Geotechnical Design Report For Widening An Existing Road

Using measurements recorded in the laboratory you will complete an ‘Interpretive Laboratory report’ following the guidelines stated in Appendix A. Please note the page limits and be aware any work shown on extra pages will not be considered for marking. Please use Font size 11, single spacing is acceptable.

You will also complete a ‘Geotechnical Design Report’ following the guidelines stated in Appendix B using the interpreted laboratory data in the ‘Interpretive Laboratory Report’. Please note the page limits and be aware any work shown on extra pages will not be considered for marking. Please use Font size 11, single spacing is acceptable.

The purpose of this report is to document subsurface geotechnical conditions, provide analyses of the expected site conditions as they pertain to the project described in this report and thereafter give a recommendation on the design and construction criteria for the roadway portions of the project. This report as well gives a geotechnical baseline that can be used in the evaluation of the existence and scope of altered site conditions. The report is intended for use by the project design engineer, bidders, construction personnel as well as the contractors.

The project aims at widening an existing road to provide maintenance of vehicle pullouts. On the south bond of the road will be retaining walls RW1 to RW3 while on the north bond will be Retaining Wall RW4 all of which will be designed in accordance with the Highways England Standards. The proposed retaining walls are as described in the proposal in the table below

Retaining Wall No.	Approximate Limits	Wall Height (ft.)	Notes
RW1	“AL1” Line Sta. 756+26 to 756+91	6	The existing 2:1 slope of the embankment fill will be used entirely for the founding of the spread footing. The actual height of the new approach fill of the wall will be retained at about 0.6 meters even though the wall will be designed to be 1.8 meters tall. The top edge of the footing closest to the wall horizontally will be a minimum of 1.2 meters away from the slope of the finish.
RW2	“AL1” Line Sta. 756+06 to 760+38	10-12	The existing 2:1 slope of the embankment fill will be used entirely for the founding of the spread footing, partially on the existing and on the new embankment. A level bench of between 0.6 to 1.2 meters feet will be constructed in front of the wall.
RW3	“AL1” Line Sta. 762+48 to 763+26	12-14	The existing 2:1 slope of the embankment fill will be used entirely for the founding of the spread footing, partially on the existing and on the new embankment. A level bench of between 0.6 to 1.2 meters feet will be constructed in front of the wall.
RW4	“AL1” Line Sta. 778+60 to 779+50	6-8	The existing 2:1 slope of the embankment fill will be used entirely for the founding of the spread footing, partially on the existing and on the new embankment. A level bench of between 0.6 to 1.2 meters feet will be constructed in front of the wall.

In order to characterize the conditions of the site subsurface, the group retained Taber Drilling to drill and sample three borings to depths that were ranging from 6.6 to 8 meters within the existing paved shoulder areas close to the proposed retaining walls. The group obtained samples of soil at various intervals using both a 3.0-inch O.D. Modified California sampler and a 2-inch O.D. Standard Penetration sampler. These samples were driven into an automatic hammer weighing approximately 140 pounds and falling at about 30 inches per blow. Bulk composite samples were also collected from the top most 0.46 to 1.5 meters of the borings conducted. By the time of completion of the drilling process, Taber Drilling backfilled the boring using neat cement grout and the surface patched using concrete (Craig 2014, p. 369).

On the south bond of the road is a pavement section that is made up of 3 to 5 inches of hot mix asphalt concrete on a 5-inch aggregate base laid over a layer of silty sand with gravel. This layer extends to 0.6 to 0.8 meters by depth below the naturally existing grade. Under the pavement section is a fill composed of very stiff to hard, lean to fat clay to the depth of about 5 meters under the naturally existing grade (Olson 2016, p. 615).  On the north bond of the road borings is a pavement section that is about 8 inches of hot mix asphalt concrete on 5 to 6.5 inches of an aggregate base laid over clay sand with gravel that extends to approximately 0.609M below the naturally existing grade. Beneath this pavement section is a fill composed of hard, lean to fat clay and high density silty sand to approximate depths of 2.4M under the naturally existing grade.

No groundwater was observed in any of the borings during the drilling process. The group reviewed the groundwater level data for the nearby dams and wells as availed in the resources from the municipal council (Radon 2017, p. 555). Based on the information extracted from these data sources, it was estimated that the levels of groundwater at the site fluctuated from about Ele. -7.6 m to -10.7 m which is as estimated as at least 15 meters below the existing surface of the ground at the site. It was noted however, that relatively shallow perched water would occur within the soils of the near surface during the seasons of spring and winter.

