Chapter 4 Cyclic Torsional Shear Compactor
4.4 Torque wrench
Fig. 4.10 shows, the torque wrenches are 600, 430, and 128 mm long. Fig. 4.10a shows the torque wrench made of stainless steel. During testing, proper wrench length was selected so that no collision between the torque wrench with the sidewall of the soil bin would occur. The torque wrench was attached to the torque loading frame to induce torsional shear on the loose fill.
The digital torque wrench shown in Fig. 4.11 and 4.12 was used to measure torque applied to the soil. The digital torque wrench has a digital torque value readout.
Accuracy in the clockwise direction was +/- 1%, and the accuracy in the counterclockwise direction was +/- 2%. Readout units included N-m, ft-lb, in-lb and kg-cm. The digital torque wrench made by OLY SCIENTIFIC Equipment Ltd. (model 921/200E) was 530 mm. The maximum operation range is 200 N-m. The square drive is 1/2 inch x 1/2 inch.
Chapter 5
Backfill and Interface Characteristics
The characteristics of the backfill, need for soil improvement experiment are introduced in this chapter. The s friction acting between the backfill and lubricated side wall is discussed. The measurement and control of soil density distribution in the backfill are also introduced.
5.1 Backfill Properties
Air-dry Ottawa sand (ASTM C-778) was used throughout this investigation.
Physical properties of the soil include Gs= 2.65, emax= 0.76, emin= 0.50, D60= 0.315 mm, and D10= 0.213 mm. Grain-size distribution of the backfill is shown in Fig. 5.1.
Major factors considered in choosing Ottawa sand as the backfill material are summarized as follows.
1. Its round shape, which avoids the effect of angularity of soil grains.
2. Its uniform distribution of grain size (coefficient of uniformity Cu = 1.5), which avoids the effects due to soil gradation.
3. High rigidity of solid grains, which reduces possible disintegration of soil particles under loading.
4. Its high permeability, which allows fast drainage and therefore reduces water pressure behind the wall.
5.2 Lubricated Side-wall Friction
To simulate the field condition of a infinite half space for the compaction constitute, the shear stress between the backfill and the side walls should be minimized to nearly frictionless. To reduce the friction between side wall and backfill, a lubrication layer fabricated with plastic sheets was furnished for all experiments. Two types of plastic sheeting, one thick and two thin plastic sheets, were adopted to reduce the interface friction. All plastic sheets were hung vertically on the side walls before the backfill was deposited as shown in Fig. 5.2.
In this study, two thin (0.009 mm-thick) and one thick (0.152 mm-thick) plastic sheets were adopted for the soil improvement experiments. Fig. 5.3. shows the variation of side-wall friction angle sw as a function of the normal stress n for the plastic sheet method (1 thick + 2 thin sheeting) used in this study. The measured side-wall friction angle with this method is about 7.5°. For all experiments in this paper, the lubrication layers were wall applied on four side walls as indicated in Fig.
5.2.
5.3 Control of Soil Density
5.3.1 Air-Pluviated Loose Ottawa Sand
To achieve a uniform soil density in the backfill, Ottawa sand was deposited by air-pluviation method into the soil bin. The air-pluviation method had been widely used for a long period of time to reconstitute laboratory sand specimens. Rad and Tumay (1987) reported that pluviation is the method that provides reasonably homogeneous specimens with desired relative density. Lo Presti et al. (1992) reported
that the pluviation method could be performed for greater specimens in less time.
Das (2010) suggested that, for granular soil deposits, the relative density Dr of 15~50 %, is defined as loose, Dr = 50~70 % is defined as medium, and Dr = 70~85 % is defined as dense. Ho (1999) established the relationship among slot opening, drop height, and density as shown in Fig. 5.4. To achieve a loose backfill, Chen (2003) adopted the drop height of 1 m and hopper slot opening of 15 mm. In this study, the drop height of 1.0 m and the hopper slot-opening of 15 mm were also selected to achieve the loose backfill. Fig. 5.5 shows the soil hopper let the sand flow through a calibrated slot opening at the lower end. A picture of the soil pluviating processes is shown in Fig. 5.6.
