4 Slope Stabilization Using EPS Geofoam at Route 23A

Transcription

1 Slope Stabilization Using EPS Geofoam at Route 23A 4.1 Introduction Geofoam introduced in recent years has provided solutions to a number of engineering problems. One of these problems is the slope stability problem. In a slope stability problem a slip surface develops between two portions of the soil as in figure 4-1. One portion tends to slip over the other making it unsafe for engineering constructions on the top or at the toe of the slope. Two main forces appear in the slope stability problem: the driving force and the stabilizing force. The driving force is due to the weight of the top portion of the slip area that tends to slip downward. The stabilizing force appears in two forms; the shear strength along the slip surface and the weight of the soil in the toe of the slope. One way to increase the factor of safety of a slope is to reduce the driving weight by replacing an amount of soil by foam. For a soil with friction, reducing the weight will result in reducing the shear strength of the soil, which in turn will reduce the stabilizing force. Foam must be placed in such a way that the overall factor of safety is increased. Two techniques of analyses are used in this study; the limiting equilibrium method and the finite difference method. Each method is used in distinct numerical simulation softwares. Results from the models in both softwares are compared for cases with and without geofoam. Softwares results are compared with Taylor charts (Taylor, 1949) for the case without geofoam. A parametric study is done to cover the practical ranges of slopes with different heights, slope inclination, soil type, geofoam shape, etc. file:///t /Test Reports/Slope Stabilization Using Geofoam at Route 23A (AE)/ch4.html (1 of 63) [9/6/2000 2:57:58 PM]

2 Figure 01 Driving and Resisting Blocks in a Failure Zone Horizontal seismic coefficient is used to represent the effect of dynamic forces on slope equilibrium. Important results of this study are presented. A case history concludes this chapter. More related research works were presented in chapter two. 4.2 Geofoam Block Alignment Generally, the cross section of a geofoam stabilized slope embankment contains layers of foam blocks, each layer being about 0.6m-0.83m (2ft-2.75ft) thick. These layers are underlined by about 0.15m (0.5ft) thick sand or filter. Reinforced concrete slab of about 0.15m (0.5ft) lays on the top of the geofoam blocks. The concrete slab acts as a load distributing layer as well as a protection for the foam. The paving material and sub base layers are laid over the top of the distributing slab. Two configurations for the distribution of foam blocks within the slope were studied. Figure 4-2 shows the traditional configuration where the foam blocks are arranged such that the side of the blocks facing the slope is inclined to an angle equivalent to that of the slope. The other edge of geofoam configuration is vertical. In this case, the soil is either excavated to its normal angle of repose by using compacted file:///t /Test Reports/Slope Stabilization Using Geofoam at Route 23A (AE)/ch4.html (2 of 63) [9/6/2000 2:57:58 PM]

3 backfill or supporting the soil with sheeting while excavating and constructing the embankment. In some cases, the vibratory driver that may be used in installation of the sheeting can make the whole slope fail or at least move because of the vibration produced. The sheeting itself may be expensive, making the geofoam solution uneconomical. Moreover the installation and the extraction of the sheeting is a time consuming process. The advantage of this configuration, though, is that the excavated space is less and the usable space for construction is more. The configuration shown in figure 4-3 requires neither sheeting nor compacted back fill. The shape of the foam fill tends to take the shape of a slip surface; hence the factor of safety will be higher for the same amount of foam, as shown later in the report. A number of parameters will affect the factor of safety of a geofoam-stabilized slope. The characteristics of the soil such as its shear strength, density, and its hydraulic conductivity constitute the most important factors. The geometry of the slope itself, such as its height and inclination also affect its factor of safety. The geometry and the position of the foam fill also control the factor of safety. Figure 02 Typical Cross Section of a Geofoam Stabilized Slope file:///t /Test Reports/Slope Stabilization Using Geofoam at Route 23A (AE)/ch4.html (3 of 63) [9/6/2000 2:57:58 PM]

4 Figure 03 Arrangement of Geofoam Blocks with Two Inclined Edges 4.3 Methods Of Analysis The Limiting Equilibrium Method GeoSlope software (GEOCOMP Corp., 1992) calculates the factor of safety and draws the slip surface of a slope. Soil Characteristics such as density, cohesion, and friction angle are used to represent soil in GeoSlope. Foam is idealized as stiff clay with very low density to insure that the slip surface does not pass through it. Horizontal seismic coefficient can be represented in GeoSlope. GeoSlope uses Bishop circular method of analysis that is capable of detecting only circular slip surface The Finite Difference Analysis Method Fast Lagrangian Analyses of Continua (FLAC) software (Itasca 1998) is used to analyze soil mechanics problems using the finite difference method. In the FLAC model, the cross section of the problem is divided into a mesh of preferably square elements. Mohr-Coulomb model is used to represent the soil. Cohesion, friction angle, dilation angle, density, shear modulus and bulk modulus are used in the model. file:///t /Test Reports/Slope Stabilization Using Geofoam at Route 23A (AE)/ch4.html (4 of 63) [9/6/2000 2:57:58 PM]

