Procedure Adopted for Field Compaction Control | Soil Engineering

In this article we will discuss about:- 1. Parameters for Compaction Specification in the Field 2. Objective of Field Compaction Control 3. Determination of In-Situ Density.

Parameters for Compaction Specification in the Field:

The parameters used to ensure effective compaction in the field include (a) relative compaction and (b) placement water content.

(a) Relative Compaction:

Determine the relative compaction using Eq. (12.7) to specify the limits of dry density to be achieved for the compacted fill with respect to the MDD obtained from laboratory IS light or heavy compaction as the case may be –

The Engineer-in-charge will specify the relative compaction to be achieved.

For cohesive soils, a relative compaction of 95% of laboratory standard Proctor test (IS light compaction test) can be achieved using either sheep’s foot rollers or, in some cases, pneumatic tired rollers. For low plastic cohesive soils, a relative compaction of 95% of modified Proctor test (IS heavy compaction test) can be achieved using pneumatic tired rollers.

For cohesionless soils, a relative compaction of the order of 98%-100% or even more of modified Proctor feat (IS heavy compaction test) can be achieved using vibratory rollers or pneumatic tired rollers.

(b) Placement Water Content:

The water content to be used for compacting the soil in the field is known as placement water content. Usually, a placement water content of ±2% of OMC obtained from relevant laboratory compaction test is adopted.

Water content different from OMC may be occasionally specified by the Engineer-in-charge to achieve an intended purpose. Placement water content less than OMC (dry of optimum) may be specified for highway embankments of cohesive soils to achieve higher shear strength and lower compressibility. Similarly, outer shells of earth dams are compacted at a placement water content dry of optimum (less then OMC) to achieve higher shear strength, higher permeability, and lower pore pressures.

Compaction of subgrades below pavements and foundation soils below floors may be done at a placement water content more than the OMC (wet of optimum) to prevent excessive swelling and to achieve lower swelling pressure. Similarly, the core of an earth dam is compacted at a placement water content more than the OMC (wet of optimum) to reduce the permeability and pre-empt swelling.

Once a placement water content is specified, which may be equal to the OMC, or dry or wet of optimum as discussed above, the water content used for compaction should be within ±2% of the placement water content or it may be within any other limits as specified by the Engineer-in-charge.

Objective of Field Compaction Control:

The following are the important objectives of field compaction control:

1. To determine the in-situ dry density and water content immediately after the compaction of each lift and to ensure that it satisfies the limits of relative compaction and placement water content as per compaction specifi­cations of the Engineer-in-charge.

2. To check and ensure that the soils from the prescribed borrow area, having the desired properties, are used for compaction.

3. To check and ensure that the required compaction energy is used in compacting the soil. This consists of ensuring that the required type of roller suitable to the soils being compacted is used as well as that the roller is of required capacity. It is also necessary to ensure that the required minimum number of passes of the roller is used for compaction of each lift. The lift thickness, which is the thickness of each compacted layer, is also to be controlled depending on the type of roller used as excessive lift thickness that leads to ineffective compaction.

4. Certain minimum number of tests are to be done in the field when the compaction is in progress:

For large fills, one test for every 1000 m² area/lift

For small fills (< 1000 m² area), two or three tests/lift

Rapid methods of water content determination are to be used because if the quality of compaction is not within specified limits of compaction specification, it is required to remove the corresponding lift and re-lay Thus, the results of field tests of each lift during field compaction are required before the next lift is started.

Determination of In-Situ Density:

The determination of relative compaction requires the following finding:

1. In-situ bulk density.

2. Field moisture content.

The in-situ density can be computed by any one of the following methods:

1. Core cutter method.

2. Sand replacement method.

3. Rubber balloon method.

Determination of water content of compacted soil can be done by any one of the following methods:

i. Proctor’s Needle Method:

Principle:

The basic principle of Proctor’s needle method is to determine the water content of compacted soil indirectly without drying the sample based on the resistance offered by the compacted soil to the penetration of Proctor’s needle.

A detailed description of the Proctor’s needle method is given below:

Proctor’s Needle:

The Proctor’s needle consists of a needle attached to a spring-loaded plunger. The needle consists of a needle point attached to the bottom of a needle shank, as shown in Fig. 12.10. The needle can be pushed into the compacted soil by pressing the loading plunger. The needle shank has graduations to read the penetration of the needle into the compacted soil.

The stem of the loading plunger has graduations to show the resistance offered by the compacted soil to the penetration of the needle. The loading plunger is calibrated to indicate the penetration resistance of the compacted soil based on the deformation of the spring, which depends on the load applied and the spring constant.

Needle points of different cross-sectional areas are supplied along with the equipment such as 0.25, 0.5, 1.0, and 2.5 cm² to use in compacted soils of increasing penetration resistance.

