Unit weight determination in soil

Janiola, Joyzelle Ann C.

July 27, 2014

CE 4A Group 3

Activity No. 4

Unit Weight Determination

Introduction

Undisturbed samples are generally taken by cutting blocks of soil, or by pushing or driving tubes into the ground. Undisturbed soil samples retain the structural integrity of the in-situ soil and have a high recovery rate within the sampler (Barnhart, 2014). Also, undisturbed soil samples allow an engineer to determine the geotechnical properties of strength, permeability, compressibility and fracture patterns among others (Geotechnical Engineering Software, 2013). Soil unit weight, also known as specific weight, is the weight per unit volume of soil. It may refer to wet unit weight or dry unit weight. Wet unit weight is the unit weight of the soil when the pores are fully or partially filled with water. While dry unit weight is the unit weight of the soil when the pores are filled only with air without any water. Moreover, soil unit weight, depends on the packing, compaction, and humidity condition of the soil. The main objective of the activity is to determine the in-place unit weight of the undisturbed soil sample obtained by pushing or drilling a thin-walled cylinder. Knowledge of the in place unit weight of undisturbed soil sample can help assess in field quality control processes whether an earth or rock fill has been compacted to the specified unit weight. Unit weight determination of undisturbed soil sample is also conducted in making comparisons and calculations related to volume and weight relationships such as void ratio or degree of saturation. For design purposes, unit weight determination is done to provide the necessary information in calculating soil loads or stresses within a soil mass or exerted by a soil mass. Another important reason to determine the unit weight of the soil is to assist in the control and management of the borrow area by providing information for calculating volume differences between excavated volume and the volume of the compacted fill areas.

Materials and Methodology A. Materials Used

 A rubber stopper, wooden rod, steel supports and the universal testing machine (UTM) was used to extract the undisturbed soil sample from the core sampler. A cutting wire (guitar string no.1) was then used to even out the base. A straight edge was also used to measure the dimensions of the sample. Lastly, an electronic balance was used to weigh the sample.

B. Preparation of soil samples

The undisturbed soil samples are taken out of the storage box and was prepared for extraction ( see Figure 1). The crumbly particles of the undisturbed sample was removed. Then a rubber stopper was pushed into the core sampler. The core sampler was then suspended, wherein its welded sides rests into the steel supports. A wooden rod was then fitted into the core sampler. The core sampler was then held tightly to prevent wobbling. The universal testing machine (UTM) was then used to push the undisturbed soil sample from the core sampler ( see Figure 2 ). The undisturbed soil sample was then seized before it falls to the base of the universal testing machine (UTM) to avoid disturbances. The samples were then wrapped with a cling wrap. The base of the soil sample was then evened out using a cutting wire ( see Figure 3). C. Determination of the unit weight

The height and the diameter of the undisturbed soil sample were measured using a straight edge ( see Figure 4 and 5 ). The soil sample was then weighed and the tare weight was recorded ( see Figure 7 ). The volume, density and unit weight of the undisturbed soil sample were then computed.

Formula for volume: πD2

Vcylinder =

h 4

Formula for density: m ρ=

V Formula for unit weight: У = ρg , where g is equal to 9.81 m/s 2

Observations/Findings/Conclusion

Table 1. Soil samp le characteristics after extraction 

Sample

Characteristics after extraction

1

It cracked and broke into two parts.

2

Side surfaces are smooth. No presence of cracks.

3

Uneven surface and has cracks.

Table 2. Soil sam ple 2

Diameter

6 cm

height

5 cm

mass

244 g

volume

141.372 cm3

density

1.725 g/cm 3

Table 1 shows the characteristics of the soil samples after extraction. The first soil sample obtained cracks and broke into two parts. The second sample has smooth side surface and does not have cracks. The third sample has jagged surface and cracks. Out of the three samples, only the second sample can be utilized for the activity. Table 2 shows the dimensions, mass, volume and density of the acceptable soil sample. Its dimensions are .06 meter for its diameter and .05 meter high and it has a mass of 244 g. The volume and density of the sample were computed using the formula mentioned above. The undisturbed sample has a volume of 141.372 cm 3 and a density of 1.725 g/cm 3. Unit weight computation of the undisturbed soil sample. У = ρg ,  where g is equal to 9.81 m/s 2

g ρ = 1.725

1 kg ×

cm3

100 cm

= 1725 kg/m 3

× 1000g

3

1m

У = 1725 × 9.81= 16922.25 N/m 3

The in-place unit weight of the undisturbed soil sample is determined to be 16922.25 N/m 3 or 16.922 kN/m 3.

References

 ASTM. 2011. Standard Test Procedures for Determining Density or Unit Weight of Soil in Place . [On-line] accessed on July 25, 2014 available at

http://www.astm.org/DIGITAL_LIBRARY/MNL/PAGES/MNL12164M.htm Barnhart, H. 2014. Disturbed Vs. Undisturbed Soil Sampling. [On-line] accessed on Jly 25, 2014 available at http://www.ehow.com/info_8747582_disturbed-vsundisturbed-soil-sampling.html Das, B.M. 2006. Principles of Geotechnical Engineering: Fifth Edition. Thomson Canada Limited, Nelson 1120 Birchmount Road Toronto, Ontario M1K 5G4 Canada. Geotechnical Engineering Software.2013.Geotechnical Engineering: The Difference Of Disturbed And Undisturbed Soil Sampling. [On-line] accessed on July 25, 2014 available at http://geotechengineeringsoftware.wordpress.com/2013/06/12/geotechnicalengineering-the-difference-of-disturbed-and-undisturbed-soil-sampling/ Wikipedia. nd. Specific Weight . [On-line] accessed on July 25, 2014 available at http://en.wikipedia.org/wiki/Specific_weight

FIGURE 1. Undisturbed soil samples.

FIGURE 2. Extraction of the sample.

FIGURE 3. Cutting the sample.

FIGURE 4. Measuring the diameter of the sample

FIGURE 5. Measuring the height of the sample.

FIGURE 6. Weighing the newspaper.

FIGURE 7. Weighing the sample