CENG30005: You are Required to Design a long Concrete gravity Retaining wall to be constructed on sand and Backfilled: Design of Geotechnical Structures Assignment, UOB, Singapore

Take the unit weight of water as gw = 10 kN/m3 .

Question 1 (a) You are required to design a long concrete gravity retaining wall to be constructed on the sand and backfilled along with its full 9m height with silty sand. While you are free to choose the quality of the concrete to be used as a retaining wall, the lumped factor of safety against bearing capacity failure should be between 2.5 and 3.0.

The silty sand soil surface will have to be inclined at an angle between 8 and 20 degrees to the horizontal. The back of the retaining wall is also designed to be inclined at an angle to the horizontal between 75 and 85 degrees. You have the freedom to choose one value of these angles within the given intervals. The retaining wall will not be embedded in the sand and the water table is at the sand level (base of the retaining wall).

The design angles of internal friction for the sand and silty sand are 33° and 36°, respectively. The design interface angles of friction, dd, between the base of the wall and the sand is 16° and between the back of the wall and silty sand backfill is 23°. The unit weights of the sand and silty sand are 19.2 kN/m3 and 18.5 kN/m3 , respectively. No surcharge applies on the top of the retaining soil.

Present the design stages you take and the assumptions you consider and comment on the final design solution.

(b) General questions:

(i) Comment on the levels of an embedded retaining wall movement required to mobilise the soil active and passive pressures and on the consequences for the retaining wall design on reaching or not these movement levels.

(ii) The backfilling process involves compaction of soil behind the retaining wall. Given the existence of the wall-backfill friction, comment on the possible consequences for the design of a retaining wall.

Question 2 (a) A tall building is to be founded on a 20 m x 20 m flexible concrete raft in an excavation 7 m deep. The total weight of the building is 132 MN. The soil profile at the site consists of 5 m of sand overlaying a stiff over-consolidated clay stratum 23 m thick. Beneath the clay is the bedrock.

Ground water conditions are hydrostatic with the water table at ground level. The saturated unit weight of the sand is 20 kN/m3 andthat of the clay is 19 kN/m3 . Four stiff clay samples were recovered from the site during a ground investigation, one from 1.5 m depth below the base of the foundation and then at 6 meters depth intervals up to the bottom of the clay layer.

One-dimensional laboratory tests conducted on these samples have shown that the coefficient of volume compressibility, mv [m2 /MN], varies almost linearly from 0.073m2 /MN for the top sample to 0.044m2 /MN for the bottom sample.

Additional triaxial tests showed that the undrained shear strength, cu, increases with depth according to the following equation: cu = 6z + 27 [kPa], where z is the depth in meters below the top of the stiff clay. It can also be assumed that Eu/cu ratio, where Eu is the undrained Young’s modulus, is about 350 and is constant throughout the clay layer.

If the displacements of the soil are the main concerns for the design of the flexible raft foundation, list the displacement types of interest and estimate them given the geotechnical data available. Assuming a fully rigid raft foundation, estimate the total and the consolidation settlements of the building. Comment on your assumptions and results for both cases, flexible and rigid raft foundations.

(b) General question: The one-dimensional settlement calculated from oedometer test results is often used to estimate settlements in three-dimensional problems, using stresses predicted by elastic theory.

Briefly indicate the theoretical justifications for this and state how the main components of settlement can be estimated when using the one-dimensional method, differentiating between normally consolidated and overconsolidated clays.

Question 3 (a) A building is to be constructed on top of a 4m deep layer of granular fill of unit weight 18kN/m3 to be placed across a site consisting of two layers of clay overlaying a bedrock. The top clay layer, 9 m deep soft normally consolidated clay, overlies 40 m deep overconsolidated clay.

For the soft normally consolidated clay, the increase of the effective stress because of the fill loading and the coefficient of volume compressibility, mv, are assumed constant (mv = 0.7 m2 /MN). The change of the thickness of the overconsolidated clay layer as a result of the placement of the granular fill is very limited and can be neglected.

The building is to be founded on piles to be installed after the granular fill is placed across the site; the piles will be smooth-sleeved through the granular fill. Considering a lumped factor of safety of 2 on the pile base resistance, design the pile to be able to support a working load of 1200kN. The effective shear strength parameters of the normally consolidated clay are c’ = 0 and φ’ = 23°. The lower clay layer has an average undrained shear strength of 120 kPa.

The unit weights of the clay layers are 16 kN/m3 for the top layer and 17.5 kN/m3 for the bottom clay layer, and the water table is at the top of the normally consolidated clay layer. Explain all the assumptions you consider and show the steps in the design process which should consider all the possible loading pile sequence situations. Comment carefully on your answer.

(b) General question: Explain the key differences and assumptions required to use total stress or effective stress pile capacity design. In each design approach, explain the key sources of uncertainty.