foundation system of burj al arab

How to Mitigate Rejected Pile?

Rejected pile can be repaired by several options. At first we have to assess the location and type of pile defects, the pile can be repaired, supplemented, can be replaced or contractor can submitted an alternate solution to keep pile in place. This proposal can be submitted as pile mitigation plan to engineer.

The test method is selected to determine location of defected area, whether this is within core of pile or near the perimeter of surrounding reinforcing cage of pile bar. There have several ways for mitigating pile once it is established that there have sufficient anomalies to reject it. Defective cast in drilled hole piles can be mitigated using following four ways:

• Excavate and recast

• Pressure grout or grouting repair

• Structural bridging

• Supplement/replacement



Excavate and recast of pile:


A rejected pile is repaired in this method by simply removal of defective concrete by mechanical method and replaced it at defective zone of pile. When such repair is conducted with 5 feet from top of pile head, the repair is termed as simple repair. 


Typically this repair method is selected to mitigate defects resulting from not wasting required amount of concrete from top of pile.However, such repair can be used for deeper defects in concrete along the length of pile. Proper shoring is provided to allow access to defective area. Contractor will propose such repair at more than 5 feet below the top of pile; engineer should consult with geotechnical professional to determine the effect on skin friction as a result of accessing to defected area of pile.


Grouting repair:


This type of repair is adopted to mitigate pile that has defects in concrete within it. These repairs, if adopted, can be executed at any depth within pile, but generally, not conducted when defects are located within five feet from the top of pile of interest, as it is considered more effective to conduct basic repair, as discussed above, at these location. Generally three type of procedures are applied in grouting repair techniques; these are

• Permeation grouting
• Replacement grouting
• Compaction grouting



Assessing feasibility of grouting:



Different operations are familiar in replacement and permeation grouting repairs. At first, it has to be ensured that defective area is accessible. The inspection pipes installed usually used in this purpose. The inspection pipe, generally, is removed using high pressure water jet at defective region of pile. Water jet cuts inspection pipe and left into pieces small enough to flush out them through top of inspection pipe. Once inspection pipe at defective area is removed, the next step is to flush this defective area with high pressure water jets. The discharge of flush is then carefully inspected for soil, chips and other contaminants. After this initial operation of washing is completed, the defective area will be evaluated by water flowing testing.

Water flowing test is required to determine nature of defective area to choose whether replacement or permeation grouting is suitable to repair. If injected water can reach into defective area under low pressure and volume of water injected is relatively high, it is considered permeation grout is the right technique for grouting repair.

When defective area of pile is found sufficiently large and permeable, it is suspected to have communication with neighbor inspection pie, and should be checked thoroughly i.e. water injected through an inspection pipe may come out through neighbor inspection pipes to ground surface. Water may flow into soil formation as well if defective area expands to edge of pile concrete. When water cannot reach to defective area of pile, replacement grouting is considered perfect technique for grouting repair. This indicates that defective area located into the pile concrete i.e. this region is embedded into soundconcrete.

After the completion of water flowing test, the defective area are typically flushed with low pressure water flow to flowout any existing loose material. A down hole camera may be used or any other method suitable to ensure no loose materials left during washing.

Permeation grouting:


Objective:

This method usually applied to repair soft tip of pile

• To enhance fractional resistance throughout the length of pile
• To deal with corrosion issues along sides of pile.


Usually this technique is adopted to repair defects associated with presence of excessive amount of settled materials, can’t be removed from bottom of drilled hole before concrete placement. Usually permeation grouting is suggested only when soil exists in discharge of washing of defective area or permeability is ensured in defective area during flowing water test.

Procedure:

The first task is to remove inspection pipe from defective pile at predetermined location ascertained by flowing water test. Then high pressure water jets are used to wash this area to flush out loose materials or any contaminant. The discharge of washing operation is then evaluated. Grout is injected under high pressure which permeates into concrete or soil formation and displaces pore water that may take place in permeable voids. Thus matrix of defective concrete/soil and cement grout is formed.

