Chapter 1 INTRODUCTION
1.3 Organization of Thesis
This thesis was divided into the following parts:
1. Review of past investigations regarding cyclic torsional shear compaction of cohesionless soils (Chapter 2)
2. Description of the National Chiao Tung University soil bin, and cyclic torsional shear compactor (Chapter 3)
3. Soil characteristic and soil density control technique (Chapter 4) 4. Testing procedure (Chapter 5)
5. Experimental results of surface settlement, volume change, and relative density distribution due to compaction (Chapter 6)
6. Conclusions (Chapter 7)
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Chapter 2
Literature Review
Das (2010) stated that the soil at a construction site may not always be totally suitable for supporting structures such as buildings, bridges, highways, and dams. For example, in granular soil deposits, the in situ soil may be very loose and perform a large settlement under loading. In such a case, the soil needs to be improved to decrease it deformability.
Sometimes the top soil layers are undesirable and must be removed and replaced with better soils on which the structural foundation can be built. The soil used as fill should be well compacted to sustain the desired structural load. Compacted fills may also be required in low-lying areas to raise the ground elevation for the construction of foundation.
To improve its engineering properties, contractors are generally required to compact the loose soils to increase their unit weights and reducing settlements. Previous studies associated with the compaction-induced effects such as the change of soil density, the volume change in the soil mass and mechanism of soils under compaction are discussed in this chapter.
2.1 Soil Improvement with Densification
Kramer (1996) defined the common soil improvement techniques to mitigate seismic hazards. Soil improvement methods were divided into four categories including, (1) densification techniques (vibrofloatation, vibro rod, dynamic compaction, blasting,
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and compaction grouting); (2) reinforcement techniques (stone columns, compaction piles, and drilled inclusions); (3) grouting and mixing techniques (permeation grouting, intrusion grouting, soil mixing, and jet grouting), and (4) drainage techniques. In this thesis, only the densification of cohesionless soil was discussed.
2.1.1 Densification Techniques
Fig. 2.1 shows two of the many possible ways that a system of equal-sized spheres can be packed. The simple cubic packing in Fig. 2.1 (a) is the loosest of the stable arrangements. The dense packings in Fig. 2.1 (b) represent the densest possible state for such a system. A dense packing of soil spheres can be reached by soil densification techniques.
2.1.2 Soil Densification with Vibratory Compactor
D’Appolonia et al. (1969) proposed the vibratory rollers are particularly useful for compacting granular soils. Fig. 2.2 shows the effects of compaction of a 8-ft lift dune sand after five passes by a vibratory roller. The low unit weight that remains in the uppermost zone is due to vibration and lack of confinement in sand. Fig. 2.3 shows the compacted unit-weight profiles for the same dune sand after 2, 5, 15, and 45 roller passes. For field compaction work, the specification requires that the granular soil be compacted to a certain minimum relative density at all depths. Determination of the height of each lift depends on the type of roller and the economic number of passes.
The method for determination of the lift height is shown in Fig. 2.4. For soils at all depths to reach a minimum relative density Dr = 75%, the lift thickness should be controlled to be less than 18 inch.
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2.2 Cyclic Simple Shear Test
The cyclic simple shear test is a convenient method for determining the shear modulus and damping ratio of soils. It is also a convenient device for studying the liquefaction behavior of saturated cohesion less soils. In Fig.2.5, Airey and Wood (1987) showed the NGI cyclic simple shear apparatus. In the cyclic simple shear test, a soil specimen, usually 20-30 mm high with a diameter of 60-80 mm, is subjected to a vertical effective stress
v and a cyclic shear stress τ, as shown in Fig. 2.6. The horizontal load necessary to deform the specimen is measured by the horizontal load cell (Fig. 2.5), and the shear deformation of the specimen is measured by the linear variable differential transformer (LVDT).2.2.1 Study of Youd
Youd (1972) reported the experimental results regarding the void-ratio reduction of sand due to cyclic simple shearing. Fig. 2.7 shows the gradual densification of sand by repeated shear displacement in a simple shear test. Each cycle of shear straining reduces the void ratio of the soil by a certain amount, although at a decreasing rate.
Decrease in volume of the sand, as shown in Fig. 2.7, can take place only if drainage occurs freely. In the figure, after 10,000 cycles, the void ratio of sand was reduced from 0.54 to 0.42. It is obvious from the figure that cyclic shearing is an effect measure to densify the cohesionless soil.