5 Elastic model is used to represent geofoam. Geofoam density, shear modulus and bulk modulus are used in the elastic model. Values of these properties were chosen so as to cover the practical range of these properties (Negussey, 1997). The boundary is selected to be far away from the expected slip surface. Failure occurs when the solution doesn t converge. The safety factor of the cross section is calculated by dividing the shear strength of the soil by the shear strength at failure. The failure slip surface can be monitored using different ways; one of which is by monitoring the zones where the shear strain increment is maximum. 4.4 Effect of Using Geofoam in Increasing the Factor of Safety Slopes with circular slip surfaces were found to have the same factor of safety and identical slip surfaces using either software, FLAC or GeoSlope. This is examined for both cases, with foam and without foam. GeoSlope and FLAC gave the same factor of safety and the same starting point of the circular slip surface for slopes (not having geofoam) as given by Taylor charts Cohesionless Soil Failure surface for homogenous cohesionless soil is generally shallow (McCarthy, 1995). It forms on the sloped edge as shown in figure 4-4. The limiting equilibrium analysis gave the same result and the same failure zone as the finite difference analysis. By using geofoam, the factor of safety and the failure surface does not change. Geofoam affects neither the driving force nor the stabilizing force as shown in figure 4-5. Figure 4-5 shows that failure zones develop in the area between the foam and the sloped edge. By replacing this zone with a more stiff soil, failure zones develop somewhere else along the sloped edge, maintaining almost the same factor of safety. file:///t /Test Reports/Slope Stabilization Using Geofoam at Route 23A (AE)/ch4.html (5 of 63) [9/6/2000 2:57:58 PM]

6 Figure 04 Shear Strain Contours in a Cohesionless Soil Slope Figure 05 Shear Strain Contours in a Geofoam-Stabilized Cohesionless Soil Slope file:///t /Test Reports/Slope Stabilization Using Geofoam at Route 23A (AE)/ch4.html (6 of 63) [9/6/2000 2:57:58 PM]

7 4.4.2 Cohesive Soil The slip surface for a general slope of a homogeneous cohesive soil is either a deep-seated circle or a toe circle (McCarthy, 1995). In both cases, a large amount of soil next to the slope edge acts in the driving force. Figure 4-6 shows the failure zones for a clay soil obtained using FLAC. The soil profile of this cross section is a cohesive soil underlined by stiffer soil. For such cases, the failure surface almost touches the stiff soil. The same failure surface is developed using GeoSlope. By using geofoam in the cross section, failure zones extend to surround the foam blocks. The surface is no more circular. In this case, the limiting equilibrium method does not predict the correct factor of safety automatically. One have to assume a certain slip surface and calculate its corresponding factor of safety and repeat that a number of times to reach the minimum value of factor of safety which may not be ever reached by this method It was found from FLAC results that the factor of safety increases by using geofoam. And it was found that failure zones surround the amount of foam as shown in figure 4-7. If the cohesive soil has a considerable amount of friction, i.e. has a high internal friction angle, then by increasing the amount of foam the failure surface will continue to form in such a way that it will surround the foam. The failure surface beyond a certain factor of safety will begin to form in the area between the foam and the slope edge. Any increase in the amount or any change in the shape of the cross section of the foam fill will never increase the factor of safety. Figure 06 Shear Strain Contours in a Cohesive Soil Slope file:///t /Test Reports/Slope Stabilization Using Geofoam at Route 23A (AE)/ch4.html (7 of 63) [9/6/2000 2:57:58 PM]

8 Figure 07 Shear Strain Contours in Geofoam-Stabilized a Cohesive Soil Slope Increasing The Factor of Safety One way to increase the factor of safety of a slope is to decrease the inclination of the slope. This may result in the reduction of the usable space. Geofoam stabilization may maintain the inclination of the slope and increase the factor of safety. In fact, the slope can still be steep with a high factor of safety at the same time as shown in figure 4-8. The more the amount of foam that is added, the larger the failure area become resulting in a higher factor of safety. The distribution of a certain amount of foam will affect the factor of safety as shown in figure 4-9. The closer is the shape of the foam fill to the slip surface shape the higher is the factor of safety. This holds good as long as the foam does not encroach upon the area of the stabilizing force. file:///t /Test Reports/Slope Stabilization Using Geofoam at Route 23A (AE)/ch4.html (8 of 63) [9/6/2000 2:57:58 PM]

9 Figure 08 Effect of Geofoam Width on the Factor of Safety for Different Slopes Figure 09 Effect of Geofoam Width on the Factor of Safety for Different Geofoam Configurations file:///t /Test Reports/Slope Stabilization Using Geofoam at Route 23A (AE)/ch4.html (9 of 63) [9/6/2000 2:57:58 PM]

10 Figures 4-10 & 4-11 show the variation of the factor of safety for both static and dynamic conditions, with the width and the depth of the foam fill. The purpose of showing the figures is to represent the benefit of using geofoam to increase the factor of safety. Figure 010 Effect of foam Width on the Factor of Safety for Different Thickness file:///t /Test Reports/Slope Stabilization Using Geofoam at Route 23A (AE)/ch4.html (10 of 63) [9/6/2000 2:57:58 PM]

11 Figure 011 Effect of Using Geofoam on the Factor of Safety for Different Horizontal Accelerations 4.5 Route 23A file:///t /Test Reports/Slope Stabilization Using Geofoam at Route 23A (AE)/ch4.html (11 of 63) [9/6/2000 2:57:58 PM]

