Chapter 2 Literature Review
2.8 Cone Penetration Test
2.8.1 Study of Jamiolkowski et al
The relationship between relative density of sand of the tip resistance qc of CPT was investigated by Jamiolkowski et al. (1985). It was found that Fig. 2.27 illustrates the variation in cone resistance for a range of relative densities for different sands.
The Ticino sand used by Baldi et al. (1986) was a clean, uniform silica sand with subangular grains and appears to have a moderate compressibility. Fig. 2.27 shows the range for five predominantly silica sands used under controlled laboratory conditions; field cases are likely to exhibit more variability. Base on the data in Fig.
18
2.27, the following relationship was established:
Dr = -98 + 66 log10 [qc / (o’)0.5] (2.11) Where qc is cone resistance (ton/m2) and o’ is vertical effective stress (ton/m2).
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. All soil improvement experiments described in this chapter and there were conducted in the soil bin of the NCTU non-yielding model retaining wall facility. This chapter introduces the soil bin, soil pressure transducer, cyclic torsional shear compactor, cone penetration facility and data acquisition system used for laboratory experiments.
3.1 Soil Bin
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 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. 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.
Assuming a 1,500 mm-thick cohesionless backfill with a unit weight
= 17.1
kN/m3, and an internal friction angle = 41
o was pluviated into the soil bin. A 4520
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 compacted soil is only 0.6 m-thick. The lateral earth pressure acting on the side wall would be lower than that due to a 1.5 m- thick backfill. As the results, the deflection of the side walls of the soil bin would be even less.
3.2 Soil Pressure transducer
To investigate the development of vertical stress σv and horizontal stress σh in the soil mass fill, eight soil pressure transducers (Kyowa BE-2KCM17, capacity = 98.1 kN/m2) buried in the compacted soil. The strain-gage-type soil pressure transducer buried in the fill is shown in Fig. 3.2. The radial extensions attached to the transducers are used to prevent possible rotation due to filling and compaction. Calibration of the soil pressure transducers are indicated in the Appendix A of this thesis. The diameter of the SPT sensing area is 22 mm.
3.3 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. 3.3 and 3.4 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 as follows.
To efficiently carry the applied cyclic shear stress from the disc to the soil, 12 steel 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 2 mm-thick, 4 mm-wide and wedge angle of the fin was 90∘. During testing, the steel fin would cut into the soil mass. To provide adequate friction between the base 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. 3.7. Table 3.1 shows the dimension and mass of the normal loading discs. As shown in Fig. 3.8, 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 is 24.3 kg. Adding 2 pieces of 19.8 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.
Fig. 3.9 and Fig. 3.10 shows the dimensions of the torque loading frame at the top of the torsional shear device. The hoist ring was screwed on top of the frame so that
22
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. 3.3.
Fig. 3.11 (a) shows, the three torque wrenches are 600, 430, and 128 mm long. Fig.
3.11 (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. 3.12 and Fig. 3.13 was used to measure the 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. Long the maximum operation range is 200 N-m. The square peg is 12.7 mm x 12.7 mm.
3.4 Cone Penetration Facility
The cone penetration facility was used to estimate the change of soil properties before and after cyclic shear compaction. This facility belong to Department of Civil Engineering, Chung Yuan Christian University (CYCU). Continuous measurements were obtained with the CPT facility. As shown in Fig. 3.14 (a) to (c), the CYCU CPT facility is composed of following these parts: (1) mini-cone penetrometer;(2) electric motor and speed control device; and (3) CPT data acquisition device (including micro signal conditioning device, DA-16 anlong to digital converter, and the lap-top
computer). A load cell was installed at the bottom of the penetrometer near the cone, to measure the cone resistance qc. After connecting the cone penetrometer to the electric motor, the mini-cone was pushed into the soil mass at a constant speed. For this study, the speed of penetration was controlled to be 5.0 mm/s. The digital signals from the load cell of the cone penetrometer were filtered and amplified by micro signal conditioning device. Then, the experimental data were digitized by the D to A converter (USB DA-16). The digital signals were transmitted to the lab-top computer (Fig. 3.14 (b)) for storage and analysis. The mini-cone used for this study has a 60 degree apex angle and a diameter of 9 mm. The standard cone has a cross section area of 1000 mm2. But the cross-section area of the mini-cone is only 63.62 mm2.
3.5 Earth-Pressure Data Acquisition System
A earth-pressure data acquisition system was used to collect and store the considerable amount of earth pressure data generated during the tests. In the Fig. 3.15 (a) and (b) the data acquisition system is composed of the following four parts: (1) dynamic strain amplifiers (Kyowa: DPM601A and DPM711B); (2) AD/DA card (NI BNC-2090); and (3) Personal Computer. The analog signals from the sensors were filtered and amplified by the dynamic strain amplifiers. Then, the analog experimental data were digitized by an A/D-D/A card. The digital signals were then transmitted to the personal computer for storage and analysis. The software LabView was used for data collection and recording.
24
Chapter 4
Soil Characteristics
The characteristics of the fill used for soil improvement experiments are introduced in this chapter. The reduction of friction between the soil and lubricated side wall is discussed. 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.
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 backfill 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. 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.
4.2 Lubricated Side-wall Friction
To simulate the field condition of a infinite half space for the compaction constitute, 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 side wall and fill Fang et al.
(2004) suggested to was 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 backfill 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
v 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 is about 7.5°. For all experiments in this paper, the lubrication layers were wall applied on four side walls of the soil bin.4.3 Control of Soil Density
4.3.1 Air-Pluviated of 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
26
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 backfill, 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, the expected relative density of soil was about 35%
by Ho (1999).
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 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% Dr
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 = ±1o,
±3o, ±5o, ±7o and ±10o. 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
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