Abstract:
A constant-head soil permeameter for determining hydraulic conductivity of earthen materials is inserted into a borehole at the desired test depth. A calibrated reservoir, disposed on the ground surface, is attached thereto with a suitable length of hose. Water is added to the calibrated reservoir and allowed to flow freely into the borehole until an equilibrium level is reached in the borehole and inside the soil permeameter. The water flowing to the permeameter is throttled by buoyant float pressure that is greatly increased by a lever-and-link valve control assembly which provides considerable mechanical advantage, thereby allowing better constant head control and much greater depths of testing than previously attained by known permeameters. A filtered vent system, backflow check valve, and seals restrict entry of soil particles and debris, thereby minimizing cleaning and maintenance of the invention. The soil permeability is determined by solving appropriate mathematical equations which utilize the equilibrium height of water, rate of water flow, and dimensions of the borehole as input parameters.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to the hydraulic conductivities of liquids through permeable materials and particularly relates to the conductivity of water through earth. It also relates to testing such conductivity from the surface of the earth to great depths beneath the surface and above the water table while preventing contamination by falling soil and debris. It more particularly relates to instruments that establish a static head of water within a borehole and maintain the water at this predetermined level by use of a float and valve system. It specifically relates to a float and valve system that provides a mechanical advantage ratio enabling use at such great depths. 
     2. Review of the Prior Art 
     It is often important to estimate the hydraulic conductivities of earthen materials in order to safely and economically develop lands for urban and agricultural uses. Hydraulic conductivity values are important considerations in design and construction of building and roadway foundations, on site sewage wastewater treatment systems, and storm water infiltration facilities. These values are important for artificial treatment of wetlands, and for estimating the rate of transport of liquid contaminants from waste disposal sites and leaking storage tanks. Hydraulic conductivity values are additionally important in design of irrigation systems and drainage of agricultural lands. 
     Soil hydraulic conductivity can be used to describe the ability of earthen materials to transmit water. Darcy&#39;s Law describes the relationship of the volume of water, moving through a cross sectional area of soil (commonly known as flux) along the hydraulic gradient of the water flow path, to the hydraulic conductivity. Under saturated conditions, such as below a water table, hydraulic conductivity is referred to as saturated hydraulic conductivity. Even though Darcy&#39;s law was originally developed to describe saturated flow, the principles of the law can be applied to water movement in partially saturated soils above the water table. 
     The determination of hydraulic conductivity under field conditions can be complicated because of the natural variation of soil properties and the specific need for which the test is being conducted. Soils typically contain multiple contrasting layers and often exhibit significantly differing hydraulic conductivity values along preferential flow paths within the soil matrix. 
     Prior art instruments developed for measuring hydraulic conductivity of soils above the water table in the field have generally fallen into three groups. The first group introduces either a ponded static (i.e., constant) or a variable (i.e., falling) head of water into the bottom of an unlined borehole below the ground surface or into a confining ring in contact with the ground surface. Instruments that establish a static head of water within a borehole maintain the water at a predetermined level, usually by use of either a float and valve system or a marriott tube system. The rate of water flow necessary to maintain a constant water level in the borehole at the predetermined level is utilized to estimate hydraulic conductivity of the soil. Methods used to measure the saturated hydraulic conductivity in a borehole utilizing a constant head of water have been referred to as the shallow well pump-in technique or constant-head well permeameter. Instruments in this first group that utilize a falling head procedure usually measure the drop of water from a predetermined level in a lined or unlined borehole as it dissipates into the soil to estimate hydraulic conductivity. 
     The second group of instruments applies water through a semi-permeable membrane to a soil surface, which is under negative pressure (tension), to measure unsaturated hydraulic conductivity. The third group of instruments utilizes various methodologies, which include electrical resistivity procedures and gas or liquid injection into the soil through penetrating probes. The instruments in the third group typically require a power source, fluid or gas pumps, multiple chambers, borehole packers, electronic data loggers, and complex analysis procedures. 
     U.S. Pat. No. 6,105,418 discloses a constant-head float valve assembly which includes a J-shaped fluid conduit for intermittently delivering water from a supply container to a borehole. As the float moves downward with dissipating water levels, a shutoff valve is contacted and thereby opened to replenish the water in the borehole. The rising water moves the float upward and away from the valve, thereby allowing pressure of the incoming water to close the valve again. 
     U.S. Pat. No. 4,561,290 utilizes a float valve assembly, connected to a water supply reservoir, to regulate water inflow and obtain a constant water level within a borehole. The float responds to a rising water level by regulating water flow through a valve and thereby maintaining a constant water level. 
     However, neither of these devices incorporates an apparatus for magnifying the vertical force of the float body that is necessary for valve regulation at large depths and flow volumes, nor do they incorporate a backflow check valve to prevent incident entry of suspended soil particles and other contaminates into the float chamber. In addition, neither of these devices includes a means for eliminating the entry of contaminants through its air equalizing passage into the interior of the device. 
     Soil hydraulic conductivity has been historically measured on a smaller scale in the laboratory, utilizing a falling or constant head of water applied to soil core samples retrieved from the field or on remolded soil samples. Laboratory centrifugal force methods are also utilized to estimate hydraulic conductivity. Laboratory measurements are often significantly at variance with in situ field measurements because of the differing methodologies and the inherent difficulty of obtaining undisturbed soil samples and replicating natural environmental and stress conditions in the laboratory. 
     It is desirable to have the capability to conduct hydraulic conductivity tests at any depth in earthen materials above the permanent water table. Such depths may range from zero to many meters below the ground surface. In addition, it is desirable to have adequate flow capacity for maintaining flow equilibrium in a wide range of soils. Clay soils often have slow permeability, whereas sandy or gravelly soils often have high permeability and, therefore, a greater equilibrium flow rate. 