5.3.2 Uniformity of Soil Density
To observe the distribution of soil density in the soil bin, the soil density cups were made. The soil density control cup made of acrylic is illustrated in Fig. 5.7. The solid circular cup wall was only 10 mm-high, so that the shear definition and volume reduction could occur in the cup during testing. A picture of the soil density cup is shown in Fig. 5.8. During the preparation of the 0.6 m thick soil specimen, density cups were buried in the soil mass at different elevations and different locations in the backfill as shown in Fig. 5.9 and Fig. 5.10. After the loose soil had been filled up to 0.6 m from the bottom of the soil bin by air-pluviation, soil density cups were dug out from the soil mass carefully. Fig. 5.11 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. 5.12. For the air-pluviated loose sand, the mean unit weight is 15.6 kN/m2, the mean relative density is Dr = 34.5 % with the standard
deviation of 2.3%. Das (2010) suggested that for the granular soil deposit with a relative density 15 % Dr 50 % is defined as loose sand. The relative density achieved in Fig. 5.12 is quite loose and uniform with depth.
Chapter 6
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; and (3) application of cyclic torsional shear. These parts will be introduced in the following sections with pictures.
The “plastic-sheets” lubrication layers were hung on the sidewall of soil bin before testing.
6.1 Specimen Preparation
Fig. 6.1 shows air-dry Ottawa sand was placed in the soil storage. Fig. 6.2 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. 6.3 shows the sand hopper was lifted by overhead crane in the laboratory. Fig. 6.4 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 the loose backfill, Fig. 6.5 (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 soil surface by the graduate student with a brush is shown in Fig.
6.6. Placement of a soil density cup on the soil surface is shown in Fig. 6.7. Fig. 6.8 shows how to check the density cup horizontal with a bubble level. Fig. 6.9 shows
density cups were buried in the soil mass at different elevations in the fill. The soil pulviation and density cup placement operations were repeated unit a backfill thickness T=0.6 m was reached.
Fig. 6.10 shows how to measure the fill surface location before loading. In the figure, a laser distance meter (Leica D3a) was placed between 2 L-shaped steel beams.
The distance between the meter (top of sidewall) and the light dot (top of fill) in Fig.
6.11 was measured by the distance meter. After compaction, the soil surface will settle, and the distance between the light dot and the meter will increase.
6.2 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. 4.2) used to apply static load has a mass of (66 kg) and circular footing diameter of 0.3 m. Fig. 6.12 illustrates the grid points for the circular vertical load application.
Fig. 6.13 shows the CTSC was hoisted with overhead crane into the soil bin. Fig.
6.14 shows the vertical static load was applied on the loose sand with either 5x5 or 4x4 formations. Fig. 6.15 shows the circular static vertical load was applied on the surface of fill with the 5x5 loading formation (Fig. 6.14 (a)). To iron the differential settlement on the soil surface, the static vertical load was applied once more with the 4x4 loading formation (Fig. 6.14 (b)). Fig. 6.16 shows, the laser distance meter was used to measure the location of soil surface after the application of vertical static load.
6.3 Application of Cyclic Torsional Shear
In this study, the cyclic torsional shear was applied on the soil surface from +5∘to -5∘. Fig. 6.17 (a) and (b) show a light dot from the laser distance meter on the
angle steel bar was used as a fixed point to the soil surface. Fig. 6.18 shows the cyclic torsional shear was applied by the operator on the loose fill to increase its density. In Fig. 6.19, 6.20 and 6.21, 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. 6.22 (a) and (b).
For the test for N=10, the torsional shear was first applied on the 4x4 loading formation for the first 5 cycles is shown in Fig. 6.23. To prevent disc penetration due to continuous shearing at the same crater, the shearing was moved to the 5x5 formation for N = 6 to 10 is shown in Fig. 6.24. Fig. 6.25 shows the soil density cup was carefully dug out of compacted soil mass. Fig. 6.26 (a) to (d) show the density cup with a spatula. Fig. 6.27 shows the brush away soil particles from base plate of density cup. Soil mass in the cup was measured with an electrical scale and the density of the compacted soil determined.
Chapter 7
Test Results
This chapter reports experimental results regarding soil densification due to static load and cyclic torsional shearing. The cyclic torque T and shearing applied on the soil surface was measured and reported. Experiments were conducted on the surface of a 0.6 m-thick soil lift. The vertical static load applied was 9.24 kPa. The settlement and relative density distribution of the soil layer due to the static load and cyclic torsional shear were measured. The loading frequency f was 0.4 Hertz, the disc rotation angle varied between +5∘and -5∘, and the number of loading cycle N varied form 1 to 40. To obtain a soil mass with a relative density greater than 70 %, experiments were conducted to soil fill with four 0.15 m-thick lifts. Each lift was compacted with the cyclic torsional compactor with q = 9.24 kPa, f = 0.4 Hertz, =
+5∘, and N = 20.
