4.2 Supporting System
4.2.2 Base Supporting Block and Base Boards
The base block used to support the steel interface plate is shown in Fig. 4.8.
The supporting block is 1 m-long, 0.14 m-wide, and 0.113 m-thick. Fig. 4.8 (b) shows
an three trapezoid grooves were caved to the face of the base supporting block. Fig.
4.5 shows the foot of the interface plate could be inserted into the groove at different distance from the model wall. 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 base boards are placed between the base supporting block and the end wall to keep the base block stable. Details of base boards are illustrated in Fig. 4.9. The base board is 1860 mm-long, 1002 mm-wide and 113 mm-thick. The surface of the top base board was cover with a layer of anti-slip material Safe-Walk.
4.3 Different Interface Inclinations
Different interface inclinations angles β = 0o, 50o, 60o, 70o and 80o associated with this investigation are shown in Fig. 4.10 to Fig. 4.14. Fig. 4.10 (a) shows the test condition for inclination angle β = 0o. Fig.4.10 (b) shows Ottawa sand was pluviated into the soil bin without the interface plate, Fig. 4.10 to Fig. 4.13 show the arrangement of model wall, plastic sheets interface plate and Ottawa sand conditions for the interface inclination angle β = 50o, 60o, 70o and 80o.
Chapter 5
BACKFILL AND INTERFACE CHARACTISTICS
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) interface characteristics between model wall and backfill; (3) side wall friction; (4) interface plate friction; and (5) distribution of soil density in the soil bin. The parameter of loose sand used for this study are summarized in Table 5.1
5.1 Backfill Properties
Air-dry Ottawa silica sand (ASTM C-778) was used as backfill. Physical properties of Ottawa sand are listed in Table 5.2 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=1.78), 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. 4.3. 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) whereφ
=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 Interface Characteristics between Model Wall and Backfill
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 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 obtained in section 5.1 and 5.2 are used for calculation of active earth pressure for
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 before the backfill was deposited 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 δsw is determined based on basic physics principles. Fig. 5.9 shows the variation of interface friction angle δsw with normal stress σ based on the plastic sheet lubrication method. 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 shear stress between the acrylic side-wall and backfill could be 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 the interface plate.Theinterface plate was used to simulate the inclined rock face show in Fig. 1.1. 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 angleδw, interface-plate friction angleδi, and sidewall friction angle δsw as a function of δ are compared in the figure. It is clear in Fig. 5.12 that, with the same unit weight, the order of 4 different friction angles is φ ＞δi ＞δ w ＞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 0.25m to 2.5m.
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.
For test 1 and test 2, 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. The locations of the cups are illustrated in Fig.5.18. Then Ottawa sand was placed layer by layer into the soil bin up to 0.61 m above the base of soil bin.
After the soil has been poured to the top, the soil cupswere 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. Experimental results thus determined are listed in Table 5.2. It is clear that the densities measured at the same elevation appears to be uniform. Standard deviations of relative density for test 1 and test 2 are 0.86% and 1.06%, respectively.
To investigate the variation of density with depth, another group of tests were conducted. As shown in Fig. 5.19, five 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 0.61m from wall base, the cups were dug out of soil mass carefully, and soil densities in the cups could be determined. The test results are summarized in Table.5.3. Standard deviations of relative density for test 3 and test 4 are 1.79 % and
1.37%, respectively. The distributions of relative density of loose sand measured at different elevations as shown in Fig. 5.20. 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
EXPERIMENTAL 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 = 0o, 50o, 60o, 70o and 80o are illustrated in Fig. 6.1. The height of backfill is 0.5 m and the air-pluviation method was used to prepare the backfill. The loose (Dr = 35%) Ottawa sand with the unit weight γ = 15.6 kN/m3 was prepared as the backfill material. Based on direct shear tests (Ho, 1999), the corresponding internal friction angle φ is 31.3o. The γ and φ values are used to calculate earth pressures based on the Jaky and Coulomb theories.
The entire study was conducted in the NUTU model retaining wall system which is described in Chapter 3.. The testing program for this study is summarized in Table 6.1.
