WO1992011526A1

WO1992011526A1 - Bearing capacity measuring means

Info

Publication number: WO1992011526A1
Application number: PCT/FI1990/000303
Authority: WO
Inventors: Anssi Lampinen; Kimmo Simomaa
Original assignee: Valtion Teknillinen Tutkimuskeskus
Priority date: 1989-09-12
Filing date: 1990-12-18
Publication date: 1992-07-09

Abstract

The invention concerns a means for measuring bearing capacity, comprising a drop weight (1), drop weight holding and releasing means (2), and a measuring apparatus (3). As taught by the invention, the means comprises a tubular body (4) inside which the drop weight (1) is provided to be movable; a loading plate (5) closing the lower end of said body; and an acceleration pick-up (6) disposed to measure the movement of the body during impact.

Description

BEARING CAPACITY MEASURING MEANS

The present invention concerns a means for measuring bearing capacity, as defined in the preamble to claim 1.

It is frequently necessary in connection with the structural courses of roads and streets, of various building foundations and also with the filling of exca¬ vations, to measure the bearing capacity of such sur- faces. The bearing capacity of roads is commonly meas¬ ured using means in which a load is applied on the structure to be measured by a load equivalent to the rear wheels of a lorry (Benkelman beam, Lacroix deflec¬ tion measuring car) , in the form of static loading with the aid of a rigid loading plate (the plate loading test) or in the form of dynamic loading on a rigid plate with the aid of a drop weight (drop weight apparatus) .

Existing means are embarrassed by one common. and significant, drawback: they are big, expensive and unwieldy in use, and therefore bearing capacity meas¬ urements on smaller objects are not feasible. An object of this kind is encountered e.g. on roads and streets in filling work on various excavations, which could by filling one course at a time, with intervening compac¬ tion and measurement using a suitable bearing capacity measuring means, be filled right away in one go in a manner precluding subsequent formation of holes or presence of residual eminences. The object of the present invention is to eliminate the drawbacks mentioned above. It is a spe¬ cific aim of the invention, to provide a novel, light¬ weight bearing capacity measuring means which enables bearing capacity measurements to be made rapidly and with adequate accuracy, and which is easily transport¬ able e.g. by carrying, from one point of measurement to another. Regarding the characteristic features of the invention, reference is- made to the claims section.

The bearing capacity measuring means of the invention comprises a drop weight of suitable size, on the order of magnitude of e.g. 10 kg, holding and re¬ leasing members for the drop weight, and a measuring instrument for measuring the subsidence of the surface under measurement due to dropping the drop weight. As taught by the invention, the means comprises a tubular body inside which a drop weight has been disposed to be substantially free to move in the direction of the tune. Furthermore, the lower end of the body is closed with a loading plate against which the drop weight is flung when it is allowed to drop freely within the body. As taught by the invention, the means comprises an acceleration pick-up, for instance secured to the body, arranged to measure the movement of the body of the means during the impact. When an acceleration pick¬ up is used, the amount of movement, i.e., the magnitude of subsidence, is found by integrating the acceleration signal to give the velocity and, further, to give the displacement. It is possible of course to use a veloc¬ ity pick-up or e.g. a seismic displacement pick-up in¬ stead of the acceleration pick-up. The measuring instrument with its acceleration pick-up is advantageously placed in the upper part of the body, above the upper position of the drop weight, in which case the greater part of the measuring equip¬ ment consists of a hollow tube closed at both ends in- side which the drop weight can move under gravity effect in one direction or the other, depending on the attitude of the tube.

It is advantageous to provide on the underside of the drop weight an elastic member, e.g. a rubber shock absorber, which prevents excessively abrupt and sharp impacts against the surface under measurement, whereby the impact will more closely correspond e.g. to those loads which cars cause on the road surface.

