Automatic hammer system for standard penetration test

Details Automatic hammer system for standard penetration test Automatic hammer system for standard penetration test

2002-08-01

2003-06-10

Smith, Scott A. (Department: 3721)

Tool driving or impacting

Automatic control of power operated means

C173S089000, C173S124000

Reexamination Certificate

active

06575253

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an apparatus for carrying out a Standard Penetration Test (SPT) to determine the penetration resistance, geological distribution and nature of the soil, and more particularly to an automatic hammer system for a standard penetration test, which enables its hammer to fall from a precise predetermined height regardless of a penetration depth of a sampler, and is able to automatically carry out sequential test procedures such as counting the number of blows by the hammer and a penetration depth of a sampler according to the number of blows.
2. Description of the Prior Art
To undertake various civil engineering works and construction works, there is a need to first determine the penetration resistance, geological structure and geological composition of the soil by checking consistency and relative density of the soil by testing the soil of an area in question. To this end, a test procedure known as the “Standard Penetration Test” is commonly used.
The standard penetration test is a representative geological surveying test for estimating soil constants such as strength, relative density and angle of internal friction of ground in question, which is carried out as follows. A hammer of 63.5 kg is raised to a height of 75 cm and then released to fall and impact a split barrel sampler (referred to merely as a sampler, hereinafter), and this procedure is repeatedly carried out until the soil is penetrated to a depth of 30 cm by the sampler. Subsequently, an N value, which is the number of blows of the hammer counted until the sampler penetrates the soil to the depth of 30 cm, is calculated, and the soil constants of the ground are obtained from the N value.
In this test, the number of blows counted until the sampler initially penetrates the soil to a depth of 15 cm is regarded as a number of preliminary blows because the soil sample is believed to be disturbed, and the number of blows counted until the sampler further penetrates the soil to a depth of 30 cm from the level corresponding to the initial depth of 15 cm is determined as the N value for the soil in question. Where the number of blows counted until the sampler penetrates the soil to the depth of 30 cm exceeds 50, a depth of the soil penetrated after the hammer gives the sampler 50 blows is measured.
As a rule, though the standard penetration test must be carried out every 1.5 m under the current ground surface, the standard penetration test is carried out only once where the same geological formation continues underground.
Referring to
FIG. 1
, there is shown the most common apparatus for use in the standard penetration test, which uses a winch.
As shown in the drawing, a frame
1
is provided at its lower portion with a winding drum
2
fixed thereto, and is provided at its upper portion with a pulley
3
. A rope
4
is wound around the winding drum
2
for several turns and wrapped around the pulley
3
to be directed downwardly. A cylindrical hammer
5
is coupled to one end of the rope
4
, and slidably inserted over a vertical guide rod
6
.
The guide rod
6
is coupled at its lower end to a drill rod
8
, which is inserted into a boring hole (not shown) which has been previously drilled. The drill rod
8
is provided at its upper end with an anvil
7
mounted thereon, on which the hammer
5
impacts, and is provided at its lower end with a sampler (not shown) coupled thereto to obtain a disturbed soil sample. The guide rod
6
is provided with a marking which indicates a maximum lifting height at a certain height from the anvil
7
.
In an operation of the winch-type apparatus, the drill rod
8
, on which the sampler is mounted, is inserted into the boring hole of the soil, and then coupled to the guide rod
6
. Subsequently, the rope
4
is pulled by an operator to raise the hammer
5
to the lifting height (75 cm), and then released to allow the hammer
5
to free fall. Consequently, the hammer
5
falls along the guide rod
6
and impacts the anvil
7
.
Therefore, the impact of the falling hammer
5
is transmitted to the drill rod
8
through the anvil
7
, so that the soil in question is penetrated by the sampler coupled to the lower end of the drill rod
8
. This procedure is repeated until the penetrated depth reaches a desired value.
However, since such a conventional winch-type apparatus for use in the standard penetration test is required for an operator to check, with his naked eye, a lifting height of the hammer
5
during every lifting procedure, it is difficult to maintain a constant lifting height throughout all the striking procedures even though the test is carried out by a skilled person. Hence, the drill rod is applied with different impact strengths throughout the striking procedures.
Furthermore, since the hammer
5
is raised by the rope
4
, frictional loss is generated between the winding drum
2
and the pulley
3
during the falling of the hammer
5
. The frictional loss varies depending on the properties and age of the rope
4
, and actual impact strength applied to the anvil
7
is reduced to a value lower than the specified value.
Therefore, the conventional winch-type apparatus is inadequate to carry out the standard penetration test, and it is difficult to assure a precise measurement of an N value and to assure reliability of test results because of various factors.
In addition, since an N value obtained by the test is in an operator's memory, and a penetration depth of the sampler is obtained by an additional measuring procedure, an operator is apt to obtain incorrect test results, and considerably different test results may be obtained depending on operators even though the tests are carried out on the same soil sample.
To overcome the above-mentioned problems, a drive hammer system for a standard penetration test is disclosed in U.S. Pat. No. 4,405,020, which is adapted to enable a hammer to consistently fall from the same height, and to minimize frictional loss generated during the falling of the hammer.
The drive hammer system is slidably supported to an outer surface of a hydraulic cylinder via a pivot arm connected to a piston rod of the hydraulic cylinder. The hydraulic cylinder is vertically mounted on a drill rig. The pivot arm is rotated to a working position and raised by the hydraulic cylinder to be positioned over an impact surface of an anvil. When the drive hammer system is positioned over the anvil, a shutoff valve is opened to allow fluid in the hydraulic cylinder to be exhausted.
In this state, by actuation of a motor mounted on the cylindrical housing, a sprocket is rotated to cause a chain to be rotated clockwise. Lifting lugs on the chain are raised along a slot axially formed at the cylindrical housing by the rotation of the sprocket. At this point, the lug comes into contact with a lower end of a hammer received in the housing. As the lug pushes the hammer up, the hammer is gradually distanced from the anvil.
When the lug reaches the sprocket and begins to move outwardly, the lug moves from under the hammer, permitting the hammer to free fall until it strikes the impact surface of the anvil. By the striking action of the hammer against the anvil, a sampler penetrates the soil, thereby allowing the anvil to be lowered. At this point, the cylindrical housing free falls by the penetration depth of the sampler, and thus is placed on a flange of a drill rod, thereby maintaining a drop height at a certain value.
The drive hammer system itself is lowered by the penetration depth after every blow so as to maintain the drop height of the hammer at a certain value. However, since the drive hammer system strikes the flange of the drill rod soon after blows from the hammer (i.e. secondary blows), the sampler further penetrates the soil.
In addition, since the hammer is adapted to be raised by the lifting lug of the turning chain and to fall by release from the lug, the hammer may be raised to a position higher than the specified height by being struck by the lug in the course of turning

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