6.1 Earth Pressure Results
6.1.1 Earth Pressure for β = 0°
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. 4.10, 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 are illustrated in Fig. 6.2. As the wall started to move, the earth pressure decrease, and eventually a limit 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 Fig. 6.2.
The ultimate experiment active pressure distribution is in fairly good agreement with that estimated with Coulomb and Rankine theories.
Fig. 6.3 shows a typical variation of horizontal earth pressure σh measured by different pressure transducer as a function of the wall movement, S/H (S : wall displacement, H : backfill height). In Fig. 6.3 the horizontal stress decreased with increasing active wall movements. The location for soil pressure transducer SPT1 through SPT9 is illustrated in Fig. 3.3. If the normal pressures at different depths are normalized by the soil unit weight γ and its depth z, the variation of σh/γz with S/H is shown in Fig. 6.4. In this figure, most of the data are concentrated. It seems possible that the active condition is reached at all depths simultaneously.
The variation of horizontal earth-pressure coefficient Kh as a function of wall displacement is shown in Fig. 6.5. The coefficient Kh is defined as the ratio of the horizontal coefficient component of total thrust toγH2 2. The horizontal thrust Ph
was calculated by summing the pressure diagram shown in Fig. 6.2. The coefficient Kh decreased with increasing wall movement until a minimum value was reached, then remained approximately constant. The ultimate value of Kh is defined as the horizontal active earth-pressure coefficient Ka,h. In Fig. 6.5, the active condition was reached at approximately S/H = 0.0035.
As shown in Fig. 6.2, the distribution of earth pressure at different wall movements is almost linear. Therefore, the point of application of total thrust, h/H should remained at about H/3 above the wall base. Experimental results in Fig. 6.6 show that these points are located at a distance of about 0.331 H ~ 0.359 H above the wall base.
For Test 0825, the distributions of earth pressure at different stages of wall movement are shown in Fig. 6.7. As the wall starts to move, the earth pressure decrease. The pressure distribution is approximately linear with depth. Although the distribution is not strictly linear, such an assumption would not be far from reality.
In Fig. 6.5, the earth pressure coefficient, Kh decreases with increasing wall movement and finally a constant total thrust is reached. For Test 0825, the active condition occurred at the wall movement of approximately S/H= 0.003. It may be
observed from Fig. 6.5 that Coulomb theories (δ =18.5o) provide a good estimate of the active earth pressure. In Fig.6.5 , data points obtained from Test 0809 and Test 0825, indicated that the experimental results were quite reproducible.
6.1.2 Earth Pressure for β = 50°
Fig. 6.8 shows the distribution of earth pressure at different stages of wall movement with presence of a stiff interface plate for an inclination angle β = 50o. Fig. 4.11 shows the steel interface plate was placed in the soil bin and dry Ottawa sand was pluviated behind the model wall. In Fig. 6.8, the measured stress at S/H= 0 is lower than Jaky’s solution. The measured earth pressure at-rest is clearly affected by the intrusion of the rough interface inclined at β = 50o. It is clear in Fig. 4.11(a) that, for the upper part of model wall, the interface plate is far from the SPT. It is reasonable to expect the measuredσh to be close to identical with Jaky’s prediction.
However, for the lower part of the model wall, the interface plate is quite close to the soil pressure transducers. As a result, the active earth pressure measured would be affected by the approaching of the interface plate.
Fig.6.9 shows the typical variation of lateral pressure as a function of active wall movement. The horizontal stress decreases with increasing wall movement, then reaches a constant value. Fig. 6.10 shows the relationship between normalized earth pressure σh/γz and wall movement S/H. It is clear in this figure, that σh measured at SPT1 to SPT9 decreases with the wall movement, then reach an active state.
Fig.6.11 presents the variation of lateral pressure as a function of active wall movement. As the wall starts to move, the lateral soil thrust decreases with increasing wall movement until a constant is reached, then remained approximate constant. The ultimate value of Kh is defined as the horizontal active earth-pressure coefficient Ka,h.
