US3610035A

US3610035A - System for determining shear strength of soil including expandable probe

Info

Publication number: US3610035A
Application number: US888734A
Authority: US
Inventors: Richard L Handy
Original assignee: University of Iowa Research Foundation UIRF
Current assignee: University of Iowa Research Foundation UIRF; Iowa State University Research Foundation ISURF
Priority date: 1969-12-29
Filing date: 1969-12-29
Publication date: 1971-10-05
Anticipated expiration: 1988-10-05

Abstract

A PROBE HAVING ELONGATED, SEPARABLE SIDE PRESSURE PLATES IS INSERTED INTO THE SOIL UNDER TEST, SEPARATING THE SOIL AS IT PENETRATES. AFTER THE PROBE IS INSERTED, A LATERAL FORCE IS APPLIED TO THE PRESSURE PLATES TO COMPRESS AND CONSOLIDATE SOIL ENGAGED BY THEM WITH A KNOWN LATERAL CONTACT PRESSURE. THE EXTERIOR SURFACES OF THE PRESSURE PLATES ARE PROVIDED WITH METAL BARBS OR TEETH TO CAUSE SHEARING WITHIN THE SOIL AS THE PROBE IS REMOVED UNDER FORCE. AS THE PROBE IS REMOVED, MEASUREMENTS ARE RECORDED OF THE RESISTING FORCE REQUIRED FOR SHEARING AND THE DISPLACEMENT OF THE PROBE. THE SEGEMENTS OF THE PRESSURE PLATES EXTENDING ABOVE THE SURFACE OF THE SOIL RELAX SO THAT THE ENTIRE LATERAL FORCE CAUSING EXPANSION OF THE PLATES IS EXERTED ON THAT FORCE CAUSING EXPANSION OF THE SOIL. THE PROBE IS PULLED OUT IN INCREMENTS, THIS DECREASING THE AREA OF THE PRESSURE PLATES IN CONTACT WITH THE SOIL. HOWEVER, THE LATERAL PRESSURE EXERTED ON THE SOIL INCREASES CORRESPONDINGLY-THIS FORMING A OF THE COHESION AND INTERNAL FRICTION OF THE SOIL WITHOUT HAVING TO FORM A BORE HOLE.

Description

Oct. 5, 1971 R L HANDY 3,610,035

SYSTEM FOR DETERMNIG SHEAR STRENGTH OF SOIL INCLUDING EXPANDABLE PROBE Filed Dec. 29, 1969 2 Sheets-Sheet 1 iV-- /f-b) 9b /9 PUMP/:No ,0k ff@ co/vsTA/vr j VOLUME L PPEssUPe MEMUP/Na PUMP MPAA/s MPa/Mmm PEcaPUA-'P la@ la? /A/L/EA/ro /c/APD L. HANDY Arroz/sys SYSTEM FOR DETERMNING SHEAR STRENGTH OF SOIL INCLUDING EXPANDABLE PROBE Filed Dec. 29, 1969 2 Sheets-Sheet 2 Oct. 5, 1971 R l. HANDY 3,610,035

I A 4 /p 5a/mace W550/MEW 5 /yp ff? 'r 4 Arron/E y;

United States Patent lO 3,610,035 SYSTEM FOR DETERMINING SHEAR STRENGTH F SOIL INCLUDING EXPANDABLE PROBE Richard L. Handy, Des Moines, Iowa, assignor to Iowa State University Research Foundation, Inc., Iowa State University, Ames, Iowa Filed Dec. 29, 1969, Ser. No. 888,734 Int. Cl. G01n 3/24 U.S. Cl. 73--101 6 Claims ABSTRACT 0F THE DISCLOSURE A probe having elongated, separable side pressure plates is inserted into the soil under test, separating the soil as it penetrates. After the probe is inserted, a lateral force is applied to the pressure plates to compress and consolidate soil engaged by them with a known lateral contact pressure. The exterior surfaces of the pressure plates are provided with metal barbs or teeth to cause shearing within the soil as the probe is removed under force. As the probe is removed, measurements are recorded of the resisting force required for shearing and the displacement of the probe. The segments of the pressure plates extending above the surface of the soil relax so that the entire lateral force causing expansion of the plates is exerted on that section which is embedded in the soil. decreasing the area of the pressure plates in Contact with the soil. However, the lateral pressure exerted on the soil increases correspondingly-thus forming a new shear plane and allowing independent determination of the cohesion and internal friction of the soil without having to form a bore hole.

