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 circular 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 0.6 m thick loose soil specimen, density cups were buried in the soil mass at different elevations and different locations in the backfill as shown in Fig. 4.9 and Fig. 4.10. After the loose soil had been filled up to 0.6 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 0.6 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 = 35.5 % with the standard deviation of 0.8%.

Das (2010) suggested that for the granular soil deposit with a relative density 15%  D_r

 50% is defined as loose sand. The relative density achieved in Fig. 4.12 is quite

loose and uniform with depth.

28

Chapter 5

Testing Procedure

The procedure to conduct the cyclic torsional shear tests are introduced in this chapter. The testing procedure can be divided into three parts: (1) specimen preparation; (2) application of vertical static load; (3) application of cyclic torsional shear; and (4) cone penetration. These parts will be introduced in the following sections with pictures. The “plastic-sheets” lubrication layers were hung on the sidewalls of the soil bin before testing.

5.1 Specimen Preparation

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

Leveling of the pluvuated soil surface by the graduate student with a brush is shown in Fig. 5.6. Four density cups were placed on each 50 mm-thick soil layer. A

total of 44 density cups were buried in the fill. Fig. 5.7 shows how check the density cup horizontal with a bubble level. Placement of a soil density cup and soil-pressure transducer on the soil surface is shown in Fig. 5.8. Fig. 5.9 shows density cups and soil-pressure transducers were buried in the soil mass at different elevations in the fill.

Eight soil pressure transducers were placed at the depths of 100, 250, 400 and 550 mm The soil pulviation and density cup placement operations were repeated unit a backfill thickness T = 0.6 m was reached.

5.2 Application of Vertical Static Load

The procedure to apply the vertical static load q on top of the air-pluviated loose sand is introduced. The cyclic torsional shear compactor (24.3 kg) and the loading discs (41.7 kg) used to apply static load has a mass of 66 kg. Diameter of the circular footing is 0.3 m and the vertical static load q = 9.2 kPa. Fig. 5.10 illustrates the grid points for the vertical load application. For the first row of static load, the center of circular load was applied at 1A, 1C, 1E, 1G and 1I.

Fig. 5.11 shows the CTSC was hoisted with overhead crane into the soil bin. Fig.

5.12 (a) shows the vertical static load q was applied on the loose sand with 5x5 formation. Fig. 5.13 shows the circular static vertical load was applied on the surface of the fill with the 5x5 loading formation.

5.3 Application of Cyclic Torsional Shearing

In this study, the cyclic torsional shear was applied on the soil surface for = ±1^o,

±3^o, ±5^o, ±7^o and ±10^o. Fig. 5.14 showed a light dot from the laser distance meter on the angle steel bar was used as a fixed point to the soil surface. Fig. 5.15 shows the cyclic torsional shear was applied by the operator on the loose fill to increase its

30

density. In Fig. 5.16, 5.17 and 5.18, with the guidance of the fixed light dot, the circular disc shears the soil from 0∘to +5°and -5°. The application of cyclic torsional shear to loose sand is shown in Fig. 5.19.

For the test for with 20 shearing cycles (N=20),after the application of vertical static load, the torsional shear was first applied on the 4x4 loading formation (Fig.

5.12(b)) for the first 5 cycles, as shown in Fig. 5.20. To prevent disc penetration due to continuous shearing at the same spots, the shearing was moved to the 5x5 formation Fig. 5.12(a) from N = 6 to 10 as shown in Fig. 5.21. For N = 11 to 20, the shearing was applied on the 4x4 formation, as shown in Fig. 5.22. Fig. 5.23 shows, after compaction the soil density cup was carefully dug out of the soil mass. Fig. 5.24 (a) to (c) shows the density cup with a spatula. Fig. 5.25 (a) to (d) shows the brush away soil particles from base plate of the density cup. Soil mass in the cup was measured with an electrical scale and the density of the compacted soil could be determined.

5.4 Cone Penetration Test

In Fig.5.26, the points of penetration in the soil bin were labeled as C1, C2, C3 and C4. The steel beam on the top of soil bin to support the CPT facility is shown in Fig.

5.27 (a). Fig. 5.27 (b) showed the beam was fixed on the soil bin with steel c-clamps.

In Fig. 5.28, the electric motor and the movable plate was fixed to the steel beam by the screw. Fig. 5.29 shows the connection of cone penetrometer to the electric motor.

Fig.5.30 shows the cone was lowered to the surface of the soil mass. During penetrating, the speed of downward penetration of the mini-cone was controlled at 5 mm/s. After reaching the penetration depth of 400 mm, the testing was terminated.

then, moved the electric motor on the movable plate to the next point and repeat the

penetration procedure. The sequence of testing would be C1, C2, C3 and C4. Test results measured by the load cell on the mini-cone were collected, stored and processed with the CPT data acquisition system.

