Abstract:
A parallel seismic tester utilizing a cone penetrometer to test the depth of a foundation or the like comprises three important elements: the cone penetrometer which houses a receiver, an impactor to impact the structure, and data gathering and analyzing equipment. The receiver may comprise a hydrophone, a geophone, or accelerometers. In the case where the receiver is a hydrophone, the hydrophone is embedded in a plastic, water filled container within the cone penetrometer head, and the head retracts prior to running tests.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to apparatus and methods for testing the depth of structures such as foundations using parallel seismic testing with a cone penetrometer to house the receiving element. 
   2. Description of the Prior Art 
   Parallel Seismic (PS) testing has been employed for such uses as determining the depth of an unknown foundation when the foundation top is not accessible or when the piles are too long and slender to be tested by echo techniques. Typically a borehole is drilled into the soil adjacent to the foundation, and the borehole is cased. In the case where the receiver is a hydrophone, the cased borehole is filled with water. In the case where the receiver is a geophone, several geophone receiver components are spaced apart in the borehole. 
   An exposed portion of the foundation is then impacted with a hammer or the like, and compression or flexural waves travel down the foundation and are transmitted into the surrounding soil. The receiver detects the transmitted signals. The depth of the foundation is indicated by a weaker and slower signal arrival below the tip of the foundation. 
   Parallel seismic testing is expensive and time consuming because the borehole must be drilled and cased (or at least braced in the case of a geophone receiver). 
   Cone penetrometers have been used to test soil conditions. For example, Hogentogler &amp; Co., Inc. builds a variety of commercially available cone penetrometer testers (CPTs) such as their Electronic Subtraction Cone CPTs. These units include cone tips each housing a strain gauge transducer and electronics for computing the detected strain and providing it to the user. Tips housing other transducers are also available. The CPT is mounted on a truck or track system, which includes, for example, hydraulic cylinders for driving the CPT cones into the earth. 
   A need remains in the art for apparatus and methods for doing parallel seismic testing in a quicker, more convenient manner. 
   SUMMARY 
   The present invention comprises three important elements:
     (1) a cone penetrometer which houses a receiver;   (2) an impactor to impact the structure; and   (3) data gathering and analyzing equipment.   

   In the case where the receiver is a hydrophone, the hydrophone is embedded in the cone penetrometer head, and is exposed to water by a retractable sleeve or openings in the penetrometer casing prior to running tests. In the case where the receiver is a geophone or accelerometers, the retracting or perforated outer casing is not required. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  (prior art) is a side schematic view of a conventional parallel seismic testing device. 
       FIG. 2  (prior art) is a side schematic view of a conventional cone penetrometer. 
       FIG. 3  is a side schematic view of a parallel seismic testing device utilizing a cone penetrometer according to the present invention. 
       FIGS. 4A–4C  show preferred embodiments of the tester of  FIG. 3 , with a variety of receivers. 
       FIG. 5A  is a plot of sample data received by the processor of the tester of  FIG. 3 . 
       FIGS. 5B and 5C  illustrate two data points in the plot of  FIG. 5A . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  (prior art) is a side schematic view of a conventional parallel seismic testing device. Foundation  101  (or some element connected to the top of the foundation, such as a pile cap) is impacted by impactor  102  (a hammer or the like). Impact hammer  102  is typically an instrumented three pound hammer producing 2000–5000 pounds of force. The instruments record (among other things) the impact time (T 0 ) of the impactor, so that the propagation time of waves  110  can be measured. An alternative hammer  102  might comprise a steel sledge hammer, three to eight pounds, with an accelerometer mounted next to the impact location to record the impact time. 
   Compressional, shear, or flexural waves  110  travel down through foundation  101  and are transmitted into the surrounding soil  112 . Borehole  104  is drilled out and the drill bit removed. Borehole  104  may be cased or braced. Receiver  103  is lowered into borehole  104 . Borehole  104  must be cased if receiver  103  is a hydrophone, because it is filled with water. It may be cased or otherwise braced if receiver  103  is a geophone, to prevent soil from caving in. The transmitted signals are received by receiver  103  and provided to a processor  105 . 
   Processor  105  analyses the signals in the time domain and identifies direct arrival times of compression and shear waves, as well as their amplitudes. Generally the tests are performed every one to three feet within bore hole  104 . Parallel seismic tests can be performed on concrete, wood, masonry, and steel foundations. Processor  105  is typically a computerized data collection system capable of collecting time domain waveforms at a sample rate of 20 microseconds per point or faster. Typical data traces are 1000–4000 points long, with one set of traces collected per hammer impact. Typically, a sets of tests are performed at each probe depth, with all waveforms averaged together (about two to eight waveforms) to complete one test set per probe depth. A test set would consist of an averaged impact time trace (for the signal start time) and an averaged receiver time trace. 
