Data Acquisition System

Due to the considerable amount of data collected by the soil-pressure transducers, a data acquisition system was used shown in Fig. 3.6. It is composed of the

DPM711B); (2) NI adaptor card; (3) AD/DA card; and (4) personal computers shown in Fig. 3.7. An analog-to-digital converter digitized the analog signals from the sensors. The digital data were then stored and processed by a personal computer.

For more details regarding the NCTU retaining-wall facility, the reader is referred to Wu (1992) and Fang et al. (1994).

Chapter 4

Interface Plate and Supporting System

A steel interface plate is designed and constructed to simulate inclined rock face near the retaining structure shown in Fig. 1.1. In Fig. 4.1, the plate and its supporting system are developed by Zheng (2008) to fit in the NCTU model retaining-wall facility. The interface plate consists of two parts: (1) steel plate; and (2) reinforcing steel beams. The supporting system consists of the following three parts: (1) top supporting beam; (2) base supporting block; and (3) base supporting boards. Details of the interface plate and its supporting system are introduced in the following sections.

4.1 Interface Plate

4.1.1 Steel Plate

The steel plate is 1.370 m-long, 0.998 m-wide, and 5 mm-thick as shown in Fig.

4.2. The unit weight of the steel plate is 76.52 kN/m³ and its total mass is 83 kg (0.814 kN). A layer of anti-slip material (SAFETY-WALK, 3M) is attached on the steel plate to simulate the friction that acts between the backfill and rock face as illustrated in Fig.

4.2 (c) and Fig. 4.3 (a). For the inclination angle β = 50^o shown in Fig. 1.2, the length of the interface plate should be at least 1.370 m. On the other hand, the inside width of the soil bin of the NCTU retaining wall facility is 1 m. In order to put the interface plate into the soil bin, the width of the steel plate has to less than 1.0 m. As a result, the steel plate was designed to be 1.370 m-long and 0.998 m-wide.

4.1.2 Reinforcement with Steel Beams

To simulate the stiffness of the rock face shown in Fig. 1.1, the steel interface plate should be nearly rigid. To increase the rigidity of the 5 mm-thick steel plate, Fig.

4.2 (b) and Fig. 4.3 (b) shows 5 longitudinal and 5 transverse steel L-beams were welded to the back of steel plate. Section of the steel L-beam (30 mm × 30 mm × 3 mm) was chosen as the reinforced material for the thin steel plate. On top of the interface plate, a 65 mm × 65 mm × 8 mm steel L-beam was welded to reinforce the connection between the plate and the hoist ring shown in Fig. 4.3 (b).

4.2 Supporting System

To keep the steel interface plate in the soil bin stable during testing, a supporting system for the interface plate was designed and constructed by Zheng (2008). A top-view of the base supporting frame is illustrated in Fig. 4.4. The supporting system composed of the following three parts: (1) base supporting block;

(2) top supporting beam;and (3) base boards. These parts are discussed in following sections.

4.2.1 Top Supporting Beam

In Fig. 4.5, the top supporting steel beam is placed at the back of the interface plate and fixed at the bolt slot on the side wall of the soil bin. Details of top supporting beam are illustrated in Fig. 4.7. The section of supporting steel beam is 65 mm × 65 mm × 8 mm and its length is 1700 mm. Fig. 4.4 shows four bolt slots were drilled on each side of the U-shape steel beam on the side wall of the soil bin. Fig. 4.6 shows the top supporting beam was fixed at the slots with bolts.

4.2.2 Base Supporting Block

The base supporting block used to support the steel interface plate is shown in Fig. 4.8. The base supporting block is 1.0 m-long, 0.14 m-wide, and 0.113 m-thick.

