OVERALL APPROACH TO ANALYSIS
Sheet piling system is an example of flexible retaining structure. This system uses sheet piles. These piles can be made up of concrete, timber, steel, and composite materials. Nowadays, steel piles are widely used in construction industry because of the proven characteristics that have been satisfying its intended purpose. In addition, there are two types of sheet pile walls. One is the cantilever sheet pile wall that supports backfill soil to a height 3m or less. The other type is an anchored sheet pile wall, also called propped sheet pile wall, used to support deep excavations and act as a waterfront retaining structure.
In lateral earth pressure analysis, Rankine theory is adopted considering the soil characteristics of backfill materials. This proposed project is an anchored sheet piling system. This type of system shall be designed because of the moving loads in the railway system 3.0m away from the deep excavation. So as to increase the stability of this wall, anchorage system is added; thus, making it an anchored sheet pile wall. Its stability relies both on passive resistance provided in the dredge line and anchorage. Unlike to cantilever sheet pile, its stability depends on passive resistance of the soil.
In analyzing sheet pile walls, the depth of embedment (D) determines the stability. To calculate, key static equilibrium condition is assumed for moment equilibrium (Budhu, 2010).
GEOTECHNICAL ENGINEERING PARAMETERS
London clay is classified categorically as stiff sedimentary clay. In London, it mostly supports tunnels and deep foundations (Resources Page: DRILCORP, 2020). This type of soil has a seasonal movement in which it swells in volume when wet and reduces in volume when dry.
In this proposed project, it provided preliminary data: from existing ground level (0.00m) to 5.0m level the soil has Standard Penetration Number, N=14, with a plasticity index, PI=40%; from 5.0m level to 20.0m level the soil has Standard Penetration Number, N=35, with a plasticity index, PI=30%.
It is assumed here that the London clay is a weathered soil and in drained conditions since the ground water level is beyond 20.0m level. Hence, the following typical soil parameters are provided (Geotechnical Information: RBKC, 2020):
For 1st layer (0-5.0m): drained unit weight, γ = 21.0 kN/m3
Angle of Internal Shearing Resistance, Ø = 22⁰
For 2nd layer (5.0m – 20.0m) drained unit weight, γ = 17.0 kN/ m3
Angle of Internal Shearing Resistance, Ø = 19⁰
These data were from a factual report of ground investigation conducted by Ground Engineering (Alan Conisbee & Associates).
FINITE ELEMENT METHOD (FEM)
Finite Element Method (FEM) is a rational and efficient method used for the analysis of sheet piles both cantilever and anchored walls. This method provides the following useful information for the design of sheet pile walls: lateral displacement profile, nodal pressures in the passive zone, bending moments at the nodes, and forces in the anchorage.
Using this analysis method, soil-wall profile is drawn out based on the given data and tentative node locations. Different plotting of profile was drawn out to show off the results of analysis and calculations: P-X coding plot, lateral earth pressure plot, nodal forces plot, shear diagram, moment diagram, and lateral displacement diagram.
ANALYSIS FINDINGS
In relation to Part A, it determined the minimum length (L) of sheet pile required for moment equilibrium. It uses the limit equilibrium analysis given the gathered soil parameters from existing data on previous initial assessment on ground conditions of London clay. Out of the calculation, there could be a need of penetration depth (D) of 8.84m to maintain the stability of the sheet pile walls.
The minimum length (L) of sheet pile should be a total of depth of excavation (6.60m) and of penetration depth (D). It sums up into 15.44m long. This figure introduces the minimum compliance to satisfy the principle of equilibrium for stability. The toe of sheet pile was considered for zero moment. The tension force (T) in anchorage and passive force (P4) provided the stability to make the toe of sheet pile no turning.
In practical manner, the minimum length (L) could be rounded up into 16.0m for easy fabrication and manufacturing of the sheet pile.
Based on the shear-moment-displacement diagrams, it could be drawn out that the anchored sheet pile wall is made stabilized. Where the maximum moment is located, the maximum displacement has the same location above the dredge line. Apart from the stability of the wall, the anchored sheet pile wall has to be design its section in order to satisfy the requirements of the shear and moment. Also, the serviceability of the sheet pile section has to be checked in terms of its lateral displacement given the minimum length against the allowable deflection a particular code may provide.
The results tend to stabilize the anchored wall; hence, the railroad has nothing to worry about despite of the moving loads caused by the rail transit.
SOIL TESTING REQUIREMENT
Considering the insufficiency of raw data provided herein, there a need to conduct actual testing both laboratory and field. Even though some data here were provided by using typical values from previous ground explorations, there is still a need to conduct for verification purposes since the proposed project is extensive. Hence, a detailed site investigation shall be conducted.
Basic soil parameters must be determined even if the soil is classified generally as London clay. The said clay soil still differs in characteristic in different stratum level.
A contractor for ground exploration must be hired. A boring activity must be conducted on site. Soil samples shall be taken for laboratory testing to determine the basic geotechnical properties: unit weight, moisture content, void ratio, etc. A vane shear test or direct shear test shall also be conducted for shearing properties of the soil such as angle of internal shearing resistance (Ø).
For verification, there is a need also to conduct Standard Penetration Test (SPT) to determine N-values of soil in each stratum. Also, the Atterberg Limits Test shall also be engaged.
In geotechnical perspective, soil characteristics are different in every location. Hence, comprehensive soil investigation is recommended for a good design practice.
DETAILED ANALYSIS CALCULATION
USING LIMIT EQUILIBRIUM ANALYSIS
Pressure Diagram
