Ground Improvement Effect Testing and Analysis: Methods, Standards, and Quality Evaluation

Ground improvement effect testing and analysis is a critical geotechnical quality assurance process that verifies whether treated ground meets design requirements for bearing capacity, deformation control, and long-term stability. This technical article provides a comprehensive overview of testing samples, testing items, in-situ and laboratory methods, applicable standards such as JGJ 79, GB 50202, and ASTM D1586, and typical engineering applications. It serves as a practical reference for geotechnical engineers, construction supervisors, and project owners seeking reliable verification of ground treatment outcomes.

Technical Overview

Ground improvement, also referred to as ground treatment or soil stabilization, encompasses a family of geotechnical techniques used to enhance the engineering properties of weak, compressible, loose, or permeable soils before superstructure construction. Common treatment methods include dynamic compaction, vibro-compaction, stone columns (granular columns), deep cement mixing, cement fly-ash gravel (CFG) piles, surcharge or vacuum preloading with prefabricated vertical drains, jet grouting, compaction grouting, and soil replacement with engineered fill.

Regardless of the method employed, the effectiveness of ground improvement must be objectively verified through systematic testing and analysis. Ground improvement effect testing determines whether the treated ground has achieved the specified bearing capacity, deformation modulus, density, strength, and uniformity required by the design. In China, the principal governing standards include JGJ 79 (Technical Code for Ground Treatment of Buildings), GB 50202 (Code for Acceptance of Construction Quality of Building Foundation), and GB 50007 (Code for Design of Building Foundation). Internationally, equivalent verification is performed in accordance with ASTM standards such as D1586, D5778, and D1196, as well as the EN ISO 22476 series for geotechnical investigation and testing.

A rigorous testing and analysis program typically combines in-situ testing, load testing, core sampling, and laboratory testing, supplemented by long-term settlement monitoring. The results are statistically evaluated against design criteria to issue a formal acceptance conclusion. Testing is normally carried out by accredited third-party laboratories holding CMA or CNAS accreditation, ensuring the independence, traceability, and legal validity of the test data.

Testing Samples

Sample selection for ground improvement verification depends on the treatment method, soil type, and verification indicators. The main sample categories are described below.

Undisturbed Soil Samples

Undisturbed samples are recovered from boreholes drilled in the treated ground after an appropriate curing or consolidation period. These samples preserve the natural structure and moisture condition of the soil and are essential for laboratory determination of compressibility, shear strength, void ratio, and consolidation parameters. Thin-wall samplers and piston samplers are used in cohesive soils in accordance with sampling practice requirements of GB 50021 and EN ISO 22475.

Cores from Cement-Stabilized Columns and Rigid Piles

For deep mixing columns, jet grouting columns, and CFG piles, cores are extracted by rotary drilling at specified ages, typically 28 days or a project-specified equivalent. Cores are used to assess continuity, homogeneity, mixing quality, and strength through visual inspection, unconfined compression testing, and, where required, point load or tensile testing on core specimens.

Disturbed Soil and Backfill Samples

Disturbed samples of treated or replaced fill materials are collected from compaction layers, sand cushions, replacement strata, or preloaded soft clay. These samples support classification tests, moisture content determination, compaction (Proctor) testing, gradation analysis, and grain density measurement, which together define the compacted state of the engineered fill.

Granular Fill and Aggregate Materials

For stone columns and vibro-compaction works, representative samples of crushed stone, gravel, or sand backfill are tested for gradation, particle shape, fines content, and relative density to confirm compliance with the specified material requirements before placement.

Testing Items

The verification program is tailored to the design acceptance criteria. Typical testing items include:

• Ultimate and characteristic bearing capacity of the treated ground or composite foundation
• Deformation modulus and compressibility modulus of treated soil layers
• Standard Penetration Test (SPT) N-values at specified depths
• Cone Penetration Test (CPT) cone resistance, sleeve friction, and pore pressure response
• Dry density, compaction degree (compaction coefficient), and relative density of fill layers
• Void ratio, moisture content, and degree of saturation before and after treatment
• Unconfined compressive strength and core recovery of stabilized columns
• Pile or column integrity for semi-rigid and rigid improvement elements
• Permeability coefficient for cutoff walls and seepage barriers
• Post-construction and staged settlement, lateral displacement, and pore water pressure dissipation
• Liquefaction resistance evaluation based on corrected SPT and CPT indices
• Uniformity of improvement across the treated plan area and depth range

Testing Methods

Ground improvement verification relies on a combination of field load testing, in-situ penetration testing, coring, and laboratory analysis. The principal methods are summarized below.

Static Plate Load Testing

Static plate load testing is the primary method for directly determining the bearing capacity and deformation modulus of treated ground and composite foundations. Loads are applied in increments to a circular or square plate, typically 0.5 to 1.0 square meter in area, and settlement is recorded at each increment until failure or a specified termination criterion. Single-pile, multi-pile composite foundation, and natural foundation load tests are performed in accordance with GB 50202, JGJ 79, and ASTM D1196. The characteristic value of bearing capacity is derived from the load-settlement curve using the specified settlement ratio or limit load criteria.

