Measurement of Soil Density - Control of Soil Density

Chapter 4 Soil Characteristics

4.3 Control of Soil Density

4.3.2 Measurement of Soil Density

To observe the distribution of soil density in the soil bin, soil density cups were made. The soil density cup made of acrylic is illustrated in Fig. 4.7. The cylindrical cup wall was only 10 mm-high, so that the shear deformation and volume reduction could occur in the cup during testing. A picture of the soil density cup is shown in Fig. 4.8.

During the preparation of the 1.5 m-thick loose soil specimen, density cups were buried in the soil mass at different elevations and different locations in the fill as shown in Fig.

4.9 and Fig. 4.10. After the loose soil had been filled up to 1.5 m from the bottom of the soil bin by air-pluviation, density cups were dug out from the soil mass carefully.

Fig. 4.11 shows the mass of the cup and soil in the cap was measured with an electrical scale.

For a 1.5 m-thick air-pluviated Ottawa sand layer, the distribution of soil density with depth is shown in Fig. 4.12. For the loose sand, the mean unit weight is 15.6 kN/m², the mean relative density is Dr = 36 % with the standard deviation of 2.0 %. Das (2010) suggested that for the granular soil deposit with a relative density 15%  Dr



50% is defined as loose sand. The loose relative density Dr = 36 % achieved by the air-pluviation method is this study (Fig. 4.12.) was quite loose and uniform with depth.

Chapter 5 Testing Procedure

The procedure to conduct the cyclic torsional shear test is introduced in this chapter.

The testing procedure can be divided into three parts: (1) specimen preparation; (2) application of vertical static load; and (3) application of cyclic torsional shearing. These parts will are illustrated in the following sections with pictures.

5.1 Specimen Preparation

Fig. 5.1 shows air-dry Ottawa sand in the soil storage. The soil was shoveled from the soil storage to the sand hopper, and the mass of the fill was measured with an electrical scale (Fig 5.2). Fig. 5.3 shows the sand hopper was lifted by the overhead crane in the laboratory. Fig. 5.4 shows Ottawa sand was deposited by air-pluviation method into the soil bin. To achieve the loose backfill, the drop height was controlled to be 1.0 m and the hopper slot-opening of 15 mm were selected. The 1.0 m-long rope next to the hopper was used to control the drop distance. Fig. 5.5 (a) and (b) show portable ladders were placed on top of the sidewalls, and a bridge board was placed between the ladders. Throughout the test, the operator stayed on the bridge board to avoid any unexpected surcharge on the soil specimen.

Fig. 5.6 shows the leveling of the pluviated soil surface by the student with a brush.

Fig. 5.7 shows density cups were buried in the soil mass at different elevations in the fill. Fig. 5.8 shows how check the density cup horizontal with a bubble level. The empty eight density cups were placed on the surface of the soil layer. The air-pluviation of soil

and density cup placement operations were repeated unit a fill thickness T = 1.5 m was reached.

5.2 Application of Vertical Static Load

The procedure to apply the vertical static load on top of the air-pluviated loose sand is introduced. The cyclic torsional shear compactor (Fig. 3.4) used to apply static load has a footing diameter of 0.45 m and the vertical static load q = 10.35 kPa. Fig. 5.9 (a), (b), (c), and (d) illustrates the loading pattern on soil surface. For applying four times of vertical static load, the 3x3 loading formation is based four points A, B, C, and D.

Fig. 5.10 shows the CTSC was hoisted with overhead crane into the soil bin. Fig.

5.11 shows the vertical static load was applied on the loose sand with four different loading pattern shown in Fig. 5.9 (a) to (d). The combination of static load footprint caused a uniform surcharge on the soil structure. Fig. 5.12 shows the soil surface of the 4 patterns of vertical static load.

5.3 Application of Cyclic Torsional Shearing

In this study, the cyclic torsional shear was applied on the soil surface with a rotation angle of +5∘to -5∘. Fig. 5.13 show that applied the CTSC with the rotation angle controller on the loose fill. In Fig. 5.14, 5.15 and 5.16, with a rotation angle indicator and controller, the rotation angle of the shearing disc could be effectively controlled to be from 0∘to +5∘and -5∘. The application of cyclic torsional shear to loose sand is shown in Fig. 5.17.