     Prior art inventions that utilize a float system alone do not provide a mechanical advantage ratio, thereby limiting testing to relatively shallow depths. Inventions utilizing the marriott tube principle to establish a constant water level are also limited to relatively shallow depths of testing. 
     A buoyant force is provided by a float in accordance with Archimedes&#39;s Principle which states that the buoyant force on a body immersed in a fluid is equal to the weight of the fluid displaced by that body. The displacement volume of any float of practical geometric shape that can fit in a small-diameter borehole is relatively small, therefore the depth at which such float can provide throttling of a valve by direct buoyant force alone is limited to relatively shallow depths and small flow rates. There is accordingly a need for an apparatus that is sufficiently rugged and versatile to measure hydraulic conductivities of soils inside a borehole at a variety of depths above the water table, ranging from shallow to deep. There is also a need for a device that can be used inside a borehole, wherein the device is subject to being struck by falling soil particles and debris, without contamination by such particles and debris through the air vent hole at its top or through water outlets at its bottom. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a simple and sturdy apparatus which functions as a constant-head soil permeameter for estimating saturated hydraulic conductivity of in situ earthen materials above the water table by establishing a constant head of water at a predetermined level in a borehole that is dug below the ground surface with ordinary hand auger equipment or with power equipment. 
     It is a further object to provide a soil permeameter that can, without incorporation of electronics, be effectively used to estimate hydraulic conductivity at desired test depths normally encountered above the water table and at depths much greater than the depths at which known devices that utilize a float system can be employed. 
     It is an additional object to provide a constant-head soil permeameter that can be effectively used to determine hydraulic conductivity within a wide range of soil permeability. 
     It is also an object to provide a soil permeameter that avoids malfunction in the field by minimizing contamination from soil particles and debris falling from the side of the borehole. 
     In accordance with these objects and the principles of the invention, the soil permeameter of this invention is an apparatus which incorporates a float and a mechanical linkage system that greatly increases the forces applied by the float to throttle water flow at the control valve. 
     The constant-head soil permeameter of this invention seeks to overcome disadvantages of other float systems by greatly increasing the buoyant force resulting from submergence of a float alone. The permeameter increases the buoyant force by use of a compound lever and link assembly, as a part of its float system, which provides a mechanical advantage ratio ranging from approximately 10:1 at full valve opening to approximately 60:1 at full valve closure. The resultant available maximum throttling force is, therefore, approximately 60 times greater than simple buoyant force at full valve closure. The effective testing depth range of the permeameter is from 15 centimeters to about 30 meters. The permeability testing range of the apparatus is from 10 −6  centimeters/second to 10 −2  centimeters/second. The range of water flow volume through the apparatus is from zero to 2000 milliliters/minute or more at depths greater than one meter. 
     This constant-head soil permeameter comprises a tubular cylinder having a top end, a bottom end, means for introducing a liquid into the top end, means for selectively closing the bottom end, and means for preventing falling debris and soil from entering the top end while enabling air to flow into and out of the cylinder, the top end and the bottom end being defined in relation to usage within a vertically disposed borehole in materials permeable to the liquid. The cylinder contains a float system that provides a mechanical advantage ratio for shutting off the introduction of liquid. 
     This float system comprises a compound lever and link assembly that functions as a valve control assembly and is hereinafter thus identified. It is particularly operative when: 
     A) the liquid is water, the materials are earthen, and the borehole has a bottom disposed above a water table in the earthen materials; and 
     B) the mechanical advantage ratio ranges from approximately 10:1 at full valve opening to approximately 60:1 at full valve closure. 
     The valve control assembly, described hereinafter with water as the liquid, comprises the following lever and link assembly: 
     A) a valve support bracket which is longitudinally disposed and rigidly supported within the cylinder, adjacent to the inner side thereof; 
     B) an actuating lever arm, having two ends, which is attached at one end to a first pivot which is attached to the valve support bracket; 
     C) a link, having two ends, which is attached at its lower end to a second pivot which is attached to but spaced apart by a selected distance from the first pivot; and 
     D) a valve seat retaining lever arm, having two ends, which is pivotally attached at one end to the valve support bracket and is pivotally attached at its other end to a pivot attached to the upper end of the link. 
     The top end of the cylinder comprises a top stopper, having an upper side and a lower side, which is rigidly attached to the cylinder and is encircled by an o-ring in sealing contact with the cylinder. The means for introducing water into the top end of the cylinder comprises a reservoir for containing water, a hose connection which is rigidly attached to the top stopper and projects outwardly from its upper side and has a bore therewithin, a hose for connecting the reservoir to the hose connection, and a valve body which is rigidly attached to the lower side of the stopper and has a bore therewithin in fluid communication with the bore within the hose connection. 
     The valve seat retaining lever arm comprises a valve seat which is attached thereto in facing relationship to the valve body and is adapted for selectively shutting off the introducing of water from the reservoir. 
     The cylinder additionally contains a buoyant float body that is axially movable within the cylinder and has upper and lower surfaces. The upper surface exerts pressure against the other end of the actuating lever arm when the float is supported by water within the cylinder. 
     The constant-head soil permeameter may be described as comprising the following lever and link assembly which provides a mechanical advantage ratio: 
     A) a valve support bracket having an upper pair and a lower pair of spaced-apart lugs attached perpendicularly thereto and projecting toward the center of the cylinder; 
     B) an actuating lever arm having one pair of spaced-apart lugs attached perpendicularly thereto at its pivot end and projecting upwardly, being attached to the lower pair by the first pivot; 
     C) a link having two pairs of the spaced-apart lugs attached perpendicularly thereto at the upper and lower ends thereof and projecting toward the valve support bracket, one pair being attached by the second pivot to the one pair of spaced-apart lugs on the pivot end of the actuating lever arm and being spaced from the first pivot by a selected distance; and 
     D) a valve seat retaining lever arm having two pairs of spaced-apart lugs attached perpendicularly thereto at the ends thereof and projecting in opposite directions, one pair being pivotally attached to the upper pair on the valve support bracket and the other pair being pivotally attached to the pair of spaced-apart lugs on the upper end of the link. 