Standard Penetration Test (SPT)

The SPT, performed per ASTM D1586 and GB 50021, drives a split-spoon sampler with a 63.5 kg hammer dropping 76 cm, and the N-value provides an index of soil density and consistency. Before-and-after SPT profiles are compared to quantify improvement depth, verify densification of granular soils after dynamic compaction or vibro-compaction, and reassess liquefaction potential using normalized blow counts.

Cone Penetration Test (CPT)

The CPT, conducted in accordance with ASTM D5778 and EN ISO 22476-1, continuously measures cone tip resistance, sleeve friction, and, in piezocone testing, pore water pressure. CPT sounding data provide continuous profiles of strength and stiffness, enabling precise delineation of the improved zone, identification of untreated weak lenses, and empirical estimation of bearing capacity and settlement parameters. CPT is particularly effective in soft clays treated by preloading and in sands improved by densification.

Core Drilling and Laboratory Strength Testing

Rotary coring of stabilized columns and rigid piles verifies column continuity, mixing uniformity, and strength development. Cores are examined for recovery rate, visible defects, and cement distribution, and cylindrical specimens are prepared for unconfined compression testing at specified ages. The measured strength is statistically compared with the design value, and the characteristic strength is evaluated in accordance with JGJ 79 acceptance criteria.

Low-Strain Integrity Testing

For semi-rigid and rigid ground improvement elements such as CFG piles and prestressed elements, low-strain stress-wave integrity testing is applied to detect necking, bulging, discontinuities, or poor pile-bottom conditions. Velocity signals induced by a hand-held hammer are analyzed in the time domain to classify each element into integrity categories, with results governed by the project specification and relevant pile testing codes.

Field Density and Compaction Testing

Compaction quality of replaced fill, cushions, and densified layers is verified by sand cone testing, water or rubber balloon methods, or nuclear density gauges in accordance with ASTM D1556, D2167, and D6938, along with GB 50021 and GB 50202 requirements. The measured dry density is compared with the maximum dry density from Proctor testing to calculate the compaction coefficient, which must satisfy the specified layer-by-layer acceptance threshold.

Instrumented Monitoring and Settlement Observation

For preloading, vacuum consolidation, and staged construction projects, surface settlement plates, layered settlement gauges, inclinometer casings, and piezometers are installed to monitor consolidation progress and lateral displacement. Monitoring data are analyzed to determine the degree of consolidation achieved, predict residual settlement, and establish the safe timing for surcharge removal or structure loading.

Applications

Ground improvement effect testing and analysis is applied across a wide spectrum of infrastructure and industrial projects, including:

• High-rise residential and commercial buildings founded on composite foundations of CFG piles, deep mixing columns, or stone columns
• Expressways, high-speed railways, and airport runways constructed over soft or loose deposits
• Port terminals and land reclamation areas treated by dynamic compaction, preloading, and vacuum consolidation
• Large-diameter oil storage tanks and silos requiring strict differential settlement control
• Industrial plants, chemical facilities, and logistics parks on treated reclaimed or collapsible soils
• Seismic liquefaction mitigation works in seismic regions, verified by corrected SPT and CPT criteria
• Seepage cutoff walls and grout curtains for dams, cofferdams, and deep excavations
• Mine tailings facilities and embankments requiring density and stability verification

FAQ

When should testing be performed after ground treatment?

The timing depends on the treatment method. For dynamic compaction and vibro-compaction in granular soils, penetration testing is typically performed after pore pressure dissipation, often within one to four weeks. For cement-stabilized columns and rigid piles, coring and strength testing are conducted at the specified curing age, usually 28 days. For preloading works, verification is based on monitored degree of consolidation, which may require several months in soft clay.

How many load tests are required for acceptance?

Under GB 50202 and JGJ 79, the number of static load tests is specified as a percentage of the total improvement elements or treated area, commonly not less than 0.5 to 1 percent of elements and no fewer than three tests per project condition, unless the project specification requires otherwise. The exact quantity is confirmed in the testing plan approved by the designer and supervisor.

Can in-situ penetration tests replace static load testing?

Penetration tests such as SPT and CPT provide continuous profiles and are excellent for evaluating uniformity and improvement depth, but they cannot fully replace direct load testing for bearing capacity determination. Most acceptance frameworks require at least a minimum number of static load tests, with penetration testing used extensively to interpolate conditions between load test locations and to check overall uniformity.

Which standards govern ground improvement effect testing?

In China, the key standards are JGJ 79 for ground treatment technology, GB 50202 for construction quality acceptance, GB 50007 for foundation design, and GB 50021 for geotechnical investigation and testing. International practice references ASTM standards including D1586 for SPT, D5778 for CPT, D1196 for plate load testing, and D1556 or D6938 for density testing, together with EN ISO 22475 and the EN ISO 22476 series.

What happens if test results do not meet design requirements?

When verification results fall below design criteria, the failure zone is delimited through supplementary testing, the probable cause is analyzed, and remedial measures are implemented. These may include supplementary compaction passes, additional or extended columns, extended preloading, local re-treatment, or design re-evaluation. The re-treated area must then undergo a full re-verification cycle before acceptance can be granted.

Who should perform the testing?

All verification testing should be performed by accredited third-party laboratories with appropriate CMA or CNAS accreditation, or equivalent national accreditation, and by qualified geotechnical testing personnel using calibrated equipment. Independent testing ensures objective data for the acceptance decision and provides legally defensible documentation of ground improvement quality.