Method Statement of Piezocone Penetration Test
Piezocone Penetration Testing (from now on represent as “CPTu”) is a quick, accurate, and efficient process for site characterization, stratigraphic profiling, soil engineering parameter evaluation, and geotechnical design for soft ground. It is also called as Static Cone Penetration Test (CPT).
An electronic steel probe is hydraulically driven in CPTu to take continuous ground resistance measurements with electronic internal sensors. Penetration depth, cone resistance, sleeve friction, and pore pressure are measured. These readings enable an understanding of ground conditions. The data is instantly transferred to a field computer and can be utilized to assess the geo-stratigraphy, soil types, water table, and engineering parameters of the ground.
EQUIPMENT AND PROCEDURE
Ground improvement is done to limit the possibility of major deformations in soft clay during static or cyclic loads. When saturated clays are subjected to static, cyclic loading caused by mechanical vibrations or an earthquake, they tend to shrink in volume due to consolidation settlement.
If drainage is not available, generated pore pressure decreases after load application, and long-term consolidation settlement occurs, increasing maintenance costs. Under the existing shear stresses, the softened soil deforms, generating lateral and vertical movement of structures and sloping land.
The prefabricated vertical drain with preload reduces produced surplus pore water pressure by providing a drainage route and achieves consolidation settlement in a short period of time.
PiezoCone penetration tests (CPTu) were utilised to determine the necessity for ground improvement in the untreated ground and the degree of improvement achieved in partially improved ground.
The PiezoCone Penetration Testing (CPTu) technique determines the mechanical and transport characteristics of soils quickly. The CPTu approach delivers continuous profiles of soil parameters that are accurate, quick, and cost-effective.
The following components comprise a CPTu system: (1) an electrical penetrometer, (2) a hydraulic pushing system with rods, (3) a cable or transmission device, (4) a depth recorder, and (5) a data collection unit.
(1) PIEZOCONE and PUSHER
The cone penetrometer is a three-channel instrumented steel probe (Envi manufacture) that measures cone tip stress (qc), sleeve friction (fs), and penetration porewater pressure (u). The front end is comprised of a 60o apex conical tip with a small lip (about 5 mm) at the top section. The penetrometers utilised were 35.7mm in diameter, with a cross-sectional area of Ac = 10 cm2 and a sleeve area of As = 150 cm2. The PiezoCone was penetrated at the typical rate of 20mm/sec using a 20-tone hydraulic pusher. The reaction was utilised by using four earth anchors.
(2) CONE RODS
Cone rods are one-meter-long hollow steel rods with tapered threads that have an outside diameter of 35.7mm. To facilitate pushing operations, a friction reducer was provided.
The friction reducer is just an extended piece of rods on the sub-connector above the penetrometer (e.g., a ring welded to the outside rod) that widens the pushed hole to a larger diameter, minimising soil contact on all of the upper rods.
Memocone Preparation
1. Remove/Unscrew the cone
2. Remove the front seal O-rings from the sleeve
3. Remove sleeve
4. Check all O-ring assemblies
5. Remove any soil or dirt
6. O-rings should be replaced as needed. O-rings on the left side were changed when they were damaged or worn, while O-rings on the right side were replaced after each push.
7. Apply O-ring lube or Envi CPT grease to all sealed seat areas
8. Check for any moisture behind the sleeve assembly
9. Grease the tip of the cone. Check that there is no air trapped in the grease and that it seeps out around the entire slot. Remove excess grease and set aside.
10. Be sure that the pore pressure chamber is clean and fill it with CPT oil to approximately ¾.
11. Sweep all inner surfaces with a small probe or dental tool to remove any void spaces or bubbles. The removal of all bubbles from the internal space between the transducer and the filter element is crucial to the data collection process.
12. Wrap a considerable piece of paper around the sleeve and use it to hold the probe. This is used to absorb excess oil while fixing the cone tip.
13. Thread the cone tip into position. The excess oil will now cause flooding, and the paper will be useful in soaking it up together with the grease that is coming from the cone tip. The cone merely has to be tightened by hand or gently using a slip joint plier or equivalent. When tightening the cone tip with a tool, take precautions to avoid damaging the groove at the cone base.
14. Unlike probes with porous pore pressure filters, the probe may now be stored in any position without losing saturation.
15. After wiping away extra grease and oil from the probe, connect the battery tube (with or without the acoustic data transmission unit). Start with the battery plug.
16. Then, depending on the type of batteries you have, fill the battery tube with 5 or 6 of them and screw it into the probe. Twist the battery tube until the two diodes on the probe are barely visible and make sure the green diode is lighted. If the red diode illuminates, the battery power is insufficient. Then, securely screw the battery tube onto the probe, and if you have the acoustic device, make sure you hear a clicking sound.
(3) DEPTH LOGGER
Depth increments were measured above ground using depth wheels during CPTu.
(4) DATA TRANSMISSION AND CABLING
The cableless (wireless) digital CPTu system was utilised to receive and store data from the PiezoCone: (a) audio-transmitted signals; and (b) data saved on a battery-powered microchip until the penetrometer was recovered at the surface. A special receiver is required uphole at the top end of the rods to capture the signals and decode them for digital output with the above infrared and acoustic transmissions.
(5) DATA ACQUISITION SYSTEM
To acquire and display the data, fully digital systems with ruggedized notebooks and micro-chip technologies with memory within the cone penetrometer itself were used.
CONCLUSION
The PiezoCone test provides information on the value of cone resistance (qc), soil friction along a side sleeve (fs), and pore water pressure (u). The water pore pressures measured are hydrostatic water pore pressure and excessive water pore pressure caused by PiezoCone cone penetration (CPTu).
The reaction of water pore pressure, which is recorded in the pore’s element, may be used to determine the kind of soil and its behaviour (drained or undrained). As a result, the soil profile may be precisely calculated. The broad interpretation of other factors such as soil layers and bearing capacity is based on empirical correlation.
REFERENCES
- U.S. National Report on CPT by Paul W. Mayne Jay A. Auxt, Georgia Hogentogler & James K. Mitchell.
- Manual for PC-Mon Data logger for CPTU Memocone type II by Environmental Mechanics AB.
- Robertson P. K., Guide to Cone Penetration Testing for Geotechnical Engineering, Gregg Drilling and Testing, Inc. 5th Edition, Nov.2012.
- Robertson P.K., Interpretation of Cone Penetration Tests – unified approach, Can Geotech. 1, 46(11): 1337-1355 (2009)
- GeoLogismiki Geotechnical Software, CPeT-IT v.1.7.6.19
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