Impact method and the device used in standard penetration test

Details Impact method and the device used in standard penetration test Impact method and the device used in standard penetration test

2000-01-12

2001-09-11

Schoeppel, Roger (Department: 3672)

Boring or penetrating the earth

Boring without earth removal

Combined with earth removal

C175S027000, C175S050000, C175S051000, C175S135000, C073S084000

Reexamination Certificate

active

06286613

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to an impact method and a device used in the Standard Penetration Test (SPT). The purpose of the point-contact impact method is to improve the impact efficiency of the SPT, while the purpose of a specially shaped anvil is to generate a point-contact impact condition. The present invention is able to improve not only the impact efficiency but also the accuracy of the SPT results.
2. Description of Related Art
Geotechnical engineers in the United States commonly use the Standard Penetration Test (SPT) in subsurface investigations for routine foundation design.
As a common practice, the routine foundation design is accomplished using the SPT results (the N-value). Almost all site investigations in some areas of the United States involve the use of the SPT. There is an American Society for Testing and Materials (ASTM) Standard Method (D1586-99) for performing the SPT entitled, “Standard Test Method for Penetration Test and Split-Barrel Sampling of Soils”. The ASTM Standard Method probably defines the SPT test for most engineering users.
The Standard Penetration Test (SPT) consists of driving a split-barrel sampling “spoon” or sampler a distance of 30 cm (12 in) after first “seating” the sampler 15 cm (6 in) by dropping a 63.5 kg (140 lb) hammer from a height of 76 cm (30 in). In field practice, the sampler is driven to a designated depth through a borehole using a long rod, and the hammer strikes the top end of the rod above the ground surface. The operator counts the number of blows that it takes to advance the sampler each of three 15 cm (6 in) increments. When the sampler has penetrated 45 cm (18 in) into the soil at the bottom of the borehole, the operator adds the number of blows for the second and third increments. This combined number is the result of the SPT and is called the “blow count” and is customarily designated as “N” or the “N value”. It directly reflects the penetration resistance of the ground or the soil under investigation.
The SPT is used as the primary soil descriptor in a geotechnical engineering (or foundation engineering) analysis and design. In most practices, the SPT is used in conjunction with other laboratory and field testing procedures and serves as an indicator of the soil profile. The SPT has been correlated with the soil's capacity to resist ground failure or excessive settlement once a new building is put on. Therefore, the N-values obtained on a specific site are very important criteria for engineers to evaluate the stability and the possible settlement of new building to be constructed. From a logical perspective, engineers have a basic understanding that a dense soil or solid ground should have a higher N value because of a higher penetration resistance. Previous experience has correlated the ground conditions of some soil types, such as sand and clay, to certain N values. The following table shows a typical correlation of the N value ranges and the corresponding ground condition.
SAND
CLAY
N Value
Ground Condition
N Value
Ground Conditions
—
—
0-2
Extremely Soft
0-4
Extremely Loose
2-4
Soft
 4-10
Loose
4-8
Firm
10-30
Medium Dense
 8-15
Stiff
30-50
Dense
15-30
Very Stiff
>50
Very Dense
>30
Hard
Previous studies have shown that the N value is closely related to the energy delivered to the rod by the hammer impact. In consequence, any significant variation in the energy transmitted to the rod from the hammer can result in a dramatic effect on the N value. For example, when the pounding effect or the energy transfer between the hammer and the rod is poor and the energy delivered to the rod is lower than what is expected, the resulting N value will be higher than it should be. This will seriously bias the engineer's judgement and lead to a faulty conclusion with regard to the subsequent design and construction. Due to the described shortcoming, an improved method with a specific device is thus proposed here to provide an effective impact technique to transmit consistent energy.
With the reference to
FIG. 8
, the conventional equipment used to perform the SPT consists of a rod (
80
), an anvil (
82
) securely mounted on the top of the rod (
80
), a sampler (
85
) attached to the bottom of the rod and a drop hammer (
86
). To carry out the SPT, the hammer (
86
) is lifted to a height of 30 in (76 cm) above the anvil (
82
) and the hammer (
86
) is supposed to “free-fall” drop to impact the anvil (
82
). The induced impact energy will then transmit through the rod (
80
) and down to the sampler (
85
). In this procedure, the stress wave of the impact can be measured by a sensor (
83
) attached to the rod with the signal received by the sensor transmitted to a computer (
84
) near by. The transferred impact energy is then calculated by the computer.
In the current practices, there is no specification for the shape and method of lifting and dropping the hammer (
86
). Various types and shapes of hammer are used, and many different methods of lifting and dropping the hammer are employed now. In the most recent ASTM D1586-99 standard, the only requirement for the hammer (
86
) is that it has a weight of 140 lb (about 63.5 kg), while the requirements for the lift-drop method only require that the hammer (
86
) be dropped in a free-fall condition at a height of 30 in (76 cm) from the anvil (
82
). As a result, significant variation in the energy transferred to the rod by the hammer impact often occurs simply because of improper operation in performing the test and, especially, some inherent mechanical problems. According to previous experience and the test results, approximately 40-90% of the ideal impact energy (475 joules) can be lost before and during the hammer impact due to various factors. In order to overcome the defects, some procedural and equipment improvements have been proposed. For example, a so call “safety hammer” is widely used in current practice. The safety type hammer is designed as a steel tube, which encloses the rod (
80
) and the anvil (
82
). Most of the weight of the hammer is contributed to the attached top cover of the tube, and the hammer impacts the anvil inside the tube. This hammer has improved the safety to workers during the SPT. The method of inside hammer impact also improves the accuracy of the hammer strike. Nevertheless, this method still leaves the energy transfer problems unsolved. Previous research has concluded that there are two main difficulties that effect the energy transfer and the energy transfer stability during the SPT:
(1) Mechanical friction during the test is inevitable in most mechanical designs.
(2) Inconsistent contact between the hammer and the anvil at impact.
In fact, the friction problem can be reduced to a very insignificant level if a good mechanical design together with proper operation can generate a nearly ideal free-fall of the hammer. The most difficult problem to overcome in the current practice is to effectively deal with the inconsistent contact problem. With reference to
FIG. 9
, the impact contact between the hammer and the anvil is suppose to be uniformly distributed on the entire anvil top, and the resultant impact force applied to the center of the anvil. Nevertheless, non-uniform contact is not uncommon in real practice, since the hammer is very likely to drop eccentrically due to imperfect mechanical control, i.e. the alignment of the hammer and the anvil and rod are not coincident. Accordingly, the resultant impact force and thus the energy transfer from the hammer to the anvil and rod are seriously affected. The efficiency and the inconsistency of energy transfer by the hammer impact may cause the results to differ greatly from SPT predictions. Hence, the present invention intends to provide an improved method and devices to mitigate and obviate the aforementioned problem.
SUMMARY OF THE INVENTION
The main objective of the invention is to provide a new impact method used in the Standard Penetration Test (SPT) to improve the impact efficiency and the energy transfer consis

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