Calibration Chart:

To use the Proctor’s needle to determine the water content, it is required to prepare a calibration chart with the soil used for compaction.

Suitable needle point is selected and attached to the bottom of the needle. The soil used for compaction is taken and its MDD and OMC are determined using either IS light compaction or heavy compaction based on the need.

While conducting the compaction test, the penetration resistance of the compacted soil in the mold is deter­mined using the Proctor’s needle of suitable needle point. The soil is compacted at the given water content in the compaction mold. After the compaction is completed, the collar is removed, the excess soil above the top of the mold is trimmed off and the surface is levelled.

The Proctor’s needle is now placed on the surface of the compacted soil in the mold and forced down by 7.5 cm into the compacted soil at a rate of about 1.25 cm/s. The penetration is checked from the graduations on the needle and the penetration resistance is read from the graduations on the stem of the loading plunger, corresponding to the position of the sliding ring. The water content and dry density of the compacted soil is determined by oven drying method as usually done in a compaction test.

The procedure is repeated by compacting the soil at different water contents, as in compaction test, and deter­mining the penetration resistance at each water content. The compaction curve is plotted as done in the compaction test between water content on x-axis and dry density on y-axis. On the same graph, the penetration resistance is plotted on y-axis for different water contents on x-axis. The area of the needle point is noted on the calibration chart.

Determination of In-Situ Moisture Content and Dry Density:

The soil used for compaction in the field, mixed with placement water content, is compacted into the compaction mold using the same compaction energy. The Proctor’s needle with the same needle point as used for preparation of calibration chart is forced into the compacted soil in the mold and the penetration resistance is determined.

The water content and dry density of the compacted soil are then read from the calibration chart corresponding to the penetration resistance.

Advantage and Limitations:

The advantage of Proctor’s needle method is that drying of soil is not required for determination of in-situ water content.

The disadvantage is that the actual in-situ compaction conditions are not truly reflected in the dry density and water content determined by this method. For in-situ compaction, soil is compacted by the appropriate roller in specified lift, whereas compaction in the mold is done using the rammer by application of blows. The water content determined from the Proctor’s needle method gives the water content and dry density of a laboratory compaction test rather than in-situ water content of field compacted soil.

Another serious limitation of the method is that water is added to the soil prior to compaction of the soil, whereas water is added in installments during field compaction.

ii. Nuclear Method:

The nuclear method is a rapid, non-destructive method for in-situ determination of bulk density and water content. As the test is non-destructive, it does not require cutting the sample from the compacted fill. Hence a large number of measurements can be made at every location and the accuracy can be verified and improved using statistical analysis.

Equipment:

Several nuclear gauges are now available; example is a Troxler nuclear gauge-3430 or 3440 model, shown in Fig. 12.11.

Principle:

In-Situ Density:

Gamma rays are emitted into soil which are scattered by the electrons in the soil and lose energy. The scattered rays are detected by the detector. Higher the density of soil, higher will be the density of electrons and fewer scattered rays reach the detector. The nuclear gauge is calibrated to read the density of the soil based on the number of scattered rays reaching the detector.

In-Situ Moisture Content:

Fast neutrons are emitted from a fast neutron radioactive source into the soil. Neutron collision with those of soil result in energy loss. The energy loss is high in neutron collisions with atoms of low atomic weight, changing the fast neutron to a slow neutron. Hydrogen present in soil water causes this energy loss. The number of slow neutrons detected by the gauge after emission of fast neutrons is counted and is proportional to the amount of hydrogen and hence water present in the soil.

There are two methods that can be used by the nuclear gauge:

1. Back-scatter method, in which source and the detector of the gauge are resting on the surface of the soil being tested, as shown in Fig. 12.12(a).

2. Direct transmission method, in which the source in the rod is extended below the gauge into the soil being tested and the detector is on the surface of the soil, as shown in Fig. 12.2(b).

The nuclear gauge is user-friendly and the interface is through simple queries in either way. The gauge should be checked for precision once in a week, or whenever there is a change in test location.

Limitations:

1. The nuclear gauge is more sensitive to the density of soil close to the surface in back-scatter method. Large size rocks or voids in the source-detector path may cause higher or lower density determination than the actual density.

2. The moisture content determination is based on the assumption that the hydrogen present in the soil is in the form of water. Hydrogen in other forms or carbon, if present in the soil, will cause higher water content than the actual water content.

3. Boron, chlorine or even small quantities of cadmium will cause lower measurement of water content than the actual value.

4. The nuclear gauge utilizes radioactive materials which are hazardous to the users unless proper precautions are taken.

Precautions:

1. Every nuclear gauge should be supplied with effective user instructions with safety procedures, including radioactive source leak tests.

2. All external neutron and radioactive sources should be kept away from the gauge to avoid effect on the precision of the measurements.