US3533283A

US3533283A - Method and instrument for determining deformability and residual stresses in foundation soils

Info

Publication number: US3533283A
Application number: US731673A
Authority: US
Inventors: Manvel Coelho Mendes Da Rocha; Jorge Neves Da Silva
Original assignee: Individual
Current assignee: Individual
Priority date: 1967-05-06
Filing date: 1968-05-06
Publication date: 1970-10-13
Anticipated expiration: 1987-10-13
Also published as: DE1759425A1; ES354141A1; DE1759425C3; SE352959B; ES353555A1; FR1561795A; JPS49254B1; GB1223342A; SE389370B; DE1759425B2

Description

Oct. 13, 1970 METHOD AND INSTRUMENT FOR DETERMINING DEFORMAB AND RESIDUAL STRESSES IN FOUNDATION SOILS Filed May 6, 1968 V N ILITY 4.'Sheets--Sheet -M. c. MENDES'DA ROCHA L 3 Oct. 13, 1970 M. c. MENDES DA ROCHA F-TAL 3,533,233

METHOD AND INSTRUMENT FOR DETERMINING DEFORMABILITY AND RESIDUAL STRESSES IN FOUNDATION SOILS Filed May 6, 1968 4 Sheets-Sheet 2 Get. 13, 1970 c MENDES DA ROCHA ETAL 3,533,283

METHOD AND INSTRUMENT FOR DETERMINING DEFORMABILITY AND RESIDUAL STRESSES IN FOUNDATION SOILS Filed May 6, 1968 4 Sheets-Sheet 5 I5 5 Ilia. 1/ Elk" 13, 1970 M. c. MENDES DA ROCHA ETAL 3,533,283

METHOD AND INSTRUMENT FOR DETERMINING DEFORMABILITY AND RESIDUAL STRESSES IN FOUNDATION SOILS Filed May 6, 1968 4 Sheets-Sheet 4 II I! Patented Oct. 13, 1970 Int. CI. (301 3/08 US. CI. 73-94 Claims ABSTRACT OF THE DISCLOSURE A method and apparatus for measuring deformability and residual stress in a foundation mass, wherein a single elongated slot is formed by cutting a plurality of contiguous narrow slots whereafter a fiat jack is inserted into the elongated slot and applies measured pressure against the faces bounding the slot whereby the faces undergo deformation which is measured by the jack.

BRIEF SUMMARY OF THE INVENTION This invention relates to methods and apparatus for measuring the deformability and residual stresses in a foundation mass.

It is increasingly recognized as very important to know the deformability of foundation masses such as soils, rock masses, and the like when designing large structures to be built on or within rock masses, such as concrete dams, bridges, large buildings, tunnels, underground power stations, etc.

Very often the deformability of these foundations is determined exclusively in laboratory tests of soil or rock samples. Recently, however, the need for in situ tests has been recognized, at least in the case of large structures involving large volumes of foundation rocks, so as to secure representative results of the true conditions in the ground, particularly as regards heterogeneity and cracks.

These in situ tests generally comprise excavating trenches or galleries, applying known forces on given areas of their walls and measuring the resulting displacements.

These forces are applied by means of rigid concrete slabs cast against the gallery wall, with which they are in contact in an area of the order, for example, of 1 square meter. By means of jacks applied to the slabs, it is possible to obtain pressures representative of those that will be applied to the ground by the completed structure. Circular steel plates with diameters of 20 or 50 cm. can be used for the same purpose.

Nevertheless, the distribution in the ground of the forces transmitted by concrete slabs or steel plates is imperfectly known, and makes the interpretation of the test results difiicult. In order to surmount this difiiculty, it is sought to obtain a distribution as uniform as possible of the loads in the ground, for which rubber plates are sometimes used between the loaded surface and the jacks.

A still more uniform distribution of pressure can be obtained by means of oil-filled deformable flat jacks. These jacks are applied in narrow slots cut in the rock mass and usually cannot be reused because the space between the jack and the slot is filled with mortar or concrete.

In deformability tests, strains are measured in the direction of the applied forces sometimes at the center of,

' sometimes outside, the loaded areas.