The grout is usually injected through one of inspection pipes. Thus success of permeation grout mainly depends on permeability of formation i.e. if pore water exists in these formations can be displaced by the pressure of grout. So pore water must be removed either through adjoining inspection pipe or into adjacent soil. Thus grouting technique requires presence of adequate confining pressure for executing grouting operations not returning grout to the ground surface.. This is the cause of not selecting permeation grouting to repair defects within 10 feet from ground surface or defects entirely within pile.

Replacement grouting:


Where to use:

• Typically adopted to repair void remain in concrete or any unconsolidated area. Thus defects related to concrete placement error can be repaired by replacement grouting

• Replacement grouting is usually suggested only when no soil is present in discharge i.e. if water flowing test indicates impermeability of defective area

• Replacement grouting is not suitable if grout finds ways to come out/escape from void area.

• If voids area encountered in pile at any side/bottom of drilled hole, replacement grouting, in general, cannot be applied for repairing pile defect.

How to do:

As in permeation grouting, defective area can be reached usually through inspection pipe provided during pile constriction. At first this pipe is taken out from the predetermined location of defect. High pressure jet of water is then used to wash out loose materials or any type of contaminate. This generally yields voids within pile concrete. The discharge of washing operation is then examined. Water used in washing of defective area must be extracted from void before injection of grout. Usually this water is displaced by compressed air. These voids are then filled with grout displacing air.


Compacting grouting:


Where to use:

The typical application is to enhance end bearing of pile by improving load carrying capacity of soil at the bottom of drilled hole as it is very difficult to apply grouting techniques at the tip of pile. This grouting method is only adopted, if pile defect is inclusive of soft tip and enhancing end bearing as desired in pile design.

Procedure:

As other grouting techniques, compaction grouting is generally injected through inspection pipe. Only bottom portion of inspection pipe is generally removed. The area under inspection pipe should be treated at top of pile. High pressure grout results a soil-grout bulb at tip of this pile. To yield a successful repair, compaction grouting should be pumped through every inspection pipe.

Difference between tip grouting and compaction grouting: 


In some projects it is preferred to perform tip grouting to improve end bearing of pile. The difference between tip grouting and compaction grouting are:

• Tip grouting is specified in the contact documents.

• There have provision for addition pipes in tip grouting and other additional equipment required for accessing grout to tip of pile.

• Compacting grouting is a mitigation measure for defective pile, not specified in contract document.


But both techniques have common drawback; there has no way to determine improvement or gain in end bearing of pile due to above operations. A particular pile may need one or more grouting techniques depending on number and type of defective areas exist within this pile.

Structural bridging:


This technique can be used to improve structural strength of defective piles. Structural bridging is chosen when engineer do not want to remove defect completely. The typical procedure is to make core into the center of defective pile followed by installation of rebar cage, structural steel section or both may be required to cast with central portion of core cut pile. The external member installed in addition to existing system may be suitable portion for redemption of structural strength of defective pile to achieve expected requirement for the project. It is sometimes expected to increase geotechnical strength of pile, in this purpose central section drilled into pile can be extended to bearing formation beneath pile tip.

Replacement and supplemental pile:


Sometimes piles may have so much defects that one may be puzzled from where repair start or have confusion actually it can be repaired. If engineer do not accept any repair method for mitigation of any pile as appropriate, provided contact specification should contain mitigation plan for pile for replacement/supplementation of such rejected pile. If there have enough space, contractor can propose for installation of supplemental piles to improve load-bearing capacity of unrepairable pile. If sufficient space doesn’t exist to accommodate supplemental piles, contractor will not have any option but to remove existing defective pile followed by installation of replacement pile.

Pile mitigation plan:


When defects in a pile significantly reduce its capacity and considered it as rejected, it is required to mitigate and contractor must enclose a mitigation plan for pile for acceptance and authorization of engineer. This plan is essential for any method of repair or replacement/supplemental piling proposed by contractor.