     The means for preventing falling debris and soil from entering the top end of the cylinder while enabling air to flow into and out of the cylinder comprises an inverted J-shaped tube, having a long portion which passes through the stopper and a short portion having a filter screen at its outer end, the filter screen being disposed to face toward the upper side of the stopper and being spaced from the upper side. 
     The means for selectively closing the bottom end of the cylinder comprises a bottom stopper, having an upper surface and a lower surface, which is rigidly attached to the cylinder, an o-ring encircling the stopper and in sealing contact with the cylinder, an axially disposed bolt attached to the stopper and extending upwardly beyond its upper surface, at least one longitudinally disposed hole extending through the bottom stopper, and a check valve disposed beneath the lower surface, whereby reverse flow of water from the borehole toward the stopper lifts the check valve and closes the hole and the bottom end. 
     This constant-head soil permeameter, adapted for operational use within a borehole in earthen materials, comprises a cylindrical housing having a top end and a bottom end which has a flow-through means for allowing water entering the top end to form a first water level within the housing and then to flow through the bottom end into the borehole to form a second water level therewithin when the second water level is lower than the first water level and having a closing means for preventing water from flowing into the cylindrical housing when the second water level is higher than the first water level. 
     This flow-through means comprises a bottom stopper which is rigidly attached to the cylindrical housing, has a countersunk bottom surface forming a downwardly extending skirt that contacts the bottom of the borehole when the permeameter is resting thereupon, has at least one longitudinally disposed hole through the stopper, and has at least one laterally extending hole through the skirt. 
     This closing means comprises a check valve guide which is axially and rigidly attached to the countersunk bottom surface of the stopper, a disk-shaped check valve which is loosely and axially fitted to the check valve guide, and a disk-shaped baffle, having a plurality of longitudinally disposed holes therethrough, which is rigidly and perpendicularly attached to the check valve guide and disposed beneath the check valve, whereby backflow of water from the borehole toward the bottom stopper passes through the plurality of holes in the baffle and lifts the check valve to block the at least one longitudinally disposed hole in the bottom stopper. 
     The rate of water flow into the borehole that is necessary to maintain the constant head is recorded at appropriate intervals during the test period. The information recorded during the test, which also includes height of constant water column, rate of flow, and borehole geometry, is factored into an appropriate mathematical equation to provide an estimate of hydraulic conductivity. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an isometric and partial cutout view of the soil permeameter in place in a borehole dug in earthen materials. 
     FIG. 2A is an isometric and partial cutout view of the upper part of the soil permeameter, showing the top stopper and the valve control assembly in its fully closed position. 
     FIG. 2B is an isometric and partial cutout view of the lower part of the constant-head soil permeameter, showing the base assembly and the check valve in its open position. 
     FIG. 3 is a sectional view of the upper part of the soil permeameter showing the top stopper, the filter vent assembly, and the air vent pathway, taken along line  3 — 3  in FIG.  2 A. 
     FIG. 4A is a sectional view of the upper part of the constant-head soil permeameter showing the top stopper, its o-ring, the filter vent assembly, the air vent assembly and the valve control assembly in its fully opened position. 
     FIG. 4B is a sectional view of the upper part of the constant-head soil permeameter showing the valve control assembly in its fully closed position, as in FIG.  2 A. 
     FIG. 4C is an enlarged sectional view of the middle part of the constant-head soil permeameter showing the valve control assembly in its fully opened position, with the force components acting at their respective pivots and the imaginary lines of action connecting the pivots. 
     FIG. 4D is an enlarged sectional view of the middle part of the soil permeameter showing the valve control assembly in its fully closed position, with the force components and the imaginary lines of action in their changed positions. 
     FIG. 5A is a sectional view of the lower part of the soil permeameter showing the base assembly and its check valve in its fully opened position, with flow arrows indicating the flow of water from the interior of the cylinder into the borehole. 
     FIG. 5B is a sectional view of the lower part of the soil permeameter, showing the base assembly and its check valve in its fully closed position and with the buoyant float body in a lowered position, in contact with a bolt attached to the base assembly, as a flow arrow indicates attempted movement of water from the borehole toward the interior of the cylinder. 
     FIG. 6 is a sectional view of the buoyant float body and the surrounding housing of the constant-head soil permeameter. 
     FIG. 7 is a plan view of the float assembly, taken along line  7 — 7  in FIG. 6, and a sectional view of the surrounding housing of the constant-head soil permeameter. 
     FIG. 8 is a plan view of the filter vent assembly and hose connection at the top end of the cylinder and a partial sectional view of the suspension bracket, taken along line  8 — 8  in FIG.  4 A. 
     FIG. 9 is a bottom view of the base assembly, taken along line  9 — 9  in FIG. 5B, of the constant-head soil pemeameter. 
     FIG. 10 is an isometric and partial sectional exploded view of the base assembly, taken generally along line  10 — 10  in FIG.  9 . 
     FIG. 11 is a sectional view of the top stopper, taken along line  11 — 11  in FIG. 4B, of the constant-head soil permeameter. 
     FIG. 12 is a sectional view of the base assembly, taken along line  12 — 12  in FIG. 5A, of the soil permeameter. 
     FIG. 13 is a chart of the mechanical advantage ratio, provided by the valve control assembly within the housing. 
