Test Method for Vertical Compressive Static Load Test of Single CFG Pile

Test Method for Vertical Compressive Static Load Test of Single CFG Pile

CFG pile (Cement-Fly ash-Gravel pile) is a typical rigid composite foundation reinforcement body, widely applied in high-rise building foundations, municipal road subgrades, industrial plant foundations and soft soil improvement projects. The vertical compressive bearing capacity of a single CFG pile is the core technical index for foundation design, construction acceptance and settlement prediction. Single pile vertical compressive static load test is the most authoritative and accurate in-situ detection method to verify the ultimate bearing capacity, deformation characteristics and working performance of CFG piles. Based on JGJ 106, GB/T 50783, ISO 22477-1 and ASTM D1143 standard specifications, this document systematically elaborates the test principle, equipment configuration, field preparation, standardized loading procedure, termination criteria, data processing and qualification judgment rules of CFG pile static load test, forming a complete and standardized technical implementation guideline for engineering inspection.

1. General Overview and Test Scope

1.1 Test Principle

The single pile vertical compressive static load test applies graded and sustained vertical static load to the pile top through hydraulic loading equipment. By monitoring the pile top settlement deformation under each load level, the load-settlement (Q-s) curve and settlement-time (s-lgt) curve are drawn to analyze the elastic-plastic deformation law of the pile body and the vertical bearing performance of pile-soil interaction. The test can accurately obtain the characteristic value and ultimate value of vertical compressive bearing capacity of a single CFG pile, and evaluate whether the pile foundation meets design and safety service requirements.

1.2 Application Scope

This test method is applicable to quality acceptance inspection, design parameter verification and process performance evaluation of CFG piles formed by vibration sinking pipe, long spiral drilling and pumping concrete pouring. It covers two test scenarios: test pile detection for design basis verification and engineering pile sampling inspection for completion acceptance, and is the mandatory authoritative detection method for composite foundation bearing capacity evaluation.

1.3 Test Classification

According to engineering purposes, the test is divided into design verification test and acceptance inspection test. The design verification test shall be loaded until the pile-soil system reaches the ultimate failure state to obtain the ultimate bearing capacity. The acceptance test shall be loaded to twice the design characteristic value to verify the safety margin of engineering piles.

2. Standard Test Equipment and Layout

2.1 Main Test Equipment

The static load test system consists of four core units: loading system, counterforce system, displacement monitoring system and data acquisition system. The loading system adopts high-precision hydraulic jack and automatic pressure maintaining oil pump to realize stable graded loading. The counterforce system adopts anchor pile counterforce or weight stacking counterforce to provide stable reaction support. The displacement monitoring system uses high-precision dial indicators or electronic displacement meters to collect pile top settlement data. All instruments shall be within the valid calibration period to ensure test accuracy and traceability.

2.2 Field Layout Requirements

The test pile shall be kept intact without artificial damage, surface cracking and pile top inclination. The pile top shall be polished flat and leveled with high-strength mortar to ensure uniform stress during loading. The counterforce support system shall be arranged symmetrically with the test pile as the center, with sufficient spacing to avoid boundary effect interference. The displacement meter support frame shall be independent of the loading and counterforce system to eliminate additional deformation interference and ensure pure settlement data of the pile body.

3. Pre-Test Preparation and Sampling Rules

3.1 Interval Time Requirement

The static load test shall be carried out after the concrete strength of CFG pile meets the design requirements and the pile-soil interaction is stable. The test interval after pile construction shall not be less than 28 days for conventional soil layers, ensuring the full development of pile side friction and pile end bearing capacity, and avoiding test deviation caused by incomplete concrete hardening and unconsolidated foundation soil.

3.2 Batch Sampling Principles

In accordance with national inspection specifications, the sampling ratio of CFG pile static load test shall not be less than 0.5% of the total number of piles in the same construction section, and the minimum sampling quantity shall not be less than 3 piles. For foundations with complex geological conditions, inconsistent construction technology or large pile quantity, the sampling proportion shall be appropriately increased to ensure the representativeness of test results.

4. Standard Test Implementation Procedure

4.1 Loading Method Selection

The slow maintenance load method is adopted as the standard test method for CFG pile static load test, which is the benchmark method for engineering acceptance and data calibration. For special working conditions such as rapid construction verification and offshore foundation detection, the fast maintenance load method can be adopted with standardized correction coefficients.