From the above, it follows that the methods so far used in deformability tests of soils and rocks are very expensive and slow, due to the need to drive a gallery or cut a slot in the rock mass to be studied. Blasting methods have the disadvantage of disturbing the structure of the soil or rock mass, possibly changing the values to be determined, however great the precautions taken.

Additionally, the volume of ground involved in the tests by the methods now in use is not as a rule sufficiently large to be representative of the properties of the whole. In practice, tests in which the loaded area exceeds 1 square meter are too slow, and the area of loading is generally not more than 0.25 square meter, and, save in special cases, such an area is obviously insufficient.

An object of the invention is'to avoid the disadvantages indicated hereinabove and to provide a new instrument by which soil deformability can be determined under the following conditions:

the surfaces to be tested can be obtained by comparatively rapid and inexpensive methods;

test surfaces can be obtained which are entirely smooth, very extensive and reaching to a considerable depth in the rock mass;

the flat jacks which apply the presure contain strainmeasuring instruments and can be easily reused;

the rock mass is not disturbed when the slot is cut.

In accordance with a method of the present invention narrow slots are successively cut by means of a disc with a diameter of, for example, 1 m.

A slight overlap in the contiguous ends of successive slots makes it possible to obtain a single slot with approximately the same length as the sum of individual slots.

The depth of the slots can exceed the radius of the cutting disc, as a cylindrical hole of suitable diameter is previously bored to accommodate a tubular support of the disc. This support, which acts as a guide for the cutting operation, can be extended by means of similar units so that slots can be cut to any depth.

In order to obtain a continuous surface for applying uniform pressures on the walls of the slot, these are extended by means of two comparatively rigid semi-cylinders with adequate dimensions which are introduced in the guide hole. It is only after this operation is completed that a flat jack containing strain measuring instruments and the fiuid under pressure is introduced in the slot to displace the walls of the slot.

From the values of the two parameters, pressure and displacement of the slot faces, it is possible by means of analytic or empirical formulas to obtain the modulus of deformability of the foundation mass.

If the distances between points, for instance at the surface, are measured before cutting the slot, the residual stress in the ground normal to the plane of the flat jack is equal to the pressure required to cancel the change in the distance between the points observed during the cutting of the slot.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a side view of a cutting instrument;

FIG. 2 is a front view of the cutting instrument;

FIG. 3 is a side view of a flat jack with a shape suitable to completely fill a slot which is cut in the material to be measured;

FIG. 4 is a side view of a -fiat jack with a different shape to be used when the slot is not to be entirely filled; and

FIG. 5 is a sectional view showing the location of a strain measuring instrument in the jack.

DETAILED DESCRIPTION The cutting apparatus comprises a supporting frame having a base 1 bolted in the ground, two guiding columns 2 connected by an upper transverse element, and two E braces between the transverse element and the base that increase the rigidity of the assembly.

Each of the two guiding columns 2 is formed with a rack 4 on a flat rear face. The rack 4 extends the entire length of the columns. A carrier 6 can be moved vertically up and down along the guiding columns by operation of a crank 5 which controls two pinions (not shown) which engage racks 4.

Carrier 6 contains a gear box 7 operated by a motor 8. This can be an electric, compressed air, or any other type of motor, although the hydraulic type is preferred due to its compact construction.

A cutting disc 9 is fitted on a shaft 10 which is mounted at the lower end of a supporting tube 11. The shaft 10 and the disc 9 thereon, is driven in rotation from the motor 8 through the gear box 7 and a double chain (not shown) or other suitable power transfer means contained within tube 11.

A slot is progressively cut in the foundation mass to be tested by the disc 9 which is advanced b the hand operated crank 5. Before cutting the rock with the disc, a guide hole is drilled in the rock with a sufficiently large diameter to permit subsequent guided passage of tube 11 when the slot is cut by the disc. The bit for drilling the guide hole may be attached to the frame and operated by the same motor 8.

By means of a crank 12 and an eccentric 13, the tube 11 supporting the disc can be given a lateral oscillatory movement which can be a rectilinear or a pendulous movement. This facilitates the penetration of the disc in the rock mass, producing a slight enlargement in the length of the slot when the slot is being cut.

