Abstract:
A method for determining allowable bearing pressures of a steel footing on variable subsurface materials whether native soils, rock, or manmade construction material is disclosed. The method continuously measures vertical displacement by an optical technique of a dismantable steel footing under the impact of a free falling steel drop weight. The load pulse of the drop weight is measured by means of a load cell.

Description:
BACKGROUND 
     Advances in site investigation techniques have afforded engineers tools to assess the suitability of shallow foundations designs with regards to the reliability under design load cases and economic considerations. Full scale, static load tests have been performed as part of research studies, but do not enjoy widespread use as acceptance criteria for constructed foundations in the profession. The operational expenses are one factor that accounts for the reluctance of engineering professionals to rely on footing load tests. Dynamic load tests provide benefits similar to a full scale load test. A dynamic test method is described using a specially designed apparatus. The apparatus described consists of an easily dismantable steel footing  100  with associated accessories for safely delivering a dynamic load and measurement of the load and footing response. All of the system components in the apparatus are capable of being reused multiple times at different locations and project sites. The associated electronic instrumentation allows direct measurement of the vertical displacement  99  and load pulse at the precision and accuracy required by the engineering profession. 
     SUMMARY OF THE INVENTION 
     The test method consists of the assembly of a steel footing  100 , anvil  200 , and safety frame  300  at the test location. Electronic instrumentation is fastened at designated points on the footing  100  and within the anvil  200 . A steel drop weight  400  is assembled by addition of steel plates  480  to a drop weight frame  430 , which frame  430  is secured together using bolts  486  welded onto a lower plate  485  of the frame. Assembly of all steps is performed by hand unaided by any special lifting equipment. The assembled drop weight frame  430  is lifted above the steel footing  100 , within the confines of the safety frame  300 , and dropped onto the anvil  200  at a height of up to 1.5 m. The load pulse generated by the free falling drop weight  400  striking the anvil  200  is measured by a load cell  220  embedded within the anvil  200 , and a vertical displacement  99  of the steel footing  100  is measured by a position sensitive detector  140 . The acquired signals from the load cell  220  and position sensitive detector  140  are used to generate a plot of load and vertical displacement  99  with time. A qualified geotechnical engineer inspects the plot. In light of other data, such as engineering properties of the subsurface material, the geotechnical engineer will determine allowable performance criteria to assess if the vertical displacement  99  is satisfactory under the imposed load. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates the setup of the primary components including the steel footing  100 , the anvil  200 , the safety frame  300 , and the drop weight  400 . The primary components consist entirely of steel sections including plate, angle, and channel sections as well as associated nuts and bolts. The secondary components consist of electronic instrumentation and acquisition devices (the position sensitive detector  140 , laser  150 , and load cell  220 ), as well as the quick release hook  410 . 
         FIG. 2  presents a plan view of the footing  100  as well as plan and section views of the steel drop weight  400 . 
         FIG. 3  presents perpendicular sectional views of the steel footing  100  through the center of the footing. These sectional views clarify the fastening points of the steel members ( 110 ,  120 ,  130 ) as well as the location of the electronic instrumentation (the position sensitive detector  140 , and load cell  220 ). 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     At the discretion of the project engineer, soil conditions at the intended test location  50  are determined by collection of soils samples by soil boring, test pit, or bucket sample. Further material testing is additionally performed to classify the subsurface material and determine strength/compressibility properties. 
     The steel footing  100  beams  120  and  130 , drop weight plates  480 , and electronic accessories are delivered to the site location. When unassembled, the maximum weight of any single steel member is 150 kg. This allows a crew of four laborers to move pieces around the site location by hand. A boom truck is a convenient vehicle to use as it allows mobilization of steel members to a project site by road and facilitates unloading of the vehicle. The steel members are placed around the intended test location as designated by the project surveyor or engineer. The steel footing  100  and drop weight  400  are assembled by hand. Footing  100  comprises upper channel (UPN) beams  130 , lower UPN beams  120 , and bottom plates  110  bolted together via bolts  135  as shown in the Figures. An optional 7 T (ton) crane  420  will facilitate the work of the labor crew but is not expressly required. The footing  100  and anvil  200  are fastened together, with anvil  200  comprising a lower steel grooved plate  210  and an upper steel plate  230  held together and fastened to the upper UPN beams  130  of the footing  100  by bolts  235  as shown in the Figures. Steel angle and channel sections are comprised of Grade 235 steel or any mild steel with a minimum yield strength of 235 MPa. Steel plate sections are comprised of Grade 509 steel or any carbon steel with a minimum yield strength of 509 MPa. The load cell  220  is placed within the anvil  200  between steel plate  230  and steel grooved plate  210 , and the anvil  200  is fastened to the footing  100  by the bolts  235 . A slab of plywood or neoprene 1″ thick (not shown) is optionally placed on top of the anvil  200  within the dimensions of the safety frame  300  to assist distribution of the load from eccentric impacts. 
