Preferred QC/QA Procedures
Deep dynamic compaction typically has performance-based criteria, but has no standard quality control or quality assurance measures. The Federal Highway Administration (FHWA) design guidance document, Lukas (1995), does include a section on construction monitoring that may serve as a quality control guide, but each project must be treated individually. A basic QC/QA program for deep dynamic compaction should always include heave and crater depth measurements after each series of tamper drops, topographic survey measurements after the ground has been leveled after each pass to assess net grade change, and if the soil conditions warrant, invasive tests such as SPT or CPT both before and after the compaction program to empirically evaluate the level of improvement achieved during the program. Dilatometer (DMT) and pressuremeter (PMT) tests can be used in the same manner as CPTs or SPTs, but there are fewer correlations between measurements and soil properties, although modulus data is helpful in conducting post-construction settlement analyses based on anticipated column loadings. Load tests can be performed in ground conditions where SPTs and CPTs may not be possible, such as in soils containing large amounts of cobbles and boulders or in landfill type materials. Surface wave measurement, such as SASW, can provide soil shear wave velocity profile in soils with obstructions. Finally, vibration monitoring should be carried out throughout the compaction process to evaluate the potential to adjacent structures and utilities.
For projects requiring the improvement of large areas, it is desirable to subdivide the total area into approval or acceptance zones that work in conjunction with subsequent phases of site construction such as fill placement or foundation construction. Completing the work with timely approval on a zone-by-zone basis means that the contractor may proceed without risk of having to return later in the project to correct deficiencies that could have been identified earlier in the project.
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 such that all applicable quality 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 deep dynamic compaction. The entries in the table are a list of the most typical items, not a list of all methods that could be used for QC/QA.
TABLE 1. TYPICAL QC/QA PROCEDURES for deep dynamic compaction
| QC or QA | Items |
|---|---|
| QC |
|
| QA |
|
QC/QA Guidelines
QC/QA programs should consider the following:
- Spacing between CPT or SPT tests (testing frequency):
- Specify at least one full post-treatment test profile (SPT or CPT) per 1,000 m² with at least four tests. This can be reduced in uniform soil conditions to one every 1,500 m², and on large sites this can be reduced to one per 2,000 m². (Dumas and Beaton 1992)
- Minimum time between the end of compaction and verification testing:
- This should be after all excess pore pressures have dissipated. This can range from days in pervious soils to months in semi-pervious soils. (Lukas 1986)
- Minimum value of penetration resistance or other measured property is based on project requirements. Maximum realistically achievable values are listed below in Table 2; however, it should be clear as to not use these maximums as the design required values. The acceptable criteria should be based on site specific performance, such as bearing capacity, static, and seismic settlement.
- Project specific requirements for minimum percentages of tests that do not meet the stated target criteria, lowest acceptable values, and their distribution throughout a site should be included. As an example, Lukas (1986) states, using relative density as a measure:
- The overall average measured values should be at least XX; e.g., a relative density of 85%, with no average of three consecutive values less than a slightly smaller value of XX; e.g., a relative density of 75%
- Tests within the upper XX feet; e.g., 15 feet, should have no values less than the specified average acceptance criterion. Below this depth no value should be less than a specified minimum value.
- Values less than a specified minimum should be reported to the Engineer for review and acceptance. Project specific requirements for minimum percentages of failing tests, lowest acceptable values, and their distribution throughout a site should be included. As an example, Lukas (1986) states, using relative density as a measure; see Table 2.
- Liquefaction induced settlement, under the site design earthquake (Magnitude, PGA, and design water table depth), should be less than XX inches and satisfy ASCE 7-22.
- Maximum levels of vibration at nearby structures of concern should generally be limited to:
- 25 mm/sec to avoid cosmetic damage in most modern buildings (Lukas 1986)
- 6.4 mm/sec for historic buildings (Lukas 1986)
- If load tests are required (usually in landfills or road embankments), the number is likely to depend on the size of the treatment area. Han (1998), Lukas (1986), and Lukas (1995) report from one to six load tests for some specific cases.
TABLE 2. TYPICALLY ACHIEVED UPPER BOUND VALUES AFTER Dynamic compaction (LUKAS 1995)
| Soil Type | Maximum Test Value: Standard Penetration Resistance (blows / 300 mm) | Maximum Test Value: Static Cone Tip Resistance, qc (MPa) | Maximum Test Value: Pressuremeter Limit Pressure (MPa) |
|---|---|---|---|
| Pervious coarse-grained soil: sand and gravels | 40 – 50 | 19 – 29 | 1.9 – 2.4 |
| Semipervious soil: sandy silts | 34 – 45 | 13 – 17 | 1.4 – 1.9 |
| Semipervious soil: silts and clayey silts | 25 – 35 | 10 – 13 | 1.0 – 1.4 |
| Partially saturated impervious deposits: clay fill and mine spoil | 30 – 40* | N/A | 1.4 – 1.9 |
| Landfills | 20 – 40* | N/A | 0.5 – 1.0 |
*Higher test values may occur due to large particles in the soil mass.
Inspections, construction observations, daily logs, and record keeping are essential QC/QA activities for all technologies. These activities help to ensure and/or verify that:
- Good construction practices and the project specifications are followed.
- Problems can be anticipated before they occur, in some cases.
- Problems that do arise are identified early, and their cause can oftentimes be addressed or remedied expeditiously.
- All parties are in good communication.
- The project stays on schedule.
Additional technology-specific details for inspections, construction observations, daily logs, and record keeping QC/QA activities are provided in the Individual QC/QA Methods section below.
Dumas, J.C. and Beaton N.F. (1992). “Dynamic compaction, Suggested guidelines for evaluating feasibility- for specifying- for controlling.” Canadian Geotechnical Conference Proceedings, p. 54-1-54-12.
Elias, V., Welsh, J., Warren, J., Lukas, R., Collin, J. G., and Berg, R. R. (2006a). “Ground Improvement Methods”- Volume I. Federal Highway Administration Publication No. NHI-06-020.
Han, J. (1998). “Ground modification by a combination of dynamic compaction, consolidation, and replacement.” Proceedings, Fourth International Conference on Case Histories in Geotechnical Engineering, St. Louis, Missouri, 341-346.
Lukas, R.G. (1986). “Dynamic Compaction for Highway Construction Volume I: Design and Construction Guidelines.” U.S. Department of Transportation, Federal Highway Administration, Washington, D.C., FHWA/RD-86/133.
Lukas, R.G. (1995). “Dynamic Compaction – Geotechnical Engineering Circular No. 1”, U.S. Department of Transportation, Federal Highway Administration, Washington, D.C., FHWA-SA-95-037.
Mackiewicz, S.M. and Camp, W.M. (2007). “Ground Modification: How Much Improvement?”, Soil Improvement, Geotechnical Special Publication No. 172, ASCE
Miller, H., Stetson, H., and Benoit, J. (2004). “DMT testing for site characterization and QA/QC on a deep dynamic compaction project.” Geotechnical Special Publication No. 126, ASCE, p. 1805-1812.
Mitchell, J.K. (1981) “Soil Improvement: State-of-the-Art,” Proceedings of the Tenth International Conference on Soil Mechanics and Foundation Engineering, Stockholm, Sweden, Vol. 4, pp. 509-565.
Schaefer, V., Abramson, L.W., Hussin, J.D., and Sharp, K.D. (1997). “Ground improvement, Ground reinforcement, Ground treatment: Developments 1987-1997.” Geotechnical Special Publication No. 69, ASCE. ASCE, New York.