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2.2.2 Study of Hsu and Vucetic
Hsu and Vucetic (2004) studied the volume decrease of dry or partially saturated sands subjected to several cycles of cyclic shear strain amplitudes c. If the cyclic shear strain amplitudesc1 are smaller than a certain threshold value called the volumetric cyclic threshold shear strain
tv (c1 <tv), their volume will not change. Such cyclic behavior is depicted schematically in Fig. 2.8. In the figure the results of three cyclic strain-controlled direct simple shear (DSS) tests conducted on dry or partially saturated specimens are sketched. The variations of shear strain over time t are presented in Fig.2.8(a). The resulting variations of vertical strain
v are presented in Fig. 2.8(b). The relationship betweenc , the permanent cyclic vertical strainvc, and the number of cycles N, is presented in Fig. 2.8(c). The cyclic vertical strain vc in Fig. 2.8(c) is taken asv at the end of cycle N, and it is also called the cyclic settlement strain.It can be seen in Fig. 2.8(c) how below certaintv the soil does not settle (vc = 0), while above it, it settles significantly (vc > 0). Accordingly, the amplitudetv represents the boundary between two fundamentally different types of volume change behavior.
Belowtv , the soil particles are not displaced with respect to each other and the soil’s mineral skeleton and volume remain practically unchanged during cycling loading.
When the soil is subjected toc >tv , the particles are displaced with respect to each other irreversibly, resulting in permanent changes of the soil’s volume and microstructure. It is clear in Fig. 2.8 that the cyclic shearing is an effective method to reduce the vertical strain of soil, and to densify the soil mass.
2.3 Cyclic Torsional Simple Shear Test
Fig. 2.9 shows the cyclic torsional simple shear device proposed by Ishibashi et al.
(1985). In this device, with a hollow cylindrical specimen 71.1 mm in outside diameter,
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50.8 mm in inside diameter, and 142.2 mm in height, can be subjected to independent variations of axial stress, inner and outer confining pressure, and torsional shear stress or strain. The device could closely simulates the in-situ stress condition.
2.3.1 Study of Ishibashi et al.
Ishibashi et al. (1985) studied the volume change of a hollow cylindrical Ottawa sand specimen subjected to cyclic torsional shearing in drained conditions. The experiments were conducted under uniform cyclic shear strains and the following conclusions were drawn. In Fig. 2.10, relationships between the induced cyclic volumetric strain the uniform cyclic shear straincyc for a given number of cycle is nearly linear. It is clear in Fig. 2.10 that the volume reduction of the soil specimen is significantly influenced by the cyclic shear strain cyc and the number of cyclic shear stress application N.
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2.4 Densification with Cyclic Torsional Shearing
2.4.1 Study of Yang
Yang (2002) used the disc-shearing instrument (Fig. 2.11(a)) at Chung-Yuan University to study the soil settlement due to cyclic torsional shearing. The diameter of the circular shearing disc was 198 mm. The diameter of the cylindrical sandy specimen was 200 mm, and the height of the soil specimen was 105 mm. The cyclic shear tests were carried out with initial relative densities from 30 % to 50 %, and normal stresses applied from 7 kPa to 150 kPa. One-way and cyclic (N=1) shear stresses were applied on Mailiao sand, Vietnam sand, and Ottawa sand. Fig. 2.11(b) shows the relative density increase ∆𝐷𝑟 due to cyclic shearing (N=1) was about twice that due to one-way shearing.
2.4.2 Study of Ren
Ren (2006) studied the soil densification due to cyclic torsional shearing. The diameter of the sandy specimen was 200 mm and the height was 105 mm. The diameter of the shear disc was 198 mm. Mailiao sand, Ottawa sand and Vietnam sand were tested with an initial relative density of 30 %. Normal stresses of 20, 60 and 100 kPa, and the shear angle 10˚, 20˚, 30˚, 60˚ and 90˚ were used for testing.
Fig. 2.12 showed the relative density of sand increased with increasing number of cyclic shear stress application N. The first 6 cycles ofτcyc application was most effective.
Fig. 2.13 showed a greater relative density increment was achieved at a shallow depth.
Less Dr increment due to the cyclic shear stress was observed at a deeper depth.
2.4.3 Study of Huang
To reduce the boundary effects due to a small soil tank, Huang (2008) used a 600
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mm-diameter, 150 mm-high soil bin. The diameter of the shearing disc was 200 mm, and the (tank diameter)/ (disc diameter) ratio was 3.0.