Fig. 4.8 (b) shows three trapezoidal grooves were carved to the face of the base supporting block. Fig. 1.2 shows the foot of the interface plate could be inserted into the groove at different distance from base of the model wall. For this study, different horizontal spacing d adopted for testing includes: (1) d = 0 mm; (2) d = 50 mm; and (3) d = 100 mm. Fig. 4.5 shows 6 pieces of base boards are stacked between the base supporting block and the end wall to keep the base block stable. The base board shown in Fig. 4.9 is 1860 mm-long, 1000 mm-wide and 113 mm-thick. The surface of the top base board was cover with a layer of anti-slip material SAFETY-WALK.

Chapter 5

Backfill and Interface Characteristics

This chapter introduces the properties of the backfill, and the interface characteristics between the backfill and the wall. Laboratory experiments have been conducted to investigate the following subjects: (1) backfill properties; (2) model wall friction; (3) side wall friction; (4) interface plate friction; and (5) distribution of soil density in the soil bin.

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, e

max= 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 effect of angularity of soil grains.

2. Its uniform distribution of grain size (coefficient of uniformity Cu=1.48), 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 of pore water and therefore reduces water pressure behind the wall.

To establish the relationship between unit weight γ of backfill and its internal friction angle φ, direct shear tests have been conducted. The shear box used has a square (60 mm×60 mm) cross-section, and its arrangement are shown in Fig. 5.2.

Chang (2000) established the relationship between the internal friction angle

φ

and unit weight

γ

of the ASTM C-778 Ottawa sand as shown in Fig. 5.3. It is obvious from the figure that soil strength increases with increasing soil density. For the air-pluviated backfill, the empirical relationship between soil unit weight

γ

and φ angle can be formulated as follows

φ = 6.43γ - 68.99 (5.1)

where

φ =angle of internal friction of soil (degree) γ =unit weight of backfill (kN/m³)

Eqn. (5.1) is applicable for γ = 15.45 ~ 17.4 kN/m³ only.

5.2 Model Wall Friction

To evaluate the wall friction angle δw between the backfill and model wall, special direct shear tests have been conducted. A 88 mm × 88 mm × 25 mm smooth steel plate, made of the same material as the model wall, was used as the lower shear box. Ottawa sand was placed into the upper shear box and vertical load was applied on the soil specimen. The arrangement of this test is shown in Fig. 5.4.

To establish the wall friction angles δw developed between the steel plate and sand, soil specimens with different unit weight were tested. Air-pluviation methods was used to achieve different soil density, and the test result is shown in Fig. 5.5.

For air-pluviation Ottawa sand, Lee (1998) suggested the following relationship:

δw = 2.33γ - 17.8 (5.2)

Eqn. (5.2) is applicable for γ = 15.5~17.5 kN/m³ only. The φ angle and δ angle

on Coulomb, and Rankine’s theories.

5.3 Side Wall Friction

To constitute a plane strain condition for model wall experiments, the shear stress between the backfill and sidewall should be eliminated. A lubrication layer fabricated with plastic sheets was equipped for all experiments to reduce the interface friction between the sidewall and the backfill. The lubrication layer consists of one thick and two thin plastic sheets as suggested by Fang et al. (2004).

All plastic sheets had been vertically placed next to both side-walls as shown in Fig.

5.6.

The friction angle between the plastic sheets and the sidewall was determined by the sliding block tests. The schematic diagram and the photograph of the sliding block test by Fang et al. (2004) are illustrated in Fig. 5.7 and Fig. 5.8. The sidewall friction angle

δ

_sw is determined based on basic physics principles. Fig. 5.9 shows the variation of interface friction angle

δ

_sw with normal stress σ based on the plastic sheet lubrication tests. The friction angle measured was 7.5°. With the plastic – sheet lubrication method, the interface friction angle is almost independent of the applied normal stress. The angle of wall friction for smooth concrete is about φ/2 to 2φ/3, with the φ = 31.3° for loose sand, the wall friction angle should be δ = 15.7°~20.9°. The shear stress between the acrylic side-wall and backfill has been effectively reduced with the plastic-sheet lubrication layer.