     The load cell  220  has a minimum capacity of 80 T. The use of a load cell  220  with a capacity larger than the anticipated load compromises the resolution and accuracy of the load measurement but reduces the possibility of deformation of the load cell housing during multiple tests under repeated impacts. The resolution of the load cell  220  is less than 0.1% of the full scale load. The accuracy of the load cell  220  is not larger than ±1% of the full scale load. The analog output of the load cell  220  ranges between 1 to 5 mV/V. 
     The position sensitive detector (PSD)  140  is fastened to the footing  100  using a 6 mm diameter stainless steel bolt. The fastening point is the flange of a beam  120  or  130  spanning the steel footing  100 . A steel nut is welded to the flange to secure the bolt fastened to the PSD  140 . The analog output of the PSD  140  ranges between 1 to 10 V. The typical resolution of the PSD  140  ranges between 500 to 800 nm. The red laser  150  is a Class II laser generating a laser pulse at a wavelength of 635 nm and at a power less than 1 mW. Shortpass and longpass optical filters with dielectric hard coatings are fastened using adhesives to the PSD  140 &#39;s housing to block light illuminating the PSD  140  at wave lengths less than 635 nm and greater than 670 nm. The red laser  150  is mounted on a short tripod  151  at a minimum distance 5 m away from the footing  100  and the laser beam  152  is aligned to shine on the PSD  140 . 
     The acquisition system (not shown) is connected to the PSD  140  and the load cell  220 . The acquisition system for the PSD  140  consists of an analog to digital converter. The acquisition system for the load cell  220  consists of an analog to digital converter using a full bridge strain module. The full bridge strain module is capable of 24 bit resolution and delivery of a suitable excitation voltage on the order of 1V to 10V to the load cell  220 . The acquisition system uses screw terminals to facilitate connection of instrumentation data cables to the acquisition terminals. The converted digital signal is acquired using a computer program on a laptop computer (not shown). Prior to commencing the test, the digital signal is checked to ensure that the instruments are responsive, and that measurements will remain within the instrumentation range. Review of the signal ensures the initial load cell  220  reading is a zero load signal. Sampling frequency of both analog signals is set to a minimum 5 kHz. 
     The acquisition system acquires converted digital signals simultaneously by using a single acquisition computer program on the acquisition laptop. 
     Once the steel footing  100  and drop weight  400  are assembled, a mobile crane  420  of a minimum 7 T capacity is mobilized to conduct the test. The crane  420  places the drop weight  400  on the anvil  200  and within the safety frame  300 . A lineman  95  attaches a release hook  410  to the drop weight  400  and the crane driver lifts the drop weight. The drop weight  400  is placed within the confines of the safety frame  300  and the release hook  410  is disengaged. The safety frame  300  ensures the drop weight  400  lands on the anvil  200  and does not rebound onto the steel footing  100 . The lineman  95  attaches the release hook  410  to the crane  420  and drop weight  400 . When the drop weight  400  is secured, the crane driver lifts the weight  400  using the quick release hook  410 . The quick release hook  410  will allow the drop weight  400  to drop in free fall onto the anvil  200 . The test arrangement prior to release of the drop weight  400  is illustrated in  FIG. 1 . 
     A seating drop at about 5 cm above the anvil  200  is performed. When the drop weight  400  is at the required height, the lineman  95  pulls the release line  96  and the drop weight  400  is released. The drop weight  400  strikes the anvil  200  and remains within the safety frame  300 . 
     An additional three drop heights are designated by the project engineer. In the absence of such an instruction, the nominal drop heights of the drop weight  400  above the anvil  200  are 0.5 m, 1.0 m, and 1.5 m. Following the completion of the third drop height, the test is concluded. The supervising engineer performs additional drops at varying heights and at their discretion. Following conclusion of the test, the equipment is unfastened and loaded into a vehicle for demobilization from the location. 
     The embodiments of the invention which are claimed are described in the following section.