Patent Application
20100018296
This invention relates to an improved penetrometer device for applying a hammering impulse force to drive a rod into the ground to a desired level.
A penetrometer is used for performing soil strength measurements in the field. The measurements obtained can be correlated with the engineering soil strength parameter such as the California Bearing Ratio (CBR), a widely accepted standard in civil engineering, or possibly with a physical soil strength parameter, such as a soil bearing strength. Soil strength measurements are used in the construction of paved and unpaved roads, airfields, and building foundations.
As an example of a prior device, a dynamic cone penetrometer (DCP) is described in U.S. Pat. No. 5,313,825, issued May 24, 1994, for performing soil strength measurement making use of a sliding hammer (one of two different weights) that is manually lifted and dropped onto a steel rod having a cone-shaped point. Each time the hammer is dropped, the rod penetrates deeper into the soil. The depth of penetration is measured with an integrated ruler and this data is later converted to an index that is then correlated to the CBR. The Dual-Mass DCP measurement meets the industry standards of ASTM D6951.
Prior devices also include an automated dynamic cone penetrometer (ADCP) which employs the same cone rod and hammer as the DCP device, but, in place of a human operator, a mechanism is used to automatically lift the DCP hammer. The hammer is then gravity dropped. The entire device is heavy and must be mounted on a trailer or other wheeled vehicle.
The disadvantages of the prior art are many. For the DCP device, repeated manual lifting of the sliding hammer causes fatigue on the part of the human operator (which in turn reduces the accuracy of the measurement because tired operator doe not lift the hammer all the way to the top) and/or requires multiple operators to avoid fatigue. Operators frequently injure themselves by getting pinched by the sliding hammer. Operators also require hearing protection because individual hammer blows are very noisy. The quality of the measurements is compromised by operator error in manually taking the depth measurements, particularly in cases where fatigue has set in. The measured data needs to be manually typed into the spreadsheet to obtain the CBR. This extra information makes the process more time consuming. The lighter of the two standard hammers is intended for use with weaker soils. However, operators without proper training may not be able to identify when the smaller hammer should be used. Furthermore, changing from one hammer to another is cumbersome and time consuming. Also, for extremely weak soils, the lighter hammer is still too heavy to produce the best possible results. The pressure wave developed in the device may also break the braze between the rod and the hammer assembly making the device useless.
For the ADCP device, the main disadvantage is the large size and mass of the unit. It must be mounted on a trailer or a small truck and this restricts the locations where it can be used. It also has the same disadvantages as the DCP device with regard to changing hammers and providing accurate measurements in very weak soils.
In the present invention, a light-weight, electronically-controlled hammering module is used to apply a repetitive hammering force under electronic control to the top end of a penetrometer rod. The hammering frequency is controlled by electronic controls to generate comparable impacts as heavy, gravity-dropped prior art devices by electronically moving a much smaller mass at a much higher frequency of impacts. In a preferred embodiment, the hammering module has a battery-powered percussive hammer that sits on top of the rod which, when activated, applies an electrically-generated impulse hammering force to the top of the rod. The hammering force is generated by electrically driving a small mass, e.g., 1 kg or less and even as low as 100 gm, at a controlled rate, e.g., of 5-50 Hz. The faster the rate of hammering, the more impact applied, so higher rates are used for stronger soils and lower rates for weaker soils. This application of force causes the cone-shaped point of the rod to penetrate into the soil at a controlled rate that is correlated with the strength of the soil. Therefore in this constant penetration mode, the impact energy is correlated to the soil strength. In the constant energy mode, however, the hammering frequency is kept constant, while the resultant penetration rate is correlated with the strength of soil.
Other objects, features, and advantages of the present invention will be explained in the following detailed description of the invention having reference to the appended drawings.
The application of the hammering force causes the cone-shaped point of the DCP rod to penetrate into the soil at a controlled rate that is correlated with the strength of the soil. The depth of penetration is determined by noting the decrease in distance between the initial height of the rod and its lower position as it descends into the ground. This distance is measured, for example, by a time-of-flight laser rangefinder (emitting a laser beam) that is mounted with the electronic module on the side of the percussive module and pointed at a suitable reflective surface placed on the ground adjacent to the location of penetration. Measurements from the laser rangefinder are transmitted by wire, or wirelessly, to a portable computer, where a software algorithm can compute the rate of penetration and in turn the soil strength profile. Control software will adjust the hammering rate to a level appropriate for the strength of the soil.