5.4 Interface Plate Friction

To evaluate the interface friction between the interface plate and the backfill special, direct shear tests were conducted as shown in Fig. 5.10. In Fig. 5.10(b), a 80

mm × 80 mm × 15 mm steel plate was covered with a layer of anti-slip material

“SAFETY-WALK” to simulate the surface of the interface plate. The interface-plate was used to simulate the inclined rock face near the wall as shown in Fig. 1.1. Dry Ottawa sand was placed into the upper shear box and vertical stress was applied on the soil specimen as shown in Fig. 5.10(a).

To establish the relationship between the unit weight γ of the backfill and the interface-plate friction angleδi, soil specimens with different unit weight were tested. Air-pluviation methods was used to achieve different soil density, and the test result is shown in Fig. 5.11. For air-pluviation Ottawa sand, Wang (2005) suggested the following empirical relationship:

δ i = 2.7γ- 21.39 (5.3)

where

δi = interface-plate friction angle (degree) γ = unit weight of backfill (kN/m³) Eqn. (5.3) is applicable for γ = 15.1 ~16.36 kN/m³ only.

The relationships between backfill unit weight γ and different friction angles are illustrated in Fig. 5.12. The internal friction angle of Ottawa sand φ, model wall-soil friction angle δw, interface-plate friction angle δi, and sidewall friction angle δsw as a function of soil unit weight γ are compared in the figure. It is clear in Fig. 5.12 that, with the same unit weight, the order of the four different friction angles is φ＞δi ＞δw ＞δsw.

5.5 Control of Soil Density

5.5.1 Air-Pluviation of Backfill

To achieve a uniform soil density in the backfill, dry 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. As indicated in Fig. 5.13, the soil hopper that lets the sand pass through a calibrated slot opening at the lower end was used for the spreading of sand. A picture showing air-pluviation of the Ottawa sand into soil bin is indicated in Fig.

5.14. Air-dry Ottawa sand was shoveled from the soil storage bin to the sand hopper, weighted on the electric scale, then pluviated into the soil bin. As indicated in Fig.

5.15, four types of slot openings (5 mm, 7 mm, 10 mm and 15 mm) were adopted by Ho (1999), and the drop height of soil varied from 0.25 m to 2.5 m.

Das (1994) suggested that the granular soil with a relative density of 15% ~ 50% is defined as loose. In this study, the drop height of 1.0 m and the slot opening of 15 mm were selected to achieve the loose backfill with a relative density of 35%.

5.5.2 Distribution of Soil Density

To investigate the distribution of soil density in the soil bin, soil density measurements were made. The soil density control cup made of acrylic is illustrated in Fig. 5.16 and Fig. 5.17. For the air-pluviated backfill, the density cups were used to measure the soil density at different elevations and locations.

In Fig. 5.18, a layer of 100 mm-thick Ottawa sand was placed in the soil bin as a soil blanket. Four density-control cups were then put into the soil bin on the surface of soil blanket. Locations of the cups are illustrated in Fig.5.18. Then Ottawa sand was placed layer by layer into the soil bin up to 0.5 m thick.

After the soil has been poured to the top, the soil cups were dug out of the backfill carefully. Soil density in the box can be found by dividing the mass of soil

in the box by the inside volume of the cup. To investigate the variation of density with depth, another group of tests were conducted. As shown in Fig. 5.19, four density control cups were put into the soil bin at different depths near the center of the soil bin. After the soil was poured into the soil bin up to 0.5 m thick from base board, the cups were dug out of the soil mass carefully, and the soil in the cups were determined. The distributions of relative density of loose sand measured at different elevations are shown in Fig. 5.20. For experimental result, the relative density average with depth was 35.4%. Standard deviation of relative density was 1.04%.

From a practical point of view, it may be concluded from these data that the soil density in the soil bin is quite uniform.

