MICROPILES QC/QA PROCEDURES - GeoTech Toolsgeotechtools.org/documents/mp_qcqa_r1.pdfmicropiles november 2012 page 1 of 30 r02 geotechnical solutions for soil improvement, rapid embankment

MICROPILES

November 2012 Page 1 of 30 R02 GEOTECHNICAL SOLUTIONS FOR SOIL IMPROVEMENT, RAPID EMBANKMENT CONSTRUCTION, AND STABILIZATION OF PAVEMENT WORKING PLATFORM

QC/QA PROCEDURES

Preferred QC/QA Procedures

The Federal Highway Administration (FHWA) provides QC/QA guidance for this technology. The documents are summarized below.

Publication Title Publication

Year Publication

Number

Available for

Download

Micropile Design and Construction 2005 FHWA-NHI-05-

039 No1

Micropile Design and Construction Guidelines

2000 FHWA–SA-97-

070 Yes2

Drilled and Grouted Micropiles: State-of Practice Review

1997 FHWA-RD-

016/019 No3

1 https://www.nhi.fhwa.dot.gov/training/course_search.aspx 2 http://isddc.dot.gov/OLPFiles/FHWA/009966.pdf 3 http://www.ntis.gov/search Construction quality is achieved by meeting established requirements, as detailed in project plans and specifications, including applicable codes and standards. Quality Control (QC) and Quality Assurance (QA) are terms applied to the procedures, measurements, and observations used to ensure that construction satisfies the requirements in the project plans and specifications. QC and QA are often misunderstood and used interchangeably. Herein, Quality Control refers to procedures, measurements, and observations used by the contractor to monitor and control the construction quality such that all applicable requirements are satisfied. Quality Assurance refers to measurements and observations by the owner or the owner's engineer to provide assurance to the owner that the facility has been constructed in accordance with the plans and specifications.

Table 1 shows the components of QC/QA monitoring programs for micropiles. The entries in the table are a list of typical items, not a list of all methods that could be used for QC/QA. Some QC procedures and measurement items may also serve as QA procedures and measurement items.

MICROPILES


QC/QA PROCEDURES

Table 1. Typical components of QC/QA monitoring programs. Topics Items

Existing QC/QA Procedures & Measurement Items

QC

Material Related

Grout and reinforcement testing

Process Control

Construction records and observations, proof testing

QA

Material Related

Load testing (ultimate, verification, and creep)

Process Control

Construction observations, inclinometer, telltale, strain gage, PDA, Statnamic testing

Performance Criteria

Material Parameters

Specific gravity and compressive strength of grout

System Behavior

Load test results

Instrument recordings (inclinometer, PDA, etc.)

Emerging QC/QA Procedures & Measurement Items

Material Related

Process Control

MICROPILES


QC/QA PROCEDURES

QC/QA Guidelines

The quality control and assurance of micropile projects is basic and reliable. In general, a comprehensive QC/QA program verifies that system components adhere to material specifications, installation methods follow execution specifications, and short-term performance specifications have been satisfied which confirm design values. A large component of quality assurance is reviewing inspection documents for completeness and consistency in construction methods and expected site conditions. Bruce and Juran (1997) and Sabatini et al. (2005) are considered to be the guidance documents for a typical QC/QA program for a micropile project. Sample testing program specifications, inspection logs, and suggested test frequencies are described in Sabatini et al. (2005).

Adherence to material specifications is verified through visual inspection logs and material tests. The installation process for every micropile must be meticulously observed with all necessary information recorded in drilling logs. The drilling logs can be used later to reveal any deviation in the installation process that may affect pile performance. “Although micropile load testing is relatively quick and inexpensive, it is not practical or economic to test every element installed. It is therefore essential that close attention is paid to the quality of the materials and the construction at all stages of the work” (Bruce and Juran 1997). During drilling inspection, the joints in micropiles should be inspected to ensure the pile materials are tight and the thread bar couplers are centered on the bar. If the thread bar couples are not centered on each bar they could slip off upon loading resulting in failure. An evaluation of the grout quality is of paramount importance because the grout serves to transfer the load from the reinforcement to the ground amongst other actions. Compressive strength testing has been traditionally conducted to confirm the strength of the grout. A detraction of this test is that it is a retrospective test. Compressive strength value from cube testing can only be established at a minimum of 48 hours after sampling and normally at 28 days. Specific gravity tests provide the benefit of rapidly assessing the conformance of the grout to design specifications prior to grouting and are preferred. It is recommended to conduct “classification” cube tests during the early stages of construction and then progress to relying wholly on specific gravity tests.