In the percussive module, the hammering force is generated by electrically driving a small mass at a controlled rate. For example, the mass can be of approximately 100 gm weight, and the rate of impulse driving can be of 5-50 Hz. The faster the rate of hammering, the more impact applied, so higher rates are used for stronger soils and lower rates for weaker soils. Thus, the rod can be driven into the soil with high frequency blows (in the range of 50 Hz) with low impulse energy (0.5-10 J).
The control software is adapted to read the depth data in real time and adjust the impulse of the hammering to keep the rate of penetration within a desired range, or to keep the rate of penetration constant. The software can consequently calculate, in real time, the penetration rate in blows per minute and compute the strength of the formation in California Bearing Ratio or some engineering property, such as soil bearing strength.
The automatically acquired depth measurements greatly reduce the amount of operator error and also speed up the determination of the soil's strength profile. The real-time penetration data are also used to instantly adjust the hammering energy to a level appropriate for the soil being tested, unlike the prior art, which requires a judgment determination and then several minutes to change from one hammer to another. Furthermore, the hammering energy can be made very small, to provide a more accurate correlation to CBR in weak soils than the prior types of DCP or ADCP hammers. The use of very small hammer impulses, which are controlled so as to maintain a constant rate of penetration, also confers on the invention the ability to measure actual engineering properties of the soil.
This invention has advantages over the prior types of penetrometers in that it is much easier to operate, requiring the user only to orient the unit upright such that the rod remains vertical, turn the unit on, and then maintain a light pressure to keep the unit vertical until the rod penetrates 2 inches into the soil. Thereafter the rod is guided vertically by the sides of the hole and no operator assistance is required. Its operation does not require the user to have superior physical strength and does not result in user fatigue. Additionally, the risk of injury to the user is much lower because the hammering mechanism is not exposed. The improved penetrometer only employs three easily assembled components. It is lightweight and portable enough to be carried by a single person, even to remote locations. It can be operated by a single person, while the prior art devices can require two or more. Use of the improved penetrometer, including set up, operation, and break down, takes significantly less time than the prior art devices. Being an automated device, the action of hammering the rod into the ground is subject to automated control, resulting in more consistent soil strength data and also less time for each individual test.
Other modifications and variations may be made in accordance with the circumstances of field use for which the penetrometer is to be employed. The control software may be encoded in a simplified form for rugged use, e.g., as a stored look-up table in read-only memory ROM, that is embedded with the electronic (control) module rather than operated on a separate computer. This would be advantageous for highly mobile use by a single operator over far-ranging distances.
The hammer mechanism can be powered pneumatically or by internal combustion, rather than battery-powered. A heavier hammer, in the range of 500-1000 gm, can be made to impact with higher energy, but at a slower rate, e.g., 1-5 Hz. This embodiment is referred to as an Accelerated Cone Penetrometer (ACP). The rod that is driven into the soil may be made of other materials, for example, titanium alloy or aluminum alloy, and other penetration point configurations may be used, such as 30° instead of DCP 60°.
The method of measuring the depth of penetration can be with any other non-contact method, such as an ultrasonic rangefinder, or it can be mechanical, making use of a wheel traveling along a guided track, or a string that is anchored to the ground and is retracted as penetration proceeds.
Instead of keeping impact energy or impulse the same and controlling the rate of penetration, an alternative approach is to keep the penetration rate constant by changing the impulse or impact energy. In this method, the impact energy or impulse could be correlated to either an engineering soil property or a physical soil property.
Instead of keeping the rate of penetration the same (or within the certain range) by controlling the impact energy or impulse, an alternative approach is to keep the impact energy or impulse the same and record the rate of penetration (as is done with the DCP and ADCP). In this method, the penetration rate could be correlated to either an engineering soil property or a physical soil property.