To include two different grain characteristics, Mailiao sand and Ottawa sand were selected as soil specimen. The initial relative density of the soil sample before shearing was 50 %. The applied vertical normal stress varied 10 to 90 kPa, the cyclic shearing angle varied from 5∘to 45∘. Fig. 2.14 indicated, for both Mailiao and Ottawa sand, the relative density of sand increased with increasing normal stress σ.
2.4.4 Study of Chen
Chen (2011) presents experimental data on the settlement and relative density change due to cyclic torsional shearing compaction. A new cyclic torsional shearing compactor was designed and constructed at NCTU. The thickness (T) of the soil after compaction was 0.6 m. The initial relative density was 34.5% ± 2.3% and the vertical pressure on the surface loading was 9.2 kPa. Fig. 2.15 showed the soil surface settlements after 1, 2, 5, 10, 20, 30 and 40 cycles of cyclic torsional shearing. It was obvious that the soil settlement increased with increasing number of cycles (N) of torsional shearing.
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.
Fig. 2.16 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.
In Fig. 2.17, cyclic torsional shearing was applied on the surface of each
0.15m-10
thick lift, and the distribution of relative density in Lifts 1 to 4. 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 %~75%
(US Navy DM-7 1982).
2.4.5 Study of Liu
Liu (2012) presents experimental data on the settlement, relative density and earth pressure due to cyclic torsional shearing compaction (CTSC). The CTSC was designed and constructed by Chen (2011) at NCTU. The vertical static load (q = 9.2 kPa) and cyclic torsional shearing were applied on the surface of the four 150 mm-thick lifts.
Then cyclic shearing was applied with rotation angles of +1°, +3°, +5°, +7° and +10° for 20 cycles. It was obvious that the soil surface settlement increased with increasing rotation angles (θ) of torsional shearing. A cone penetrometer was used to measure cone resistance qc with depth in the compacted soil mass. Based on the test results, the following conclusions were drawn.
Fig. 2.18 showed that the variation of surface settlement with the disc rotation angle from 0° to +10°. After 20 cycles of torsional shearing with the rotation angle of
= ±10° on the surface of the four 150 mm-thick lifts, the average surface settlement was 38.2 mm (volumetric strain = 6.4%). The surface settlement due to the static load q was 19.0mm. The extra surface settlement due to the torsional shearing compaction was about 19.2 mm. It is obvious that the cyclic torsional shearing compaction (static plus cyclic loads) is an effective method to densify loose soil.
Fig. 2.19 showed that relative density distribution for θ = 0° to +10°. In the figure, the relative density of compacted fill increased with increasing disc rotation angle θ.
With static load q = 9.2 kPa and the lift thickness of 150 mm, after 20 cycles of torsional
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shearing with angle
of ±5°, the relative density achieved was 72 to 84%. The
compacted relative density increased with increasing angle.Fig 2.20 showed the distribution of normalized qc for the compacted fill. For the loose fill, the qc/ qc,loose was 1.0. In Fig 2.20, the cone resistance ratio qc/ qc,loose increased from 4.6 to about 9.0 due to cyclic torsional shear compaction. Test results showed the effect of static vertical load and the cyclic torsional shearing on the cone resistance of soil were quite obvious.
2.5 Requirements of Soil Improvement
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. The relative density of soil is defined as :
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)
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reported that for dense soils the value of Dr was 65 to 85 % as shown in Table. 2.1. In the compaction requirements and procedure, US Navy Design Manual NAVFAC DM-7 (1982) reported that DM-70 to DM-75 % relative density can be obtained by proper compaction procedures. For coarse-grained, granaler well-graded soil, vibratory compaction generally is the most effective procedure.
2.6 Maximum Index Density and Unit Weight of Soils Using a Vibratory Table
ASTM Test Designation D 4253 – 93 (2007) provided a test method for determining the maximum index density/unit weight of cohesionless, free-draining soils using a vertical vibrating table.
The maximum index density/unit weight of a given free-draining soil is determined by placing either oven-dried or wet soil in a mold, applying a 2-lb/in2 (13.78 kPa) surcharge (dead weight) to the surface of soil, and then vertically vibrating the mold, soil, and surcharge (see Fig. 2.21). Without the surcharge on the soil surface, a low unit weight zone might remain in the upper most part of the compacter soil due to lack of confinement in sand. Use either an electromagnetic, eccentric, or cam-driven vibrating table having a sinusoid-like time-vertical displacement relationship at a double amplitude of vertical vibration (peak-to-peak) of about 0.013 in. (0.33 mm) for 8 min at 60 Hz, or 0.019 in. (0.48 mm) for 10 min at 50 Hz. The maximum index density/weight is calculated by dividing the oven-dried mass/weight of the densified soil by it volume (average height of densified soil times area of mold).