     FIG. 14 is a chart containing permeameter test depth curves for the constant-head soil permeameter of the invention undergoing permeameter flow rates from zero to 2,000 ml/min. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As shown in FIG. 1, the constant-head soil permeameter of this invention comprises calibrated reservoir  15  which is disposed on ground surface  101  near borehole  11 , cylindrical housing  19  which is lowered into borehole  11 , lifting and/or support means for housing  19 , means for water delivery from reservoir  15  to housing  19 , means for venting air from housing  19 , means for providing a mechanical advantage ratio for shutting off water flow into housing  19 , and means for preventing debris and fallen earth particles from entering housing  19 . Inside of housing  19  are valve control assembly  17 , float assembly  18 , base assembly  60 , and portions of flow control assembly  59 . 
     Housing  19  consists of a tubular cylinder suitable for isolation and protection of interior components of the permeameter. The lifting and/or support means for suspension and stabilization of cylindrical housing  19  comprises cable  12 , which is secured at its lower end through hole  99 , as seen in FIGS. 4A,  4 B of suspension bracket  13 , at snap connection  71  and at its upper end to any suitable anchoring mechanism above ground surface  101 . Hose connection  28  provides the entry port of water into housing  19  during tests. The means for water delivery from reservoir  15  to housing  19  comprises hose  14 , which has a suitable length for the testing depth. The permeameter may rest on bottom  11   b  of borehole  11  or may be supported at any desired height above bottom  11   b  by cable  12 . 
     Top stopper  16 , as shown in FIGS. 2A,  3 ,  4 A, and  4 B, provides a rigid mounting base for valve control assembly  17 , suspension bracket  13 , filter vent assembly  21 , and hose connection  28  as additional portions of flow control assembly  59 . Top stopper  16  incorporates an o-ring  30  to provide a seal between stopper  16  and housing  19 , thereby preventing soil particles and debris from entering the invention in the annular space between top stopper  16  and housing  19 . 
     Hose connection  28  and valve body  44  are hydraulically connected and secured through top stopper  16  by commercial pipe  57 , as shown in FIGS. 4A,  4 B, and  11 . Suspension bracket  13  and valve control assembly  17  are additionally secured to top stopper  16  by commercial bolt  55  and commercial nut  56 . Top stopper  16  is secured to housing  19  by commercial machine screws  43 , as seen in FIGS. 2A and 11. 
     Filter vent assembly  21 , as shown in FIGS. 2A,  3 ,  4 A,  4 B, and  8 , comprises commercial pipe nipple  51 , commercial pipe elbows  52 , filter housing  53 , filter screen  31 , and filter retaining snap ring  54 . Pipe nipple  51  is threadably fastened to top stopper  16  in a manner that allows free movement of air through vent pathway  68  in filter vent assembly  21  and top stopper  16 . Filter vent assembly  21  is constructed as an inverted J-shape to discourage entry of soil particles into the cylinder chamber through gravitational action while allowing free passage of atmospheric gas and excluding soil particles and other debris. Because filter screen  31  faces downwardly and is spaced from the upper side of stopper  16 , there is substantially no opportunity for soil and debris to pass through screen  31  into vent pathway  68 , whereby contamination of the apparatus is substantially impossible. 
     Valve control assembly  17 , as shown in FIGS. 2A,  4 A, and  4 B, comprises valve body  44 , valve seat  45 , valve seat retaining lever arm  38 , valve support bracket  35 , stabilizing bracket  48 , link  37 , and actuating lever arm  36 . Valve support bracket  35  and stabilizing bracket  48  are fastened together by commercial bolts  49  and  46  and by commercial nuts  50 . 
     Valve support bracket  35 , actuating lever arm  36 , link  37 , and valve seat retaining lever arm  38  comprise pairs of spaced apart and perpendicularly extending support lugs, shown in FIGS. 2A,  4 A, and  4 B. Each lug pair has at least one pivot which comprises a hole drilled completely through its lug pair and a pivot alignment pin inserted completely through each drilled hole of the lug pair, thereby serving as an axis of rotation. The pivot alignment pins are crimped on the outsides of the lugs to ensure retention. These pivot alignment pins are parallel to each other and provide a nearly frictionless connection between actuating lever  36 , link  37 , valve seat retaining lever arm  38 , and stationary valve support bracket  35 . 
     Each lug pair has at least one pivot, except for actuating lever arm  36  which comprises two pivots on its single lug pair. The pivots allow actuating lever arm  36 , link  37 , and valve seat retaining lever arm  38  to move freely in a plane parallel to the longitudinal axis of stationary valve support bracket  35 . 
     Actuating lever arm  36 , which comprises a single lug pair  90 , rotates axially around pivot  39 , which is also connected to lug pair  91  of valve support bracket  35 . Valve retaining lever arm  38 , which comprises two lug pairs  94  and  95  extending in opposite directions, rotates axially around pivot  42  which is also connected to lug pair  96  of valve support bracket  35 . Link  37 , which comprises two lug pairs  92  and  93  extending in opposite directions, is connected at pivot  40  to actuating lever arm  36  and at pivot  41  to valve retaining lever arm  38 . Link  37  rotates axially around both pivots  40  and  41  in response to the rising and lowering of float assembly  18 . 
     Valve support bracket  35  provides a rigid stationary connection between valve seat retaining lever arm  38  and actuating lever arm  36 . Link  37  provides a movable rigid connection between valve seat retaining lever arm  38  and actuating lever arm  36 . Valve control assembly  17  is shown in a fully opened position in FIG.  4 A and in a fully closed position in FIGS. 2A and 4B. 
     Float assembly  18 , as shown in FIG. 6, comprises a buoyant float body  72 , upper and lower float end guides  27 , commercial threaded rod  58  and commercial nuts  50  and  100 . Water flow channels  23  are disposed opposite to each other on the perimeter of float body  72  and extend longitudinally through float body  72  and both float end guides  27 , as seen in FIGS. 2A and 7. 