     FIG. 2  (prior art) is a side schematic view of a conventional cone penetrometer tester (CPT)  201 . CPT unit  204  is a van which houses and transports the CPT equipment  201 , including hydraulic cylinders, mounted on a framework, driving push rods  203 , which are threaded together as needed to achieve the desired depth. Push rods  203  drive the CPT cones (probe tips)  202  into the earth  112 . Instrumented cone  202  is driven into the soil  112  to be tested. The instruments might determine pore pressure, tip resistance, and sleeve resistance for bearing and skin friction value determination. CPT  201  can also be used in a seismic piezocone test, wherein the earth is impacted and compressional and shear wave energy is measured by accelerometers or geophones in the cone. A plastic casing can be installed by pushing a dummy tip to the desired location, and then leaving the internal casing in the ground as the rods  203  are withdrawn. 
     FIG. 3  is a side schematic view of a parallel seismic testing device utilizing a cone penetrometer  301  according to the present invention. Rather than drilling a borehole and casing or bracing it, the cone penetrometer directly delivers the receiver  302  to the right depth. The cone  310  housing receiver  302  is steadily driven into the soil generally parallel to the shaft  303  to be measured. In this patent, the terms “shaft” and “foundation” are used interchangeably, and are defined to include foundations, piles, piers, caissons, footings, or other element of which the depth is to be measured. The shaft to be measured is typically formed of concrete, timber, steel, and/or masonry. 
   In one specific embodiment which has been implemented, a Hogentogler &amp; Co. Electronic Subtraction Cone including a Seismic Electronic Cone Penetrometer was pushed into soil adjacent to a foundation element to be tested with a Hogentogler CPT unit mounted on Caterpillar tracks. The CPT used two double acting hydraulic cylinders coupled by a platen that pushed and pulled the push rods connected to the cone. 
   Periodically, as the cone  310  is being driven downward into the soil, foundation  303  is impacted by impactor  304  (a hammer or the like). Compressional, shear, or flexural waves  110  travel down through foundation  303  and are transmitted into the surrounding soil  305 . The transmitted signals are received by receiver  302  and provided to a processor  306 . Processor  306  analyses the signals in the time domain and identifies direct arrival times of compression and shear waves, as well as their amplitudes. 
     FIG. 4A  shows a side schematic drawing illustrating one preferred embodiment of testing device  301 , which utilizes a hydrophone  302 A for receiver  302 . Periodically during the time cone  310  is being driven into the soil, the pushing element pauses and allows metal cone penetrometer tip  307 A to open and withdraw slightly to uncover plastic inner casing  308 . Inner casing  308  is filled with water surrounding hydrophone  302 A. Shaft  303  is impacted and hydrophone  302 A measures the arrival time of the generated waves in the soil. Then tip  307 A lowers and surrounds casing  308  and cone  310  continues its journey into the soil. 
     FIG. 4B  shows a second embodiment which utilizes a geophone  302 B as the tip transducer to act as the receiver. A geophone measures movement or vibrations of the surrounding earth, for example by using the motion of a spring supported coil in the field of a permanent magnet to generate an output signal.  FIG. 4C  illustrates a third embodiment of the present invention which includes an accelerometer  302 C as a receiver. An accelerometer measures acceleration, for example by measuring the displacement of a mass connected to a spring. In the case where a geophone or an accelerometer is used, tip  307 B,  307 C does not generally need to be retracted while the measurement is made. The movement (pushing) of cone  310  may be paused while each measurement is made, or the measurements may be taken while the cone is moving. 
   In all cases, receiver  302  is detecting the arrival of waves  110  which have travelled down shaft  303  and transmitted through the soil. The amount of time between the impact and the detection of the wave is used to detect where the shaft ends, as is shown in  FIG. 5 . 
     FIG. 5A  is a plot of sample data received by processor  306 . Arrival time T increases slowly with depth until the end of foundation  303  is reached. Then arrival time increases much more quickly. As shown in  FIG. 5B , time T 1  is measured before the end of the shaft is reached, so it is on the shallow part of the curve. As shown in  FIG. 5C , time T 2  is measured after tip  302  has extended beyond the end of the shaft, so it is on the steep part of the the curve. Other analysis may also be performed, including amplitude and phase of signals sensed above, at and below the foundation bottom to determine its depth.