32

Chapter 6

Test Results

This chapter shows experimental results regarding soil densification due to static vertical load and cyclic torsional shearing. The static vertical load applied of the fill was q = 9.2 kPa. The cyclic torque and shearing applied on the soil surface was measured and reported. The surface settlement, distribution relative density, vertical and horizontal stresses and cone resistance of the compacted fill due to the static vertical loading and cyclic torsional shearing were investigated. The rotation angle

±1^∘, ±3^∘, ±5^∘, ±7^∘ and ±10^∘, and the number of loading cycle were set to be N = 20. To obtain a soil mass with a relative density greater than 70%, compaction was applied on the fill surface for 0.15 m-thick lifts.

6.1 Static Load Test

To separate the densification effects due to static and cyclic loadings, in this section, the surface of four 0.15 m-thick soil lifts was compressed with the static vertical loading only. Effects of soil densification such as the surface settlement, change of relative density, vertical and horizontal stresses and cone resistance in the compressed fill are investigated.

6.1.1 Surface Settlement Due to Static Load

The surface settlements of the four 0.15 m-thick compressed soil lifts due the static

weight of the compactor were discussed. The initial relative density of the loose fill was 35.5% (see Fig. 4.12). The applied static normal stress was q = 9.2 kPa. To achieve a uniform settlement, the vertical static loading was applied on the surface with the 5×5 formation (see Fig. 5.12 (a)). Fig. 6.1 showed the settlement measurement was carried out with the laser distance meter placed on top of the steel beam. The surface settlement measured at the centers of disc loading disc was shown in Fig. 6.2. For the four 150 mm-thick soil lifts, the accumulated minimum and maximum settlements were 15.0 and 22.3 mm. The average settlement was 19.0 mm, which was about 3.2% (volumetric strain ε_v = 3.2%) of the soil thickness. It is obvious that static vertical loading is an effective method to compact the loose fill. To limit the scope of this thesis, only q = 9.2 kPa was used throughout this study. It should be mentioned that the vertical strain distribution in the soil lift may not be uniform.

6.1.2 Density Change Due to Static Load

To investigate the density distribution in the compressed fill, density cups were buried in the soil mass at different elevations and locations in the four 0.15 m-thick soil lifts (see Fig 5.9). For the un-compacted loose soil, the average relative density was about 35.5%. In Fig. 6.3, Chen (2011) reported that, for a 0.6m-thick fill, after the application of static vertical load q, the density increase more near the surface and the density increase less at greater depths. The induced density change decreased with increasing depth. Fig. 6.4 shows, after applying the static vertical load 9.2 kPa on each lift, the relative density of fill increased. This static vertical loading represents the weight of the cyclic torsional shear compactor. On the average, the relative density increase was about 26.4% from 35.5% to 61.9%. It should be mentioned that the distribution of density is not uniform with depth. However, the relative density

34

achieved Dr = 61.9% is not enough to achieve the target of dense sand (Dr = 70%-85%).

6.1.3 Stress Change Due to Static Load

For comparison purposes, at the beginning of this study, experiments were conducted to investigate the stresses in an uncompacted loose fill. Air-pluviation method was adopted to prepare the fill and the relative density achieved for the loose sand was 35.5 %. Fig. 6.5 shows the location of soil pressure transducers to measure the distribution of vertical and horizontal stress with depth. SPT2, SPT3, SPT6 and SPT7 were buried in the soil mass to measure



v and SPT1, SPT4, SPT5 and SPT8 were used to measure h. The vertical stress v measured in the soil mass was shown in Fig. 6.6. In this figure, the vertical stress v increased with increasing depth z. Test data were in fairly good agreement with the traditional equation v = z. In this study, unit weight  was 15.6 kN/m³ for the loose sand. The distributions of horizontal earth pressure h with depth were shown in Fig. 6.7. In the figure, the earth pressure profile induced by the 0.6 m-thick loose fill was approximately linear and was in good agreement with the Jaky’s equation. Mayne and Kulhawy (1982), Mesri and Hayat (1993) reported that Jaky’s equation was suitable for backfill in its loosest state. From a practical point of view, it was concluded that for a loose fill, the vertical and horizontal earth pressure in the soil mass can be properly estimated with the equation



v = z and Jaky’s equation, respectively.

To investigate the change of stresses due to static load, the loose fill was placed in four 0.15m-thick lifts as shown in Fig. 6.5. Static load q was applied each lift on the surface with the 5×5 formation (see Fig. 5.12) and then removed. Fig. 6.6 shows the vertical stress profile after the static vertical loading. It is clear in the figure that the vertical overburden pressure v can be properly estimated with the equation v = z.

