The SPTMAN by JF Strainstall is an easy to use and portable system that is used to calculate hammer energy.
The SPTMAN system measures the actual energy transferred from the drive hammer to the drive rods and calculates the hammer energy coefficient by comparison with the theoretical potential energy.
The heart of the system is a rugged, portable analyser with solid state memory. The system is supplied with a standard 54mm diameter instrumented SPT rod, as well as a waterproof carry case for added protection.
Tests are carried out using as easy to follow menu display which enables hammer energy and energy ration to be displayed in real-time.
SPTMAN hammer energy tester: Frequently asked questions
Q: What is SPT hammer efficiency correction used for?
A: The SPT hammer energy measurement can be used to more accurately compare SPT hammer N values as the actual energy imparted by differ slightly from hammer to hammer.
Q: What is SPT hammer calibration?
A: This in practice is the measurement of the actual SPT hammer energy imparted to the top of the drill string and a comparison made with the theoretical energy for a standard SPT hammer to produce an SPT hammer energy ratio.
Q: What is SPT hammer energy ratio?
A: The energy ratio is the difference between the measured energy and the theoretical energy (473J). The measured energy will always be less due to friction and energy loss on impact. Typically for trip hammers the value is 60-70% and for window samplers 70-80%.
Q: What are the standard SPT trip hammer key dimensions?
A: The standard weight of an SPT hammer (falling weight) is 63.5KG and the standard drop height is 760mm, and this should generate a theoretical energy of 473J.
Q: How long should the SPT rod string be to carry out a test?
A: Ideally between 10-12m to allow the energy to be gathered before the base echo, in a cased borehole and with a gravel plug at the base.
Q: How is SPT hammer energy calculated?
A: The energy of an SPT hammer is calculated by using an instrumented rod and measurement equipment (i.e. the SPTMAN system) to measure the acceleration and strains developed in the instrumented rod, and then by calculation deriving the energy produced – effectively a form of stress wave analysis similar to used in pile driving analysis.
Q: Why do we do an SPT test?
A: BS EN ISO 22476-3 states the energy ratio of equipment carrying out Standard Penetration Tests has to be known “if N values are to be used for quantitative evaluation of foundations or for comparisons of results".
Q: What is the difference between a cone penetrometer test (CPT) and a standard penetrometer test (SPT)?
A: The CPT method pushes a steel cone into the ground and uses an electronic measuring system to record tip resistance and sleeve friction. As the cone is pushed into the ground, the soil responds with differing degrees of resistance and the resistance is recorded using force sensors in the tip. The SPT method is probably the most frequently used method of subsurface drilling investigation testing, and measures the number of blows for a standard hammer to drive the sampler into the ground at set depths.
Q: What is N value in SPT test?
A: The number of blows required to penetrate the first 15 cm is called “seating drive” and the total number of blows required to penetrate the remaining 30 cm depth is known as the “standard penetration resistance”, or otherwise, the “N-value”.
Q: How long does the SPT hammer calibration take?
A: Typically once the hammer is set up on the test drill string, about 15-20 minutes.
Q: How often should I carry out an SPT hammer calibration?
A: It is recommended that measurements are carried out annually, unless equipment has been modified or damaged, or as requested by the client. It is often the case that where multiple rigs are required on large contracts, SPT equipment to be used are calibrated at the start.
System service and calibration
The measurement of SPT hammer energy is now recommended for all Standard Penetrometer Test (SPT) systems.
BS EN ISO 22476-3 states that the hammer energy ratio has to be known and have a certificate of calibration available if N values are going to be used for the quantitative evaluation of foundations or for the comparison of results.
This standard also recommends that checks should be carried out on a six monthly basis. JF Strainstall recommends that it is also good practice to carry out a calibration at the start of any large contract.
Benefits of the SPTMAN hammer energy tester:
- Instant hammer energy measurement and transfer coefficient calculation
- Rugged and lightweight system
- Storage for over 700 results
- Complies with the relevant parts of both BS EN ISO 22476-3 and ASTM D 4633-10
- Backlit LCD screen for working in dark environments
- JF Strainstall's calibration service available
The SPTMAN analysis software is supplied as standard, and adapts a user friendly, Windows based platform for downloading data, analysing results and producing calibration certificates.
The certificate includes plots of force, velocity, acceleration and displacement; as well as measured energy, theoretical energy and energy ratio. Full details of the rig and site can also be shown.
The SPT hammer energy measurement method is based on the propagation of waves in long elastic cylinders. When the SPT rod is struck with a falling weight, the rod section is deformed (enlarged) and this enlargement travels down the pile to the base of the rod where it is reflected back.
The force transmitted to the rods is calculated by measuring the axial strain in the rod over a period of time and taking the modulus and cross section of the rod into account by:
F(t) = Aa x Ea x εm(t)
The particle velocity of the measurement section is determined by integrating data from the accelerometers over a period of time. The energy which passes into the drive rods can be determined from the force and velocity data using:
E(t') = t∫0 F (t) v (t) dt
SPT hammer calibration service
JF Strainstall offers SPT hammer calibration services whether on-site or at one of our specialist testing facilities located in the UK and Ireland.
It is recommended that SPT hammers are calibrated at 4 monthly intervals to ensure efficiency and comply with the latest EN Eurocode specifications.
JFTS' SPT hammer calibration service uses the latest SPTMAN system, designed specifically for SPT hammer energy measurements. We carry a range of adaptors to suit a variety of hammer threads and all of our equipment is fully calibrated to national standards ensuring high accuracy.
Calibration's can be completed at our specialist testing facilities and are fully compliant with BS EN ISO 22476-3 and ASTN D 4633-05. We provide free stamping to uniquely mark your hammers and certificates are available on the same day.
We also provide on-site testing services as well as equipment and training support, should you decide that you want to do your own testing.