Short-term performance measures of Case 1 micropiles include static load proof and creep tests, which are conducted during construction. Production micropiles are subject to proof tests. The information gained from proof tests is used to verify acceptable micropile capacity and consistent installation procedures. Typically, proof tests load the micropile to 150% of the design load. Ultimate and verification tests are static load tests performed prior to production pile installation and are used to verify or develop design values. Should ultimate and verification tests disclose unanticipated behavior, the design of production micropiles can be modified. Ultimate tests are run to geotechnical failure and verification tests load the micropile to 200 to 250% of

MICROPILES


QC/QA PROCEDURESthe design load. Ultimate, verification, and proof tests may consist of compression, tension, or lateral loads. Tension tests are usually less expensive because the ground may be used in place of reaction piles or tie-down anchors. Tension load tests can often be used with similar results as compression load tests. Creep tests are performed as part of ultimate, verification, and proof tests. The effective bond length and apparent elastic length of the micropile can be estimated using static load test results. Although not commonly used, telltales can be installed at discrete depths on the micropile to provide information about micropile axial displacement with depth during a static load test. Strain gauges can also be mounted along the micropile to provide information about the axial load distribution with depth. Sabatini et al. (2005) recommend load testing be performed on Case 1 micropiles used for slope stabilization; however, “required micropile side resistance has relied exclusively on contractor experience in similar ground.”

Hammer-based integrity tests, for example PDA and Statnamic tests, are significantly cheaper to perform and have been used to determine micropile capacity. In an effort to reduce project cost, PDA and Statnamic tests can be incorporated into the testing program in place of static load tests. However, the use of hammer-based integrity tests are less common than static load tests, require considerable interpretation, and may not always be appropriate because of the damage which can be sustained by the micropile as a result of the test.

Inclinometers can also be used as part of short term monitoring of slope stabilization projects. Inclinometers are especially important for monitoring Case 2 micropiles where it is less meaningful to test each individual micropile because the micropiles and the soil act together as a composite soil-pile mass. Long-term monitoring is typically reserved for slope stabilization projects.

MICROPILES


QC/QA PROCEDURES

Individual QC/QA Methods

QC/QA Method: Reference(s):

Drilling Inspection Armour et al. (2000) Bruce (2003b) Bruce and Juran (1997) Cadden et al. (2004) DFI-ADSC (2002) Gibler et al. (2005) Gomez et al. (2008a, 2008b, 2004) Sabatini et al. (2005)

Method Summary

Typical tasks performed during a drilling inspection include:

Record soil log for each hole: this entails maintaining a drilling log for each hole which classifies the drill cuttings, observation of hole cleanout, rate of penetration, and includes elevation of bedrock encountered, strata change, and ground water table.

Confirm stability of each hole: the drilling log should confirm the stability of each drilled hole and describe any methods which might be utilized to maintain hole stability. If temporary casing is used, the casing type and length should be recorded. Casing should be accompanied by a coupon for strength testing.

Check tolerances: the final depth of the hole, alignment of the hole, angle of the drilling rig, etc. should be recorded.

Record drilling rate and other general observations: this includes describing any unusual installation behavior, flush returns, connections, and any deviation from the intended parameters.

Location of the piles: an as-built drawing of the site showing the locations of the piles should be included.

For projects where limited information is available about the subsurface conditions, maintaining a detailed drilling operation log is especially crucial. For sites with limited information, cuttings can be used to assess in real time whether the encountered soil stratum is suitable for developing the required bond strength. The drilling rate (Measurement While Drilling or MWD) is an invaluable tool that can be used to confirm that the soil encountered during drilling is similar to

MICROPILES


QC/QA PROCEDURESthe soil encountered where other micropiles were installed. Bruce (2003b) provides a thorough explanation of what a proper drilling inspection should address.

Accuracy and Precision

Drilling inspection is a powerful quality control method. Drilling inspection records accurately and precisely represent the drilling process as it occurs in the field.

Adequacy of Coverage

Drilling inspection records are kept for every micropile installed.

Implementation Requirements

Maintaining records is a simple and necessary process.

General Comments

Drilling inspection records do not provide actual strength parameters. In general, every hole drilled during micropile installation is an excellent source of information about the ground. Drilling inspection records are used to verify subsurface conditions throughout the project. Drilling inspection records are used to confirm adherence to design specifications and ensure consistent construction methods.

