geomechanics

Objectives:

After completion of this assignment students will have an understanding of the application

of numerical modelling in geotechnical design. The assignment will provide an opportunity

for students to use WALLAP software to numerically model a simple geotechnical problem

and to use hand calculations to understand the limitations and assumptions involved in

solving such problems. Also this exercise will provide an opportunity to identify and simulate

different stages involved in the construction sequence of large geotechnical structures.

Description of the problem:

The side elevation of the retaining wall shows a typical solution adopted for a retaining wall

in a dive structure. The dive will be formed by excavating the existing (M1) motorway.

The retaining wall consists of 750mm diameter, cast-in-paced bored piles spaced at 2.0m,

along the length of the dive. Piles are connected to each other by a 1050mm wide and a

800mm deep reinforced concrete upper capping beam. The top of the motorway is at RL

175.00. When the excavation progresses to RL 171.00, a 40mm diameter ground anchor

(Steel Grade 500N) in a 150mm diameter hole will be installed between piles. A lower

capping will be constructed after that and the concrete is allowed to gain the full strength. It

can be assumed that only one ground anchor will be sufficient to resist a single pile in the

retaining wall.

When the excavation reaches up to the soffit of the lower capping beam (RL 171.00), rock

will be stepped by 3.60m. When the excavation gradually progresses with 2.0m to 2.50m

benches, rock face will be strengthened with shotcrete and additional rock nails. (It can be

assumed that the rock face is stable). The total depth of excavation is approximately

12.50m. Water table can be assumed as below the final excavation level and sufficient

amount of subsoil drainage is provided to drain any water accumulated on the rock face.

The dive structure will be constructed adjacent to M1 motorway and it will be subjected to

normal traffic and construction related traffic loading. It can be assumed that the surcharge

loading is equivalent 40 kPa. The construction sequence is as shown in the elevation

drawing.

Q1 Draw sketches to show different construction stages.

Q2 Calculate the total pressure distribution using AS 5100 approach (Hand calculation).

Q3 Calculate the total pressure distribution using AS 4678 approach (Hand calculation).

Q4 Calculate the anchor force, and pile bending moment and shear force based on AS 5100

approach (Hand calculation).

Q5 Carryout numerical modelling using WALLAP.

Q6 Plot the total pressure distribution using WALLAP.

Q7 Compare Q6 results against Q2 and Q3 pressure distributions. Please discuss the

difference between these calculations.

Q8 Check anchor force, pile bending moment and pile shear force in Q4 against WALLAP

results. Please discuss your results.

Q9 If a Finite Element Analysis is performed for the same problem, would you expect better

results than those given by WALLAP? Please justify your answer.