7.1 Applied Cyclic Torsional Shearing
Fig. 7.1 showed the cyclic torque applied on the soil surface was measured with a digital torque meter. For the disc rotation angle changing between +5∘and -5∘, the torque measured at N = 1, 2, 10, and 20 was shown in Fig. 7.2, 7.3, 7.4 and 7.5, respectively. In Fig. 7.2, for N = 1 the applied torque varied between 48.6 to -44.2 N-m. In Fig. 7.5, for N = 20 the applied torque varied between 50.2 to -56.0. Fig. 7.6 showed the applied torque T as a function of number of cycle N. Test results indicted that the applied torque increased slowly with increasing number of cycle. On the
average, From N = 1 to 20 the applied torque increased from 47.4 to 51.85 N-m. The measured torque increased about 9.4 %.
Fig. 7.7 showed the 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. 7.8 shows the maximum shear stress as a function of N. Test results indicted that maximum shear stress increased slowly with increasing N value. On the average, From N = 1 to 20 the maximum shear stress increased from 8.94 to 9.78 kPa. The applied shear stress increased about 9.4 %. With increasing cycles of shear stress application, the soil density of compacted soil increased, therefore its stiffness and shear strength increased.
7.2 Compaction of a 0.6 m-thick Lift
In the experiments, the surface of a 0.6 m-thick single soil lift was compacted with the static vertical load (dead-load of the compactor) and cyclic torsional shearing.
Effects of soil densification were indicated with the surface settlement and relative density change of the compacted soil fill.
7.2.1 Settlement Due to Static Vertical Load
The surface settlement of the 0.6 m-thick soil lift due the weight of the compactor was reported. The initial relative density of the loose fill was 34.5 %. The applied normal stress was = 9.24 kPa. To achieve a uniform settlement, the vertical static loading was first applied on the surface with the 5×5 formation (see Fig. 6.14), and then applied on the 4×4 formation. Fig. 7.9 showed the settlement measurement was carried out with the laser distance meter. The surface settlement measured at the
centers of loading disc was as shown in Fig. 7.10. For the 600 mm-thick soil lift, the minimum and maximum settlements due to = 9.24 kPa were 12.8 and 17.5 mm. The average settlement was 14.9 mm, which was about 2.5% 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 = 9.24 kPa was used throughout this study. It should be mentioned that the vertical strain distribution in the soil lift was not uniform.
The vertical stress transmitted to a deeper location would be less, therefore the density increase at a deeper location was expected to be less significant.
7.2.2 Settlement Due to Cyclic Torsional Shearing
After the application of the static loading, cyclic torsional shearing was applied on the surface of the soil specimen. The shearing was applied on the circular areas of the 4×4 formation for N = 1 to 5, 11 to 20, and 31 to 40. The shearing was applied on the 5×5 formation for N = 6 to 10, and 21 to 30. Fig. 7.11 showed the surface settlements after the first cycle of shear stress application. The measured surface settlement varied from 17.5 to 20.8 mm, and the average value was 19.2 mm.
Fig. 7.17 showed the surface settlements after 40 cycles of shear stress application.
The measured surface settlement varied from 32.4 to 35.8 mm, and the average value was 33.8 mm. The measured settlement values were relative uniform.
Fig. 7.18 showed the soil surface settlements after 1, 2, 5, 10, 20, 30 and 40 cycles of cyclic torsional shearing. It was obvious in the figure that the soil settlement increased with increasing number of cycles of torsional shearing. In the figure, the average settlement due to the static vertical loading was about 14.9 mm. After 40 cycles of torque application, the average settlement was 33.8 mm. The extra settlement due to the dynamic shearing cycles was about 18.9 mm, which was greater
than the settlement due to the static vertical load. It was obvious that cyclic the torsional shearing is an effective method to compact loose cohesionless soil.
Fig. 7.19 showed the variation of surface settlement with the number of cycle N.
In the first 2 cycles of torque application, surface settlement increased significantly.
However, after N = 20, the major part of settlement has accomplished, soil particles were sheared and reached a densely-packed condition. Therefore, it was difficult to increase the settlement any further with more cyclic shear application.