BACKGROUND The present invention relates to measuring shear strength of loose or sandy soil; more particularly, it relates to in situ determination of shear strength of such soil.

The determination of the soil parameters of cohesion and internal friction is essential for the application of classical soil mechanics theory to construction and stability problems.

Two distinct approaches have been developed for measuring soil shear strength. One of these approaches encompasses triaxial and direct shear tests conducted in a laboratory on undisturbed samples of soft material, and is a difficult task. Conventional recovery apparatus includes piston samplers and freezing chambers.

One commonly used method of obtaining a sample is by pushing a thin-wall cylindrical tube called a Shelby tube into the soil. Several types of laboratory tests may be performed, in which the soil is subjected to a pressure and then a load is applied to cause shearing.

In a known direct shear test, a short cylinder of the undisturbed sample is loaded axially with a vertical load, and sheared on a plane normal to the load in a shear box. That is, the top half of the shear box is made to slide laterally with respect to the bottom half; and the necessary force to effect shear is measured.

A more accurate laboratory test is the triaxial test in which a cylinder of soil is sealed in a flexible membrane and confined laterally with uid pressure. A vertical load is applied until the sample fails or breaks. ln this test, the orientation of the shear plane must be determined, and cohesion and internal friction must be calculated using Mohr theory.

In either of these cases of laboratory testing, normal procedure requires taking two or three samples and determining shear strength at different values of confining The probe is pulled out in increments, thus.

ice

pressure in order to define the Coulomb relationship from which cohesion and internal friction may be determined.

Normally, these laboratory tests may take at least two to three days before results are achieved, and they are relatively expensive. In situ shearing tests have the advantage that the results may be determined almost immediately, further, the cost per test is reduced.

In addition to the problems in obtaining samples of firm soil, the recovery of undisturbed samples of soft 0r loose soil raises additional problems. Obtaining sub-aqueous samples is still more diliicult because the original sediment is in a high pressure environment; and the difference in elastic rebound between the sample and the recovery apparatus will cause disturbances in the recovered sample. Further, gases dissolved in the sample will escape from solution and expand and thus further disturb the sample. The sample could, of course, be enclosed within a pressure vessel; but this unduly complicates the recovery equipment as well as the laboratory tests on the sample. Thus, in testing soft soil or subaqueous sediment, it is highly desirable to conduct the tests in situ.

Two commonly used types of in situ testing include probing and vane shear. Heretofore, most in situ testing methods have had the disadvantage that cohesion and internal friction were not capable of being separately measured.

In the vane shear test, a bladed vane is pushed into sediment and a torque is applied to cause circumferential shearing. However, application of a torque requires a base for reaction. It cannot be applied through a cable without some type of laterally disposed anchor. Although in subaqueous applications a limited torque might be applied through the use of a large-diameter vane messenger and a maximum resisting torque could be recorded by instrumentation, the vane shear test suffers from another limitation-namely, that it principally measures cohesion. Attempts have been made to measure internal friction angle, but these have had limited success.

In the use of probes for in situ testing, a simple approach is to register maximum depth of penetration for a given insertion force. If the test is rapid, for example by using an impact force, the penetration resistance is a function of viscosity, pore-water pressure, and inertia effects-all of which combine to resist penetration. Thus, the resistance force due to cohesion and internal friction tend to be obscured and is not capable of being separated from the combined resistance force.

The penetration resistance test yields an empirical measurement of soil-bearing capacity, and the exact relationship depends upon the kind of soil. That is, a standard penetration test blow count of 40 in a clay indicates a substantially different bearing capacity than a count of 40 in sand. The reason for this is that the penetration test results are influenced by both cohesion and internal friction.

If an inserted probe is pulled, a value may be determined for skin friction on the probe; however, there is generally no determination on the lateral pressure of the soil on the probe; and, hence, the internal friction of the soil cannot be determined.