Subsurface Exploration Programs

The following laboratory tests were completed for soil samples that were obtained from the exploratory borings:

• Moisture content and dry density test
• Plasticity index test
• Direct shear test/ triaxial tests

Moisture content and dry density test is used in the establishment of the relationship between maximum dry density of the soil and the optimum moisture content that is obtainable from the soil compaction (Sanglerat 2013, p. 415). This was obtained from a standard proctor test. The soil compaction curve that is illustrative of the correlation is important in the determination of the maximum water content at which it is possible to attain optimum dry density of the soil through compaction. In this test it is impossible to directly determine the dry density hence the bulk density and the moisture content are first obtained in order to calculate the dry density from the formula  where

Various soil samples were found to be compacted at different water contents and the compaction curve as illustrated below is derived by plotting dry density against water content (Donaghe 2012, p. 751).It is possible to relate the percentage air void to the dry density Assessment and Suitability of Standard Plan Wall Design.

Based on the information provided by Highways England Standards regarding standards and quality measures, the spread footing for Retaining Wall RW1 will be founded within the exiting 2:1 embankment fill slope entirely without any bench in front of the wall (Watson 2013, p. 568). The actual height of the new approach fill of the wall will be retained at about 0.6 m even though the wall will be designed to be 1.8 m tall. The top edge of the footing closest to the wall horizontally will be a minimum of 1.2 m away from the slope of the finish.

In relation to the subsurface conditions experienced in the borings that were drilled within the same road approach fill, I anticipated that the spread footing foundation material would consist of very stiff to hard lean to fat clay fill. The unconfined compressive strength of the strength of the fill is approximately 0.9 m or even higher depending on the results of the pocket penetometer test (Helwany 2010, p. 245). This compressive strength exceeds the shear strength that is developed using the properties of the granular soil that is used for the design of the standard plan design of the wall.

The DMRB Volume 5: Section 1, Part 3: TA 79/99 Amendment No 1 can be used for Retaining Wall RW1 based on the limited retained fill height, the very stiff to hard soil conditions and the planned distance of 1.2 m distance from the footing at anticipated levels of foundation.

Based on the information provided by Highways England Standards regarding standards and quality measures, the existing 2:1 slope of the embankment fill will be used entirely for the founding of the spread footing, partially on the existing and on the new embankment. A level bench of between 0.6 to 1.2 m will be constructed in front of the wall (Acton 2013, p. 114). In relation to the subsurface conditions experienced in the borings that were drilled within the same road approach fill, I anticipated that the spread footing foundation material would consist of very stiff to hard lean to fat clay fill. Taking into consideration that a new filled will be placed, more conservative standard plan properties of soil would be used to evaluate the foundation bearing as well as the lateral capacity for footings that are embedded in the sloping ground (Hoddinott 2010, p. 189).

Subsurface soil conditions

In order to establish the required passive resistance/ passive wedge against the footing foe face, it is recommended that the minimum footing embedment of the wall be increased to 0.9 m under the finish grade in front of the walls (Koseki 2017, p. 598). It is also important having the top edge of the footing placed at least 1.8 m horizontally from the slope face of the finish. We evaluated the bearing resistance of the spread footing as outlined in the Strength and Extreme Limits States using the various available methods as outlined in the Bridge Design Specifications document (Editors 2008, p. 258). For the purposes of the analysis, the following were used:

• Modified bearing capacity factor for the footing on the sloping ground
• Soil properties of the standard plan that is a friction angle of 34 degrees and a unit weight of 120 pcf
• Effecting loading conditions and footing dimensions for the standard wall height of 3.6 m and 4.8 m respectively in order to take into account the variation in the proposed heights of the walls
• 45 as the geotechnical resistance factor used for strength limit resistance analysis as well as a geotechnical resistance factor of 1.0 as for the extreme limit cases.

From the analysis, it was found out that the bearing resistance for the proposed wall was in excess of the provided Strength and Extreme Limit loads as indicated on the standard plan for wall heights of 16 feet and/or less (Editors 2008, p. 361).As long as the recommended footing embedment and footing distance from the face of the slope is incorporated into the design of this road, Highways England Standards can be used in this analysis as can be noticed in the above analysis.

Based on the information provided by Highways England Standards regarding standards and quality measures, the existing 2:1 slope of the embankment fill will be used entirely for the founding of the spread footing, partially on the existing and on the new embankment. A level bench of between 0.6 to 1.2 m feet will be constructed in front of the wall (Aysen 2009, p. 965). In order to establish the required passive resistance/ passive wedge against the footing foe face, it is recommended that the minimum footing embedment of the wall be increased to 0.9 m under the finish grade in front of the walls.

It is also important having the top edge of the footing placed at least 6 feet horizontally from the slope face of the finish (Craig 2014, p. 126). The unconfined compressive strength of the strength of the fill is approximately 0.9 m or even higher depending on the results of the pocket penetometer test. This compressive strength exceeds the shear strength that is developed using the properties of the granular soil that is used for the design of the standard plan design of the wall. As long as the recommended footing embedment and footing distance from the face of the slope is incorporated into the design of this road, Highways England Standards can be used in this analysis as can be noticed in the above analysis.