In an advantageous embodiment of the invention the holding and releasing member belonging in the upper part of the body consists of a magnet, its control, i.e., releasing of the drop weight, being arranged by means of an electric pulse which is delivered with a release push button external to the body. It is however equally possible to apply mechanical means for releas¬ ing/securing the drop weight. When a magnet is used for releasing member, dropping the drop weight is accomplished with an elec¬ tric pulse, with which the zero resetting of the appa¬ ratus is also accomplished, e.g. with a suitable delay, counted from the moment of release. In this way, reset- ting is achieved clearly after the weight has been set free and before it strikes against the loading plate, and the recoil from the release of the weight will have died out completely and will not cause any measuring error. The means here described is meant to be port¬ able so that it can be carried in the terrain, which implies that normally no external electricity supply is available. The release pulse, e.g. 24 V, which the re¬ lease magnet requires is advantageously obtained e.g. from three 9 V press-button batteries connected in ser¬ ies, which store a release charge e.g. through a 600 ohm resistance e.g. in a 2200 microfarad electrolytic capacitor, which is discharge through the solenoid of the magnet at the moment of release. The rapidly dying pulse shape appearing across the magnet terminals is also appropriate to be used towards the base zero-set¬ ting of the electronics just before impact. The measur¬ ing electronics is advantageously constructed applying low-consumption CMOS technique, and a liquid crystal display (LCD) is advantageous in the role of display means.

The body consists, advantageously, of a tube of some light material, for instance an aluminium tube. The apparatus itself will thereby have the lowest pos¬ sible weight and the drop weight alone has a large enough mass. Therefore, the inertia of the rest of the measuring apparatus has no great influence on the meas¬ urement.

The body advantageously comprises at least one handle, suitably two separate carrying handles, by which the measuring apparatus can be transported and by which it can be upended with ease so that the drop weight is made to slide inside the body, from the lower to the upper position.

In an advantageous embodiment of the invention the acceleration pick-up is in the upper part of the body, surrounded with a suitable, thin and elastic layer, e.g. a rubber sheath, which appropriately damps excessive, sharp motions acting on the pick-up and thus increases the uniformity and accuracy of the measure¬ ments. In an advantageous embodiment of the invention holes have been provided in the tubular body, through which air can flow as the drop weight is moving within the tube. The drop weight may likewise contain suitable air passages running parallel to its direction of motion. It has been found in practice, however, that the drop weight falls substantially with complete free¬ dom through the distance of about 1 m in the tube and against the loading plate even in the absence of air holes, provided that the drop weight has been dimen- sioned so as to leave a gap of a few millimetres be¬ tween itself and the tubular body.

In an advantageous embodiment of the invention the loading plate is exchangeable, or it is possible to attach to it, e.g. on its outside, plates with various areas, whereby different kinds of measurement can be made using one drop weight. It is equally possible to use, instead of said plates, bodies of various shapes which represent load modes encountered in actual situa¬ tions, such as a horseshoe, when it is desired to meas¬ ure bearing capacities of a horse race track, for in¬ stance. The advantage of the invention over bearing capacity measuring means of prior art is in the first place seen in the simple and light-weight construction of the present means, which enables its easy and fast use, and transportability, also in small and cramped applications.

The invention is described in the following in detail, referring to the attached drawing, wherein:- Fig. 1 presents schematically and in elevational view, a bearing capacity measuring means according to the invention, and

Fig. 2 presents the means of Fig. 1, in top view.

A bearing capacity measuring means according to the invention, depicted in the drawing, comprises a circular cross-section, tubular body 4 closed at the lower end by a loading plate 5 and at the top end by the measuring apparatus 3. Between these is left a sub¬ stantially free, rectilinear tubular cavity with a length about 80 to 90% of the overall length of the means. Inside the tube has been disposed s cylindrical drop weight 1 having a diameter slightly less than the inner diameter of said cavity, and on the underside of which has been affixed a rubber pad, serving as resili¬ ent member 7. On the bottom of the measuring apparatus are affixed holding and release members 2, in the pres- ent embodiment consisting of a magnet 4 with high enough attraction to be able to hold the drop weight 1 stationary.

The measuring apparatus 3 comprises an accel¬ eration pick-up 6, braced against the body of the meas- uring means, and hereby it measures the movement of the means as the drop weight, after free fall, strikes against the loading plate 5. On the cover 10 of the means is provided a display means 11, and a power switch 13 and release button 12. The measuring means further comprises two carrying handles 9, one in the upper part and the other in the lower part of the measuring means.