U.S. Pat. No. 3,427,871 of Handy and Fox entitled Bore-Hole Soil Testing Apparatus discloses apparatus for in situ testing of soils for determining shear strength. The apparatus of that application includes a device which is lowered into a preformed bore-hole, and the device includes two pressure surfaces which are movable relative to each other for engaging opposite sides of the bore-hole wall in response to a pressurized fluid being transmitted to cylinder and piston rod units within the device. The piston end of the unit is connected to one pressure surface, and the cylinder end is connected to the other pressure surface.

With a constant radial pressure exerted on the pressure surfaces, a shearing force is applied by pulling the device axially of the bore-hole. After a first shearing force is measured, the hydraulic pressure extending the pressure surfaces is increased so that a second shearing force may be applied and measured. The data points relating shearing stress at failure to the normal pressure on the shear plane (that is, the pressure exerted b'y the hydraulic fluid) defines the classical Coulomb relationship from which cohesion and internal friction may be determined, as described in that patent.

SUMMARY The present invention contemplates inserting an elongated probe into soft or sandy soil or subaqueous sediment. The probe includes separable pressure plates which are inserted in retracted position and which separate the soil as the probe is inserted. After the probe has been inserted, a force is transmitted to the pressure plates moving them laterally apart thereby compressing and consolidating soil contacted by the pressure plates of the probe with a lateral contact pressure.

A pulling force is exerted on the probe to remove it. The pressure plates are provided with metal barbs or teeth to cause shearing within the sediment as the probe is removed. The maximum force required to remove the probe a given increment (Le. to cause shearing) is recorded. As one alternative of the invention, the segments of the pressure plates which extend above the soil surface relax so that the entire expansion force is exerted on the segment of the probe buried beneath the soil. In other Words, as the probe is retracted incrementally, the net force expanding the pressure plates remains constant whereas the area of the pressure plates decreases thereby increasing the expansion pressure on the plates and compressing the soil to form a new shear plane.

Cohesion and internal friction of the sediment may be obtained from the record of the exerted retraction force, as explained in more detail herein.

Thus, the present invention provides a simple and practical method of determining in situ soil shear strength in soft soil while allowing independent determination of cohesion and internal friction, which have been a major shortcoming of prior testing methods.

Further, the present invention greatly facilitates the in situ determination of soil shear strength by obviating the need for drilling a bore-hole (which, of course, is difficult to do in most sandy soil or soft sediments beneath water). Further, one set of recorded data points, as will be clear from the detailed description below, yields suicient information to dene different strata that exist beneath that top soil level. In the case of testing subaqueous bore-holes, a problem would be encountered in relocating the hole in order to insert a device.

yOther features and advantages of the instant invention will be apparent to persons skilled in the art from the following detailed description of a preferred embodiment accompanied by the attached drawings.

THE DRAWINGS FIG. 1 is a partially schematic view of a system used for the practice of the present invention;

FIGS. 2 and 3 'are vertical and horizontal cross section views of the probe of FIG. 1 in a closed position;

FIGS. l4 and 5 are vertical and horizontal cross section views of the probe of FLG. 1 in an open position; and

6-9 are shear diagrams of resultant measurements illustrating the determination of cohesion and internal friction in typical soils according to the present invention.

DETAILED DESORIPTIO'N Turning now to FIG. l, the illustrated embodiment comprises two principal componentstripod supporting means, generally designated 10k and including legs 10a, and the penetrating probe generally designated 11. The

legs 10a are supported on a base plate 10b adapted to be placed on the surface of the soil to be treated and defining a central aperture 10c through which the probe 11 is urged to penetrate the soil.