For the test with N = 20, the soil surface after the torsional shear for the 3x3 loading formation (Fig. 5.9 (a)) for the first 5 cycles is shown in Fig. 5.18 (a). To prevent disc penetration due to continuous shearing at the same location, the shearing was moved to

another 3x3 formation (Fig. 5.9 (b)) for N = 6 to 10. The soil surface after shearing compaction for N = 6 to 10 is shown in Fig.5.18 (b). Fig. 5.18 (c) and (d) show that the soil surface after the loading pattern for N = 11 to 15 (Fig. 5.9(c)) and N = 16 to 20 (Fig.

5.9 (d)).

To determine the relative density of soil in the cup, Fig. 5.19 shows the density cup was carefully dug out of compacted soil mass. Fig. 5.20 (a) to (c) show the scraping of soil toward the edge of the density cup with a spatula. Fig. 5.21 shows the brush away of soil particles from the base plate of cup. Soil mass in the cup was measured with an electrical scale and the relative density of the compacted soil could be calculated.

Chapter 6 Test Results

This chapter showed experimental results regarding soil densification due to static load and cyclic torsional shearing. The vertical static load applied of the fill was q = 10.35 kPa. The cyclic torque T and shear stress 𝜏_𝑚𝑎𝑥 applied on the soil surface was measured and calculated, respectively. Experiments were first conducted on the surface of a 1.5 m-thick soil lift. The surface settlement S and relative density Dr distribution of the soil layer due to the static load and cyclic torsional shear were measured. The rotation angleof the shearing disc varied between +5∘and -5∘, and the number of loading cycle N varied from 1 to 40. In the second part of this chapter, to obtain a soil mass with a relative density greater than 70 to 75%, experiments were applied on the fill for five 0.30 m-thick lifts. Each lift was compacted with the cyclic torsional shear compactor with q = 10.35 kPa, = ⁺5∘, and N = 20.

6.1 Static Load Tests on a 1.5 m-thick Lift

To separate the densification effects due to static and cyclic loadings, in this section, the surface of a 1.5 m-thick soil lifts was compressed with the static vertical loading q only. Effects of soil densification such as the volume change, change of relative density in the compressed fill were investigate.

For this test, a 1.5 m-thick lift was prepared by air-pluviation method. Fig 6.1 (a) and (b) showed that the density cups were buried in the soil mass at different elevations and locations in a 1.5 m-thick fill. Fig 6.2 showed that measure points A to I for surface

settlement. The surface settlements of a 1.5 m-thick compressed soil lift due to the static load of the compactor were investigated. The initial relative density of the loose fill was 36 ± 2% (see Fig. 4.12.) The applied static normal stress was q = 10.35 kPa. To achieve a uniform settlement, the vertical loading was applied on the surface with four different 3x3 formations as indicated in Fig. 5.9.

6.1.1 Volume Change Due to Static Load

Fig. 6.3 showed the settlement measurement was carried out with the laser distance meter placed between two steel beams. The surface settlements were measured at measure at points A to I indicated in Fig 6.2. Fig. 6.4 (a) showed that surface settlement of Lift1 due to static load, the minimum and maximum values were 14.1 mm and 17.4 mm. The average of surface settlement was 15.4 mm.

To express the dimensional volume change characteristics, the volume change data was normalized by dividing the volume change ∆V by the original volume V0 to obtain the volumetric strain v. The fill in the soil bin does not allow any lateral deformation.

Only vertical compression was allowed for volume change. The horizontal cross-section of soil mass was kept a constant A. The volumetric strain v of the soil mass is defined as: induced volumetric strain v was about 1.02 %. It is obvious that static vertical loading is an effective method to compress the loose fill. To limit the scope of thesis, only the vertical stress q = 10.35 kPa was used throughout this study.