     Base assembly  60 , as shown in FIGS. 2B,  5 A,  5 B,  9 , and  10 , comprises bottom stopper  20  which provides a rigid mounting body for o-ring  30  which is in sealing contact with housing  19 , commercial bolt  29 , check valve  32 , check valve guide  34 , baffle  25 , longitudinally disposed holes  24  through stopper  20 , laterally disposed holes  26  in the skirt of bottom stopper  20 , and commercial nuts  56  and  98 . Check valve  32  moves freely in a vertical direction on check valve guide  34 . Bottom stopper  20  is secured to housing  19  by commercial machine screws  43 , as shown in FIGS. 2A,  11 , and  12 . 
     Check valve  32  rests on baffle  25  and remains open during normal operation, as illustrated in FIG. 5A, when water is flowing through base assembly  60  to borehole  11 . However, check valve  32  rises into contact with countersunk bottom surface  20   c  and closes holes  24  to prevent backflow, as seen in FIG. 5B, if forced upward by reverse water flow, shown by flow arrow  69  through hole  33  in baffle  25 . 
     Valve control assembly  17  and float assembly  18  provide flow control of water from reservoir  15  to maintain a constant head of water in borehole  11 . The constant head of water is established by the preset level of the permeameter within borehole  11  and the resultant equilibrium of the pressure head induced by the height of water  70  in reservoir  15  and the rate of water absorption  47  into earthen material  11   a , as depicted in FIG.  1 . The force provided by any float assembly to effectively stop or throttle the flow at a valve must be sufficient to exceed hydrostatic pressures produced by the height of water  70  through hose column  14  and resultant pressure of water flow at control valve components  44  and  45  in the constant-head soil permeameter. 
     As the depth of testing increases, the increasing hydrostatic pressure at control valve components  44  and  45  of valve control assembly  17  requires progressively greater water displacement by float assembly  18  to throttle and maintain flow equilibrium. Other constant head devices utilizing a float alone with a float displacement equivalent to displacement of float assembly  18  become fully submerged and, therefore, ineffective at deep depths. In addition, the float assemblies of other constant head devices, not having a mechanical advantage means, displace a greater volume of water than the present invention at any given depth while maintaining equilibrium, thereby causing a correspondingly greater transient rise of the water level, H, within the borehole. This complicates determining the constant height of water for permeability test determinations. 
     The entire constant-head soil permeameter of this invention is constructed of stainless steel except for: a) o-rings  30 , check valve  32 , and valve seat  45  which are made of neoprene; b) top stopper  16 , bottom stopper  20 , baffle  25 , and upper and lower float end guides  27  which are made of polycarbonate plastic; and c) float  72  which is made of a closed-cell foamed plastic. 
     Explanation of Forces Exerted within Valve Control Assembly  17   
     The mechanical advantage ratio that is necessary for hydraulic testing at considerable depths is provided by the lever and link action of valve control assembly  17 . A force along a line of action is required to make any body rotate about an axis. The perpendicular distance from the line of action of the force to the axis of rotation is the moment arm of the force and the product of the force and the moment arm of the force is the torque. As seen in FIGS. 4C and 4D, buoyant force  74 , which has a line of action parallel to the longitudinal axis of valve support bracket  35 , can be resolved into force components  75  and  76 , which are, respectively, perpendicular and parallel to imaginary line  77  which joins the point of application of force  74  to the axis of actuating lever arm  36 . Line  77  is, therefore, a moment arm of force  75  about pivot  39 . The torque applied at pivot  39  is equal to the product of force  75  and the length of moment arm  77 . Force  76  is directed toward the axis at pivot  39  and does not cause rotation. 
     As actuating lever arm  36  rotates around pivot  39 , every point on actuating lever arm  36 , including pivot  40  on lug  90 , sweeps out the same angle at any time. The torque produced at pivot  39  from force  75  results in force  78  at pivot  40 , which acts in a line of action perpendicular to moment arm  79 . Force  78  is proportional to the ratio of the length of moment arm  77  to the shorter length of moment arm  79 , thereby greatly exceeding force  75 . Force  78  can be resolved into force  81  and force  80  along moment arm  79 . 
     Force  81  lies along line of action  87 , which is an imaginary line connecting pivot  40  and pivot  41  of link  37 , all parts of which act as a rigid plate. Force  80  is directed toward pivot  39  and does not cause rotation. Forces  81  and  80  can be determined by two-dimensional equilibrium equations. The maximum ratio of force  81  to force  78  is achieved just before pivot  40  and line of action  87  of force  81  move across line  73 . Line of action  87 , however, is prevented from crossing line  73  by bolt  46 , which limits the rotational travel of link  37 . 
     Force  81  can be resolved into force components  82  and  83  which are, respectively, perpendicular and parallel to imaginary line  84  which joins the point of application of force  81  at pivot  41  with pivot  42 , which is the axis of rotation of valve retaining lever arm  38 . All parts of valve retaining lever arm  38 , similarly to link  37  and actuating lever arm  36 , act as a rigid plate. Line  84  is, therefore, a moment arm of force  82  about the axis at pivot  42 . 
     The torque applied at pivot  42  is equal to the product of force  82  and the length of moment arm  84 . Force  83  is directed either toward or away from the axis at pivot  42 , depending on the degree of closure of valve control assembly  17 , and does not cause rotation in either case. Force component  82  is nearly superimposed on line of force  87 , as seen in FIG. 4C, because this line of force is nearly perpendicular to moment arm  84  at the fully open position. 
     As valve retaining lever arm  38  rotates around pivot  42 , every point on valve retaining lever arm  38 , including valve seat  45 , sweeps out the same angle at any time. Force  86  is perpendicular to moment arm  85  and is a result of the torque at pivot  42  acting at the length of moment arm  85 . Force  86  is proportional to the ratio of the length of moment arm  84  to the shorter length of moment arm  85 , thereby greatly exceeding force  82  which applies the initial torque. Force  86  can be resolved into component force  88  that is perpendicular to face  89  of valve seat  45  and another force (neither shown nor numbered) that is parallel thereto. 