12 4.5.1 Background Geofoam is used in an unstable 100m long on route 23A east of the Village of Jewett Center in the Town of Jewett in Greene County (Jutkofsky, 1998). In 1966 in order to meet the current safety requirements and future traffic needs the New York State Department of Public Works reconstructed Rte 23A between the Village of Hunter and Jewett Center in Green County. Shortly after the reconstruction, a 100-m-long section of the roadway embankment began moving laterally towards the creek, as shown figure A scarp showed up on the north side of the cross section as shown in photo 4-1. By frequent patching a normal grade was maintained through the following years. Although the average annual movement was less than m it was a traffic hazard and maintenance costing. In 1978, a subsurface exploration program began. The general subsurface profile up to a maximum explored depth of 21 m, consist of compact gravelly silt (clayey); over layered clayey silt (silty clay); underlain by clayey silt gravelly. Ground water was found at 1.5 to 5m below surface. In 1979, a cluster of 22 horizontal drains was installed in a fan-shaped pattern along the toe of the slope in an attempt to reduce the movement by lowering the water table. At the same time, a monitoring program began. Inclinometer A, shown in figure 4-12, was installed near the center of the failure area to determine the zone and the rate of movement. Inclinometer reading was taken for the following 14 years. Photo 01 Main Crack Before 1996 file:///t /Test Reports/Slope Stabilization Using Geofoam at Route 23A (AE)/ch4.html (12 of 63) [9/6/2000 2:57:59 PM]

13 Figure 012 Positions of the Slope Indicators As the results of the inclinometer showed progressive movement, the horizontal drain treatment was considered unsuccessful. That was partly due to the nature of clayey soils and also their low strength. In 1994, a permanent treatment was pursued. The weight reduction by using EPS geofoam to reduce the driving force and using a sand filter to provide a positive drainage with lowering the water table to strengthen the soil was chosen as a solution as shown in figure This could be accomplished with fast reconstruction, without property- taking from the adjoining homeowners and maintaining the same geometry of the slope. A 15m long sheeting was required to handle two construction problems; retaining surrounding soils during excavation and functioning as a safety barrier for the detour traffic while construction. The excavation level was chosen to be the 100-year flood level to prevent of the uplifting of the geofoam. That made the excavated depth equal to 5m. A typical cross section in figure 4-13 shows 0.6m crushed stone - filter below the 5 layers of foam. Besides functioning as a drainage blanket the horizontal sand layer established a clean stabilized working platform over the soft saturated soils. 0.6m crushed stone acts as a filter extended between the foam blocks and the sheeting as shown in figure 4-2. The main purpose of this arrangement was to lower the water table to the bottom drainage layer. The vertical layer functioned as a protection to the foam from damage as the sheeting was extracted, it filled the voids when the sheeting was vibrated out. file:///t /Test Reports/Slope Stabilization Using Geofoam at Route 23A (AE)/ch4.html (13 of 63) [9/6/2000 2:57:59 PM]

14 Figure 013 Typical Cross Section in Route 23a Photo 02 The crushed stone filter file:///t /Test Reports/Slope Stabilization Using Geofoam at Route 23A (AE)/ch4.html (14 of 63) [9/6/2000 2:57:59 PM]

15 Photo 03 Pouring the concrete slab Photo 04Backfilling over the foam side file:///t /Test Reports/Slope Stabilization Using Geofoam at Route 23A (AE)/ch4.html (15 of 63) [9/6/2000 2:57:59 PM]

16 The foam was type VIII Expanded Polystyrene with nominal density of 20 kg/m3. The blocks were 0.6m X 1.2m X 2.4m. The minimum shear strength of such Type was between 159kpa (ASTM D732). The minimum compressive strength at 10% deformation according to ASTM D1621 was 90 kg/m3. A 0.1 m concrete slab was placed on the top of the 3m height foam blocks to serve as a protection from petroleum spilling and to distribute the traffic load on the foam. In August 1995 inclinometer B, shown in figure 4-13, was installed. In October sheeting driving began. Excavation started after about 70 percent of the sheeting was instaled. On November 20 the foam blocks were placed. Backfilling began after several geofoam courses were placed. Earth fill was placed over the stepped face of the block mass. This operation continued as geofoam courses were added up to the finish grade. Only the earth fill was compacted. By December , the first half of the blocks was in place and ready to receive the concrete slab as shown in photo 4-3. The graded sub base crushed stone was placed after concrete curing and completing backfilling as shown in photo 4-4. The sheeting was completely removed in January Placing of 0.23 m asphalt pavement was completed in April The total thickness of the base and the sub base ranged from to 0.6m to 1.2m because of the road banked and inclined geometry. Figure 4-14 shows the construction sequence. Figure 014 Construction Time Line file:///t /Test Reports/Slope Stabilization Using Geofoam at Route 23A (AE)/ch4.html (16 of 63) [9/6/2000 2:57:59 PM]

17 4.5.2 Field Results Figures 4-15 through 4-26 show the results obtained from inclinometer A. The inclinometer gives the displacement in two directions, the main direction and the secondary direction. The main direction is the North/South direction as shown in figure The secondary direction is the East/West direction, which is the direction out of plane of figure The total displacement is calculated by taking the square root of sum of squares of the displacement in the main and secondary directions. The angle that the total direction makes with the North/South direction is 6 degrees based on the maximum North/South displacement and its correspondence in the East/West direction. The results were obtained for a 20.72m depth each 0.6m from the top surface, elevation , all the way down to elevation m. The results were obtained from September 4 th 1979 up to October 7 th It must be kept in mind the involved maintenance that occurred in the roadway cross-section and the change in the water table level as well as the creek level during this fourteen years period. The twelve figures show that there is a movement but of a slow rate. An overall failure did not occur. The factor of safety can be considered to be 1.0 or slightly less. Figure 4-15 is the North/South direction creep. The movement for zone between Elevation m and m can be considered zero with no creep effect. Up to 0.6m above this zone, i.e. between elevations m and m, displacement increases with time. The top zone between elevations m and elevation 439.8m has the same trend of increasing displacement. Thus a slip surface can be considered passing between the two zones and at elevation at inclinometer a position. A number of explanations can be given for the displacement values for the last point. For example, it could be due to the effect of a sudden drop in the water level or an error in the reference level. Figure 4-16 shows the East/West direction creep. The displacement values are generally a tenth of those in the North/South direction. Two zones appear in this figure. Below elevation m, the soil has less creep. Above this zone, the displacements are almost equal for all monitored elevations with more creep effect. The last two readings show different trends from the previous readings and again, this could be due to the error in the reference level or a sudden change in the applied load such as change in the water table. file:///t /Test Reports/Slope Stabilization Using Geofoam at Route 23A (AE)/ch4.html (17 of 63) [9/6/2000 2:57:59 PM]