6.1.2 Relative Density after Static Load

To investigate the relative density distribution in the compressed fill, density cups were buried in soil mass at different elevations and locations as shown in Fig. 6.1. (a) and (b). For the un-compacted loose soil, the initial relative density was about 36 % (see Fig 4.12). Fig. 6.5 showed the distribution of relative density with depth due to the application static vertical load = 10.35 kPa on the 1.5 m-thick lift. The segmental line was obtained by connecting data point closet to the average Dr for at the depth. It is obvious show that the relative density increase at the top of the lift. The relative density increase was most apparent in the upper 0.45m of the lift, was equal to the diameter of the shearing disc. However, in the lower part the lift the relative density did not enough to reach the target value of Dr = 70 to 75 % required by NAVFAC DM-7 (US Navy 1982).

6.1.3 Relative Density Increase Ratio

To investigate the effects of the cyclic torsional shear compaction, the relative density increment ∆𝐷_𝑟 = 𝐷_𝑟,𝑁− 𝐷_{𝑟,𝑙𝑜𝑜𝑠𝑒} is defined, where 𝐷_𝑟,𝑁 = the relative density due to compaction with N cycles of shearing, 𝐷_{𝑟,𝑙𝑜𝑜𝑠𝑒} = the relative density of loose sand. Fig 6.6 (a) showed that the distribution of relative density increment due to static load with depth. In the figure, near on the top of the fill, the relative density increased significantly. Little Dr increase was observed near the bottom of the 1.5 m-thick fill.

To study the effects due to cyclic torsional shearing compaction, a new index was defined in this section. The relative density increment was normalized by the relative density of loose sand. The Relative Density Increase Ratio, RDIR, was defined as:

NAVFAC DM-7 (1982) reported that the relative density of 70% to 75% can be obtained by proper compaction procedures. In this study, the initial relative density of the loose fill is 36% (see Fig 4.12). Based on Eqn. (6.2), the target of range of soil improvement corresponding to Dr = 70% and 75 % would be 0.94 to 1.08, respectively, the RDIR due to static load was far from the required relative density increase ratios.

6.2 Cyclic Torsional Shear Compaction on a 1.5 m-thick Lift

In the experiments, the surface of a 1.5 m-thick single soil lift was first compressed with the static vertical load (dead-load of the compactor), and then compacted with cyclic torsional shearing. The effects of soil densification were demonstrated with the surface settlement and relative density change of the compacted fill.

6.2.1 Measurement of Applied Torque

Fig. 6.7 showed the torque applied on the soil surface was measured with a digital torque meter. Fig 6.8 showed that relationship between the measured torque T’ and applied torque T. Fig 6.9 showed the difference between the CTSC with extension tube or without extension tube. For the rotation angleof shearing discchanging from +5

∘to -5∘, the torques measured at N = 1, 5, 10, 15, and 20 were shown in Fig. 6.10, 6.11, and 6.12, respectively. In Fig.6.8, for N = 1 the applied torque varied between 67.8 to -65.8 N-m. In Fig. 6.12 (b), for N = 20 the applied torque varied between 69.7

to -70.2 N-m. Fig 6.13 showed the applied torque T as a function of the number of cycle of cyclic torsional shearing. Test results indicated that the applied torque did not change with increasing number of shearing cycles. Test Results also indicated that the application extension tube (see Fig. 6.5 (b)) did not affect the transmission of torque the torque loading device to the shearing disc.

Fig. 6.14 showed the how to determine the maximum torsional shear stress max at the edge of the shearing disc due to the applied torque T. A linear distribution of shear stress from the center to the edge of the disc was assumed. Fig. 6.15 showed the maximum shear stress with increasing number of cycle of torsional shearing.

6.2.2 Volume Change Due to Cyclic Torsional Shear Compaction

Fig 6.16 showed that, after the application of the static loading, cyclic torsional shear was applied on the surface of soil fill with the 3x3 formation for N = 1, 2, 3, 4, 5, 10, 15, 20, 30, 40. The applied vertical stress was 10.35 kPa. The rotation angle of the shearing disc varied between +5˚ to-5˚. The diameter of the shearing disc was 0.45 m.