As compared with the v for loose sand the measured,v increased slightly, probably because the compressed fill had a slightly higher density (see Fig. 6.4). It is clear in the figure that the static vertical load did not result insignificant residual stress in the vertical direction. It may be concluded that the effect of static loading on the vertical pressure v was insignificantly. Fig. 6.7 shows the horizontal stress was also increased slightly after the application of the static vertical loading.

6.1.4 Cone Resistance Change Due to Static Load

Cone penetration tests were conducted to investigate the change of soil properties due to static loading on the loose fill. The fill is 600 mm-thick as shown Fig. 6.5. The bottom of the soil bin is a solid steel plate. Due to boundary effects, the cone resistance may suddenly increase, when the penetrometer approached the bottom of the soil bin. For this study, cone penetration was conducted for z = 0 to 400 mm. For Fig. 6.8 shows the cone resistance for the loose soil varied from 0 to 300 kPa. After static vertical loading, the cone resistance varied from 0 to 1681.2 kPa. It is obvious that static vertical loading significantly increase the tip resistance of the compressed fill.

Jamiolkowski et al. (1985) were reported that could make cone resistance converted to relative density. Fig 6.9 shows the cone resistance of static load transformed to the relative density by Jamiolkowski theory. It could be found the test result and the theory were agreement in the loose fill. However, it was not agreement after compressed. It was probability that the soil mass not big enough.

6.2 Applied Cyclic Torsional Shearing

Fig. 6.10 shows the torque applied on the soil surface was measured with a digital torque meter. For the number of loading cycle N = 20, the torque measured at  = ±1^∘,

36

±3^∘, ±5^∘, ±7^∘and ±10^∘was shown in Fig. 6.11. In figure, for different

angles, the

applied torque varied between -53 to 47.8 N-m. Fig. 6.12 shows the applied torque T as a function of shearing angle

. Test results indicated that the applied torque does

not change with increasing shearing angle. On the average, for  = ±1^∘, ±3^∘, ±5^∘, ±7^∘ and ±10^∘, the applied torque was about ±47.7 N-m. The variation of torque for the



angle between 0° and ±1° should be studied in the future.

Fig. 6.13 indicates how to determine the maximum torsional shear stress



max at the edge of the shearing disc due to the applied torque. A linear distribution of shear stress from the center to the edge of the disc was assumed. Fig. 6.14 shows the

6.3.1 Surface Settlement Due to Cyclic Torsional Shearing

After the application of the static loading, cyclic torsional shearing was applied on the surface of each lift. The cyclic shearing was applied on the circular areas of the 4×4 formation for N = 1 to 5, and 11 to 20. The shearing was applied on the 5×5 formation for N = 6 to 10 (see Fig. 5.12). Fig. 6.15 shows the surface settlements of the 0.6 m-thick fill after the disc rotation angle at ±1^∘, ±3^∘, ±5^∘, ±7^∘ and ±10^∘. In the figure, the surface settlement increased with increasing shearing angle  of torsional shearing. In the figure, the average settlement due to the static vertical loading q = 9.2

kPa was about 19 mm. After the cyclic disc rotation

 = ±10° and N = 20 cycles of

torque application, the average settlement was 38.2 mm. The extra settlement due to the torsional shearing cycles was about 19.2 mm, which was more than the settlement due to static vertical loading. It was obvious that cyclic torsional shearing (static and cyclic load) is an effective method to compact loose soil. Fig. 6.16 shows the variation of surface settlement from 19 to 38.2 mm with the disc rotation angle from 0° to ±10°

6.3.2 Density Change Due to Cyclic Torsional Shearing

Fig. 6.17 to Fig. 6.21 show the distributions of relative density due to cyclic torsional shearing angle of

= ±1

^∘, ±3^∘, ±5^∘, ±7^∘ and ±10^∘. Fig. 6.19 shows the compaction could effectively increase the relative density up to 70% at all depths at the shearing angle  = ±5°. However, the soil densification was effective only for the 150 mm-thick soil lift. To increase the effective compaction depth from 0.15 m to 0.30 m, it is possible that the radius of the compaction disc has to increase from 0.15 m to 0.30 m. .