MICROPILES


QC/QA PROCEDURES


Reinforcement Inspection Armour et al. (2000) Bruce and Juran (1997) Cadden et al. (2004) DFI-ADSC (2002) Sabatini et al. (2005)

Method Summary

A thorough inspection should be performed on the reinforcement upon arrival and immediately before installation in order to verify adherence to design specifications such as type and bar size. Records should be kept which detail conformance to specified pile material, dimensions, and condition of the bar. The condition of the bar can be assessed by a visual inspection, which examines the reinforcement for any defects in workmanship, damage by handling, and signs of corrosion. Bruce and Juran (1997) states that “a light coating of surface rust on the steel is normal and indicates that oil and grease are not present” and “deep flaky corrosion or deep pitting of the steel is cause for rejection”. The reinforcement should be delivered with mill certificates. If not, a sample should be subjected to tensile and chemical testing to verify the reinforcement quality. Prior to installation, the reinforcement should have a clean surface. During installation of the reinforcement the location of centralizers (if used) should be recorded and it should be confirmed that the bar was lowered to the bottom of the hole without interference. After installation, the total pile length and bond zone length should be recorded for every micropile.


Confirming the quality of the materials for use in the project is a straightforward process.


Reinforcement inspection records are kept for every micropile installed.


Confirming the quality of the reinforcement is a straightforward process requiring minimal training or experience

MICROPILES


QC/QA PROCEDURES

General Comments

Once the reinforcement is in place, it is difficult and costly to extrude and replace. Most reinforcement materials can be obtained with certificates. Reinforcement inspection records are used to confirm adherence to design specifications. This is a critical process to ensure successful micropile installation.

MICROPILES


QC/QA PROCEDURES


Grout Inspection: Grouting Observations Armour et al. (2000) Bruce (1997b) Bruce and Juran (1997) Cadden et al. (2004) DFI-ADSC (2002) Gibler et al. (2005) Gomez et al. (2008a, 2008b) Sabatini et al. (2005)

Method Summary

Sabatini et al. (2005) lists what must be observed and recorded as part of grout inspection. Volume 3 of Bruce and Juran (1997) provides a full description of these inspection procedures that should be performed during grouting operations.

• Verify hole cleanliness (the hole should be flushed with water or air to remove any loose debris)

• Observe dry cement for indications of hydration

• Verify water/cement ratio and grout mix design (the grout should not contain lumps or exhibit any evidence of poor mixing)

• Verify that all grouting equipment is in good working order

• Record grout volumes and grout pressures with time for each micropile (should also be recorded for post grouting)

• Observe quality of grout at ground surface (i.e., when hole is full of grout)

Grout pressure should be measured as close to the point of injection as possible. The pressure gauge should always be monitored during grouting. The grout pressure and take needs to be monitored to ensure complete filling of hole and in an effort to prevent excessive heave or fracturing in the soil or rock.


Confirming the quality of the grout for use in the project is a straightforward and highly precise process.

MICROPILES


QC/QA PROCEDURES


Grout inspection records are kept for every micropile installed. The frequency of cube testing and checking specific gravity of the grout varies based on project conditions.


Confirming the quality of the grout is a straightforward process requiring minimal training or experience. Cube testing and checking the specific gravity are commonly performed and well understood procedures.

General Comments

Grouting observation records are used to confirm adherence to design specifications. “Care in execution of the grouting process can greatly impact the quality of the completed micropile and its load carrying capacity” (Sabatini et al., 2005).

MICROPILES


QC/QA PROCEDURES


Grout Inspection: Specific Gravity Armour et al. (2000) Bruce (1997b) Bruce and Juran (1997) Cadden et al. (2004) Gomez et al. (2008a, 2008b) Sabatini et al. (2005)

Method Summary

A specific gravity test is performed as part of grout inspection and is used to confirm the water content (i.e. water-cement ratio) of the grout. Confirming the water-cement ratio of grout is an important quality control measure because water content is the prime determinant of grout properties and strength. Specific gravity is determined with a calibrated mud balance following the American Petroleum Institute (API) Recommended Practice (RP) 13b-1 “Recommended Practice Standard Procedure for Field Testing Water-Based Drilling Fluids”.


Confirming the specific gravity of grout for use in the project is a straightforward and highly precise process.


The specific gravity should be measured at least once per micropile or batch.


Measuring specific gravity of the grout is quick and inexpensive.

General Comments

Measuring the specific gravity prior to and during grouting verifies that the grout is being prepared as the design specifies. Specific gravity testing is timely, simple, and provides real-time results. In addition, specific gravity testing is a more reliable indicator of grout consistency and quality because the test is more sensitive to water-cement ratio. For these reasons, specific gravity is preferred to cube testing.