18 Figure 015 In Plane creep at Different Elevations Figure 016 Out of Plane Creep at Different Elevations file:///t /Test Reports/Slope Stabilization Using Geofoam at Route 23A (AE)/ch4.html (18 of 63) [9/6/2000 2:57:59 PM]

19 Total creep measured by inclinometer A is shown in figure The maximum displacement is equal to 0.2m, which is equal to what was measured in the North/South direction. By comparing figure 4-17 with figure 4-15 we can say that the effect of the out of plane movement can be neglected. Figures 4-18, 4-19 and 4-20 show the vertical distribution of the horizontal displacement on three different instants in the 14-year period. Figure 4-18 is for the North/South direction, figure 4-19 is for the East/West Direction and figure 4-20 is the resultant. The displacement was initially zero in For the three figures, it can be seen that the movement below elevation m is negligible. A sudden increase started at elevation m. Figure 017 Total Creep at Different Elevations file:///t /Test Reports/Slope Stabilization Using Geofoam at Route 23A (AE)/ch4.html (19 of 63) [9/6/2000 2:57:59 PM]

20 Figure 018 In Plane Movement in 14 Years file:///t /Test Reports/Slope Stabilization Using Geofoam at Route 23A (AE)/ch4.html (20 of 63) [9/6/2000 2:57:59 PM]

21 Figure 019 Out of Plane Movement in 14 Years Figure 020 Total Movement in 14 Years Movements above elevation m are almost equal. Movements in the East/West Direction are a tenth of that in the North/South Direction. Results at three instants were presented in these figures. For the East/West, movement was towards the east up to April 1982 and after that it moved towards the west. The maximum movement was less than 2 cm. Movements in the North/South direction was towards the south throughout the 14 years (i.e. towards the creek). Figures 4-21, 4-22 and 4-23 represent the displacement rate with time for the North/South, the East/West and the resultant directions respectively. The values in East/West direction are one half that in the North/South Direction. There was an increase in the displacement rate from 1997 to 1986 when a peak of m/year was reached. Between 1986 and 1992, there was a reduction in the creep rate. file:///t /Test Reports/Slope Stabilization Using Geofoam at Route 23A (AE)/ch4.html (21 of 63) [9/6/2000 2:57:59 PM]

22 Figure 021 In Plane Displacement Rate at Different Elevations Figure 022 Out of Plane Displacement Rate at Different Elevations file:///t /Test Reports/Slope Stabilization Using Geofoam at Route 23A (AE)/ch4.html (22 of 63) [9/6/2000 2:57:59 PM]

23 Figure 023 Total Displacement Rate at Different Elevations After 1992, the results gave a large increase in the displacement rate where the rest of the results in this year are not plotted. As mentioned before, this is an indication that the factor of safety is equal to or slightly less than 1.0, as in this case any small changes in the applied loads can jump the situation from stable to unstable equilibrium or vise versa. Figures 4-24, 4-25 and 4-26 show the increment strain distribution over a 20m height for the North/South, East/West and the resultant directions. The increment strain distribution is for the period between 1979 and For the three directions, there are large strains in the 2.44m thick zone between elevations m and m. Both the total and the North/South strain increment exceeded a 20% strain value. The east/west direction reached only a 3.5%strain increment. Strain values were calculated by dividing the difference in displacement at the ends of the 0.6m deep intervals by the thickness. The peak strain for the three directions is at elevation file:///t /Test Reports/Slope Stabilization Using Geofoam at Route 23A (AE)/ch4.html (23 of 63) [9/6/2000 2:57:59 PM]

24 Figure 024 In Plane Shear Strain in 14 Years Figure 025 Out of Plane Shear Strain in 14 Years file:///t /Test Reports/Slope Stabilization Using Geofoam at Route 23A (AE)/ch4.html (24 of 63) [9/6/2000 2:57:59 PM]

25 Figure 026 Total Strain in 14 Years Results obtained from inclinometer B are shown in figures 4-27 to The results were for the period from August 1995 to June Parts of the results were obtained during the construction period, August 1995 to April Figures 4-27, 4-28 and 4-29 show the creep behavior for the three directions the North/South, the East/West and the resultant respectively. The results were shown for different elevations. For all three directions, the displacement increase from zero to a certain level until construction is done after which there was no increase in the displacement values. Both the North/South and the East/West directions show comparable displacement values. The North/South direction reached a value of 8cm while the East/West reached 7cm displacement during construction. file:///t /Test Reports/Slope Stabilization Using Geofoam at Route 23A (AE)/ch4.html (25 of 63) [9/6/2000 2:57:59 PM]