Secured to the top of the legs 10a there is a cylindrical member 10d. Transverse plates 10e and 10f are secured to the cylindrical member 10d to dene a cylindrical cavity 10g which receives a piston 10h to which is attached a rod 10i. An O ring 10j seals the periphery of the piston 10h with the interior surface of the cylinder wall 10d; and the piston is adapted for reciprocation therein. The chamber-defining Wall 10d and rod 101' form a hydraulic cylinder unit which has a stroke approximately equal the length of the probe 11, or at least of the depth to which the probe is designed to penetrate. Fluid (preferably oil) under pressure is forced into the cylinder cavity 10g through a conduit 10k from a source including an oil pump and means for measuring the volume of the pump, schematicaly designated by the block 10L. Fluid forced from the pump through conduit 10k will force the piston head 10h and its associated rod 10i downwardly. Rod 10i is received in suitable apertures 1n the transverse plates 10e and 10f and it reciprocates through these plates as the piston moves. The volume of the iluid is measured and recorded by any suitable means, but preferably, by a strip chart recorder 10u. A return conduit 10m' communicates the lower section of the cylinder with the suction port of the pump to return the liquid to the pump when the piston is lowered. If the pump is reversed, conduit 10m becomes the feed conduit and 10k the return conduit. The hydraulic unit 1s thus a double-acting cylinder and piston rod unit.

The probe 11 is attached to the lower end of the rod 10i. by means of a threaded connection with a second cylinder-defining member 19 having generally inverted cup` shape. The rod 10i is hollow to provide a conduit whlch communicates with the interior of the member 19. The upper edge of rod 10i is connected by means of a 4flexible conduit 12a to a second pump mechanism schematlcally designated 12b and provided with a conventional pressure release valve (not shown) to insure that the pressure of the oil in conduit 10i remains constant.

Turning now to the friction 11, it includes rst and second

pressure side plates

16 and 17 which are elongated vertlcally. and which provide the lateral contacting surfaces which engage the soil under tests and exert a lateral pressure thereon.

The

pressure side plates

16 and 17 are provided at their lowermost ends with tapered shoe portions 16a and 17a respectively for facilitating penetration of the soil under test in a stabbing or piercing action in which the soil is separated and consolidated laterally-as distinguished from compressing or compacting the lowermost soil in a vertical direction, as would be the case with a blunt end or cylindrically-shaped probe.

As seen slightly exaggerated in FIG. 1 for purposes of illustration, when the

side pressure plates

16 and 17 which are embedded in the soil, they are closed as seen in solid. But the side pressure plates are opened (chain line) when the probe is drawn above the level of the soil. Further, the

side pressure plates

16 and 17 are laterally flexible so that when the lateral restraining force caused by the soil is released, the entire force causing expansion of these side pressure plates is transmitted only to that portion of the probe buried in the soil. Thus, as the probe is withdrawn, the pressure plates will separate for that portion which is above the soil. 'The

plates

16 and 17 are provided with barbs 16b yand 17b respectively for engaging the soil.

Referring now to FIGS. 2 and 3, the

side pressure plates

16 and 17 are attached adjacent to the lower open edge of the cup member 19 by means of horizontal rods 18 secured to the interior sides of that member and extending through receiving apertures in the

side pressure plates

16 and 17. Thus, the

side pressure plates

16 and 17 are free to slide along the rods 18; however, when the member 19 is raised or lowered, the

side pressure plates

16 and 17 are raised or lowered with it. As best seen in FIGS. 3 and 5, thecross section of the

plates

16 and 17 has a crescent shape to maximize the contact area while minimizing the volume of soil displaced upon insertion.

The second pneumatic cylinder and piston rod unit 19 includes "a housing 19a which defines a piston chamber 1917 which Fs ealingly receives a piston 19C. To the lower end of the vpiston 19e` there is secured a piston rod 19d extending centrally of and downward into the probe 11. The lower portion of the piston rod 19d is connected to the upper portion of a ladder 21 to move the same downwardly when oil under pressure is forced into the piston chamber 19b via input conduits 10i from the source of constant pressure oil 12b.

The ladder 21 acts as a force-transmission member; and it extends in a plane which is parallel to the direction of lateral expansion of the

side plates

16 and 17. A top rung on the ladder 21 is designated by reference numeral 22; and mounted beneath the rung 22 and extending perpendicular to the plane of the page of FIG. 2 is an arm 23 having a wedge-shaped cross section providing inclined bearing surfaces 23a and 23b. As seen more clearly in FIG. 5, the wedge-shaped arm 23 extends beyond the transverse width of the ladder 21. A first shoulder member 25 is secured to the concave surface of side plate 15, and it defines a Vertical central aperture 26 for receiving the the ladder 21. First :and

second rollers

28 and 29 `are journalled in the shoulder member 25 for engaging the bearing surface 2317 of the arm 23 on each side of ladder 21.