The bearing capacity measuring means of the invention is operated as follows. When measuring the bearing capacity of a substantially horizontal surface, the measuring means is placed in vertical position on this surface, with the loading plate 5 against the point to be measured. With the drop weight 1 in its upper position, as in Fig. 1, the means is switched on with the power switch 13, and thereafter the release button 12 is pressed to produce a release pulse acting on the magnet 8, which releases the drop weight 1, which is thus allowed to drop freely within the tube towards the loading plate 5. The impact of the drop weight against the loading plate 5 causes downward de¬ pression of the surface that is being measured, at the same time causing the body of the means to descend an equal amount, this displacement being measured with the aid of the acceleration pick-up 6 in a manner known in itself in the art. Since as a rule the surface under measurement is resilient, the surface, and at the same time the means, will rebound a certain distance upward. Advantageously, as taught by the invention, the meas¬ urement is accomplished as a continuous measurement so that in one measurement is measured the total displace¬ ment of the body of the means during the impact, that is, the permanent as well as the elastic compaction of the test surface during impact. The user has the option of getting as measurement output e.g. only the highest values of the permanent and elastic compactions, or the curve pattern of the whole compaction event during impact, in other words, the compaction during impact as a function of time.

When a repeat measurement is desired, the measuring means is held by the carrying handles 9 and inverted, whereby the drop weight 1 is returned into contact with the magnet 8 and made adherent thereto. The measurement may then be repeated. Zero-setting of the means is provided e.g. in connection with the re¬ lease button 12 so that on pressing this button, i-e-, just before the drop weight strikes against the loading plate 5, the means will be reset to zero.

In the foregoing, the means has been described in detail with reference to an advantageous structural design thereof, while various embodiments of the inven¬ tion are feasible within the scope of the inventive idea delimited by the claims following below.

Claims

1. A means for measuring bearing capacity, or modulus of elasticity, comprising a drop weight (1) , drop weight holding and releasing means (2) , and a measuring apparatus (3) for measuring the depression of the surface under measurement caused by dropping the drop weight, characterized in that the means comprises a tubular body (4) to be placed in upright position upon the surface to be measured, inside said body being provided a drop weight (1) to be substantially freely movable in the longitudinal direction of the body; a loading plate (5) closing the lower end of said body, the drop weight being disposed to strike against this loading plate; and an acceleration pick-up (6) , dis¬ posed to measure the displacement of the body during impact.

2. Measuring means according to claim 1, characterized in that the measuring apparatus (3) with its acceleration pick-up (6) has been placed above the upper position of the drop weight (1) .

3. Measuring means according to claim 1 or 2, characterized in that the means comprises a resilient member (7) placed e.g. on the underside of the drop weight or on the topside of the loading plate, e.g. a rubber spring.

4. Measuring means according to any one of claims 1-3, characterized in that the holding and re¬ leasing member (2) consists of a magnet (8), control of this magnet, or release of the drop weight, being arranged to take place by means of an electric pulse.

5. Measuring means according to any one of claims 1-4, characterized in that the body (4) consists of a tube of light-weight construction, yet sturdy, e.g. an aluminium tube.

6. Measuring means according to any one of claims 1-5, characterized in that the body (4) compris- es at least one, preferably two carrying handles (9) .

7. Measuring ^• means according to claim 6, characterized in that one carrying handle (9) is locat¬ ed at a distance from the lower end of the body (4) so that when carried by this handle the means is balanced with the drop weight (1) in its lower position, against the loading plate (5) .

8. Measuring means according to any one of claims 1-7, characterized in that on the upper end of the measuring means, on its cover (10) , is provided a display means (11) for displaying the depression, and a weight release button (12) .

9. Measuring means according to any one of claims 1-8, characterized in that the acceleration pick-up is surrounded with a thin, resilient layer, e.g. a rubber sheath.

10. Measuring means according to any one of claims 1-9, characterized in that the drop weight (1) and/or the body (4) comprises air passages to enable free air flow while the weight is being dropped.

11. Measuring means according to any one of claims 1-10, characterized in that the release pulse going to the magnet (8) is obtained from a capacitor charged through a resistance, said capacitor being dis- charged through the solenoid of the magnet at the moment of release.

12. Measuring means according to claim 11, characterized in that the pulse dying out rapidly in the solenoid is utilized towards resetting the elec- tronics of the means immediately prior to impact.

13. Measuring means according to any one of claims 1-12, characterized in that the acceleration pick-up (6) is arranged to measure the displacement of the body during impact as a substantially continuous function.

14. Measuring means according to any one of claims 1-13, characterized in that on the underside of the loading plate (5) is attached an impact body repre¬ senting the true load acting on the object of measure¬ ment, such as e.g. a horseshoe.