26 Figure 027 In Plane Creep at Different Elevations after 1995 Figure 028 Out of Plane Creep at Different Elevation Before 1995 file:///t /Test Reports/Slope Stabilization Using Geofoam at Route 23A (AE)/ch4.html (26 of 63) [9/6/2000 2:57:59 PM]

27 Figure 029 Total Creep at Different Elevations Before 1995 Figures 4-30, 4-31 and 4-32 represent the movement distribution all over the 12m depth from the surface for the North/South, the East/West and the resultant direction respectively. Three dates are shown in this figure; November 1995 was during construction, August 1996 was when the first readings were available after construction and the third date is one for which the most recent reading is available. Three zones can be determined in these figures; the first zone is below elevation 426.1m, the second zone is between elevations 426.1m and 429.1m and the third zone is above elevation 429.1m. No movement showed up in the lower zone for the 4-year period during and after construction. The top zone has uniform movement all over the height of the zone during and after construction. The movement in the top zone is occurred during the construction period and none showed up after construction. The middle zone has linear movement distribution all over the zone height. It acts as a transition zone between the two other zones. All the movement in this zone occurred during construction. file:///t /Test Reports/Slope Stabilization Using Geofoam at Route 23A (AE)/ch4.html (27 of 63) [9/6/2000 2:57:59 PM]

28 Figure 030 In Plane Movement after 1995 Figure 031 Out of Plane Movement after 1995 file:///t /Test Reports/Slope Stabilization Using Geofoam at Route 23A (AE)/ch4.html (28 of 63) [9/6/2000 2:57:59 PM]

29 Figure 032 Total Movement after 1995 Figures 4-33, 4-34, and 4-35 show the displacement rate for the North/South, East/West and the resultant directions respectively. After the construction is done in April 1996, the displacement rate is equal to zero for the three directions. During construction a displacement rate of 2mm per day was reached in the North/South direction and 1.4mm/day in the East/West direction. The peak for both directions did not occur at the same time. The Displacement rate was calculated by dividing the difference in two successive dates by the duration between them and was plotted at the initial date. So the zero values of the displacement rate can be extended to June 1999 in the three figures. file:///t /Test Reports/Slope Stabilization Using Geofoam at Route 23A (AE)/ch4.html (29 of 63) [9/6/2000 2:57:59 PM]

30 Figure 033 In Plane Displacement Rate at Different Elevations after 1995 Figure 034 Out of Plane Displacement Rate at Different Elevations after 1995 file:///t /Test Reports/Slope Stabilization Using Geofoam at Route 23A (AE)/ch4.html (30 of 63) [9/6/2000 2:57:59 PM]

31 Figure 035 Total Displacement Rate at Different Elevations after 1995 Figures 4-36, 4-37 and 4-38 represent the strain distribution obtained from inclinometer B in the North/South, the East/West and the resultant direction respectively. The results are for a 4 years period. Three dates were presented in these figures during construction and just after construction and three years after construction. A maximum strain of 3.8% was obtained in the North/South direction. This amount was obtained during construction, from August 1996 to June Figures 4-39 through 4-50 show the same results for figures 4-27 through 4-38 with the results for the period after April 1996, i.e. after the construction is ended. Figure 4-51 shows the Creep trend over 20 years. file:///t /Test Reports/Slope Stabilization Using Geofoam at Route 23A (AE)/ch4.html (31 of 63) [9/6/2000 2:57:59 PM]

32 Figure 036 In Plane Strain at Different Elevations after 1995 file:///t /Test Reports/Slope Stabilization Using Geofoam at Route 23A (AE)/ch4.html (32 of 63) [9/6/2000 2:57:59 PM]

33 Figure 037 Out of Plane Strain at Different Elevations after 1995 Figure 038 Total Strain at Different Elevations after 1995 file:///t /Test Reports/Slope Stabilization Using Geofoam at Route 23A (AE)/ch4.html (33 of 63) [9/6/2000 2:57:59 PM]

34 Figure 039 In Plane Creep after Construction Figure 040 Out of Plane Creep after Construction file:///t /Test Reports/Slope Stabilization Using Geofoam at Route 23A (AE)/ch4.html (34 of 63) [9/6/2000 2:57:59 PM]

35 Figure 041 Total Creep after Construction file:///t /Test Reports/Slope Stabilization Using Geofoam at Route 23A (AE)/ch4.html (35 of 63) [9/6/2000 2:57:59 PM]

36 Figure 042 In Plane Displacement after Construction Figure 043 Out of Plane Displacement after Construction file:///t /Test Reports/Slope Stabilization Using Geofoam at Route 23A (AE)/ch4.html (36 of 63) [9/6/2000 2:57:59 PM]

37 Figure 044 Total Displacement after Construction Figure 045 In Plane Displacement Rate after Construction file:///t /Test Reports/Slope Stabilization Using Geofoam at Route 23A (AE)/ch4.html (37 of 63) [9/6/2000 2:57:59 PM]

38 Figure 046 Out of Plane Displacement Rate after Construction Figure 047 Total Displacement Rate after Construction file:///t /Test Reports/Slope Stabilization Using Geofoam at Route 23A (AE)/ch4.html (38 of 63) [9/6/2000 2:57:59 PM]

39 Figure 048 In Plane Strain after Construction file:///t /Test Reports/Slope Stabilization Using Geofoam at Route 23A (AE)/ch4.html (39 of 63) [9/6/2000 2:57:59 PM]