A second shoulder member 30 similar to the shoulder 25, is attached to the interior surface of the side plate 16; and it too, defines a center aperture 31 for receiving the ladder 21 and first and

second rollers

32 and 33 for engaging the bearing surface 23a of the arm 23 on each side of the ladder 21.

First and second end plates identified in FIG. 3 by

reference numerals

35 and 36 are coupled to the sides of the

shoulders

25 and 30 by means of horizontally-elongated slots (see reference numerals 37 in FIG. 2) for receiving studs 39 thereby permitting lateral expansion of the

side plates

16 and 17 while defining a limit to this lateral expansion FIG. shows the side plates opened and the limiting function of slots 37.

It will be appreciated that as the ladder 21 is forced downward, the transverse wedge-shaped arm 23 will be forced between rthe associated pair of rollers mounted within the

shoulders

25 and 30 and thereby force the pressure plates 16 and l17 from each other in opposite directions. As schematically illustrated in FIG. l, there are a number of these same arrangements of transverse wedgeshaped arms (23) engaging rollers within supporting shoulders (25 and 30) attached to the

side plates

16 and 17 and spaced vertically along the probe 11.

Since the ladder 21 is a rigid force-transmitting member, it will urge each of the shoulder-pairs apart (see FIG. 4) if a constant distributed restraining force is exerted on the sides of the probe; however, if the restraining force is greater along one section of the probe (as in the case when a portion of the probe embedded in soil) the flexibility of the

side plates

16 and 17 will cause a lateral yielding of the probe at the portion not embedded in soil; and hence, the full expansion force will be applied to the position of the pressure plates which is embedded in the soil. In practice, when pressurized gas expands the piston and cylinder unit 19, that portion of the probe 11 which is embedded in the soil will have a constant force (determined by the regulated pressure of the CO2 in chamber 19b) forcing the pressure plates in lateral expansion. The corresponding pressure exerted by the plates on the sediment, of course, is a function of the contacting surface area which depends on the depth to which the probe penetrates. Further, as the probe is retracted, it Will be noted that the normal pressure will increase since the expansion force remains a constant and the total contacting area of the pressure plates decreases.

OPERATION With the above-described apparatus, the present invention contemplates embedding the probe 10 in soil so that it penetrates and separates the soil as it penetrates. Although the illustrated embodiment is designed for tests to be taken on land, it will be appreciated by persons skilled in the art that the method presently to be described is equally suitable for subaqueous measurements. The main features of the disclosed apparatus will also be suitable for an underwater environment, and in such case it is contemplated that the probe would be provided with a cylindrical shroud to stabilize it during descent to insure a perpendicular insertion relative to the horizontal. Further, a line would be secured to the top of the rod l10i for recovery of the apparatus.

After the bottom plate 10b of the tripod support is resting on the soil to be tested, the pump of means 10L is energized to force oil into line 10k to force piston 10h downward to insert the probe. During insertion, the depth of penetration is determined by the volume of oil pumped by the means 10L, knowing the volume of line 10k and cylinder 10g. The cylinder and piston rod unit 19 is then actuated to exert a predetermined force on the ladder 21. As the ladder 21 is forced downwardly, the wedge-shaped members 23 will bear against their associated

rollers

28, 29, 32 and 33, to urge the pressure plates :16 and 17 laterally apart. Thus, the downward force of the piston rod 19d is transmitted into a lateral force to urge the pressure plates apart and to compact the soil adjacent their outer surfaces. Since there are a number of these structures spaced along the direction of elongation of the probe 1'1, the two pressure plates will be urged apart in substantially parallel relationship, and those portions of the pressure plates which are retracted from the soil will not be restrained laterally by the soil so they will relax slightly. Nevertheless, because the

pressure plates

16 and 17 remain parallel, the entire downward force exerted by the cylinder 19 will be transmitted as a lateral pressure onto the compacted soil as long as the probe is even partially embedded in the soil (until the very tip alone is embedded, of course).

