Method and device for pore pressure sounding
BACKGROUND OF THE INVENTION embodiment 
     This invention relates to a method and a device for pore pressure sounding. For earlier methods of sounding soil layers, various types of sounding equipment have been used, in which the appliance is fitted with a sounding probe. In order to find out the structure of a soil layer, a registration has been made of the penetration force. 
     PURPOSE AND SUMMARY OF THE INVENTION 
     The purpose of the present invention is to provide an entirely new method of sounding soil layers, in which variations in permeability in different soils are used for determining the structure (stratigraphy) of a soil layer. 
     This is done by using a method of pore pressure sounding according to which a sounding probe containing a filter and a pressure transducer is continuously driven down into a soil layer with simultaneous registration, via the filter and pressure transducer, of the pore pressure generated by the intrusion of the sound. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention will be described below with reference to the accompanying drawings showing an embodiment of the device for carrying out the method described above, in which FIG. 1 shows schematically a complete device for pore pressure sounding, and 
     FIG. 2 shows a section through the sound. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows an embodiment of the device for pore pressure sounding. The device includes a frame 1 which supports a screw jack 2. The screw jack 2 includes a screw 3 and a worm gear 4. By rotating an axle 5 which forms part of the worm gear, the screw 3 can be made to move vertically upwards or downwards. At the lower end of screw 3 a sounding rod 6 is attached, which bears a sound 7 at its lower end. 
     The movement of screw jack 2 and sounding rod 6 is monitored by a recording instrument 8. This instrument includes a separate pulse generator 9, which is connected to the drive axle 5 of the worm gear 4. The pulse generator 9 is connected to a recorder 10, which is also connected to the sounding rod 6. The pulse generator 9 delivers pulses to a step motor (not shown), which drives a strip of paper in the recorder 10. The measurement signals from the sound 7 are registered on this paper. The number of pulses per time unit delivered by the pulse generator is dependent on the speed of rotation of the drive axle 5. As there is a well defined relation between the vertical movement of the screw 3 and the rotation of the drive axle, it is therefore possible to obtain a well defined relation between the length of paper driven through the recorder 10 and the depth to which the sound 7 is driven. 
     FIG. 2 shows at a larger scale an emboidment of the sound 7 shown schematically in FIG. 1. As shown in FIG. 2, the sound 7 has an upper cylindrical part and a lower conical part. At the lower end of the conical part of the sound 7 there is a filter 11 of stainless steel mesh. This filter 11 is connected via a duct 12 in the sound 7 to a pressure transducer 13. This pressure transducer 13 is then connected to the recorder 10 via an electric cable 14 in the sounding rod 6. 
     The method of pore pressure sounding with the device described above is as follows: The sound 7 is driven downwards at a constant speed by of the screw jack 2 into the soil layer to be investigated. The penetration of the sound 7 into a soil layer generates an expansion pressure. This expansion pressure is composed of a generated pore pressure due to fluids in the soil pores moving to and from the probe and a generated effective pressure due to pressure from the soil particles. The filter 11 conveys the total pore pressure present in the soil, which consists of the sum of the generated pore pressure and the pore pressure in the surrounding undisturbed soil, to the pressure transducer 13, thereby producing a signal which passes via the electric cable 14 to be registered by the recorder 10. 
     As different generated soils have different permeability, different pore pressures will be encountered at different levels during the sounding. In normally consolidated clay, for example, a high excess pore pressure will be encountered, as clay has a low permeability. Sand, on the other hand, has a high permeability, which means that the generated pore pressure is insignificant compared to that encountered in normally consolidated clay, for example. The sound 7 is driven down continuously through a soil layer with simultaneous recording of the generated pore pressure via the recording instrument 8. 
     If the pore pressure sounding described above is made in a clay which contains permeable layers of, for example, sand, these will be recorded as sudden drops in generated pore pressure as the sounding probe moves through these layers. The generated pore pressures, which are continuously recorded, are thus transient and a function of the character of different soil types, especially of permeability. By studying the recorded variations in generated pore pressure, it is therefore possible to obtain a picture of the structure (stratigraphy) of a soil layer. 
     This invention is not limited to the embodiment described above. For example, the sound can have a modified shape differing from that shown in FIG. 2 without departure from the idea of the invention. Also, the penetration and recording devices can have another construction than the one shown. However, the condition which must apply to these devices is that the penetration of the sound be done continuously with simultaneous registration of generated pore pressures at a speed which is sufficient to ensure that the generated or transient pore pressures are recorded. The invention can thus vary within the scope of the following patent claims.