In Fig. 6.11, the active condition was reached at approximately S/H = 0.003.
In Fig. 6.8, as the wall starts to translate, the earth pressure start to decrease. This non-linear earth pressure distribution causes the total thrust to act at to higher location.
Fig. 6.12 shows h/H reaches a constant value which is about 0.40 H ~ 0.42 H above the base of the wall.
For Test 0815, the distribution of earth pressure at different stages of wall movement for β = 50o is shown in Fig. 6.13. As the wall started to move, the earth pressure decrease and eventually a limiting active pressure was reached. The variation of Kh with S/H for Test 0814 and Test 0815 are summarized in Fig. 6.11. It can be seen from the figure that the two sets of test data concentrate in narrow strip. It can be concluded that the experimental results are highly reproducible.
6.1.3 Earth Pressure for β =60°
Fig. 6.14 shows the earth pressure distributions corresponding to different stages of wall displacements for the interface inclination angle β = 60°. At S/H = 0, the measured σh was significantly lower than Jaky’s solution, especially the σh
measured near the base of wall. It may be observed in Fig. 4.12 (a), with increasing β angle, the horizontal distance between the model wall and interface plate was reduced.
Fig.6.15 shows the typical variation of lateral pressure as a function of active wall movement. The horizontal stress decreases with increasing wall movement, then reaches a constant value. Fig. 6.16 shows the relationship between normalized earth pressure σh/γz and wall movement S/H.
For β = 60°, the variation of earth pressure Kh with wall movement is shown in Fig. 6.17. The earth-pressure coefficient value Kh decreased with increasing wall movement until a constant value is reached. In Fig. 6.17 the active condition was reached at approximately at S/H = 0.003. Referring to Fig. 6.14, at S/H = 0.003 the active earth pressures measured near the base portion of the wall is much lower than Coulomb’s prediction. The measured active earth pressure is clearly affected by the interface plate inclined at β = 60°. It is reasonable to expect the point of application of the active thrust would be located at a position higher than h/H = 0.333. Fig. 6.18 shows the experiment points of application the active thrusts were located at about 0.40 H ~ 0.43 H above the wall base.
For Test 0818, Fig. 6.17 shows the pressure distribution at various movement stages. The measured active earth pressure was lower than Coulomb’s solution especially the pressure measured near the base of wall. This is most probably because the active earth pressure is affected by the intrusion of the inclined interface plate.
6.1.4 Earth Pressure for β =70°
The pressure distributions at various wall movements for β =70° are shown in Fig.
6.20. At S/H = 0, the measured earth pressure at rest was lower than Jaky’s prediction, especially at the lower part of the model wall. This is because the interface plate is very close to the soil pressure transducers as shown in Fig. 4.13.
Fig. 6.21 shows the variation of horizontal earth pressure σh measured by different pressure transducer as a function of the wall movement. It is clear from the data shown in Fig. 6.21 that the horizontal stress decreases with increasing active wall movements. The variation of σh/γz with S/H is shown in Fig. 6.22.
Fig. 6.23 shows the variation of Kh with active wall movement for β = 70°. The coefficient Kh decreases with increasing wall movement. The wall movement needed for Kh to reach an active state is about S/H = 0.0035.
The variation of the location of to the active soil thrust with wall movement is shown in Fig. 6.24.Without the interface plate (β = 0°), the point of application h/H of the earth resultant is located at about 0.33H above the base of the wall. With the interface angle β = 70°, the earth pressure does not increase linearly with depth. This active earth pressure distribution shown in Fig. 6.20 causes the location of the total thrust to rise to a higher location. Experimental result in Fig. 6.24 shows the point of application of the active thrust was located at about 0.41 H ~ 0.43 H above the wall base.
Fig. 6.25 illustrates the distributions of earth pressure at different stages of wall movement for Test 0824. The active earth pressure measured near the base of the wall
Fig. 6.25 illustrates the distributions of earth pressure at different stages of wall movement for Test 0824. The active earth pressure measured near the base of the wall