The lateral pressure may be calculated by observing the penetration depth and knowing the contact area of the pressure plates. Next the pressure in the inserting hydraulic cylinder and piston rod unit is released, and the pump 10L reversed to force oil into line 10m to withdraw the probe. This causes pressure in the cylinder 10g to be built up to a point suicient to cause shearing of the soil. The pressure in the line 10m is recorded on the recorder 1011 in relation with the volume measurement of means 10L. Vertical displacement or movement of the probe is monitpred by knowledge of the volume of oil forced from the'time the pump is energized and the constant rate at which it pumps. The use of a constant rate pump produces a linear strain rate. Thus, strain is noted directly on the strip recorder 10L since pressure in line '10m is directly related.

The metal teeth or barbs .16b and '17b cause shearing within the sediment as the probe 10 is removed. Further, Water pore pressure is dissipated (through the use of

porous pressure plates

16 and 17, if necessary) sufiiciently to allow consolidation and formation of a dense layer of soil on the pressure plates. As the probe 11 is withdrawn, the force generated by the hydraulic unit 19 causing lateral expansion of the pressure plates 16 and 1-7 (by means of the transverse wedge-shaped arms 23 connected to the rungs of the ladder 21 and bearing against shoulders fastened to the pressure plates) is transmitted solely to that portion of the probe which is buried beneath the soil.

It will be noted that once shear has started it will continue until built-up pressures in the line to cylinder 10g are dissipated, after which shear will cease until pressures have built up suiciently to re-initiate movement. Hence, the retraction of the probe 11 is intermittent (or incremental) rather than a constant, steady withdrawal. Thus, a shearing occurs and then the probe is halted whereupon the portion of the

side plates

16 and 17 emerging from the soil expand so that the entire expansion force is transmitted to the portion of the side plates beneath the soil thereby increasing the pressure exerted by the probe pressure plates on the sediment in forming a new shear plane.

THEORY OF OPERATION The fundamental equation for determining the shear strength of soil, as derived by Coulomb, is as follows:

S=c+ (N-u) tan where S is the shearing strength,

c is cohesion (or cohesion shearing strength), u is the pore water pressure, and

tangent gb is the coeiicient of internal friction.

S, c, and N all have units of force per unit area, such as pounds per square inch. The term c represents the previously-described cohesion, and tan p is the internal friction.

On land, the above equation forms the basis for calculations of parameters such as stability of slopes, foundation ybearing capacity, bearing capacity of piles, etc. In subaqueous structures, these parameters include predictions of slope stability, pile bearing capacity and depth of sinking and force for retrieval of objects on the bottom. In order to perform such calculations, three soil parameters must be known-cohesion, internal friction and pore Water pressure. Pore water pressure is often ignored if the test loading conditions approximate those anticipated in the field, as in the case of in situ testing. The shear strength of soft soil and bottom sediments is of direct engineering importance for founding stable marine cables structures, retrieving objects, etc.

In an ideal clay, pore water pressures dissipate so slowly there is no apparent internal friction. That is, u equals N in the equation. Hence, the force resisting shear is directly proportional to the contact area regardless of the applied normal pressure N. An idealized plot of the contact area A vs. resisting force F, for this case is illustrated in FIG. 6. It will be appreciated that the contact area of the probe is directly related to the depth of the probe. As mentioned, if the initial insertion of the probe is incomplete, the maximum contact area A0, may be obtained from an inspection of probe side plates, or there may be an accessory provided with the probe for marking the top level of the sediment. Data points 41 are plotted on the graph, with the initial data point 41a being obtained with knowledge of the maximum depth of penetration of the probe, and subsequent data points being obtained from a knowledge of the amount of probe movement between two successive applications of peak resisting force, for example, between the data points 41a and 4|1b in FIG. 6. =It will be observed that the line delined by the data points 41, 41a, and 41b is a straight line passing through the origin and defining an angle, r, with the ordinant. The cohesion is equal to the maximum resisting force Fc divided by the initial contact area A0. Hence, tan r=c (cohesion).

