Torque Loading Device - Cyclic Torsional Shear Compactor

Chapter 2 Literature Review

3.2 Cyclic Torsional Shear Compactor

3.2.3 Torque Loading Device

The entire torque loading device consists of two components, namely: (1) Torque shaft and connecting frame, and (2) Torque wrench.

1. Torque Shaft and Connecting Frame

Fig. 3.13 and 3.14 show the dimensions of the torque loading frame at the top of the torsional shear device. The hoist ring was placed on top of the frame so that torsional shear compactor be lifted and lowered by the overhead crane in the laboratory. Two hexagon caps were fixed on the arms of the connecting frame, which enable the torque wrench to be hooked up to the connecting frame. The applied torque was transmitted from the torque wrench, to the connecting frame, then to the torque shaft and shear disc as illustrated in Fig. 3.2

Fig 3.15 (a) and Fig 3.15 (b) the show the dimensions of the extension tube. It can use to connect the CTSC and the connecting frame to lengthen the height of the CTSC.

The mass of extension tube is 4.80 kg, which can be the surcharge weight of the CTSC.

Fig 3.16 show the CTSC is connected by the extension tube. It can use to compact the deep of the soil bin when the CTSC cannot shear with two torque wrenches.

2. Torque Wrench

Fig. 3.17 (a) shows, the torque wrench is 430 mm long. Fig. 3.17 (b) 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. 3.18 and Fig. 3.19 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 12.7 mm x 12.7 mm.

Without any normal loading disc, the mass of the CTSC frame is 49.4 kg. Adding 5 pieces of 19.80 kg, 1 piece of 9.6 kg, 1piece of 4.8 kg, 1 piece of 1.55 kg, 3 pieces of 1.05 kg and 1 piece of 0.5 kg loading normal discs, the total mass of the entire CTSC became 168.0 kg.

The weight of the entire CTSC is equal to 1.65 kN. The diameter of the shearing disc is 0.45 m, and the area of the bottom of the shearing disc is 0.159 m². The vertical pressure acting on the surface of the fill due to the weight of the CTSC is q = weight / area = 10.35 kPa. For all tests, the vertical pressure of 10.35 kPa, which is equal to the ultimate bearing capacity of the loose sand under circular shearing disc, was used throughout the investigation. It should be mentioned that this thesis is intended to report the preliminary experimental results obtained from a light-weight cyclic torsional shear compactor.

Chapter 4 Soil Characteristics

This chapter introduces the properties of the fill, and the reduction of friction between the soil and lubricated side wall. The control and measurement of soil density distribution in the fill are also introduced.

4.1 Soil Properties

Air-dry Ottawa sand (ASTM C-778) was used throughout this investigation. Table 4.1. showed that physical properties of the soil include Gs

= 2.65, e

max = 0.76, emin = 0.50, D60 = 0.39 mm, and D10 = 0.26 mm. Grain-size distribution of the soil is shown in Fig. 4.1. Major factors considered in choosing Ottawa sand as the fill material are summarized as follows.

1. Its round shape, which avoids the effect of angularity of soil grains.

2. 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.

4.2 Side-wall Friction

To simulate the field condition of a infinite half space for compaction, the shear stress between the fill and the side walls of the soil bin should be minimized to nearly frictionless. To reduce the friction between sidewall and fill Fang et al. (2004) suggested to use a lubrication layer fabricated with plastic sheets. 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 soil was deposited as shown in Fig. 4.2.

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. 4.3 shows the variation of side-wall friction angle sw as a function of the normal stress

 for the plastic sheet

method (1 thick + 2 thin sheeting) reported by Fang et al. (2004). The measured side-wall friction angle with this method was about 7.5°. For all experiments in this paper, the lubrication layers were applied on four side walls of the soil bin.

4.3 Control of Soil Density

4.3.1 Air-Pluviation of Loose Sand

To achieve a uniform soil density in the fill, Ottawa sand was deposited by air-pluviation method into the soil bin. The air-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. For the air-pluviation method, Fig. 4.4 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. 4.5. To achieve a loose fill, Chen (2003) adopted the drop height of 1.0 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 fill. In Fig. 4.6, under such a condition, Ho(1999) indicated that the expected relative density of soil was about 35%.

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

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