Chapter 6 Test Results

This chapter reports the experimental results regarding effects of an adjacent inclined rock face on the active earth pressure against a retaining wall filled with loose sand. The rock face interface inclination angles β = 0^o, 50^o, 60^o, 70^o, 80^o, 90^o and spacing b = 0, 50 mm, 100 mm are illustrated in Fig. 1.2 to Fig. 1.4. The height of backfill H is 0.5 m and the air-pluviation method was used to prepare the backfill.

The loose Ottawa sand (Dr = 35%) with the unit weight γ = 15.6 kN/m³ was prepared as the backfill material. Based on direct shear tests (Ho, 1999) the internal friction angle φ for the loose backfill would be 31.3^o. The γ and φ values are used to calculate earth pressures based on the Jaky and Coulomb theories. The testing program is listed in Table 6.1

6.1 Horizontal Earth without Interface Plate

The variation of lateral earth pressure as function of active wall movement was investigated. After the loose backfill and had been placed into the soil bin as shown in Fig. 6.1 (a) (b), the model wall slowly moved away from the soil mass in a translation mode at a constant speed of 0.015 mm/s. No compaction was applied to the loose backfill.

Distributions of horizontal earth pressure σh measured at different stages of wall displacements S/H (S : wall displacement, H : backfill height) for Test 1215-3 and 1229-1 are illustrated in Fig. 6.2 and Fig. 6.3. As the wall started to move, the earth pressure decrease, and eventually a limit limiting active pressure was reached.

The pressure distributions are essentially linear at each stage of wall movement.

Active earth pressures calculated with Rankine and Coulomb theories are also

indicated in the figure. The ultimate experiment active pressure distribution at S/H = 0.04 is in fairly good agreement with that estimated with Coulomb and Rankine theories.

The variation of horizontal earth-pressure coefficient Kh as a function of wall displacement is shown in Fig. 6.4. The coefficient Kh is defined as the ratio of the horizontal component of total soil thrust Ph to

γ H

² 2. The horizontal soil thrust Ph

was calculated by summing the pressure diagram shown in Fig. 6.3. The coefficient Kh decreased with increasing wall movement S/H until a minimum value was reached, then remained approximately a constant. The ultimate value of Kh is defined as the horizontal active earth-pressure coefficient Ka,h. In Fig. 6.4, the active condition was reached at approximately S/H = 0.00375~0.004. In Fig. 6.4, it may not be an easy task to define the point of active wall movement Sa. For a wall that moved away from a loose sandy backfill in a translational mode, Mackey and Kirk concluded the wall displacement required to reach an active state is Sa = 0.004 H.

The Sa values recommended by Mackey and Kirk (1967), Bros. (1972), Fang and Ishibashi (1986) Fang et al. (1997) were summarized in Table 2.2 and illustrated in Fig. 6.4. In this study the active wall movement is assumed to be Sa = 0.004 H.

It may be observed in Fig. 6.4 that the Coulomb theory (δ = φ/2) provide a good estimate of the active earth pressure. In the actual design of retaining walls, the wall friction angle δ is generally assumed to be between φ/2 (smooth concrete) and φ (rough stone) (Sowers, 1979). The steel piles against the following clean sand : δ = 17° (NAVFAC DM-7.2, 1982) The model wall used for this study is made of steel.

For this reason, δ = φ/2 was used in the theoretical Coulomb solutions in this study.

In Fig. 6.3, the distribution of earth pressure with depth at different wall movements is nearly linear. As a result, the point of application of the total thrust should act at about H/3 above the wall base (h/H = 0.333). Test results in Fig. 6.5 show that the point of application of soil thrust are located at about 0.33 H ~ 0.36 H above the wall base.

6.2 Horizontal Earth Pressure for b = 0

Fig. 6.6 to Fig. 6.9 show the steel interface plate was placed in the soil bin for β = 50°, 60°, 70° and 80°, and dry Ottawa sand was pluviated behind the model wall for b = 0. It should be mentioned that, to clearly show the position of the interface plate, the picture in Fig. 6.6 was taken without the plastic-sheet lubrication layer.