     Force component  88  of force  86  has a line of action through the center of and perpendicular to face  89  of valve seat  45 . Force  88 , applied at the surface of valve seat  45 , provides the force necessary to throttle or stop fluid flow from valve body  44 . The mechanical advantage ratio of force  88  to buoyant force  74  ranges from approximately 10:1 at full valve opening (FIG. 4C) to approximately 60:1 at full valve closure (FIG.  4 D). The mechanical advantage increases as a result of the cumulative mechanical advantages of actuating lever arm  36 , link  37 , and valve retaining lever arm  38 . As can be seen in FIG. 13, the mechanical advantage ratio becomes larger at an increasing rate as pivot  40  and line of force  87  approaches, but does not cross, line  73 . 
     Within the confines of housing  19 , the distance between pivots  39  and  40  is the principal factor controlling amplification of the mechanical advantage ratio beyond 60:1. If the distance of 7.75 mm between pivots  39  and  40 , as in the preferred embodiment herein described, is reduced, the torque about pivot  39  is increased in accordance with the ratio of the length of moment arm  77  to the length of moment arm  79 , thereby correspondingly increasing resultant force  78 . In addition, reduction in distance between pivots  39  and  40  simultaneously increases resultant force component  81  along line of action  87 . However, this increased mechanical advantage and increased resultant force comes at a cost because the valve does not open as much as formerly and the maximum fluid flow is less. 
     The lugs and lever arms are quite rigid while using the preferred 14 gauge stainless steel materials, with a significant safety factor at a depth of even 40 meters. Consequently, the mechanical advantage ratio can be further increased, and the testing depth can thereby be significantly increased beyond 30 meters. The maximum depth of testing for the preferred embodiment described herein is limited to some undetermined depth greater than 40 meters because of limitations imposed by float capacity, fluid pressure, and turbulence created by the incoming water. 
     Valve control assembly  17  controls the water flow through the permeameter. At the beginning of a typical hydraulic conductivity test, water flows into hose connection  28  of valve control assembly  17  as shown by flow arrow  61  in FIG.  4 A. Valve control assembly  17  is initially in a fully open position, thereby allowing water flow through valve body  44  and through the opening between valve body  44  and valve seat  45  as shown by flow arrow  62  in FIG.  4 A. 
     This water falls on upper float end guide  27  and passes through water flow channels  23  of float assembly  18  and holes  24  of base assembly  60  and continues to flow beneath bottom stopper  20  and into borehole  11 , as indicated by flow arrows  64 ,  65 ,  66  and as seen in FIG.  5 A. 
     The water rises at equal corresponding levels in borehole  11  and inside housing  19 . As the water level continues to rise, nut  100  of float assembly  18  strikes heel  67  of actuating lever arm  36 , which is pivotally connected to valve support bracket  35  at pivot  39 , and initiates upward rotation of actuating lever arm  36  around pivot  39 . As float assembly  18  continues to rise, forcible contact at heel  67  of actuating lever arm  36  is transferred from nut  50  to float end guide  27 , which maintains continuous sliding contact until partial or full valve closure is attained, as seen in FIGS. 1,  2 A, and  4 B. 
     As actuating lever arm  36  rotates upwardly around pivot  39 , link  37 , which is pivotally connected to pivot  40 , revolves around pivot  40  and transfers the buoyant force provided by float assembly  18  to pivot  41  of valve seat retaining lever arm  38 . This arm  38  is pivotally connected to valve support bracket  35  at pivot  42 . It consequently revolves upwardly and progressively closes the opening between valve body  44  and valve seat  45 . The mechanical advantage imparted by actuating lever arm  36 , link  37 , and valve seat retaining lever arm  38  increases with progressive valve closure. 
     Line  73  is an imaginary straight line passing through the centers of pivot  39  and pivot  41 , as depicted in FIG.  4 C. As float assembly  18  rises, heel  67  of actuating lever arm  36  slides from nut  100  onto upper float end guide  27  and continues to slide across guide  27  as actuating lever arm  36  revolves upwardly around pivot  39 , thereby moving pivot  40  and link  37  toward line  73 . The maximum mechanical advantage is attained just before pivot  40  reaches line  73 . Pivot  40 , however, is prevented from crossing line  73  by bolt  46 , which limits the travel of link  37 . The present invention provides a mechanical advantage ratio of approximately 60:1 at full valve closure. 
     Float assembly  18  provides the buoyant force required by valve control assembly  17  to throttle water flowing through channel  97  in hose connection  28 , pipe  57 , and valve body  44 . Float assembly  18  moves freely inside housing  19  and is maintained in alignment by both float end guides  27 , as shown in FIGS. 4A,  4 B,  6 , and  7 . The upward travel range of float assembly  18  is limited by contact with valve control assembly  17  and the downward travel range is limited by contact with bolt  29 . Prior to application of water during the hydraulic conductivity test, threaded rod  58  of float assembly  18  is at rest on bolt  29 , as shown in FIG.  5 B. 
     Water added to reservoir  15  during a typical test flows freely around float assembly  18  through flow channels  23  that are disposed longitudinally on float body  72  and float end guides  27 , as well as in the annular space between float body  72  and housing  19 , as shown in FIG.  7 . Bolt  29 , which limits downward travel of float assembly  18 , prevents float assembly  18  from resting on the upper surface of bottom stopper  20 , thereby allowing water applied during the test to flow freely through holes  24 , as illustrated in FIGS. 5A and 12 by flow arrows  63 ,  64 ,  65 , and  66 , through bottom stopper  20  and also allowing the water to contact the entire lower surface of float assembly  18  when the water is rising inside housing  19 . 