An idealized shear diagram for the case in which the sediment is sand is illustrated in FIG. 7. In this case, the cohesion is zero; and, as the device is retracted, the reduction in contact area will be compensated by an increase in the normal pressure, and the total shearing resistance remains constant, as illustrated by the vertical straight line 43 dened by the data points 43a.

In the case of a sand, since cohesion equals zero, the total restraining force is equal to the normal force times the coeiicient of the internal friction or, the internal friction equals the normal force divided by the lateral expansion force W, supplied by the pneumatic unit 19 and a source of regulated pressure for the apparatus shown.

In FIG. 8, the data points 45 define a line 46 which does not pass through the origin and which is not vertical. Hence, the total restraining force is composed partly of a friction force and partly of `a cohesion force, that is to say, the sediment is a sand-clay. The component of the total resisting force caused by `cohesion is indicated Fc in FIG. 8; and that component of the total force which is caused by internal friction is designated FW These values are obtained by extending line 46 until it intersects with the `abscissa, and then constructing a vertical line 47 to intersect the horizontal line 48 representative of the total resisting force. The cohesion is then:

C=Fc/A0 and internal friction tan =F/ W EXAMPLE FIG. 9 illustrates an idealized shear diagram for the case of a layered sediment wherein the upper layer is denoted layer 1 and the lower layer Ias layer 2. In the example, it is assumed that the initial contact area is 5l) sq. in. per foot of probe length; and the expansion force W, is 200 pounds. The data points 49 define a line 50, the tangent of which is Fcl/A o. The initial contact area is the depth of the probe times the contact area per unit length of the probe.

It will be observed that the shear diagram of FIG. 9 is divided into the top layer and the lower layer, but the inventive concept need not be limited to two layers. It will be appreciated that the beginning of each successive layer is marked by its own shear diagram defined by a point of maximum contact area and its maximum restraining force. The two points illustrating this in FIG. 9 are denoted 5-1 and 52'. To complete the calculation, tan 2=F2/200=l30/200; hence =33. The cohesion for the lower layer, /(12) (50-)=0.3 p.s.i.

For the top layer, the internal friction is zero since the line 52 delined by the data points 53 passes through the origin, and cohesion c=45/(8)(501)=0.1 p.s.i.

Since the probe is actually inserted into the sediment, some disturbance is inevitable, and the extent of the disturbance will depend upon the volume displaced by the instrument in relation to the area tested, Hence, the probe is preferably designed with a crescent or diamond-shaped cross section to minimize volume displacement while maximizing plate contact area. This is the reason for the above-described insertion or penetrating motion being more of a cutting or piercing action with lateral compressions and displacements of the soil rather than a punching in to cause vertical compression and displacement. Expansion of the probe causes a further lateral compression and consolidation at the probe surfaces which is desirable. Simultaneously, tension occurs at the edges, but this is insignicant as long as water flows through the center of the probe to equalize pressure.

PORE PRESSURE The sequence of application of normal pressure is a stepwise increase; and as disclosed in the above-identified patent of Handy and Fox, if pore pressure becomes excessive, it may be measured as described therein. However, measurement of pore pressure is probably less irnportant in marine soil mechanics than in land investigations where excess pore pressure during testing will more or less dissipate during the slower loading of construction. In other words, in foundation engineering the consolidation drained test may be very pertinent, whereas in a marine landslide, an undrained test condition appears justifiable and may give more meaningful data. Two methods of correcting for pore pressure are to minimize pore PIQSQRIIQ, and t0 measure it.V A correction should be made if only the maximum and minimum excess pore pressure are measured and recorded by pressure transducers and instruments fitted near the top and bottom of the probe, thus avoiding use of electrical cables. Alternatively, transducers and electrical lines might be used to give a continuous record of pore pressure.

Stress-strain curves may be drawn either by recording the maximum penetration of the probe and assigning strain increments to each test point, based on oil volume. The resulting graph which might appear is illustrated in FIG. 7.