During testing, the lubrication layers were hung vertically between the acrylic side-wall and backfill. Fig. 6.10 to Fig. 6.17 show the distribution of earth pressure at different stages of wall movement with presence of a stiff interface plate for an inclination angle β = 50^o,60°, 70° and 80^o. In Fig. 6.10 and Fig. 6.11 (β = 50°), the measured horizontal stress at S/H = 0 is lower than Jaky’s solution at lower H/3 of the wall. At the wall movement S/H = 0.004, the active earth pressure is less than that of Coulomb’s solution at the lower H/3 of the wall. In Fig. 6.6(a), for the upper part of the model wall, the interface plate is relatively far from the SPT. It is reasonable to expect the measured σh to be close to Coulomb’s prediction. However, for the lower part of the model wall, the interface plate is relatively close to the soil pressure transducers. As a result, the active earth pressure measured would be affected by the approaching of the interface plate.

In Fig. 6.12 to Fig. 6.15 (β = 60^o and 70°), the measured σh was significantly lower than Jaky’s solution at S/H = 0. At S/H = 0.004, the σh measured at lower H/2 of wall was lower than Coulomb’s solution. It may be observed in Fig. 6.7 and Fig.

6.8, with increasing β angle (β = 60° to 70°), the horizontal distance between the model wall and interface plate was reduced. In Fig. 6.16 and Fig. 6.17 (β = 80°), the measured at-rest pressure distribution is not linearly with depth. and it is significantly less than the Jaky solution at S/H = 0. Fig. 6.9 shows, for β = 80°, the interface plate was quite close to the wall surface. The amount of backfill soil sandwiched between the rock face and the wall was very little. In this Fig. 6.16 and Fig. 6.17, the earth pressure slightly decreased with the active wall movement.

Fig. 6.18 to Fig. 6.21 presented the variation of lateral soil thrust as a function of active wall movement for β = 50°, 60°, 70°, and 80°. As the wall started to move, the lateral soil thrust decreased with increasing wall movement until a stable value is reached, Kh then remained approximately a constant. The ultimate value of Kh is defined as the horizontal active earth pressure coefficient Ka,h. For b = 0, the active condition was observed at approximately S/H = 0.004.

The Fig. 6.22 to Fig. 6.25 showed the variation of the point of application of the soil thrust as a function of active wall movement for β = 50°, 60°, 70°, and 80°.

At the active wall movement of 0.004 H, for β = 50^o, 60^o and 70^o, the (h/H)a values reached 0.35, 0.38 and 0.40, respectively. For β = 80^o, experimental results showed that the active soil thrust was located at about 0.46 H ~ 0.47 H above the base of the wall. As compared to Fig. 6.4 and Fig. 6.5 (without interface plate), it is clear that the magnitude and the point of application of the active soil thrust are significantly affected by the presence of the nearby rock face.

6.3 Horizontal Earth Pressure for b = 50 mm

Fig. 6.26 to Fig. 6.30 showed the steel interface plate was placed in the soil bin for β = 50°, 60°, 70°, 80° and 90° and dry Ottawa sand was pluviated behind the model wall. In the figures ,the horizontal spacing between the base of the interface plate and the base of the wall b = 50 mm. Fig. 6.31 to Fig. 6.40 showed the distribution of earth pressure at different stages of active wall movement with the presence of a stiff interface plate with the inclination angle β = 50°, 60°, 70°, 80°

Fig. 6.26 to Fig. 6.30 showed the steel interface plate was placed in the soil bin for β = 50°, 60°, 70°, 80° and 90° and dry Ottawa sand was pluviated behind the model wall. In the figures ,the horizontal spacing between the base of the interface plate and the base of the wall b = 50 mm. Fig. 6.31 to Fig. 6.40 showed the distribution of earth pressure at different stages of active wall movement with the presence of a stiff interface plate with the inclination angle β = 50°, 60°, 70°, 80°

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