     Float assembly  18  rises with the rising water and displaces a volume of water equal in weight to the weight of float assembly  18 . Float assembly  18  continues to rise in response to the rising water level and strikes heel  67  of actuating lever arm  36  and initiates upward rotation of actuating lever arm  36  around pivot pin  39 , as illustrated in FIG.  4 A. As float assembly  18  continues to rise, contact at heel  67  of actuating lever arm  36  is transferred to upper float end guide  27 , which maintains continuous sliding contact until valve throttling control or full valve closure is attained, as seen in FIG.  4 B. Float assembly  18  becomes partially submerged in proportion to the buoyant force required to throttle water flow from the contact orifice of valve body  44  by valve seat  45 . 
     As float assembly  18  rises initially, valve seat  45  almost contacts valve body  44  to close channel  97  and stop the flow of water from reservoir  15 ; then valve seat  45  lowers slightly to establish an equilibrium fluid level in borehole  11  with only a slight fluctuation. 
     It is desirable to prevent inadvertent backflow entry of water, which may contain suspended soil particles or other debris, into the permeameter. Potential for reverse water flow, as shown by flow arrow  69  in FIG. 5B, may occur if the permeameter is placed in a borehole already containing water, if the borehole is advanced further after initial testing and water is not removed, or if the sidewall of the borehole collapses during the test and displaces a sufficient volume of water to cause backflow. Check valve  32  remains closed by pressure differential as long as the water level remains higher in the borehole than in the chamber of housing  19 . 
     During normal operation, water flows through holes  24  in base assembly  60 , as shown by flow arrow  63  in FIGS. 2B and 5A, then into the space above check valve  32  and around the annulus between baffle  25  and bottom stopper  20 , as shown by flow arrow  64 . Water continues to flow through lateral holes  26  of base assembly  60  into the annulus between housing  19  and borehole  11 , as shown by flow arrow  65 , and into the cavity below baffle  25  as shown by flow arrow  66  in FIG.  5 A. 
     Baffle  25  and check valve  32  physically block direct entry of loose soil and other debris into the chamber of housing  19  when the permeameter of the invention is initially placed in the borehole. Bottom stopper  20  also incorporates o-ring  30  to provide a seal between bottom stopper  20  and housing  19 , thereby further preventing entry of suspended soil particles and debris. Bottom stopper  20  is countersunk at its bottom  20   c  to leave a narrow circular rim  20   b  having a bottom edge  20   a , as shown in FIG. 2B, thereby minimizing the contact area with borehole bottom  11   b  and providing negligible smearing or blockage of the absorptive soil surface at the bottom of the borehole. 
     During field operations to determine hydraulic conductivity, an unlined borehole  11  is drilled into the earth to a desired test depth with a suitable drilling or digging device to remove earthen materials and provide an approximately level surface at the bottom of the borehole. The constant-head soil permeameter of the invention is then lowered in a vertical position by cable  12  to rest upon bottom  11   b  of borehole  11 , as shown in FIG.  1 . Water is poured into reservoir  15  and flows by gravity through hose  14  and bore  97  in hose connection  28  into valve control assembly  17 , as shown by flow arrow  61  in FIG.  4 A. Valve control assembly  17  is initially in a fully open position, thereby allowing water to flow, as shown by flow arrow  62 , through the opening between valve body  44  and valve seat  45 . 
     Water then flows onto and around float assembly  18  through channels  23 , as seen in FIGS. 2A and 7, into the annular space between float assembly  18  and housing  19 , and into the lower part of housing  19 . Water next flows through holes  24  in bottom stopper  20 , as shown by flow arrow  63  in FIGS. 2B and 5A. During normal test procedures, check valve  32  is in its open position which allows water to flow freely through holes  24  in bottom stopper  20  into the space above check valve  32  and around the annulus between baffle  25  and bottom stopper  20 , as shown by flow arrows  63  and  64  in FIG.  5 A. Water then continues to flow through lateral holes  26  in skirt  20   b  into the annulus between the housing  19  and the perimeter of borehole  11 , as shown by flow arrow  65  and into the cavity below baffle  25  as shown by flow arrow  66 . 
     Filter vent assembly  21  allows exhausting of air as water rises inside the chamber within housing  19  and maintains atmospheric pressure equally inside and outside of housing  19  within borehole  11  at all times; this pressure equalization between level  22  within housing  19  and height of water H within borehole  11  is essential for maintaining equal water levels inside and outside of housing  19 . Filter screen  31  of filter vent assembly  21  also stops entry of loose soil particles into housing  19 . 
     Water rises freely at equal levels within the constant head permeameter and in the annular space between cylindrical housing  19  and the borehole sides until float assembly  18 , which is buoyed by the rising water, engages valve control assembly  17 . Water flow through valve body  44  of valve control assembly  17  is progressively throttled by valve seat  45  as float assembly  18  continues to rise until water level  22 , as seen in FIG. 1, is approximately attained. After a suitable period of time that may vary from several minutes to one-half hour or more depending on soil characteristics, while water from borehole  11  is being transported radially into the surrounding soil matrix  11   a , as shown approximately by permeation arrows  47 , equilibrium water levels H and  22 , which are equal, are attained. 
     The wetting-front continues to develop radially from borehole  11  as water levels H and  22  are maintained above the bottom of borehole  11  during the testing period. Water moves radially from the borehole through interparticle pores and along voids and fissures that are unique to any particular borehole in response to pressure induced by the constant head of water H, gravitational forces, and capillary forces within the earthen materials. The saturation that occurs within the wetting front during the test period is sometimes referred to as field saturation because some of the voids and pores may contain entrapped air and thereby reduce the potential flow that may occur under fully saturated conditions below the water table. An approximate steady state flow is attained in soil matrix  11   a  after a period of initial saturation and equilibrium is developed. Water level H is the resultant equilibrium level maintained by the permeameter of the invention in response to water absorption by soil and a pressure head of water level  70  in reservoir  15 , as illustrated in FIG.  1 . Once equilibrium of flow is approximately attained, reservoir  15  is filled approximately to the initial level  70  in preparation for recording test data. 
