# Effect of deep excavation of soft ground on support pile - 6

Size

- Hole

- strut system

- Distance of pile to excavation pit

- Piles

Parameter

- Soil

- Wall

- Struts

- Piles

Create

geometric pattern

Material

Boundary conditions

Meshing 2D and 3D

Initial stress setting

Soil

Ground deformation

Wall

- Horizontal displacement

- Bending moment

Piles

- Horizontal displacement

- Bending moment

Activate stakes

Activating the retaining wall

Dig and activate the strut

Input data

Calculate

Output results

Figure 3.1 – Analytical procedure

Shown in Figure 3.1, PLAXIS has three main parts: input, computation, and output. The input generally consists of 5 stages: model generation, input materials, boundary conditions, 2D and 3D element meshing, and initial stress generation. Create a geometric model that requires the size of the excavation, the support system, and the distance between the excavation and the pile. The input data needs to know about the parameters of the soil, earth retaining wall, support system and piles. Calculation is a process that includes: determining construction steps such as activating piles and diaphragm walls, conducting excavation and installing struts. The output part is the final process that will give us the results of the behavior of the model. In this study we need to know about the surface settlement of the soil, the horizontal displacement of the wall, the moment in the wall, the displacement. horizontal position of the pile and moment in the pile,

3.2. Analysis of the influence of piles inside the excavation pit corresponding to the actual construction

3.2.1. Basic features of the project

Construction of 15-storey residential area, District 8 - City. Ho Chi Minh City has 15 floors and 1 basement. The work uses prestressed centrifugal concrete pipe pile foundation to support the superstructure. The 6.0m-long Larsen pile is used to block the excavation pit during the construction of the basement. The excavation pit is 52.4m×33.9m in size, constructed by open excavation method.

Construction steps:

 Step 1 : Construction of prestressed concrete pipe piles with diameter of 600mm (including 3 segments, each length is 12m).

 Step 2 : Construction of Larsen pile wall.

 Step 3 : Dig the soil to a height of -1.8m (relative to the natural ground (MDT).

 Step 4 : Dig the soil to a height of -3.8m (compared to MTN) to construct the foundation. Simultaneously dig trenches and drainage ditches. Anchor piling and anti-skew construction.

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Figure 3.2 – Overall plan of excavation pit construction

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Figure 3.3 – Construction site of excavation pit

Figure 3.4 – Details of piling anchor cable

Figure 3.5 – Section after construction of piles and walls of Larsen

Figure 3.6 – Section after construction digging to a depth of -1.8m compared to the MN

Figure 3.7 – Section after construction excavation to a depth of -3.8m compared to the MTN

Figure 3.8 – Detail of anti-skew in tunnel and cable anchorage outside of tunnel

3.2.2. Material parameters and models

3.2.2.1. Parameters of land used in the model

Based on the geological survey records, the author analyzes and selects the stability and deformation parameters of the ground to simulate the entire excavation in the PLAXIS 3D Foundation program.

3.2.2.2. Larsen sheet wall specifications

Larsen FSP-IV pile wall system is built by vibrating hammer driven to a height of -5.35m (compared to MTN). Based on specifications from the manufacturer of the pole

Larsen FSP - IV, we calculate the parameters of the wall in the simulation using PLAXIS 3D Foundation software.

Filling soil, 1m . thick

Flowing clay mud, 25m . thick

SPT N = 0

Clay mud mixed with plastic sand lens, 8.5m . thick

SPT N = 3

Fine sand in medium compact state, 13.9m . thick

SPT N = 17

Solid state lightning, >11.6m . thick

SPT N = 49

Figure 3.9 – 3D model of geological layers

Table 3.1 – Parameters of Larsen poles from the manufacturer

FSP - IV

 Cross-sectional area cm 2 Weight/m kg/m Moment of inertia cm 4 Elastic modulus cm 3 On the wall 242.5 190 38600 2270

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Figure 3.10 – Dimensions of Larsen pile type IV

 Parameter Symbol Filled earth Grade 1 Mud – State run Grade 2 Mud of clay sandwiched with sand lens – State pliable Grade 3 Sand – Medium compaction Grade 4 Lightning – Status hard Unit Thickness - 1.0 25 8.5 13.9 11.6 m Material model Model MC MC MC MC MC - Material behavior Type Drained Undrained Undrained Drained Undrained - Natural weight  (unsat.) 18 14.5 16.6 19.5 20.5 kN/m 3 Saturated density  (sat.) 18 14.5 16.6 19.5 20.5 kN/m 3 Permeability coefficient k x =k y =k z 8.64e-2 8.64e-6 8.64E-5 8.64E-3 8.64E-6 m/day y Modulus E ref 10000 2500 6500 26000 74000 kN/m 2 Poisson's coefficient ' _ 0.25 0.33 0.3 0.25 0.25 - Adhesive force c'/c u 0.1 9 11 3 52 kN/m 2 Internal friction angle / u _ _ 28 4 6 28 18 o Expansion angle  0 0 0 0 0 o Contact stress reduction factor R inter first first first first first -

Table 3.2 – Ground parameters using Mohr – Coulomb model (MC)

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Date published: 09/07/2022
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