40 Figure 049 Out of Plane Strain after Construction Figure 050 Total Strain after Construction file:///t /Test Reports/Slope Stabilization Using Geofoam at Route 23A (AE)/ch4.html (40 of 63) [9/6/2000 2:57:59 PM]

41 Figure 051 Creep Trend Before and after utilizing Geofoam Four Extensometers were installed between geofoam layers to detect and measure any movements of the blocks and between the blocks as shown in figure Extensometers A and B were located between the vertical stone drainage gallery and the blocks. Extensometers C and D were located within the Blocks. Figure 4-39 shows the results of Extensometer B and D. Movement occurred during the construction period. Extensometer B moved about 6 cm towards the Greek while Extensometer D moved 2.5 cm only. The movement is believed to be the closing of the gaps between the blocks. The difference in the value can be explained to be due to the position of the Extensometer within the geofoam mass. More gaps are required to be closed for the case of Extensometer B. After Construction no movement was measured by both Extensometers. Extensometers A and C measured zero movement. file:///t /Test Reports/Slope Stabilization Using Geofoam at Route 23A (AE)/ch4.html (41 of 63) [9/6/2000 2:57:59 PM]

42 Figure 052 Positions of the Extensometers Figure 053 Extensometers B & D Readings file:///t /Test Reports/Slope Stabilization Using Geofoam at Route 23A (AE)/ch4.html (42 of 63) [9/6/2000 2:57:59 PM]

43 4.5.3 Numerical Analysis Rt23a Geofoam stabilized cross-section is numerically modeled. A finite difference mesh is shown in figure The problem is simulated as an in plane problem. The dimensions of the cross-section are chosen such that the boundaries are away from the stressed zones. A plastic model, Coulomb-Mohr is used to simulate the soil. The model can be represented using the shear modulus, the bulk modulus, the friction angle, the cohesion and the density of the soil. After applying the loads and the boundary conditions, the numerical analysis software is run in a large strain mode. Large strain mode will adjust the dimensions of the mesh after each solution step to take account of the deformations that will occur. Although failure and large deformations may occur, the solution will continue until either equilibrium occurs with a new grid dimensions or the ratio of any rectangular element reaches a ratio of 10. In the later case the solution will stop running. The problem can be solved in a small strain mode. In the small strain mode the dimensions of the grid will not change during stepping. In the case of no failure, implying very small movements, convergence will occur. The solution will stop if there are large deformations in the dimensions of the elements. Large strain mode can converge even after deformations of the order of 10 meters. Figure 4-55 shows the grid distortion for the cross-section before 1996 as well as the boundary of the cross section before starting the solution. The grid is magnified 60 times. The maximum displacement is 8 cm. This is a static equilibrium situation. No creep effect is encountered in this solution. Slightly reducing the strength the displacement will be in the order of meters and may not converge. That means the factor of safety equals one for the situation before The failure surface can be figured from figure 4-55 by tracing the skew rectangle elements where excessive shear strain takes place. Small deformation can be seen in the first three rows where the soil in these rows is stiffer than the upper rows. The upper line of the exaggerated grid profile shows settlement in the road area as well as high deformation in the north side of the road where the scarp occurred. file:///t /Test Reports/Slope Stabilization Using Geofoam at Route 23A (AE)/ch4.html (43 of 63) [9/6/2000 2:57:59 PM]

44 Figure 054 The Finite Difference Grid Displacement vectors are shown in figure The directions of the arrows represent the direction of the movement. The movements are due to two reasons; the own weight and the effect of the sloped edge. Only vertical movements occurred near the north vertical boundary implying that the boundary was far enough from the zone of the slope effect. On the other side very small diagonal arrow shows up on the south vertical boundary. Although moving that boundary few meters to the south will increase the accuracy of the results the current solution gives good results with reasonable solution time. The failure surface can be determined by the large change in the length of the two adjacent arrows. file:///t /Test Reports/Slope Stabilization Using Geofoam at Route 23A (AE)/ch4.html (44 of 63) [9/6/2000 2:57:59 PM]

45 Figure 055 Exaggerated Grid Distortion before 1996 file:///t /Test Reports/Slope Stabilization Using Geofoam at Route 23A (AE)/ch4.html (45 of 63) [9/6/2000 2:57:59 PM]