The tubular support of the disc 11 can be extended by means of similar elements fitted with suitable transmission systems so that its length is adapted to correspond with the desired depth of the slot.

After successive slots have been cut to form an elongated slot of desired length, a jack is inserted into the slot to apply a measured pressure on the walls bounding the slot in order to measure the corresponding deflection of the walls whereby the deformability of the foundation mass can be measured.

The jacks by which uniform pressure is applied to the slot walls are essentially constituted of two flatted mildsteel flexible sheets 14 welded along their edges to define an interior space. Any other high strength very deformable material can be used instead of mild steel.

Flat jacks intended to fill the entire area of the slot have the shape of a rectangle with a semi-circular end (FIG. 3). If the flat jacks need not fill the entire area, they can be smaller and have other shapes, such as that shown in FIG. 4. The latter are used when the edges of the slots are not to be disturbed by applied forces.

Flat jacks of the former type include a flap 15 on the upper edge which protects the operators against projections of fluid under pressure in case of a rupture in the jack.

Both types of jacks are fitted with a nipple 16 for the inlet of fluid under pressure and an outlet 17 for electric cables of the strain measuring instruments.

Each strain measuring instrument is applied at suitable points between the faces of the flat jack, as shown in FIG. 5, and consists of two deformable steel blades 18 each fixedly secured at its opposite ends to a respective plate. Electrical resistance strain gauges 19 are secured on the blades 18 and are incorporated in a Wheatstone bridge circuit and connected with an external reading instrument,

4 where changes in the distance between the faces of the fiat jack can be directly read.

It should be noted that the disclosed method and apparatus may be modified without departing from the scope and spirit of the invention as defined in the appended claims.

What is claimed is:

1. A method of measuring deformability and residual stress in a foundation mass, said method comprising cutting a plurality of contiguous narrow slots in a foundation mass so that a single elongated slot is formed and applying forces to the faces bounding said elongated slot to determine the deformability thereof.

2. A method as claimed in claim 1, wherein said forces are developed by inserting relatively displaceable plates into said elongated slot and applying the plates with measuredforce against said faces.

3. A method as claimed in claim 2 comprising boring a hole in the foundation mass prior to the cutting of said slots, and guiding the cutting of the slots by said hole.

4. A method as claimed in claim 1, wherein strains result in said mass from said forces, comprising measuring the strains to determine the residual stress in said mass.

5. Apparatus for applying forces to faces bounding an elongated slot for measuring deformability and residual stress in a foundation mass, said apparatus comprising a jack including two deformable flatted sheets having peripheral edges secured together to define an enclosed interior space, means for pressurizing said sheets against said faces, and means within said space for measuring the displacement of the sheets as the foundation mass deforms under the pressure applied to the sheets.

6. Apparatus as claimed in claim 5, wherein said sheets have a shape corresponding to the entire face of the elongated slot.

7. Apparatus as claimed in claim 6, wherein said jack has an upper edge with a protection flap thereat, said means for pressurizing said sheets comprising an inlet at said flat edge for admission of a pressure fluid which is introduced into the space between said sheets, said means for measuring the deformability of the foundation mass including electrical means within said space and an outlet at said flat edge.

8. Apparatus as claimed in claim 5, wherein said means for pressurizing said sheets comprises means for introducing a pressure fluid into the space between the sheets.

9. Apparatus as claimed in claim 5, wherein said means in said space for measuring displacement of the sheets comprises a flexible element coupled to both sheets to be stressed as the sheets undergo relative displacement, and strain measuring means mounted on said flexible element.

10. Apparatus as claimed in claim 5 in combination with means for cutting said elongated slot in the foundation mass, said jack being of a size operatively related to said cutting means for being insertable into said slot to apply forces to the faces thereof and measure deformability thereof.

References Cited UNITED STATES PATENTS 3,l42,173 7/1964 Pfann 7388.5 3,364,737 1/1968 Comes 73151 3,419,923 1/1969 Cowan 5348 JERRY W. MYRACLE, Primary Examiner U.S. Cl. X.R. 7384