MICROPILES


QC/QA PROCEDURES


Grout Inspection: Cube Testing Armour et al. (2000) Bruce and Juran (1997) Gibler et al. (2005) Gomez et al. (2008a, 2008b) Sabatini et al. (2005)

Method Summary

Cube testing is traditionally performed as part of grout inspection and is used to verify the unconfined compressive strength of the grout. Cube testing can take place to confirm acceptability of the grout mix as part of verification, ultimate, and proof load tests. A grout sample is molded into cubes and after a specified number of days each cube is loaded in compression to failure according to AASHTO T106/ASTM C109 “Standard Test Method for Compressive Strength of Hydraulic Cement Mortars.” Tests are typically performed after 3, 7, and 28 days of set time. The required strength will vary depending on the project.


Confirming the unconfined compressive strength of the grout for use in the project is a straightforward but not always accurate process. Curing and handling conditions are critical and can have major impact on the results obtained.


The frequency of cube tests varies depending on the project. Sabatini et al. (2005) recommends “one set of three grout cubes be taken for every 10 micropiles installed, or every day for each grout plant in operation, whichever occurs more frequently.”


Measuring unconfined compressive strength of the grout is a simple procedure. However, the time required for the sample to set up needs to be considered in the timeframe of construction processes.

MICROPILES


QC/QA PROCEDURES

General Comments

Measuring the unconfined compressive strength of the grout verifies the acceptability of the grout mix design, which is critical to successful micropile implementation. Quality grout allows the micropile to satisfactorily support the anticipated service load. Because cube testing is a retrospective test, specific gravity testing is typically preferred. Cube testing should be performed early in the project to verify the correct mix design and then replaced with specific gravity tests correlated to the results obtained from the cube tests.

MICROPILES


QC/QA PROCEDURES


Ultimate Load Test Armour et al. (2000) Bruce and Juran (1997) Gibler et al. (2005) Gomez et al. (2008a, 2008b) Sabatini et al. (2005)

Method Summary

Ultimate tests are conducted before production pile installation in order to obtain information about the performance of micropiles at the project site given methods of construction, materials, etc. This information is then incorporated into the final design. For this reason, ultimate tests are technically not a QC/QA method. For projects that utilize ultimate tests, the relationship of ultimate tests to other static load tests, verification and proof tests, and the contribution of ultimate tests to selected installation methods should be understood. Ultimate tests are similar to verification tests, with the exception that ultimate tests are intended to be run to geotechnical failure. Failure is defined as the inability to maintain a constant load without excessive micropile movement or sudden loss of ability to support the load. The micropiles are individually tested using a static load which is applied in compression, tension, or laterally and movement of the micropile head is measured for each load increment. The micropile is loaded and unloaded in increments until the micropile demonstrates failure. Ultimate tests on micropiles allow the Contractor to demonstrate and optimize his specific means and methods. Typical load testing equipment includes: a hydraulic jack and pump; reaction and reference beams; pressure gauges and load cells; dial gauges; and a wire with mirror and scale (Sabatini et al., 2005). A creep test is performed as part of the ultimate test.


Ultimate testing accurately and precisely assesses the ultimate strength of the micropile given the specified design and construction processes.


Depending on the site and project conditions, ultimate tests may not be required.

MICROPILES


QC/QA PROCEDURES


The requirements of ultimate testing are significant. The amount of time required to perform ultimate tests are greater than proof or verification testing. As a result, performing ultimate tests are more expensive than proof or verification tests.

General Comments

Ultimate tests, also referred to as pre-production tests, offer a detailed range of information about the performance of the micropile system in the project environment. The results of an ultimate test can be used to provide an opportunity to verify the strength capacity of the micropile, establish load-deformation behavior, identify causes of pile movement, workmanship, method of construction, suitable interaction of materials, and verify the appropriateness of the assumed rock/soil shear strength to be used in design of production micropiles. The average ultimate bond strength and the ultimate pile capacity can be calculated based on the results of an ultimate test. The ultimate bond strength values can be used to confirm the ability of the micropile to support an expected service load. When conducting an ultimate test it must be verified in advance that the geotechnical capacity of the pile will be exceeded before the structural capacity of the pile exceeds 80% ultimate. Ultimate tests are not required for all projects and are typical for only large projects for which a design phase test program can be justified.