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Chapter 3
Experimental Apparatus
To investigate the effects of cyclic torsional shear compaction on the relative density of a cohesionless soil mass, the instrumented non-yielding model retaining wall facility at National Chiao Tung University (NCTU) was used. All soil improvement experiments described in this chapter were conducted in the soil bin of the NCTU non-yielding model retaining wall facility. This chapter introduces the soil bin, cyclic torsional shear compactor used for laboratory experiments.
3.1 Soil Bin
The soil bin shown in Fig. 3.1, which was fabricated with steel plates with inside dimensions of 1,500 mm ×1,500 mm ×1,600 mm. The model wall in Fig. 3.1. is 1.5 m-wide, 1.6 m-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 walls could be neglected when the soil bin is filled with cohesionless soil. 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 welded horizontally around the box to further increase the stiffness of the box.
Assuming a 1.5 m- 3,
o 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 model wall material. The estimated deflection of the model wall would be only 1.22 × 10-3 mm.
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Therefore, it can be concluded that the lateral movement of the 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.
3.2 Cyclic Torsional Shear Compactor
In previous studies Chen (2011), and Liu (2012) showed that cyclic torsional shearing compaction is an effective method to improve the engineering properties of loose sand. However, since the diameter of the shearing, disc was only 300mm, the compaction was effective for only the relatively-thin top soil layer.
Fig 3.2 shows, under the application of the same vertical pressure p, the settlement S1 of a full sized footing of width b1 in a structure will always be greater than the settlement S2 of a smaller test plate of width b2. This is because the depth to which vertical pressure of the same intensity p will penetrate is a function of the width b of the footing.
The effective depth of compaction plays an important role in field earthwork. The effects of compaction with a smooth-wheel vibratory roller can easily reach an effective depth of 0.3 m. In this study, the effective depth of compaction was increased by adjusting the diameter of the shearing disc D up to 450 mm.
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). To increased effective compaction depth, the diameter of the shearing disc
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was enlarge to 450 mm. Fig 3.3. and Fig 3.4. show the cyclic torsional shear compactor.
The entire cyclic torsional shear compactor consists of four components, namely: (1) shearing disc; (2) surcharge weight; and (3) torque loading device. The design and construction of cyclic torsional shear compactor is introduced as follows. The new compactor was designed by the author of thesis.
3.2.1 Shearing Disc
Fig. 3.3 shows the disc diameter is 450 mm, and the steel base disc is 25 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. 3.5. Fig. 3.6 shows the steel radial fin was 3 mm-thick, 6 mm-wide and the wedge angle of the fin was 90∘. Under the vertical pressure, the steel fin would bite into the soil mass. To provide adequate friction between the disc and the soil, the bottom of the shearing disc was covered with a layer of anti-slip frictional material called Safety-Walk (3M). The slip resistant tape was attached to the disc bottom on the fan-shaped areas between the steel fins as shown in Fig. 3.7.
3.2.2 Surcharge Weight
1. Ultimate Bearing Capacity of a Circular Footing
Vesic (1973) proposed three failure modes of shallow foundations, which included general shear failure, local shear failure and punching shear failure. Fig. 3.8 showed a strip foundation with a width of B resting on the surface of soil, and the nature of bearing capacity failures Fig 3.8 illustrated the relationship between the load per unit area q and the foundation settlement for three failure modes. The load per unit area of
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foundation at which shear failure in soil occurred was called the ultimate bearing capacity.
Vesic (1973) proposed a relationship for the mode of bearing capacity failure of foundations on resting on sands (shown in Fig. 3.9). The mode of failure was affected by the relative density of sand, depth of foundation embedment and the effective footing width. In this study, the initial relative density of loose sand was 36 % (see Fig. 4.12 in chapter 4), the static was applied on the surface of sand (Df = 0). To determine the failure mode of the circular loading disc used in this study, with Dr = 36%, Df = 0, and B* = B = the diameter of shearing disc. In fig. 3.9, the point was located between the punching and local shear failure zone.
Terzaghi (1943) suggested that for a continuous foundation, the failure surface in
Terzaghi (1943) suggested that for a continuous foundation, the failure surface in