GB2443647A

GB2443647A - Method and apparatus for determining the plastic limit of soil

Info

Publication number: GB2443647A
Application number: GB0622407A
Authority: GB
Inventors: Vinayagamoothy Sivakumar
Original assignee: Queens University of Belfast
Current assignee: Queens University of Belfast
Priority date: 2006-11-10
Filing date: 2006-11-10
Publication date: 2008-05-14
Also published as: GB0622407D0; WO2008056170A1

Abstract

An apparatus for determining the Plastic Limit of a soil sample, said apparatus comprising a sample holder for holding a sample of soil, a penetration tool mounted above said sample holder for reciprocal movement along a substantially vertical axis, whereby a tip of the penetration tool can penetrate into a soil sample contained in the sample holder, wherein loading means are provided for applying a predetermined vertical load to the penetration tool, control means being provided for controlling the operation of said loading means whereby the Plastic Limit of the soil can be determined as the water content of a soil sample at which a predetermined penetration depth of the tip of the penetration tool into the soil sample is achieved upon application of said predetermined vertical load for a predetermined time period.

Description

Method and arparatus for determininri the Plastic Limit of Soil The

present invention relates to a method and apparatus for determining the physical properties of soil and in particular to a method and apparatus for determining the Plastic Limit of soil.

The physical properties of a fine-grained soil can be defined by predefined consistency limits, which are used to categorise the state of a soil into liquid, plastic, semi-solid, and solid. The limits are defined in terms of the water content of the soil and are traditionally known as Atterberg Limits or Index Limits. The measurement of such consistency limits is an essential component in any ground engineering investigation, regardless of the type and size of construction. The most commonly used consistency limits are: Liquid Limit (LL), which defines the water content at which the behaviour of the soil changes from liquid state to plastic state, and Plastic Limit (PL) which defines the water content of the soil at which the behaviour of the soil changes from plastic to semi-solid.

Consistency limits provide useful information regarding engineering properties such as shear strength, shrinkage characteristics, and settlement. Consistency limit tests are the foremost approach available to fully understand the fundamental behaviour of the soils and the only way to distinguish the soil types, i.e. whether silt or clay. Clay particles are plate-like in shape, while silt particles are not. Since the size of the particles is small and cannot be detected by the naked human eye, the only feasible method to separate the silt particles from clay particles is to use index tests.

The determination of the Liquid Limit of a soil sample is carried out by means of a mechanical method widely known as the cone penetrometer or fall-cone test. The Liquid Limit is evaluated by determining the value of water content of the soil to allow 20mm penetration of a cone weighing BOg with a tip angle of 3O. The test is carried out sing a standard test device with good repeatability characteristics and hence the opportunity for human error is small.

The Plastic Limit of a soil sample is typically evaluated by determining the water content of the soil when a thread, made by rolling the soil on a glass plate, breaks up at diameter of 3 mm. This known test is specified in British Standard BS 1377:Partl:1990.

The accuracy of the abovementioned method for determining the Plastic Limit of a soil sample relies heavily upon the expertise of the operator performing such a test and thus the method is prone to human error and poor repeatability and hence low accuracy. Well-documented drawbacks of this method and its subjective nature include considerable operator judgement, such as pressure applied during rolling, speed of rolling and geometry of the thread. Vague guidelines on test procedure, friction between hand, soil and glass plate, risk of contaminants within the soil sample and the lack of any direct measurement of soil strength, together contribute to devalue the standard thread rolling method and create demand for a more controlled laboratory procedure.

A known method attempt to provide a more accurate method for determining the Plastic Limit of a soil sample is disclosed in GB 2,297,849.

Such document discloses a method of forcing a penetration cone in to the soil sample using mechanical means and measuring the load applied to the cone as a function of displacement to evaluate the Plastic Limit.

However the method involves a considerable amount of preparation time and requires complex instrumentation to accurately measure the load I. applied to the cone. Thus adoption of such proposed procedure is difficult in standard laboratories.