7.2.3 Relative Density Change Due to Static Vertical Load
To investigate the density distribution in the compacted soil, density cups were buried in the soil mass at different elevations and locations in the 0.6 m-thick soil lift as shown in Fig. 7.20 and Fig. 7.21. For the un-compacted loose soil, the average relative density was about 34.5 %. Fig. 7.22 showed, after applying the static normal stress 9.24 kPa, the distribution of relative density increased. This static normal loading represents the weight of the cyclic torsional shear compactor. At the depths z
= 50, 100 and 150 mm, the relative density increased from 34.5 to 52.6, 47.8 and 40.5%, respectively. The segmental line was obtained by connecting data points closest to the average value for the depth. It was apparent that the effect of density increase was obvious in the top 150 mm (radius of the circular loading disc R) of fill.
However, below the depth of 150 mm, the density increase due to the static surface loading was less obvious.
7.2.4 Relative Density Change Due to Cyclic Torsional Shearing
Fig. 7.23 to Fig. 7.29 showed the distribution of relative density due to cyclic torsional shearing from N=1 to 40. In Fig. 7.23 for N = 1, at the depth of 50, 100 and
150 mm, the relative density increased to 61.6, 61.2 and 54.0, respectively. Below the depth of 150 mm (disc radius R), the density increase was less significant.
In Fig. 7.27 for N = 20, at the depth of 50, 100 and 150 mm, the relative density increased to 87.1, 83.5 and 71.3, respectively. Below the depth of 150 mm (disc radius R), the measured relative density was less than 70 %. It was apparent in the figure that, for = 9.24 kPa, = + 5∘, and N = 20, the cyclic torsional shear compaction could effectively increase the relative density up to 70 %. However, the soil improvement was effective only for the top 150 mm (disc radius) of soil.
Fig. 7.30 showed the relative density distributions of the compacted specimen for N = 1 , 2, 5, 10, 20 and 40. Test results showed that the density distribution increased with increasing number of cycles of torsional shearing. 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. 7.30, if N = 20 is selected to save the compaction effort, the corresponding effective-depth of compaction would be 0.15 m. It should be mentioned that the effective depth of compaction could be influenced by the applied normal stress , angle of disc rotation , and number of shearing cycle N. Further study should be carried out regarding these parameters.
7.3 Compaction of Four 0.15 m-thick Lifts
In the field, it is often necessary to compact the entire soil mass to a required minimum relative density. For this study, a 0.6 m-high dense fill was accomplished by compacting four 0.15 m-thick (effective depth of compaction) lifts on the surface with the cyclic torsional shear compactor. The applied vertical stress was 9.24 kPa and the
number of shearing cycle was 20. Fig. 7.31 showed soil density cups were buried at different elevations in Lift 1. The distribution of relative density in Lift 1 was shown in Fig. 7.23. The initial relative density of soil was 34.5%. After cyclic shear compaction, at the depth of 50 mm, 100 mm, and 150 mm, the average relative density was 82.5, 77.1 and 71.0 %, respectively. The relative density in the 0.15 m-thick lift 1 was successfully increased to above 70%.
Fig. 7.33 showed density cups were buried at different elevations in Lift 1 and 2.
Each lift was compacted on the surface with the CTSC. The distribution of relative density in Lifts 1 and 2 after compaction was shown in Fig. 7.34. Fig. 7.35 showed density cups were buried at different elevations in Lifts 1 to 3. After compaction on the surface of each lift, the relative density distribution in Lifts 1 to 3 was shown in Fig. 7.36.
Fig. 7.37 showed soil density cups buried at different elevations in lifts 1 to 4.
Cyclic torsional shearing was applied on the surface of each lift. The distribution of relative density in Lifts 1 to 4 was shown in Fig. 7.38. Test results revealed that the trend of pressure distribution in each 0.15 m-thick lift was similar. The average relative density achieved in each lift was greater than the required value of 70 %. The entire sandy fill had been successfully compacted to the required density The proposed cyclic torsional shear compaction appears to be an effective method for soil improvement.
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 of compaction of 0.3 m. Although the compaction with the cyclic torsional shear compactor is less noisy and induce less vibration, the effective depth of compaction of 0.15 m might double the number of lifts in the field. However, the laboratory experimental investigation shown in this thesis is only preliminary. The effective
depth of compaction in construction can be enlarged by properly adjusting the radius of the shearing disc R, the applied normal stress during construction.