The cyclic torsional shear was applied on the 3x3 loading pattern. Fig. 6.17 (a) showed the surface settlement after the first cycle of torsional shearing application. The measured surface settlement varied from 17.6 to 21.6 mm, the average value of surface settlement was 19.4 mm. Fig. 6.26 (a) showed the surface settlement after 40 cycles of cyclic torsional shearing application. The measured surface settlement varied from 34.3 to 39.8 mm, and the average value was 37.4 mm. The extra settlement due to the cyclic torsional shearing compaction was about 22 mm, which was more than the settlement due to static vertical loading. Fig 6.27 (a) show the measured surface settlement of the 1.5 m-thick fill after the application cyclic torsional shearing cycles of 1, 2, 3, 4, 5, 10,

15, 20, 30 and 40 cycles. In the figure, the surface settlement increased with increasing number of



max application. Fig. 6.28 (a) showed the variation of surface settlement from 19.4 to 37.4 mm with increasing value from static load to N = 40.

Fig 6.17 (b) showed the volumetric strain after the first of cycles of torsional shearing application. The volume change was normalized by the initial soil volume by Eq. (6.1), the value of the volumetric strain due to the first cycle of



max varied from 1.17 % to 1.44%, the average value was 1.29 %. Fig. 6.26 (b) showed the volumetric strain after 40 cycles of varied from 2.28 % to 2.65 %, the average value was 2.49 %.

Fig 6.27 (b) showed the volumetric strain of the 1.5 m-thick fill after application of cycle shearing for 1, 2, 3, 4, 5, 10, 15, 20, 30 and 40 cycles. In the figure, the volumetric strain increased with increasing number of cycles of cyclic torsional shearing. Fig 6.28 (b) showed the variation of surface settlement and volumetric strain with number of cycle N of shearing. It should be mentioned that, for 1.5 m-thick soil fill, the densification due to shearing compaction occurred only at the upper most part of the soil mass. Little volume change occurred at the lower part of the fill. Therefore, the volumetric strain of the entire soil body may not be very significant.

In Fig 6.28 (a), in the first 5 cycles of cyclic torsional shearing application, the surface settlement was increased significantly. However, after 20 cycles, the major part of settlement was accomplished, soil particles were sheared and reached a densely-packed condition. As a result, it was difficult to increase the surface settlement any further with more cyclic shear application. Thus, N = 20 may be the optimal number for cyclic torsional shearing construction.

6.2.3 Relative Density Distribution after Change Due to Surface Compaction

Fig 6.29, 6.30, 6.31 showed the distribution of relative density due to cyclic torsional shearing for N = 5, 10 and 20, respectively. It is clear in these figures, the soil density increase due to static load and shearing compaction was most obvious in the upper of the lift. There was little density increase at the bottom of the 1.5 m-thick lift.

To achieve the required relative density Dr = 70 to 75 %, for the entire soil mass, several strategies were proposed: (1) enlarging the diameter of shearing disc D (to influenced depth); (2) reducing the lift thickness of fill layers, (for example, from T = 1.5 m to T

= 0.3 m); (3) increase the applied torque T and the cyclic torsional applied shear stress



max accordingly.

Fig. 6.32 showed the distribution of relative density after the application of cyclic torsional shearing for N = 0 (static load), 5, 10, 20. In the figure, the relative density of the compacted fill increased with increasing number of cycles of torsional shearing application. The US Navy design manual (NAVFAC DM-7.2 1982) described that for coarse-grained, granular well-graded soils, 70 to 75 % relative density can be obtained by proper compaction procedures. In this study, the range Dr = 70 to 75 % is selected as the minimum required relative density. In Fig. 6.31, N = 20 was selected as suggested by Fig. 6.28 (a), the corresponding effective-depth of compaction would be about 0.30 m. The effective depth of compaction and the the number of cycles of compaction during construction could be reduced by properly adjusting the applied rotation angle

, and the normal load q. Further study should be carried out regarding these factors.

6.2.4 Relative Density Increase Ratio

Fig 6.33 (a), 6.34 (a), and 6.35(a) showed that the distributions of the relative density increment with depth after cyclic compaction for N = 5, 10 and 20, respectively.