MICROPILES


QC/QA PROCEDURES


Verification Test Armour et al. (2000) Bruce and Juran (1997) DFI-ADSC (2002) Gibler et al. (2005) Gomez et al. (2008a, 2008b) Sabatini et al. (2005)

Method Summary

Verification tests are conducted on pre-production micropiles installed following the same processes anticipated for production micropiles. Results from verification tests are used to confirm the adequacy of the design and construction process. At a minimum, one verification test is required for each significantly different soil or rock stratum where bond strength will be engaged. The micropiles are individually tested using a static load which is applied in compression, tension, or laterally and movement of the micropile head is measured for each load increment. The micropile is loaded and unloaded in increments until the specified maximum test load is reached. The micropile is loaded to 200 to 250% of the design load during verification tests. Because verification tests are carried to high loads, sacrificial micropiles are tested, which are not incorporated into the final structure. Three primary acceptance criteria must be met during verification testing of a micropile: rate of pile head movement at the test load, total pile head movement at the design load, and total pile head movement under sustained load (i.e. creep). If failure occurs during verification load testing, the design and/or the construction procedure must be adjusted to provide an acceptable capacity. Typical load testing equipment includes: a hydraulic jack and pump; reaction and reference beams; pressure gauges and load cells; dial gauges; and a wire with mirror and scale (Sabatini et al. 2005). Creep tests are performed as part of the verification test.


Verification testing accurately and precisely assesses the design and construction quality of a micropile project.


Depending on the site and project conditions, more or less tests are required. On a typical project, at least one verification test is performed prior to production pile installation. Additional

MICROPILES


QC/QA PROCEDURESverification tests are required if significant subsurface variation exists in the soil stratum relied on for bond strength and/or projects where a large number of micropiles are installed.


The requirements of verification testing are not excessive, although the amount of time required to conduct a verification test is greater than proof testing. As a result, verification testing is more expensive than proof testing. Verification testing is a required quality assurance procedure for all micropile projects.

General Comments

Verification tests are performed to demonstrate the adequacy of design and construction method given similar subsurface conditions. “The results of a verification test can be used to provide an opportunity to verify the strength capacity of the micropile, establish load-deformation behavior, identify causes of pile movement, and verify the appropriateness of the assumed rock/soil shear strength used in design” (Gibler et al. 2005). These results are then used to determine the ability of the micropiles to carry an expected service load.

MICROPILES


QC/QA PROCEDURES


Proof Test Armour et al. (2000) Bruce (1997b) Bruce and Juran (1997) DFI-ADSC (2002) Gibler et al. (2005) Gomez et al. (2008a, 2008b) Sabatini et al. (2005)

Method Summary

Proof testing is conducted during construction on a percentage of installed production micropiles, which are intended to be incorporated into the structure if the test load does not cause failure. Proof tests provide verification of a consistent construction process, design adequacy, and material quality. The micropiles are individually tested using a static load which is applied in compression, tension, or laterally. The load is applied in increments until the specified maximum test load. Proof tests use a maximum test load which typically corresponds to at least 150% of the design load. Acceptance criteria are defined by a specified acceptable pile head movement at the design load and total pile head movement under a sustained load (i.e. creep). If failure occurs during proof load testing, the design and/or the construction procedure must be adjusted to address the exposed deficiencies. The results from proof tests can be used to determine the bond strength of the micropile. Additionally, equations can be used to determine the micropile effective bond length and the free length. Typical load testing equipment includes: a hydraulic jack and pump; reaction and reference beams; pressure gauges and load cells; dial gauges; and a wire with mirror and scale (Sabatini et al. 2005). A creep test is performed as part of the proof test.


Proof testing accurately and precisely assesses the construction quality of micropile installation.


Proof tests are considered to provide a high amount of coverage. In general, it is recommended to test at least 5% of the production micropiles. Depending on the site and project conditions, more or less tests are required. Table 7-3 in Sabatini et al. (2005) recommends a minimum number of test micropiles for proof testing.

MICROPILES


QC/QA PROCEDURES


The requirements of proof testing are not excessive. Proof testing is typically required as a quality assurance procedure for micropile projects.

General Comments

Proof tests are performed as part of verifying consistent construction procedures throughout the project and the micropile strength meets the required minimum design load. These results are then used to determine the ability of the micropiles to carry an expected service load.

MICROPILES


QC/QA PROCEDURES


Creep Test Armour et al. (2000) Bruce (1997b) Bruce and Juran (1997) Gomez et al. (2008a, 2008b) Sabatini et al. (2005)

Method Summary

A creep test is performed by measuring the movement of the micropile at a constant load over a specified period of time. Creep tests are conducted as part of ultimate, verification, and proof tests at specified load increments. Creep is potentially a concern in organic and soft cohesive soils with liquid limit greater than 50, in creep susceptible rocks (e.g., shale or weathered mélange), and in soils with a plasticity index greater than 20 (Sabatini et al. 2005).