This there is a demand for a simplified, user friendly, inexpensive and versatile equipment that could be used to evaluate the Plastic Limit of a soil sample in any standard geotechnical laboratory.

In one embodiment the load applying means for applying a predetermined vertical load to the penetration tool comprises a pneumatic or hydraulic piston cylinder arrangement. Preferably said control means comprises valve means for controlling the application of a predetermined air or fluid pressure to an upper side of the piston of said piston cylinder arrangement.

Preferably the control means includes timer means to enable the control means to automatically deactivate the loading means when said I. predetermined vertical load has been applied on said penetration tool for a predetermined time period.

Preferably said load applying means can be disconnected from the penetration tool or deactivated to permit the penetration tool to penetrate a soil sample in the sample holder under its own weight whereby the apparatus can be used to determine the Liquid Limit of the soil sample.

Preferably the tip of the penetration tool is defined by a conical member, preferably having a cone angle of 300.

A method of determining the Plastic Limit of soil comprising the steps of:-i) providing a test apparatus comprising a sample holder for holding a sample of the soil, a penetration tool mounted above said sample holder for reciprocal movement along a substantially vertical axis, whereby a tip of the penetration tool can penetrate the soil sample, loading means for selectively applying a predetermined vertical load to the penetration tool; ii) placing the soil sample in the sample holder; iii) operating the loading means to apply a predetermined vertical load on the penetration tool for a predetermined period of time; iv) measuring the penetration depth of the tip of the penetration tool into the soil sample following step (iii); v) recording the water content of the soil sample; vi) placing a further soil sample in the sample holder, said further soil sample having a water content different from that of the previous soil sample; vii) repeating steps (iii) to (vi) to determine the water content of the soil at which the penetration depth of the tip of the penetration tool equals a predefined penetration depth, said predefined penetration depth being the penetration depth of the tip of the penetration tool for a known soil sample at its Plastic Limit when the predetermined vertical load is applied to said penetration tool for said predetermined time period.

Preferably the method comprises the further step of initially calibrating said test apparatus by placing a soil sample known to have a water content equal to the Plastic Limit of the soil sample in the sample holder, determining the load required to be applied to the penetration tool to achieve said predefined penetration depth when said load is applied for said predetermined time period, said determined load being recorded as said predetermined vertical load of step (iii).

Said predefined penetration depth may be between 10mm and 30mm, preferably between 15mm and 25mm. In a preferred embodiment the predefined penetration depth is 20mm.

The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a perspective view of a known test apparatus for determining the Liquid Limit of a soil sample; Fig. 2 is a schematic view of a modified test apparatus according to an embodiment of the present invention for determining both the Liquid Limit and the Plastic Limit of a soil sample.

A known apparatus for determining the liquid limit of a soil is illustrated in Fig 1 and comprises a stand 1 having a base 2 for supporting a sample cup 3, a penetration tool being mounted on the stand 1 for reciprocal movement along a vertical axis above the sample cup 3, the penetration tool comprising an elongate rod 4 passing though a guide bore mounted on the stand 1, a penetration cone 5 being provided at a lower end of the rod 4, said penetration cone 5 having a cone angle of 30g. The penetration tool has a mass of 80g. A dial gauge 6 is mounted on the upper region of the stand in contact with an upper end of the rod 4 to measure the penetration depth of the penetration cone.

As standard, the determination of the Liquid Limit of a soil is defined as the water content at which the penetration cone of 80g mass (0.78N weight) and 30 tip angle penetrates 20mm when released from an initial point at which the cone tip is in contact with the soil surface when the penetration cone is applied to the soil for a predetermined time period (usually ten seconds).

In order to control the time period during which the soil is exposed to the weight of the penetration cone, a timer 7 is used to control a magnetic clamp means 8, energisable to lock the elongate rod 4 of the penetration tool within the guide bore.