In these figures,

the relative density increment D

r observed near the top of the fill were greater than that near the bottom. Fig 6.36 (a) showed that the distributions of the relative density increment Dr with depth for various number of shearing cycles. In Fig.

6.33 (a) for N = 5, at the depth z = 0.3 m, the relative density increment

D

r after 20 cycles of cyclic torsional shearing compaction was 22 to 28 %. These values were less than the required relative density Dr = 70 to 75 %. However, after 20 cycles of shearing application, in Fig.6.35 (a), the relative density of compacted soil reached the required values. Thus test results indicated that cyclic torsional shearing was an effective method for compacting the upper 0.3 m fill.

Fig. 6.33 (b), 6.34 (b), and 6.35 (b) showed that the distribution of the relative density increase ratio with depth after cyclic torsional shearing compaction. On the top of lift (depth = 0 to 0.3m), the ratio reached the range of RDIR = 0.94 to 1.08. This means that, the relative density of the loose fill must increase about 94 to 108 % to reach the required state. In Fig. 6.35 (b), after 20 cycles of shearing loading, the RDIR varied from 0.94 to 1.03. Fig. 6.36 showed that at different depth, that RDIR mostly increased with increasing number of cycles of shearing compaction.

6.3 Compaction on Five 0.30 m-thick Lifts

In the field, it is often necessary to compact the entire soil mass to a requirement minimum relative density. In this study, a 1.5 m-thick fill was accomplished by compacting five 0.30 m-thick lifts with the cyclic torsional shear compactor (CTSC).

The applied vertical load q was 10.35 kPa, and the number of cycle shear stress application N was 20.

6.3.1 Compaction of Lift One

Fig. 6.37 illustrated the thickness of soil fill was 0.3 m and soil density cups were buried at different elevations in Lift 1. Fig. 6.38 showed the surface settlement and volumetric strain after the application of the static load on lift 1. In Fig. 6.38 (b), the minimum and the maximum of volumetric strain of the soil were 4.4 and 7.6 %, and the average value was 5.9 %. Fig. 6.39 shows the surface settlement and volumetric strain after 20 cycles of shearing compaction. In Fig. 6.39 (b), the minimum and the maximum of the volumetric strain were 8.9 and 11.3 %, and the average value was 9.8%. It apparently indicated that cyclic torsional shearing compaction is an effective method to compact the soil in Lift 1. The extra volumetric strain due to the cyclic torsional shearing compaction on Lift 1 was 3.9 %. Test results indicated that static compression alone was not sufficient to compact the soil fill.

The distribution of relative density after 20 cycles of shearing compaction was shown in Fig. 6.40. After the static compression and cyclic torsional shearing compaction, the relative density increased significantly. The relative density in Lift 1 increased from about 36 %values mostly to above 70 %.

Fig. 6.41 showed the relative density increment and relative density increase ratio after 20 cycles of shearing compaction. Fig. 6.41 (a) showed the relative density increment successfully increased reached the target zone (for Dr = 70~75%)

6.3.2 Compaction of Lift Two

Fig. 6.42 showed soil density cups were buried at different elevations in lifts 1 and 2. Both lifts were compacted on the surface with the CTSC. Fig. 6.43 (a) and Fig.6.44 (a) showed the accumulated settlement after static load and cyclic torsional shearing compaction, respectively. In Fig 6.44 (b), the accumulated volumetric strain after 20

cycles of shearing compaction range from 9.8 to 11.4 %, the average volumetric strain was 10.5 %.

Fig. 6.45 showed that distribution of relative density with depth after 20 cycles of shearing compaction on the top of lift 2. In the figure, the relative density in the lift 2 obviously reached the target zone of Dr = 70 to 75 %. In the two 0.3 m-thick compacted lifts, most of the measured relative densities were above to 70 %. The relative density increment ∆𝐷_𝑟 and relative density increase ratio RDIR after 20 cycles of shearing compaction on the top of lift 2 were shown in Fig. 6.45 (a) and (b). Test results indicated that cyclic torsional shear compaction was an effective method to compacte the

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