Creep testing is a trusted and commonly performed procedure.


The results of a creep test can be used to represent other micropiles of the same design in a project which were installed in similar subsurface conditions and construction processes.


The requirements of creep testing are not excessive.

General Comments

Creep tests can be used to assess whether the service loads can be supported safely with an acceptable amount of micropile movement. The results of the creep test can be used as failure criterion for the load test. The test data should be plotted on a semi-log plot in order to interpret data correctly.

MICROPILES


QC/QA PROCEDURES


Apparent Elastic/Effective Bond Length Armour et al. (2000) Bruce et al. (1993) Gibler et al. (2005) Gomez et al. (2008a, 2008b) Sabatini et al. (2005)

Method Summary

A micropile can be partitioned into two zones: the cased or free length in the upper part of the micropile and the bond length in the lower. A portion of the bond length is referred to as the effective bond length wherein the load applied to the micropile is transferred to the surrounding ground. A portion of the entire micropile length is called the elastic length, which is the length of the micropile that undergoes elastic shortening under a load.


The values obtained are approximate.


The apparent elastic length and/or the effective bond length should be determined for every micropile load tested (ultimate or verification).


Determining the apparent elastic length and the effective bond length requires simple mathematical understanding and can be completed as part of a standard static load test.

General Comments

The effective bond length can be used to estimate the ultimate capacity of the pile. The apparent elastic length is used to confirm the length over which elastic movements occur to allow for a more accurate measurement of elastic movement and total pile group settlement. These values can be used to assess a micropile’s ability to satisfactorily support its service load.

MICROPILES


QC/QA PROCEDURES


PDA Bruce and Juran (1997) Cadden et al. (2004) Gomez et al. (2004) Sabatini et al. (2005)

Method Summary

Pile Driver Analyzer (PDA) testing is typically reserved for driven piles. However, methods exist which allow PDA testing to be used to test micropile capacity. PDA testing is a time efficient method that has been shown to provide similar results as static load tests. Because static load tests require more time and resources, it may be more economically desirable to use PDA tests for projects requiring a significant number of static load tests. Using PDA tests, valuable information can be gathered quickly to modify the design if necessary. However, PDA testing on micropiles has limitations. Because micropiles are not driven, a pile hammer is necessary to perform PDA testing, and damage sustained by the micropile is a concern. For this reason, there is a maximum capacity to which the micropile can be loaded to during PDA testing. The maximum testing capacity and any additional precautions that must be taken are a function of the design of the micropile. Another disadvantage of PDA testing is that creep, tension, and lateral loading tests cannot be performed.


PDA testing is not used on a regular basis to test micropiles. As a result, sufficient field experience using PDA on micropiles does not exist. General procedures and recommendations for PDA testing on micropiles are not yet developed.


PDA testing provides a reasonable assessment of the capacity of the installed micropile. PDA testing is rapid and inexpensive compared to full-scale static load testing and therefore provides a means to approximate the static capacity of a relatively large number of production piles at a relatively low cost (Sabatini et al. 2005).

MICROPILES


QC/QA PROCEDURESImplementation Requirements

PDA testing requires specialized equipment and experienced testing professionals to conduct the test and analyze the results. Interpreting results of micropiles with varying cross sections is very difficult.

General Comments

PDA tests are performed as part of verifying adequate construction procedures throughout the project. The results of a PDA test can be used to determine the strength capacity of the micropile as a quality control measure. These results are then used to determine the ability of the micropiles to sustain a service load. The shortfalls and special considerations mentioned above make PDA testing on micropiles difficult. As such, PDA tests are rarely performed on micropiles in common practice.

MICROPILES


QC/QA PROCEDURES


Statnamic Testing Cadden et al. (2004) Sabatini et al. (2005)

Method Summary

Statnamic testing can be used to evaluate the capacity of micropiles. Static load tests require more time and resources than Statnamic tests. For this reason, it may be more economically desirable to incorporate Statnamic testing in place of some static load tests in a full-scale load test program. Because of the ability to test a significant number of micropiles in a short period of time, valuable information can be quickly gathered to modify the design if necessary. Sabatini et al. (2005) describes the Statnamic test as follows:

“The Statnamic test method uses solid fuel burned within a pressure chamber to rapidly accelerate upward the reaction mass positioned on the pile head. As the gas chamber increases, an upward force is exerted on the reaction mass, while an equal and opposite force pushes downward on the pile.”