 X (90, 20), Y (-60,-20)

C=(90+-60)/2=15MPa

From the above calculations

From Mohr’s Circle:X

tan 2?p1=20/75

2?p1=14.93?; ?p1=7.47?

tan 2?s1=75/20

2?p1=75.07?; ?p1=37.5?

Estimation of the wall dimension

Step 1

Determination of the number of blocks required

Length of wall (in inches)/length of a block=blocks per course

=4375/11.5= approximately 380 blocks per course

Step 2

Determination of the number of courses

Height of wall (in inches)/height of a block=number of courses

200/4=50 courses

From above information on the description of the project, there is no water behind the retaining wall and the calculations will assume no surcharge. The calculation will be in compliant with the Eurcode standards and requirements as regards strength of soils and other properties. Assuming the material of the retaining wall to be mass concrete and using an embankment height of 5.0M, the calculations are as follows;

1.0 1.1 Table 2 Cl 1.4321 Table 1 Cl. 1.4922 Table 3 Cl. 2.2.8 Cl. 3.2.5 Cl. 3.2.6 Table Case A 1400 mm thick Serviceability Ultimate Cl. 2.2.8, 3.2.6 Case B 1400mm thick Serviceability Ultimate Comment

Mass Concrete Wall H=5.0m, compact medium retaining wall with no water pressure. Soil properties of compact sand Wall friction Active Earth pressure coefficient Dense Medium sand, moderate grading: Assuming the sub-angular soils particles SPT ‘N’=20 at a depth of 5m A=2?, B=2? Overburden pressure=5.0×18.5=92.75 kN/m2: N,/N=1.7 So N,=1.7*20=34, so C=4.8? Wall friction design tan=tan=0.670 or 0.675 (M=1.2) design tanδ=tan 20? or 0.75*design tan=0.364 or 0.503 design 10kN/m² surcharge on the soils retained behind the wall Trying 1400mm thick wall Surcharge=0.242*10*5²=60.5 Active pressure= Wall friction=45.5*0.364=16.562 Wall self-weight=5.0*1.40*17=119 TOTAL Load eccentricity=55.6/145.4 Net bearing pressure at toe=145.4/1.4+6*55.6/1.42-0.6*18.5=263 kN/m2. Allowable pressure for dense sand=600 kN/m2 Check ultimate allowable bearing pressure to CIRIA C516 Effective breadth of base B’=1.40-2*0.382=0.635m Average bearing pressure=145.4/0.635=228.9 kN/m2 Horizontal/Vertical FH/FV=45.5/145.4 design 2 bearing cap.=11.1*28.7*0.476+0.5*18.5*0.635*37.1*0.324=231.5kN/m2 Sliding: base friction coefficient=or 0.75 design tan0.675 or 0.503. vertical load × friction=145.4×0.503=73.1kN/m2 sliding force=45.5kN/m2 Unplanned excavation to the wall front and no surcharge behind the wall Depth of excavation=10% of clear height of 5.0M Self-weight=5.0*1.4*17 Active pressure=1/2*5*5*18.5*0.242 Wall friction=35.8*0.364 Toe bearing pressure=141.8/0.895=158.4 kN/m2 Load eccentricity=35.8/141.8=0.252m Effective breadth=1.40-2*0.252=0.895m Bearing capacity=q’Nqiq+ FH/FV=35.8/141.8 Iq=0.558, Bearing capacity=4.81*.0558*28.7+0.5*18.5*0.895*37.1*0.418=205.6kN/m2 Sliding force=35.8 kN/m2 Resistance=141.8*0.503=71.3 kN/m2 Case A is critical due to the surcharge from both sides of the wall hence 1400mm thickness of the wall is required. In case of a 10 kN/m2 surcharge on the rear side of the wall, Euro code Standards require a thinner wall as compared to the traditional CP2 that can also be used in the calculation.

density= 18.5kN/m2 , δ=20? design tan=0.670 design tanδ=0.364 Ka=0.242 P         W     Ia      M 35.8         1.333  47.8 9.7           2.0      19.4         128.8  -         16.6 -0.70 -11.6 45.5 145.4          55.6 e=0.382m 263<600 kN/m2 OK 231.5 kN/m2>228.9 OK 45.5<73.1 kN/m  OK P    W    Ia        M       35.8 1.333 47.8      128.8   -        13.0  -0.70   -9.1 35.8 141.8         38.7 220<600 kN/m2 OK 158.4<205.6 kN/m2 OK 35.8<71.3 kN/m2 OK

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