46 Figure 056 Displacement Vectors before 1996 Three zones are shown in figure The zone with small circles has tension failure. The zone with the mark (*) is a shear failure zone. No failure occurred in the rest of the cross section. A scarp occurred north of the road in the tension failure zone. The failure surface will pass through the tension and shear failure zones and as can be seen from the flattered shear failure zone next to the lower stiff layer, it will not be a circular surface. Figure 4-58 shows the shear strain distribution before The values are due to static equilibrium, which means that no creep effect is taken into consideration. Comparing this figure with figure 4-23, it can be seen that at horizontal distance equals to 26m where inclinometer A is located, maximum strain occurs at elevation 429.5m for both figures. Again, the difference in values is equal to that due to the creep effect. The maximum shear strain in the cross section occurs at distance 42m and is too close to the stiffer lower layers. An inclinometer may give very high readings if installed in this location. The failure surface can be located by passing a line through the points of highest strain at each vertical section. Figure 4-59 shows the horizontal displacement contours. The failure surface can be easily identified by the contour B, the first nonzero contour. Again, it is not a circular surface, and the creep effect is not included in these results. Comparing the in plane horizontal distribution in figure 4-17 with a vertical section at horizontal distance 26m, the position of inclinometer A, one can find that both distributions start with a zero value and continue up to 427m-428 m. A rapid increase of the horizontal displacement with height occurs in the upper 3 meters. The horizontal displacement does not change mush above an elevation of 431m. The vertical displacement contours are shown in figure The movement is the result of the own weight and the slope effect. The maximum settlement will occur beneath the road. Before 1996, it was essential to repave the settled portion of the road to maintain leveled driving surface from time to time. Creep effect was the main reason of this progressive settlement. The vertical parts of the contours show that a horizontal section through this portion of the cross-section will have a sloped vertical movement where the maximum value will be on the north side. As the soil is not compressible, this can only occur by allowing lateral movement towards the creek. Two material models are used for the case after 1996 as shown in figure Elastic model is used for foam. Young s Modulus, Poisson s ratio and the density are the parameters used in the elastic model. Coulomb-Mohr is used for soil. No interface elements between the geofoam blocks and the soil or between the foam blocks are modeled in this solution. From the results that are shown later, shear stresses are too low to reduce slippage. To reach the case after 1996 cross-section, construction sequence is modeled in this solution. After reaching equilibrium using the soil cross-section, the sheet pile is added to the model, excavation is done and the drainage filter, foam blocks and back filling are placed finally followed by removal of sheet pile. In each construction step, the solution has to reach equilibrium before proceeding to the following step. file:///t /Test Reports/Slope Stabilization Using Geofoam at Route 23A (AE)/ch4.html (46 of 63) [9/6/2000 2:57:59 PM]

47 Figure 4-62 shows the displacement vectors after excavation. A lateral movement of 0.6m of the sheet pile was reported. It was noticed that several inches of settlement occurred behind the sheet pile as soil was removed. In the FLAC model, 0.2m settlement occurred just after excavation. The exact value from the field is not known as they paved the road behind the sheet pile to maintain safe driving environment. After removing the sheet pile in the FLAC model, the solution was allowed to reach convergence to study the geofoam-stabilized slope. Figure 4-63 shows the shear strain distribution in the cross section. The maximum value reached is 0.1% compared to the 0.2% reached in figure In both figures, the shear strain can be considered zero. Inclinometer B can be located at a distance of 35.5 m from the edge of the model. Figure 057 Failure Zones before 1996 file:///t /Test Reports/Slope Stabilization Using Geofoam at Route 23A (AE)/ch4.html (47 of 63) [9/6/2000 2:57:59 PM]

48 Figure 058 Shear Strain Contours Before 1996 Figure 059 Horizontal Displacement Contours Before 1996 file:///t /Test Reports/Slope Stabilization Using Geofoam at Route 23A (AE)/ch4.html (48 of 63) [9/6/2000 2:57:59 PM]

49 Figure 060 Vertical Displacement Contours Before 1996 Figure 061 Material Models file:///t /Test Reports/Slope Stabilization Using Geofoam at Route 23A (AE)/ch4.html (49 of 63) [9/6/2000 2:57:59 PM]

50 Figure 062 Displacement Vectors after Excavation Figure 063 Shear Strain Contours after 1996 file:///t /Test Reports/Slope Stabilization Using Geofoam at Route 23A (AE)/ch4.html (50 of 63) [9/6/2000 2:57:59 PM]

51 Figure 4-64 shows the horizontal displacement contours after The maximum value of horizontal displacement is 0.5 cm. Figure 4-35 gives the same results all over the body of the inclinometer with the peak at its top. The horizontal movement in the numerical model shows up near the sloped surface. Figure 4-65 shows the vertical displacement contours after The maximum value is 1.8 cm. The settlement is due to the elasticity of both the soil and the Foam. The main load for this settlement is the fill on the top of the foam as the own weight of the foam is negligible. Ground water is considered in the numerical solution. FLAC calculates the pore pressure. Figure 4-66 shows the pore pressure contours before The Drain blanket changes the water table level as shown in figure 4-67 and new pore pressure contour is calculated. Figure 064 Horizontal Displacement Contours after 1996 file:///t /Test Reports/Slope Stabilization Using Geofoam at Route 23A (AE)/ch4.html (51 of 63) [9/6/2000 2:57:59 PM]

52 Figure 065 Vertical Displacement Contours after 1996 Figure 066 Pore Pressure Contours Before 1996 file:///t /Test Reports/Slope Stabilization Using Geofoam at Route 23A (AE)/ch4.html (52 of 63) [9/6/2000 2:57:59 PM]

53 Figure 067 Pore Pressure Contours after 1996 Figures 4-68 through 4-71 show the stresses in three directions: vertical direction, in plane horizontal direction and out-of-plane direction. All stresses are shown for the two cases (total stresses and effective stresses). In-plane shear stress is also shown. The figures are shown for the two cases before and after The total horizontal in plane stresses in the foam zone is reduced after 1996 as shown in figure 4-68 compared to figure For the same zone, the effective stresses are the same (figures 4-70 and 4-71). The total Vertical stresses in the foam zone and below it is reduced after 1996 as shown in figure 4-72 compared to figure This is due to the effect of the lightweight fill. In front and on the back of the foam blocks the stresses are the same before and after The same distribution can be observed in figures 4-74 and 4-75 but with reduction in the stresses because of the effect of the pore water pressure. The total and the effective shear stress contours are identical, as Mohr circle at each point will have the same radius for both cases. Figure 4-76 shows the in plane shear stress for the case before The maximum value reached for the zone between the weak and the stiffer soil is 35 kpa. For the case after file:///t /Test Reports/Slope Stabilization Using Geofoam at Route 23A (AE)/ch4.html (53 of 63) [9/6/2000 2:57:59 PM]

