Non-Working Pile Testing and Static Compression Load Test

Purpose of Non-Working Piles

Non-working piles are constructed at the beginning of the project to evaluate soil bearing capacity and determine the maximum settlement. These piles are loaded up to twice the working load. Based on the test results, the pile length may be adjusted. If the measured settlement exceeds the allowable limit, the pile length will be increased accordingly.

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Static Compression Load Test

The static compression load test provides insight into how the pile responds to loading. Settlement is measured at different load increments to determine the appropriate pile design length. The maximum applied load is typically twice the working load (2 × working load) or as specified by local standards. The following steps outline the testing procedure:

A. Steel Cage Preparation

• The steel reinforcement cage is prepared in accordance with the approved shop drawings.
• The spiral reinforcement extends the full length of the cage to prevent concrete bursting at the pile top under applied loads.

B. Strain Gauge Installation

• Strain gauges are fixed to the steel cage at four levels: near the pile head and near the pile toe.
• Each level contains four strain gauges, with an accuracy tolerance of 1 µ strain.

C. Pile Casting and Preparation

• The pile is cast, and the pile head is leveled to ensure a flat surface for jack installation during testing.

D. Loading System

• Compression loads are applied directly to the pile using a properly designed reaction system.
• Hydraulic jacks are used for load application.

E. Load Measurement

• Pressure gauges measure the applied load.
• All pressure gauges must be calibrated before testing, with valid calibration certificates available.

F. Loading Sequence

• The compression load is applied in increments, starting from 0% and increasing up to 200%.
• The pile is tested only after the concrete has attained at least 95% of its required strength.

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Test Arrangements

Two main test arrangements are used for applying the load:

1. Kentledge System

• Concrete blocks are used to balance the applied load.
• This method requires a significant number of blocks and occupies a large space.

Figure 1

2. Self-Reaction System

• Used in locations where space is limited.
• Strands are inserted and grouted into the ground, then fastened to a crown beam to transfer the applied load.

Figure 2

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Test Instrumentation and Setup: 

Pile Steel Cage Preparation

• The spiral reinforcement extends to the pile’s top level to prevent concrete cracking or bursting under test loads.
• some times, four levels of vibrating wire strain gauges are attached to the reinforcement cage, each with four equally spaced gauges.
• Strain gauges measure the pile strain at different levels, with readings taken near the cut-off level and at the pile bottom.

Load Calculation Formula:

$$\text{Load} = \text{strain} \times \left( (\text{Area of Steel} \times E_s) + (\text{Area of Concrete} \times E_c) \right)$$

Reference Beam

• After the concrete reaches the required strength, the test area is excavated and leveled.
• A reference beam is installed around the pile and securely fixed to prevent movement during testing.
• This beam serves as a reference for measuring pile displacement.

Displacement Transducer (if specified)

• An electrical device used to measure pile shaft movement in a single direction.
• Three displacement transducers are installed at equal spacing around the pile to monitor displacement.
• Transducers must be periodically calibrated.

Dial Gauges

• Dial gauges are used to measure pile settlement.
• These must be periodically calibrated as per the manufacturer’s recommendations.

Hydraulic Jacks

• Loads are applied to the pile head using hydraulic jacks.
• For high-load tests, multiple jacks may be used.
• A thick steel plate is placed between the pile head and the jacks to prevent damage.
• Jacks must be in good condition and calibrated before use.

Load Cell

• A transducer used to verify the applied load on the pile.
• Load cells require periodic calibration.

Figure 3

Figure 4

Figure 5

Test Failure Criteria

A pile is considered to have failed if:

• Settlement exceeds 1% of the least pile dimension.
• Residual settlement after rebound exceeds 6 mm.

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