Chapter 2 Literature Review
2.5 Assessment of Relative Density
ASTM Test Designation D-4253 (2007) provide a procedure for determining the minimum and maximum dry unit weights of granular soils. These unit weights can be used to determine the relative density of soil compacted in the field. The term relative density is commonly used to indicate the in situ denseness or looseness of a granular soil. Relative density is defined as
( 2.1 ) Where e = in situ void ratio of the soil, emax = void ratio of the soil in the loosest state, emin = void ratio of the soil in the densest state.
Das (2010) reported that the value of Dr may vary from a minimum of 0 % for very loose soils to a maximum of 100 % for very dense soils. Soils engineers qualitatively
describe the granular soil deposits according to their relative densities. In-place soils seldom have relative densities less than 20 to 30 %. Compacting a granular soil to a relative density greater than about 85 % is difficult. Lambe and Whitman (1969) reported that the value of Dr was 65 to 85 % for dense soils as shown in Table. 2.1.
NAVFAC DM-7 (1982) reported the relative density was 70 to 75 % can be obtained by proper compaction procedures.
Chapter 3
Experimental Apparatus
To investigate the effects of cyclic torsional shear compaction on the relative density of in a cohesionless soil mass, the soil bin at National Chiao Tung University (NCTU) was used. This chapter introduced the soil bin. All soil improvement experiments described in this their were conducted in the soil bin of the NCTU non-yielding model retaining wall facility.
3.1 Soil Bin
The model wall shown in Fig. 3.1 is 1,500 mm-wide, 1,600 mm-high, and 45 mm-thick. To achieve an at-rest condition, the wall material should be nearly rigid. It is hoped that the deformation of the model wall could be neglected when the soil bin is filled with cohesionless soil. As indicated in Fig. 3.1, twenty-four 20 mm-thick steel columns were welded to the four sidewalls to reduce any lateral deformation during loading. In addition, twelve C-shaped steel beams were also welded horizontally around the box to further increase the stiffness of the box.
The soil bin is designed to minimize the lateral deflection of sidewalls during testing. In Fig. 3.1, the soil bin was fabricated of steel plates with inside dimensions of 1,500 mm ×1,500 mm ×1,600 mm.
Assuming a 1,500 mm-thick cohesionless backfill with a unit weight = 17.1 kN/m3, and an internal friction angle = 41o was pluviated into the soil bin. A 45 mm-thick solid steel plate with a Young’s modulus of 210 GPa was chosen as the wall
material. The estimated deflection of the model wall would be only 1.22 × 10-3 mm.
Therefore, it can be concluded that the lateral movement of the model wall is negligible.
The end-wall and sidewalls of the soil bin were made of 35 mm-thick steel plates.
Outside the steel walls, vertical steel columns and horizontal steel beams were welded to increase the stiffness of the end-wall and sidewalls. If the soil bin was filled with dense sand, the estimated maximum deflection of the sidewall would be 1.86 × 10-3 mm. From a practical point of view, the deflection of the four walls around the soil bin can be neglected.
For this study, the thickness of the soil to the compacted in 0.6 m. The lateral earth pressure acting in the side wall would be much lows than that due to a 1.5 m- thick backfill. As a results, the deflection of the side walls of the soil bin can be achieved.
Chapter 4
Cyclic Torsional Shear Compactor
To enhance an effective soil compactor with less noise, and less vibration, a cyclic torsional shear compactor (CTSC) was developed at National Chiao Tung University (NCTU). Fig. 4.1 and 4.2 show the cyclic torsional shear compactor. The entire cyclic torsional shear compactor consists of four components, namely: (1) shearing disc; (2) normal loading discs; (3) torque loading frame; and (4) torque wrench. The design and construction of cyclic torsional shear compactor are discussed in this chapter. All of the experiments mentioned in this thesis were conducted with the NCTU cyclic torsional shear compactor, which is briefly introduced in the following sections.
4.1 Shearing Disc
Fig. 4.1 shows the disc diameter is 300 mm, and the steel disc is 15 mm-thick. To efficiently carry the applied cyclic shear stress from the disc to the soil, 12 radial steel fins were carved on the bottom of the shearing disc as shown in Fig. 4.3. Fig. 4.4 shows, the steel radial fin was 2 mm-thick, 4 mm-wide and wedge angle of the fin was 90∘. During testing, the steel fin would bite into the soil mass. To provide adequate friction between the bottom of the disc and the soil, the bottom of the shearing disc is covered with a layer of anti-slip frictional material called SAFETY WALK (3M). The SAFETY WALK was attached to the disc bottom on the fan-shaped areas between the steel fins as shown in Fig. 4.5.
4.2 Normal Loading Discs
For the compaction of cohesion less soil in the field, Duncan et al. (1991) summarized the dynamic total force due to five different types of soil compactor. For vibratory-plate soil compactors, assuming the contact pressure between the plate and soil was uniform, the total (static + dynamic) cyclic pressure applied to the soil surface varied from 32.4 to 101.0 kN/m2. For rammer-plate soil compactors, the total pressure applied varies from 72.2 to 175.6 kN/m2. Several small hand tampers used in the field are illustrated in Fig. 4.6. The mass of the hand tampers varies from 50 kg to 80 kg. Assuming the mass of the hand-operated compactor is 66 kg, and the radius of the compaction disc is 0.36 m. The static normal load acting on the soil surface would be 9.24 kN/m2, if the contact pressure between the plate and soil was uniform. For this study, the normal pressure of 9.24 kPa was used throughout the investigation.
Table 4.1 shows the dimension and mass of the normal loading discs, which is made of iron. As shown in Fig. 4.7, the outside-diameter normal loading discs is 290 mm, the diameter of the screw rod hole is 21.6 mm, the diameter of the torque shaft hold is 43 mm, and the diameter of the hoist screw hole is 10.25 mm. Without any normal loading disc, the mass of the CTSC frame is 24.3 kg. Adding 2 pieces of 19.80 kg and 2 pieces of 1.05 kg loading discs, the total mass of the entire CTCS is 66.0 kg.
It should be mentioned that this thesis is intended to report on the preliminary experimental data obtained from a light-weight cyclic torsional shear compactor.
4.3 Torque Loading Frame
Fig. 4.8 and 4.9 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 torque frame, which enable the torque wrench to be hooked up to the torque frame. The applied torque was transmitted from the torque wrench, to the torque frame, then to the torque shaft and shear disc as illustrated in Fig. 4.1.
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
centers of loading disc was as shown in Fig. 7.10. For the 600 mm-thick soil lift, the