54 1996 the same spot has a 30 kpa stress as shown in figure The factor of safety of the slope will be 35/ as a factor of safety would have been increased by redistribution of the same foam amount in the cross section as mentioned earlier. The out of plane stresses behaved like the in plane horizontal stresses. As shown in figures 4-78 and 4-79 the total stresses are less before 1996 for the geofoam zone. For the same zone, the effective stresses are a little less before 1996 (figures 4-80 and 4-81). Figure 068 Horizontal Stress Contours Before 1996 file:///t /Test Reports/Slope Stabilization Using Geofoam at Route 23A (AE)/ch4.html (54 of 63) [9/6/2000 2:57:59 PM]

55 Figure 069 Horizontal Stress Contours after 1996 file:///t /Test Reports/Slope Stabilization Using Geofoam at Route 23A (AE)/ch4.html (55 of 63) [9/6/2000 2:57:59 PM]

56 Figure 070 Effective Horizontal Pressure Contours before 1996 Figure 071 Effective Horizontal Stress after 1996 file:///t /Test Reports/Slope Stabilization Using Geofoam at Route 23A (AE)/ch4.html (56 of 63) [9/6/2000 2:57:59 PM]

57 Figure 072 Vertical Stress Contours before 1996 Figure 073 Vertical Stress Contours after 1996 file:///t /Test Reports/Slope Stabilization Using Geofoam at Route 23A (AE)/ch4.html (57 of 63) [9/6/2000 2:57:59 PM]

58 Figure 074 Effective Vertical Stress Contours before 1996 Figure 075 Effective Vertical Stress Contours after 1996 file:///t /Test Reports/Slope Stabilization Using Geofoam at Route 23A (AE)/ch4.html (58 of 63) [9/6/2000 2:57:59 PM]

59 Figure 076 Shear Stress Contours before 1996 Figure 077 Shear Stress Contours after 1996 file:///t /Test Reports/Slope Stabilization Using Geofoam at Route 23A (AE)/ch4.html (59 of 63) [9/6/2000 2:57:59 PM]

60 Figure 078 Out of Plane Stress Contours before 1996 Figure 079 Out of Plane Stress Contours after 1996 file:///t /Test Reports/Slope Stabilization Using Geofoam at Route 23A (AE)/ch4.html (60 of 63) [9/6/2000 2:57:59 PM]

61 Figure 080 Out of Plane Effective Stress Contours before 1996 Figure 081 Out of Plane Effective Stress Contours after 1996 file:///t /Test Reports/Slope Stabilization Using Geofoam at Route 23A (AE)/ch4.html (61 of 63) [9/6/2000 2:57:59 PM]

62 4.6 Summary The factor of safety of slopes can be increased using geofoam stabilization for both static and dynamic conditions. A number of parameters including the amount and the distribution of foam blocks, soil characteristics, geometry of slope affect the factor of safety of a geofoam stabilized slope. The effect of amount and distribution of foam on the factor of safety are controlled by the type of soil. Using geofoam blocks in cohesionless soil slopes, where shallow failure surface occurs, will not affect the factor of safety at all. Both the finite difference analysis and the limiting equilibrium analysis give the same factor of safety and the same failure surface for general type of soil slopes having circular slip surfaces with as well as without foam. Software simulation results match with those given by Taylor charts for circular slip surface when foam is not used. Modeling geofoam blocks as cohesive soil with a very high cohesion value using the limiting equilibrium analysis is same as modeling geofoam blocks as elastic material using finite difference analysis. For cohesive soil, the failure surface of geofoam-stabilized slopes is deep, either surrounding the whole amount of foam or forming on the slope side of the foam. For geofoam stabilized cohesive soil where failure surface tends to develop only on one side of the foam, increasing the amount and the distribution of foam will increase the factor of safety of the slope until the slip surface occurs. Any further amount of foam will not increase the factor of safety of the slope. For geofoam stabilized cohesive soil slopes in which the failure surface develops on one side of the foam and ends on the other side of the foam, increasing the amount of foam increases the factor of safety up to a certain amount. Beyond this point, the slip surface starts to form only on one side of the slope. Any further amount of foam will not increase the factor of safety of the slope. For cohesive soil with an amount of friction particles, the failure surface of geofoam-stabilized slopes is shallow and develops on the slope side of the foam. Any increase in the amount of the foam in this case does not affect the factor of safety. In general, the more the amount of foam, the more the factor of safety as long as the slip surface does not develop on the slope side of the foam. For the same amount of foam, the closer is the foam to the slope side the higher is the factor of safety, as long as the failure surface does not develop on the foam slope side. For the same amount of foam, the closer is the distribution of the foam to the circular shape (i.e. with two inclined edges) the higher is the factor of safety, so long as the failure surface does not develop file:///t /Test Reports/Slope Stabilization Using Geofoam at Route 23A (AE)/ch4.html (62 of 63) [9/6/2000 2:57:59 PM]

63 on the slope side of the foam. Parameters affecting the factor of safety of geofoam-stabilized slopes under static loading will affect the factor of safety under dynamic loading. Geofoam stabilization leads to a steeper slope whose factor of safety is higher. A steep slope also increases the usable space. Geofoam Research Center 220 Hinds Hall Syracuse University Syracuse,New York Copyright 2000 Geofoam Research Center. All Rights Reserved. file:///t /Test Reports/Slope Stabilization Using Geofoam at Route 23A (AE)/ch4.html (63 of 63) [9/6/2000 2:57:59 PM]