     After initial flow equilibrium is attained, the steady state flow of water absorbed by the soil is determined by recording at discrete time intervals the dropping water levels observed at graduations on reservoir  15 . The optimum recording interval varies with the soil type and permeability and is determined by the user. For example, the optimum recording interval for highly permeable sandy soils may be approximately 5 minutes, but for slowly permeable clayey soils may be one-half hour or more. The total time during which observations are recorded may typically vary from on-half hour to 2 hours or more. The flow rate is derived from observations recorded during the selected time period. Level H of water in the borehole may be determined from direct observations or by the use of FIG. 14, which determines level  22  as a function of water flow rate and depth of the permeameter below ground surface  101 . The estimated hydraulic conductivity is determined by factoring the steady state flow rate, water depth, and borehole geometry into an appropriate analytical solution. 
     Solution to Hydraulic Conductivity Values 
     The depth of water, indicated as level H in the borehole, may be determined from direct observations by use of a measuring tape or may be estimated by use of the Flow Volume/Test Depth Chart in FIG.  14 . Test depth curves for placement of the permeameter below ground surface  101  range from 0.5 to 30.0 meters, as shown in FIG.  14 . The test depth curves of FIG. 14 represent the mean of all observations, 95% of which are within +/−0.5 cm as determined by using an embodiment of the present invention. The test depth curves denote the height of water in the borehole if the permeameter rests on the bottom of the borehole. Alternatively, the permeameter can be suspended at any desired distance above the bottom of the borehole, and the suspended distance beneath rim  20   a  can be added to the height determined in FIG. 14 to obtain H. 
     The chart in FIG. 14 can be used to estimate the depth of water within the borehole at any flow rate of the invention ranging from zero to 2000 ml/min. For example, if the permeameter is placed on the bottom of the borehole, the depth of the borehole is 10.0 meters, and the flow rate is 500 mL/mm., then the estimated depth of the static water level H is approximately 15.2 cm. Where test depths are intermediate to the depth curves of FIG. 14, an appropriate interpolation is made. 
     The estimated hydraulic conductivity is determined by factoring the steady state flow rate of water into the soil, height of water within the borehole, and borehole geometry into an appropriate analytical solution. One example of an analytical solution has been developed by R. E. Glover (Zangar, 1953). This equation, suggested by Amoozegar and Warrick (1986) for use where the distance between the bottom of the borehole and an impermeable layer is at least twice as large as H, is: 
     
       
           K   S   =Q [sin  h   −1 ( H/r )−( r   2   /H   2 +1) 0.5   +r/H] (2π H   2 )  [Equation 1] 
       
     
     Where 
     K S =Saturated hydraulic conductivity, 
     Q=Steady-state flow rate of water into the soil, 
     H=Constant height of water in a cylindrical borehole, indicated as level H, and 
     r=Radius of the cylindrical borehole. 
     Use of this equation is illustrated in the two following examples. 
     EXAMPLE 1 
     A cylindrical borehole  11  with diameter of 9.5 cm is augured to a depth of 0.6 meter. It is desired to establish a minimum height H of water equalling 25 cm above the bottom of the borehole, so that the permeameter is suspended 10 cm above the bottom of the borehole. During the test, in which volumetric readings of falling water levels in reservoir  15  are recorded at discrete time intervals spanning a two-hour period, it is determined that the steady-state flow rate of water Q into soil  11   a  is 5 ml/min. The constant height H of water is, therefore, 25.1 cm (15.1 cm from FIG. 14, plus 10 cm of suspended height). The radius r of borehole  11  is 4.75 cm, and the saturated hydraulic conductivity, K S , from Equation 1 is 3.2×10 −5  cm/sec. This is a low hydraulic conductivity value, typical of silt and clay soils. 
     EXAMPLE 2 
     A circular borehole  11  with diameter of 9.5 cm is augured to a depth of 10.0 meters. It is desired to establish a minimum height H of water equalling 25 cm above the bottom of the borehole and to suspend the permeameter at a height of 10 cm above the bottom of the borehole. During a test period of one-half hour, it is determined that the steady-state flow rate of water Q into soil  11   a  is 900 ml/min. The constant height H of water is, therefore, 25.1 cm (15.1 cm from FIG. 14, plus 10 cm of suspended height). The radius r of the borehole is 4.75 cm and the saturated hydraulic conductivity from Equation 1 is 5.8×10 −3  cm/sec. This is a high hydraulic conductivity value, typical of sandy soils. 
     In the event that water covers bottom  11   b  of borehole  11  at the time of inserting the permeameter in borehole  11 , check valve  32  of base assembly  60  closes and stops water and suspended soil particles from entering housing  19 , as seen in FIG.  5 B. It is desirable to prevent inadvertent entry of water, which may contain suspended soil particles or other debris, into the permeameter. 
     This situation may occur if seepage water enters the borehole after it is drilled or if the borehole is advanced to a deeper depth after an initial test has been performed and the remaining water has not been removed during drilling or has not drained completely away into the soil. Water must be removed from the borehole if the initial water levels exceed the equilibrium height of the permeameter. If the water level is a result of seepage or groundwater inflow, the test procedure is invalid because the permeameter is designed to measure hydraulic conductivity as a result of outflow to the soil. Potential reverse water flow may also occur if the sidewall of the borehole collapses during the test and displaces a sufficient volume of water to cause backflow. 
     Because it will be readily apparent to those skilled in the constant-head soil permeameter art that innumerable variations, modifications, applications, and extensions of the principles hereinbefore set forth can be made without departing from the spirit and the scope of the invention, what is hereby defined as such scope and is desired to be protected should be measured, and the invention should be limited, only by the following claims.