Responsibility of engineer:

The responsibility of engineer are as below:

• To arrange acceptance tests; based on these results make decision about whether reject or accept defects within pile

• To inform contractor about findings of results in writing enclosed with copy if test results

• Once it is established that pile is rejected, to consult with FTB to determine whether additional testing will be required. Generally, this report will suggest to conduct additional testing, if performed tests are not considered enough. Additional test results will also be supplied to contractor. Sometimes contractor may be asked to conduct additional tests.

• To use design data with acceptance test results to select proper mitigation measure which may be geotechnical, structural or for corrosion reasons. If any pile need to be mitigated consult with pile mitigation committee, designer, corrosion engineer and geoprofessionals to decide an acceptable mitigation measure discussed above. It should be noted that supplemental or replacement piling, structural bridging are considered as non-standard mitigation.

• In case of non-standard mitigation, the problem should be addressed to related professionals

• Pile mitigation plan should be reviewed thoroughly to ensure ensure that all components listed in contract specification are included, if any of them is omitted, return the proposal for mitigation for resubmission to contractor.

• Once engineer is satisfied with mitigation plan, he should send it to local authority like FTB (Foundation Testing Branch) and pile mitigation plan review committee to have clearance about technical review.

• When authority reject the plan, it has to be returned to contractor or recommendation is positive he can authorize the plan.

Preparation of pile mitigation plan:

It is the responsibility of contractor to prepare and submit pile mitigation plan for review and authorization to engineer. Contractor uses results of pile acceptance test results and design data supplied by engineer. Sometimes it is required to use recommendation of non-standard mitigation committee. The essential information, that are required in such plan are as follows: 


1. Designation of rejected pile with its location on foundation plan.

2. Review of geotechnical, structural and corrosion design parameters of rejected pile. Pile design data is supplied to contractor as a form; contractor must address design parameters in plan ensuring he/they realize why mitigation of a pile is required and submitted mitigation plan address defects or flaws in pile to satisfy design requirements.

3. Mitigation work should be placed with description of step-by-step procedures, inclusive of drawing, if required, so that engineer has idea about what types of methods and equipment are proposed by contractor to mitigate pile and to determine whether proposal is adequate for mitigation to satisfy design requirements.

4. Assessment of effectiveness if mitigation plan addressing geotechnical, structural and requirements of corrosion design of rejected pile.

5. Methods for restoration or preservation of surrounding earthen materials. In case of basic repair, the mitigation measures may disturb surrounding soil which may affect load bearing capacity developed by skin friction along the length of pile. Such disturbance in any mitigation method should be considered in proposed mitigation measures especially on shin friction bearing capacity of defective pile. 


6. Must include a list of facilities affected during mitigation measure including methods used to protect this facilities with equipment required during mitigation. The existing facilities may be utilities above and around pile. This ensures contractor has considered all facilities while developing such mitigation proposal.

7. Number and name of personnel should be listed with their qualification and an estimate of materials for mitigation plan should be included. These ensure who are employed to mitigate the pile, their experience in this work and whether they are familiar with such mitigation work and when to store sufficient amount of materials on site.

8. According to contract specification in shop drawing, the valid contact number, full name of structure, district-county, post-code, contractor’s name if assigned subcontractor’s too. Plan should have signature with seal of registered civil engineers of state, which ensures that such plan is produced or reviewed by qualified engineers to ensure all requirements.

9. An assessment of rejected pile in respect of size and nature of anomalies based on reports of acceptance test, concrete placement log and other information supplied or collected by contractor. It helps to decide approved repair method. As an example, size of anomaly often used to determine whether excavate and repair or grout repair is adequate to restore capacity of pile. When nature is concerned, an unconsolidated area within concrete of pile or inclusion of soil at side of pile determines whether excavated and repair method or grout repair is suitable to repair such pile.

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