In the example of FIG. 9, allowing minutes per point, a total testing time would be one hour plus the time for lowering and raising the instrument.

Thus, with the inventive principle and apparatus disclosed herein, a complete shearing diagram for a layered sediment beneath a body of water may be obtained in a relatively short time as compared to the heretofore long laboratory testing times. Further, all the advantages of in situ testing are obtained without the need for drilling a separate borehole and placing an instrument within it. In addition, a complete shearing diagram as a function of depth is obtained thereby identifying a sublayer within the sediment.

It will be apparent that a hydraulic piston and cylinder rod until could replace the dead weight messenger in forcing the ladder down, but this would require the addition of hydraulic feed lines, whereas in its simplest form, the illustrated embodiment has only one cable connected between the probe and the ship.

Having thus described a preferred method and apparatus for carrying out my invention, it will be obvious to persons skilled in the art that various other probes (for instance, expandable packer types) may be substituted for that which has been described while continuing to practice the method. Further, the particular probe described may be used to accomplish other purposes. It is, therefore, intended that all such equivalents and substitutions be covered as they are embraced within the spirit and scope of the appended claims.

l claim:

1. In a method of determining cohesion and internal friction of a soil in situ, the steps comprising: inserting a probe into the soil without a borehole and with a minimum of compression in the direction of insertion, said probe including tWo elongated separable pressure plates frictionally engaging said soil throughout the inserted length of the probe; urging said pressure plates apart with a constant force to exert a known pressure on soil engaged by said pressure plates, thereby cornpacting said soil laterally; inducing a retraction force on said probe sufiicient to cause a shearing of said soil; partially removing said probe from the soil during the exertion of said retraction force to thereby reduce the contact area of said pressure plates against said soil so that the expansion force increases the lateral pressure applied to said soil; measuring said retraction force; then inducing a second retraction force on said probe sufficient to cause a second shearing of the soil; and measuring said second retraction force.

2. A system for determining the shear strength of soil comprising: a probe insertable in the soil under test without a borehole and including first and second elongated, laterally separatable pressure plates for penetrating the soil, each plate including an outer surface for frictionally engaging soil under test; a first set of bearing members connected in longitudinal space relation along one of said pressure plates; a second set of bearing members connected in longitudinal space relation along the other of said pressure plates, a bearing member from each of said sets forming an opposing pair of bearing members; forcing means engaging said bearing members for forcing opposing pairs of said bearing members laterally apart, thereby compacting the soil in contact with said pressure plates, said probe being adapted to transmit all of said applied force to the soil engaged by said pressure plates, thereby increasing the normal pressure applied to the soil as said pressure plates are withdrawn from the soil; means for retracting said probe to cause shearing of the soil; and means for measuring the shearing force.

3. The system of claim 2, wherein said bearing members each include roller members, rotatable about a horizontal axis and rotatably mounted to the interior of an associated pressure plate in opposing relation to a bearing member mounted on the interior of the other pressure plate, and wherein said forcing means includes a frame, and a plurality of wedge members rigidly connected to said frame and arranged between opopsing pairs of said roller members, whereby as said frame is forced in one direction, said wedge members urge said pairs of bearing members apart in a transverse direction to exert lateral pressure on said soil.

4. The system of claim 3, wherein said forcing means includes fiuid pressure means in said probe and having a rod connected to said frame to urge the same in said one direction upon the appliction of fluid pressure.

5. The system of claim 2, further comprising hydraulic means for inserting said probe into soil under pressure.

6. The system of claim 2, wherein the cross section of said pressure plates transverse of the direction of insertion isformed in the shape of a crescent to minimize displacement of soil laterally of said probe as said probe enters said soil while maximizing the lateral Contact area of said plates.

References Cited UNITED STATES PATENTS 2,603,967 7/1952 Carlson 73-101 2,709,368 5/1955 Wolpert 73-84 X 2,833,144 5/1958 Miller et al 73--84 3,427,871 2/1969 Handy et al. 73-101 X 3,481,188 12/1969 Mori 73-84 RICHARD C. QUEISSER, Primary Examiner E. I. KOCH, Assistant Examiner U.S. Cl. XR. 73-84