inches [50 mm] of driving. However, the Engineer will in no case approve the continuation of driving at practical refusal for more than 12 inches [300 mm]. When the required pile penetration cannot be achieved by driving without exceeding practical refusal, use other penetration aids such as jetting or Preformed Pile Holes.
Have you reached Practical refusal? 20 blows per inch
For every foot the pile is driven, we are getting a foot closer to the Cutoff Elevation. Cutoff Elevation is "The finished elevation of the pile top as shown in the plans."
If the pile is driven past the Cutoff elevation, a splice will be required in most cases to get back up to the planned Cutoff elevation. So, as the pile approaches Cutoff, we may want to take advantage of performing a Set-Check, as prescribed following:
Set-checks and Pile Redrive:
(a) Set-checks: In the event that the Contractor has driven the pile to approximately 12 inches [300 mm] above cut-off without reaching the required resistance, the Engineer may require the Contractor to interrupt driving at least 15 minutes prior to performing a set-check. A set-check consists of ten hammer blows or 10 inches [250 mm] or more of driving. Provide an engineer's level or other suitable equipment for elevation determinations to determine accurate pile penetration during the set-checks. There will be no separate payment for an initial set-check. In the event the result of an initial set-check is not satisfactory, the Engineer may direct additional set-checks. For each additional set-check ordered by the Engineer within 72 hours from the end of original driving, the Contractor will be paid an additional quantity of 10 feet [3 m] of Piling. The Engineer may accept the pile as driven when a set-check shows that the Contractor has achieved the minimum required pile bearing and has met all other requirements of this Section...
Is the pile within 2 feet of Cutoff?
Next we will review how to calculate this target.
This is the same calculation that you performed in the "Begin Pile Driving" section for the "Stop for Set check Target". Remember:
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Though not one of the questions for "When to Stop", it is important that an Inspector know how to determine Length Driven.
Length driven is not a required length to know in the newest specifications, however, there will be projects let for years that are designed with the 2000 or earlier versions of the specifications that includes this. For those projects, you will need to know how to determine length driven.
Using the illustration below, we know:
Length Drive (LD) = Cutoff Elev. - Tip Elev.
Using the illustration below and the information on the next page, we know:
LD = [Cutoff Elev. - (Tip Elev.)]/Correction Factor]
[See correction factor table, figure, definitions and formula from above battered pile example]
Therefore:
LD = [+10.00' - (-20.00')]/.949
LD = 30.00'/.949
LD = 31.61'
Don't forget to use the + & - signs for elevations. Also, always do the work in the brackets ( ) or [ ] before doing calculation.
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