The present invention is based upon the realisation that by using a certain force applied to the standard 80g, 3O penetration cone the Plastic Limit can be found at a depth of 20mm penetration. Simply incorporating a heavier cone into the known test apparatus would be impractical due to difficulties in handling the cone.

Therefore, in accordance with an embodiment of the present invention, the known test apparatus is modified to apply a predetermined vertical load onto the to the existing 80g penetration cone.

As illustrated in Fig 2, such load is provided by means of a piston cylinder arrangement 10 placed above the standard penetration cone device that is used for evaluating the Liquid Limit, to selectively apply an additional loading force to the penetration cone 5.

As illustrated in Fig 3, the cylinder 11 is fixed with respect to the stand 1 of the test apparatus and a piston 12 is reciprocally mounted within the piston to divide the cylinder into upper 13 and lower 14 chambers. A diaphragm 15 is mounted between the piston 12 and cylinder 11 to provide a gas tight seal between the upper and lower chambers.

The piston 12 is attached to or formed with a vertically arranged piston rod 1 6a, 1 6b extending from either end of the cylinder 11. The upper chamber 13 of the cylinder is selectively connectable to a regulated pressure source as will be described below. To eliminate the build up of pressure during loading of the penetration cone, the lower chamber 14 of the cylinder is vented to the atmosphere.

The depth of the penetration is read by digital dial gauge 6 mounted above the piston 12 and in contact with an upper end 16a of the piston rod.

Pressurised air is selectively supplied to the upper chamber 13 of the cylinder 11 via a pressure regulator 17 and a control valve 18. A pressure sensor and display 19 is provided to enable the user to determine the pressure applied.

A control means incorporating a timer may be provided for automatically controlling the opening and closing and the duration of opening of the control valve.

In order to determine the Plastic Limit of a soil sample, the apparatus is first calibrated using a soil sample for which the water content at the Plastic Limit is known.

A typical calibration sample comprises kaolin, for which the Plastic Limit is well known and equivalent to approximately 34% water content.

Calibration tests are performed in order to evaluate the pressure required to achieve a penetration of 20mm at a water content of 34% when the penetration cone is loaded for a period of 15 seconds. This pressure is then recorded and applied in subsequent tests to determine the Plastic Limit.

A method of using the calibrated test apparatus will now be described.

Initially soil specimens are passed through a 425pm sieve and the sieved materials are stored in air-tight containers.

To evaluate the Liquid Limit of the specimen, the known evaluation method is used, in which about 200g of the soil is mixed with de-aired water to produce a thoroughly mixed workable paste. The paste is then placed into an air-tight container to allow the sample to equilibrate over a period of 24 hours.

A proportion of the sample is packed into the standard metal sample cup 3 and the penetration of the penetration cone 5 is measured after a predetermined period (typically 10 second) from the release of the penetration cone 5 from its initial position in which the cone tip rests on the flat surface of the sample within the cup 3.

The procedure is repeated at different water contents in order to establish a relationship between penetration and water content. The water content at the Liquid Limit is defined as that for which a penetration depth of 20mm is achieved.

To evaluate the Plastic Limit of the soil specimen the followings steps are carried out.

Approximately 300g of dry material is mixed at four different water contents. Typically the samples are stored in air-tight containers and allowed to equilibrate for a period of 24 hours. The wet material is then packed into the sample cup 3 and levelled with a straight edge. If the soil is saturated, in theory the amount of pressure applied on the soil should not influence the bulk density or the void ratio of the soil and this has been independently verified by compressing the wet materials in the cup to range of pressures.

The cup 3 with the sample is placed under the penetration cone 5 and the gauge 6 is zeroed when the tip of the cone 5 was just in contact with the surface of the sample. The valve 18 located between the cylinder 11 and the regulated air supply is then opened, which in effect applies an instantaneous load on the penetration cone 5 and results in penetration into the soil. The valve is closed, automatically by the control means if provided, and the penetration is terminated after a predetermined time, typically 15 seconds.