4.2 Graded Loading Rules

The maximum test load is determined according to twice the design characteristic bearing capacity. The total load is divided into 10 equal grades for graded loading. The first and second load grades are double the single graded load to accelerate the compaction of pile-soil gap. Subsequent loads are applied grade by grade with equal single load. Unloading is carried out in reverse graded mode, with double graded load for each unloading level, and the unloading interval is shortened correspondingly.

4.3 Data Reading and Stability Standards

Under slow maintenance load mode, the settlement data shall be read at 5min, 15min, 30min, 45min and 60min after each grade of loading, and then read every 30min until the settlement is stable. The relative stability standard is that the settlement increment per hour is less than 0.1mm and lasts for not less than 2 consecutive hours. After each grade of unloading, the rebound data shall be read at fixed time intervals until the rebound deformation is stable.

5. Test Termination Judgment Criteria

The static load test can be terminated when any of the following standard conditions is reached, in strict accordance with JGJ 106 and GB/T 50783 specifications:

Condition 1: The settlement deformation of the current load grade is more than 5 times the settlement of the previous grade, showing obvious sharp settlement mutation and pile foundation failure characteristics.

Condition 2: The current grade settlement exceeds twice the previous grade settlement, and the deformation still fails to reach stability after 24 hours of load maintenance, indicating continuous plastic failure of pile-soil system.

Condition 3: The total pile top settlement exceeds 40mm, and the deformation rate is continuously accelerated, which meets the foundation failure judgment standard.

Condition 4: The test load has reached the predetermined maximum design load, and the pile body deformation is stable without failure signs.

Condition 5: Abnormal conditions such as excessive counterforce displacement, equipment failure and safety hazards occur during the test.

6. Post-Test Data Processing and Curve Analysis

6.1 Basic Data Sorting

Sort out all graded load values, real-time settlement data and rebound data, eliminate abnormal interference points caused by external vibration and manual operation errors, and form complete and continuous test original data records.

6.2 Characteristic Curve Drawing

Draw two core analysis curves: load-settlement (Q-s) curve and settlement-time logarithm (s-lgt) curve. The Q-s curve reflects the overall bearing and deformation characteristics of the pile; the s-lgt curve judges the stability of deformation and the occurrence of accelerated failure.

6.3 Bearing Capacity Determination Rules

The ultimate vertical compressive bearing capacity of a single pile is determined according to the inflection point of the Q-s curve and the mutation point of the s-lgt curve. If the curve has an obvious steep drop inflection point, the load corresponding to the inflection point is taken as the ultimate bearing capacity. If the curve changes gently without obvious failure inflection point, the maximum test load value is taken as the verification basis. The characteristic value of bearing capacity is half of the measured ultimate bearing capacity, which shall not be lower than the design specified value.

7. Common Test Abnormalities and Correction Measures

Excessive Initial Settlement
Causes: Uncompacted pile top, incomplete consolidation of foundation soil, gap between pile top and bearing plate. Correction: Strictly level the pile top, pre-compact the foundation before test, and ensure full contact of the loading system.

Unstable Deformation Data
Causes: External site vibration, unstable counterforce system, unindependent displacement support. Correction: Isolate the test site vibration interference, fix the counterforce device, and adopt independent displacement monitoring frame.

Abnormal Rebound Deformation
Causes: Incomplete concrete strength development, local damage of pile body. Correction: Extend the maintenance period, eliminate unqualified test piles, and re-sample for testing.

8. Engineering Qualification Judgment Standards

After the static load test, if the measured characteristic value of vertical compressive bearing capacity of a single CFG pile is not less than the design requirement, the pile deformation is stable, and no sudden failure occurs during the loading process, the single pile is judged qualified. If individual test piles are unqualified, expand sampling and re-inspection shall be carried out. For unqualified batches, construction process adjustment, reinforcement treatment or batch rejection judgment shall be implemented to ensure foundation engineering safety.

9. Technical Summary

Single pile vertical compressive static load test is the core and authoritative detection method for CFG pile foundation quality evaluation. Standardized field layout, graded loading procedure, stable data acquisition and scientific curve analysis are the key to ensure accurate test results. The test results can truly reflect the pile-soil cooperative bearing performance, ultimate deformation resistance and engineering safety margin of CFG piles, provide reliable technical basis for foundation acceptance, construction quality evaluation and structural safety design, and effectively avoid foundation settlement, deformation exceeding limit and structural safety hazards in subsequent engineering operation.