Statnamic testing can be conducted as lateral or compressive loads and can be performed on battered and vertical micropiles. “The magnitude and duration of the applied load and the loading rate are controlled by the selection of piston and cylinder size, the fuel mass, fuel type, the reaction mass, and the gas venting technique” (Sabatini et al. 2005). Several projects have been completed where Statnamic testing was conducted (Sabatini et al. 2005). Cadden et al. (2004) states that a reasonable estimation of soil strength parameters were obtained using the Statnamic tests on micropiles. However, information about creep behavior of the micropile cannot be gathered from a Statnamic test. Statnamic testing may not be possible in areas which are difficult to access. Data from Statnamic tests may lose accuracy for length to pile diameter greater than 30, but reasonably consistent results can still be found.


Statnamic testing is not used on a regular basis to test micropiles. As a result, sufficient field experience using Statnamic on micropiles does not exist. Published test data does not exist to verify the effectiveness of Statnamic testing as a quality assurance method for micropile projects.

MICROPILES


QC/QA PROCEDURES


Statnamic testing is rapid and inexpensive compared to full-scale load static load testing and therefore provides a means to approximate the static capacity of a relatively large number of production piles at a relatively low cost (Sabatini et al. 2005).


Statnamic testing requires specialized equipment and experienced testing professionals to conduct the test and to interpret the results. However, because the load duration is longer than with PDA tests, stress wave formations can be ignored allowing for a simpler analysis. Statnamic testing does not require reaction piles or a reaction system, resulting in a shorter set up time.

General Comments

Statnamic tests are performed as part of verifying adequate construction procedures throughout the project. The results of a Statnamic test can be used to determine the strength capacity of the micropile. These results are then used to determine the ability of the micropiles to sustain a service load. As mentioned above, Stanamic testing of microiles is not well-documented nor has sufficient experience been obtained to make it a commonplace tool. Statnamic testing should not be intended to control a micropile testing program.

MICROPILES


QC/QA PROCEDURES


Telltales Armour et al. (2000) Sabatini et al. (2005)

Method Summary

Axial displacement can be measured using telltales at discrete elevations in a micropile after installation or during load testing. Telltales are metal or fiberglass rods anchored in the micropile grout at the tip of the rod at various elevations. The remainder of the rod is sheathed in order to allow free movement of the micropile (Sabatini et al. 2005). The free end of the telltale protrudes above the soil surface, by which movement of the telltale can be measured. Movement of the telltale is an indication of axial displacement of the micropile at the location where the telltale is embedded. Multiple telltales can be installed in a micropile to provide an idea of the micropile’s axial displacement with depth.


Telltales can be relied upon to indicate how the micropile is moving beneath the soil surface. However, care must be taken to avoid damage to the telltales during micropile installation as this will provide inaccurate information about axial displacement with depth. Even when installed correctly, telltales can be difficult to interpret.


Telltales provide a reasonable assessment of the micropile’s ability to sustain a service load with minimal displacement with the adequacy of coverage that all load tests provide.


The installation of telltales requires considerable experience in order to avoid damage and erroneous results.

General Comments

Telltales provide a reasonable assessment of the load distribution over the length of the micropile. Telltales are very susceptible to construction damage and should be treated with great care and suspicion. Because of this, telltales are not commonly used in micropile projects.

MICROPILES


QC/QA PROCEDURES


Strain Gauge Armour et al. (2000) Bruce and Juran (1997) Cadden et al. (2004) Sabatini et al. (2005)

Method Summary

Strain gauges can be mounted on the reinforcing steel at different elevation to determine the axial load distribution and to evaluate micropile bending moments with depth.


The accuracy and precision of the strain gauges is dependent on avoiding damage to the gauge and connecting wires during installation.


A large number of strain gauges are required to provide an accurate assessment of strain with depth.


Great care must be taken during installation to avoid damage to the gauge and connecting wires.

General Comments

If used, vibrating-wire-type strain gauges are recommended. Strain gauges, as well as other instrumentation, are not common tools for testing and monitoring micropiles. Strain gauges should not be intended to control a micropile quality assurance program.

MICROPILES


QC/QA PROCEDURES


Inclinometer Armour et al. (2000) Bruce (1997b) Bruce and Juran (1997) Sabatini et al. (2005)

Method Summary

Inclinometers are installed in the soil and are read at an established time interval in order to record lateral movements with depth.