Possible human error in evaluating Plastic Limit using the method and apparatus of the present invention is considerably reduced compared to previous methods.

From tests performed, it has been concluded that at an applied load of 55N on the cone 5 with a tip angle of 3O, leads to a reliable determination of Plastic Limit.

Various modifications and variations to the described embodiment of the inventions will be apparent to those skilled in the art without departing from the scope of the invention as defined in the appended claims. Although the invention has been described in connection with a specific preferred embodiment, it should be understood that the invention as claimed should not be unduly limited to such specific embodiment.

Claims

Claims 1. An apparatus for determining the Plastic Limit of a soil

sample, said apparatus comprising a sample holder for holding a sample of soil, a penetration tool mounted above said sample holder for reciprocal movement along a substantially vertical axis, whereby a tip of the penetration tool can penetrate into a soil sample contained in the sample holder, wherein loading means are provided for applying a predetermined vertical load to the penetration tool, control means being provided for controlling the operation of said loading means whereby the Plastic Limit of the soil can be determined as the water content of a soil sample at which a predetermined penetration depth of the tip of the penetration tool into the soil sample is achieved upon application of said predetermined vertical load for a predetermined time period.
2. An apparatus as claimed in claim 1, wherein the load applying means for applying a predetermined vertical load to the penetration tool comprises a pneumatic or hydraulic piston cylinder arrangement.
3. An apparatus as claimed in claim 2, wherein said control means comprises valve means for controlling the application of a predetermined air or fluid pressure to an upper side of the piston of said piston cylinder arrangement.
4. An apparatus as claimed in any preceding claims, wherein the control means includes timer means to enable the control means to automatically deactivate the loading means when said predetermined vertical load has been applied on said penetration tool for a predetermined time period.
5. An apparatus as darned in any preceding claim, wherein said load applying means can be disconnected from the penetration tool or deactivated to permit the penetration tool to penetrate a soil sample in the sample holder under its own weight whereby the apparatus can be used to determine the Liquid Limit of the soil sample.
6. An apparatus as claimed in any preceding claim, wherein the tip of the penetration tool is defined by a conical member.
7. An apparatus as claimed in claim 6, wherein the conical member has a cone angle of 300.
8. A method of determining the Plastic Limit of soil comprising the steps of:-i) providing a test apparatus comprising a sample holder for holding a sample of the soil, a penetration tool mounted above said sample holder for reciprocal movement along a substantially vertical axis, whereby a tip of the penetration tool can penetrate the soil sample, loading means for selectively applying a predetermined vertical load to the penetration tool; ii) placing the soil sample in the sample holder; iii) operating the loading means to apply a predetermined vertical load on the penetration tool for a predetermined period of time; iv) measuring the penetration depth of the tip of the penetration tool into the soil sample following step (iii); v) recording the water content of the soil sample; vi) placing a further soil sample in the sample holder, said further soil sample having a water content different from that of the previous soil sample; vi) repeating steps (iii) to (vi) to determine the water content of the soil at which the penetration depth of the tip of the penetration tool equals a predefined penetration depth, said predefined penetration depth being the penetration depth of the tip of the penetration tool for a known soil sample at its Plastic Limit when the predetermined vertical load is applied to said penetration tool for said predetermined time period.
9. A method as claimed in claim 8, wherein the method comprises the further step of initially calibrating said test apparatus by placing a soil sample known to have a water content equal to the Plastic Limit of the soil sample in the sample holder, determining the load required to be applied to the penetration tool to achieve said predefined penetration depth when said load is applied for said predetermined time period, said determined load being recorded as said predetermined vertical load of step (iii).
10. A method as claimed in claim 8 or claim 9, wherein said predetined penetration depth is between 10mm and 30mm.
11. A method as claimed in claim 8 or claim 9, wherein said predefined penetration depth is between 15mm and 25mm.
12. A method as claimed in claim 8 or claim 9, wherein said predefined penetration depth is 20mm.