Fig. 6.22 shows the density distribution for  = 0° to ±10°. In the figure, the relative density of compacted fill increased with increasing disc rotation angle

. The US

Navy design manual (NAVFAC DM-7.2) described that for coarse-grained, granular well-graded soils with less than 4 percent passing No. 200 sieve, 70 to 75 relative density can be obtained by proper compaction procedures. In this study, Dr = 70% is selected as the minimum required density. In Fig. 6.22 shows, with static load q = 9.2 kPa and the lift thickness of 150 mm, the relative density Dr = 70% can be achieved with 20 cycles of torsional shearing with angle

 greater than ±5°. Fig. 6.23 shows

increasing shearing angle  would make the relative density more focused. It was also

38

found the shearing angle  greater than ±5°, the relative density Dr = 70% could be achieved. It should be mentioned that the effective depth of compaction could be influenced by the applied normal stress q, angle of disc rotation, and number of shearing cycle N. Further study should be carried out regarding these parameters.

6.3.3 Stress Change Due to Cyclic Torsional Shearing

The change of vertical stress



v and horizontal stressh in the fill due to cyclic torsional shearing are reported in this section. Fig. 6.24 to Fig. 6.28 show the measured after 20 cycles of cyclic torsional shearing at shearing angle

 = ±1

^∘, ±3^∘,

±5^∘, ±7^∘ and ±10^∘. In Fig. 6.24, little



v change was observed between the loose fill and compacted sand. Fig. 6.25 to Fig. 6.28 also show no significant



v change after torsional shearing compaction. The vertical earth pressure v was affected neither by the cyclic torsional shearing



cyc, nor by the shearing angle

 as shown in Fig. 6.29.

Fig. 6.30 shows the normalized v with shearing angle . From  = 0° to 10°, the v

/z ratio is a constant 1.0. Based the test results, the vertical earth pressure in the fill was not changed by the cyclic shearing compaction.

Fig. 6.31 to Fig. 6.35 show the distribution of horizontal stress after cyclic torsional shearing. Fig. 6.31 shows, after the compaction at the shearing angle

 = ±1° for 20

cycles, as compared with that for uncompacted soil, the horizontal earth pressure



h

increased a little. From Fig. 6.32 to 6.35, it was found that the horizontal pressure increased with increasing shearing angle

. Fig. 6.36 shows the distribution of

horizontal stress



h after cyclic shearing with  = ±1^∘, ±3^∘, ±5^∘, ±7^∘ and ±10^∘. It was clear that the horizontal earth pressure in the compacted fill increased with increasing shearing angle

. Fig. 6.37 shows normalized 

h with the shearing angle

. It was

obviously that the h / Ko

z ratio increased with increasing shearing angle for  = 0°

to ±5°. For an angle  greater than ±5°, the curve of



h / Ko

z vs.  became flat. It

was obviously that the cyclic shearing compaction would influence the horizontal earth pressure in the compacted fill.

6.3.4 Cone Resistance Change Due to Cyclic Torsional Shearing

The change of CPT cone resistance in the fill due to cyclic shearing compaction is studied in this section. Fig. 6.38 to Fig. 6.42 shows the test result of cone resistance after 20 cycles of shearing at different the shearing angles. The cone resistance for loose sand and statically loaded sand are also shown in the figures. Fig. 6.38 shows, the cone resistance q_c has increased after compaction at shearing angle

 = ±1°. Fig.

6.39 to Fig. 6.42 show the tip resistance q_c at shearing angles of ±3°, ±5°, ±7° and

±10°. As compared with that for loose fill, statically loaded fill, the cone resistance increased signification due to compaction. Fig. 6.43 shows the cone resistance increased significantly after cyclic shear compaction. Fig 6.44 showed the distribution of normalized q_c for uncompacted loose fill. For loose soil, the q_c / q_c,loose was 1.0. In Fig 6.44, cone resistance ratio increased to about 4 times due to the static loading q = 9.2 kPa. After the application of static and cyclic shear compaction, the cone resistance increased up to about 6 times. The effects of static vertical load and cyclic shearing on the cone resistance of soil were quite obvious.

Fig. 6.45 to Fig. 6.49 show the cone resistance at different depths were converted to the relative density of soil with the method proposed by Jamiolkowski et al. (1985).

In these figure, the calculated relative density of loose fill was compared with that measured in the lab. In fig 6.48, the test values were neared the calculated relative density at the shearing angle  = ±7° at the depth z = 200 mm to 300 mm. The cone resistance was very low at the depth z = 0 mm to 200 mm because of the extremely

40

low overburden and confining pressure. With such a thin soil fill (H = 600 mm), it may not be a good idea to predict the distribution of soil density with the penetration test.

The effective depth of compaction plays an important role in field earthwork.

Compaction with a smooth-wheel vibratory roller can easily reach an effective depth

Compaction with a smooth-wheel vibratory roller can easily reach an effective depth

在文檔中 反復扭剪夯實試驗之扭剪角度對砂土密度及應力之影響 (頁 44-0)