This method provides a reliable depiction of lateral movements with depth.


Inclinometers are able to provide a reasonable area of coverage. The greater the number of inclinometers, the more information can be retrieved about lateral extent of movements below the surface.


Once installed, inclinometers are simple to take readings from. However, obtaining reasonable results requires experience and knowledge of reading inclinometers.

General Comments

Commonly, inclinometers are used to monitor movement of slopes stabilized by micropiles or embankment support projects. Inclinometers can also be used to obtain micropile lateral movement data with depth during a lateral load test. Inclinometers, as well as other instrumentation, are not common tools for testing and monitoring micropiles. Inclinometers should not be intended to control a micropile quality assurance program.

MICROPILES


QC/QA PROCEDURES

References

Armour, Groneck, Keeley, and Sharma (2000) “Micropile Design and Construction Guidelines Implementation Manual.” Federal Highway Administration, Publication No. FHWA –SA-97-070, June 2000.

Bruce, D.A. (1997b). “Ground Improvement, Reinforcement, and Treatment: Developments 1987-1997”, Chapter 2.6 on Micropiles, Proc. of Sessions Sponsored by the Committee on Soil Improvement and Geosynthetics of the Geo-Institute of the American Society of Civil Engineers, Logan, UT, July 17-19, Ed. by V.R. Schaefer, Geotechnical Special Publication No. 69, pp. 151-175.

Bruce, D.A. (2003a). "The Basics of Drilling for Specialty Geotechnical Construction Processes." Grouting and Ground Treatment, Proceedings of the Third International Conference, Geotechnical Special Publication No. 120. Edited by L.F. Johnsen, D.A. Bruce, and M.J. Byle, American Society of Civil Engineers, New Orleans, LA, February 10-12, pp. 752-771.

Bruce, D.A. and Juran, I. (1997). "Drilled and Grouted Micropiles: State of Practice Review, Volumes I, II, III, and IV." Prepared for the Federal Highway Administration, Publication Nos. FHWA-RD-96-016. –017, -018, and –019, July.

Cadden, A., J. Gómez, D.A. Bruce, and T. Armour. (2004). "Micropiles: Recent Advances and Future Trends," Current Practices and Future Trends in Deep Foundations, Geotechnical Special Publication No. 125, ASCE Geo-Institute GSP Honoring Dr. George Goble, GeoTrans Conference, Los Angeles, CA, July 27, 27 pp.

Deep Foundations Institute & ADSC-IAFD Micro Pile Committee, Thomas D. Richards Jr., P.E., DFI Committee Chair; Tom Armour, ADSC Committee Chair (2002). “Guide to Drafting a Specification for Micropiles.” DFI publication #TM-MP-1.

Gibler, P., Bruce, D.A., and Hadzariga, M. (2005). "Quality Assurance Issues Related to the Installation of High Capacity Micropiles, Richmond-San Rafael Bridge Seismic Retrofit Project, California," Geo3 GEO Construction Quality Assurance/Quality Control Conference Proceedings, Editors D.A. Bruce and A.W. Cadden, Dallas/Ft. Worth,TX, November 6-9, pp. 570-592.

Gómez, J.E., Rodriguez, J., Mikitka, J., Keough, L., and Robinson, H.D. (2008a). “Bond Strength of Hollow-Core Bar Micropiles.” 6th International Conference on Case Histories in Geotechnical Engineering, Arlington, VA, August 11-16, 2008. Paper No. 8.05c.

MICROPILES


QC/QA PROCEDURES

Gómez, J.E., Rodriguez, J., Mikitka, J., and Robinson, H.D. (2008b) “Hollow Core Bar Micropile-Design Parameters Interpreted from 404 Load Tests.” Proceedings of the 33rd Annual and 11th International Conference on Deep Foundations, 2008, New York, NY, USA, (DFI)

Gómez, J.E., Cadden, A., and Webster, O.C. (2004) “Micropile Foundation in Karst: Static and Dynamic Testing Variability,” Proceedings: Fifth Annual Conference on Case Histories in Geotechnical Engineering, New York, New York, April 2004.

Sabatini et al. (2005). “Micropile Design and Construction (Reference Manual for NHI Course 132078).” Federal Highway Administration, Publication No. FHWA-NHI-05-039, December 2005.

Documents

MICROPILES QC/QA PROCEDURES - GeoTech Toolsgeotechtools.org/documents/mp_qcqa_r1.pdfmicropiles november 2012 page 1 of 30 r02 geotechnical solutions for soil improvement, rapid embankment