In
With each rotational cycle of the crankshaft 22, the free mass is pulled back to start the cycle and released to impact the anvil 28 and penetrometer rod. The frequency can be increased by increasing the motor rotational velocity. The impact energy is a function of the free mass and its velocity. In this manner, a small free mass, such as of 1 kg or lower (even as small as 0.1 kg), can deliver as much or more impact energy to the rod as the prior art DCP device by multiplying the frequency of impacts in a given amount of time. The free mass movement is essentially dictated by the air flow in the reciprocating system. When the reciprocating system is closed, the loss or addition of air to the system is essentially negligible and the system can be considered closed. In its closed state the free mass motion is couple to that of the cylinder. When the system is opened to outside air, the free mass is decoupled from the cylinder's motion and then travels until it strikes the anvil.
In
In
The Dynamic Cone Penetrometer (DCP) device of the prior art uses a heavy mass such as 8 kg that must be mechanically lifted and gravity-dropped to generate impacts/percussion. Because the hammer is gravity-dropped, the impact frequency is limited by the time it takes the hammer be lifted and dropped, typically a distance of 30 inches. The prior art SAPPER device, which automates lifting of the 8 kg DCP hammer can achieve a maximum practical impact frequency of the order of only 0.5-2 Hz (depending on how fast the hammer can be lifted).
In contrast, the present invention employs a light-weight, electronically controlled hammering module in which the hammering frequency is controlled by electronic controls to generate comparable impacts by moving a much smaller mass at a much higher frequency. This offers reductions in both the overall weight and size of the device. In the above-described embodiments of the electronically controlled hammering module, the impact frequency and total impact energy delivered can be readily controlled by varying motor or coil voltage and current only. Thus, an embodiment of the system shown in
The light-weight, electronically controlled hammering module can be used to test soil strength in real time and with high precision. As compared to the SAPPER/DCP device which uses a constant energy per blow to drive the rod into soil a depth that is measured to infer geotechnical soil properties, the electronically controlled hammering module can be operated in two modes: energy controlled and rate controlled. In the energy controlled mode illustrated in
In the energy controlled mode, the light-weight, electronically controlled hammering module can maintain a constant energy per blow and fix the frequency (like the Dynamic Cone Penetrometer (DCP)) to measure the soil penetration rate (inch/blow or inch/sec). After appropriate correlations, the penetration rate can then be reported in terms of a standard California Bearing Ratio (CBR). In the rate controlled mode, the light-weight, electronically controlled hammering module can maintain a constant soil penetration rate (like the Static Cone Penetrometer (SCP)) and measure the energy/force required. The energy/force measured can then be correlated to other geotechnical soil properties, such as bearing strength.
If the hammering module is vehicle deployable, the deployment system can use a load cell for maintaining a certain preload on it. The percussive energy, frequency and penetration rate could then be kept constant, while the preload can be increased or decreased. The force required to push the cone, while maintaining constant percussive energy and frequency could then be converted to bearing strength, akin to measuring bearing strength from a Static Cone Penetrometer. This approach is not possible with DCP device.
The above chart illustrates a penetration controlled mode when robotically deployed using a Z-axis drive and a load cell. The Z-axis drive maintains a constant penetration rate, while the load cell measures the penetration force required to maintain the constant penetration rate.
While certain embodiments and improvements have been described above, it is understood that many other modifications and variations thereto may be devised given the above description of the principles of the invention. It is intended that all such modifications and variations be considered as within the spirit and scope of this invention, as defined in the following claims.
This is a continuation-in-part application from U.S. patent application Ser. No. 11/756,604 filed on May 31, 2007, by the same inventors, which claimed the priority of U.S. Provisional Application No. 60/804,076 filed on Jun. 6, 2006, entitled “Percussive Cone Penetrometer and Accelerated Cone Penetrometer”.
The subject matter herein was developed for ERDC as part of the “Rapid In-Situ Soil Characterisation System”, funded through the Department of Defense SBIR Phase I program. The U.S. Government retains certain rights in the invention.
| Number | Date | Country | |
|---|---|---|---|
| 60804076 | Jun 2006 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 11756604 | May 2007 | US |
| Child | 12555776 | US |
