Geotechnical Design Assignment Sample

Introduction

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1.1 Aim:

The main aim of this test is to get the value of parameters of shear strength without measuring the pressure of pore water.

1.2 Theory

Shear strength components are total stress, effective stress, and pressure of pore water.

1.3 Apparatus

The apparatus needed for the compilation of this test are given below in a listed manner-

• Transparent chamber and triaxial cell.
• Apparatus for maintaining and applying the desired pressure of the fluid.
• Dial gauge
• Proving ring for the measurement of additional axial forces.
• Rubber membrane.
• Membrane stretcher(Lee1a et al 2019).
• Rubber rings.
• for the determination of moisture content non-corrodible and air-tight containers.
• Weight balance.
• Split mold, wire saw, trimming knife, metal scale, and straight edge of metal.
• Heat oven.
• Compression machine.

1.4 Sample preparation:

The sample should be formed like a cylinder. The diameter of this cylinder should be equal to 38 millimeters. The ratio of height and diameter of this specimen should be two.

1.4.1 Undisturbed specimen

The internal diameter of the undisturbed soil sample is the same as the required internal diameter of the soil sample. Then it is extruded by a sample extruder and further it si pushed into the split mold. A trimmed knife is used then for trimming the ends of the specimen. Then it is taken out from the split mold.

1.4.2 Disturbed specimen

The disturbed specimen is obtained by the compaction of the soil sample at the required water content and dry density in the big mold. This sample is then extracted by sampling tubes.

1.5 Procedure

The procedures of the “uniaxial triaxial test” are given below in a step-by-step manner-

• Measurement of length, mass, and diameter of this specimen properly.
• Cover triaxial cell’s pedestal with the help of an end cap and the valve of drainage is kept closed. After that, this specimen is placed over the end cap. A membrane stretcher is then used to use the rubber membrane around the sample(Wei et al 2019). After that rubber rings are used for sealing the membrane end.
• The sample is then placed in the loading machine.
• Fluid is admitted to operating into this cell and the pressure of this fluid is brought to the required value.
• The loading machine is adjusted in such a way that, the ram of loading can contact the top of this specimen. The initial reading is then noted.
• After that, the axial compressive load is applied to the sample. Hence The sample fails within 5 to 20 minutes. The readings of proving rings and gauge readings are then noted down. The loading is continued further until the sample fails with maximum stress.
• After the failure of the specimen, this sample is taken out of the cell and the fluid is drained off. The rubber membrane is removed and the nature of the failure is noted down(Wu et al 2021). After that, the weight balance is bused for the determination of the weight of the sample, and further determination of moisture content is performed.
• The test is repeated for more specimens under various cell pressures.

1.6.1 Calculations and observation

The data regarding soil parameters has been assumed in few cases where appropriate data are missing.

• Soil type: loose alluvium
• Preparation procedure of soil: Disturbed specimen
• Specimen’s initial length: 120mm
• Specimen’s initial diameter: 20 mm
• Area of sample: 314 mm^2
• Volume: 37680 mm^3
• Constant of proving ring: 0.01
• Strain rate: 0.5
• Determination of moisture content (initial)
• Cell pressure observation
• Determination of moisture content (final)
• Plotting of a graph between deviator stress and axial strain to obtain the deviator stress at the level of failure.
• Calculation of specimen
• Axial force(P)= Constant of proving ring* rading of proving ring(kgf)
• Axial strain= ?L/L_o
• Revised Area= (cm^2)
• Axial stress= 8Kn/ m^2
• Plot the conventional failure sketch by using Mohr’s circles to obtain the parameters of shear strength
• The cohesion of the soil sample= 2.6
• The angle of internal friction= 20 degrees

2.0 Square footing design

In this section a square column footing is designed. For designing square footing various aspects should be considered. As per Eurocode, the design of the foundation has been performed. Here some assumptions have been done for the calculation of the depth of foundation and size of the foundation.

The column is assumed to be axially loaded(Islam et al 2020). The size of the column is taken as 400mm x 400 mm. The “safe bearing capacity” of soil is considered 200 KN per meter square. There are two loads that are applied on the column footing. One is permanent load and another is variable load. The value of permanent load and variable loads are 1505 KN and 1100 KN respectively. Therefore the total load that is acting on the footing is (1505+1100) KN which is 2605 KN. C20 grade of concrete has been assumed here. Therefore the “compressive strength” of this concrete grade is 20 Newton per millimeter square. The steel grade has been chosen S450. Therefore the strength of steel members is 450 Newton per millimeter square.

• Step 1: (Load calculation)

The total load on the column (W) = 2605 KN = 2605 * 10^3 N

Footing’s self-weight = 10% of the column load = (2605 * 10^3) * 10% = 2605 * 10^2 N

Hence total load is = 2865500 N = 28655 * 10^2 N

Factored load is (W_u) = 42982 * 10^2 N

Ulitimate “bearing capacity” of soil is = 2 * 200 * 10^3 N/ m^2 = 400 * 10^3 N/m^2

Area of this footing =(4298200/400000) meter square = 10.76-meter square

Therefore one side of square footing is √10.76 meters that are 3.28meters.

Hence it is considered that the one side of this footing is 3.5 meters.

Therefore the size of the footing is 3.5m * 3.5m

• Step 2: (Net ultimate upward pressure of soil)

P_u = ((1.5*W) / B^2)

In this above equation, B means the size of square footing

Here W is the value of load excluding 1.5 factors. The value of W is 2605*10^3 N

After putting all the values in the equation the ultimate upward pressure of soil is coming about 318979 N/ m^2

• Step 3: (Depth of footing)

Bending moment about critical section,

B.M= P_u*B/8(B-b)^2

B.M= 318979* 3.5 / 8 (3.5-0.4)^2 N.mm = 1341107*10^3 N.mm

We know that, B.M = 0.138 f_ckb d^2

1341107*10^3= 0.138*20*3500*d^2

Or, d= 372.59 mm (Assume d= 500 mm)

3.0 Determination of pile diameter and length

Let the diameter of the single pile be d meters.

According to the analytical method,

Q_up= Q_eb+Q_sf

Here Q_up means the total load that is acting on the pile =16000 KN

It is assumed that the pile can resist the overall load by its end-bearing property(Dhatrak et al 2018). Therefore the resistance of skin friction can be considered as zero. All the resistance is given by the pile by its end surface area.

Hence for that, Q_eb should be calculated. The equation for the calculation of Q_eb is given below-

Q_eb = q_b* A_b

Here q_b is 9C, where C signifies the cohesion of the soil sample. Therefore the value of this factor is 900 KN per meter square

A_b can be calculated by π*r^2

Therefore 16000= 900*π*r^2

Or, r^2 = 5.6 m

or, r= 2.36 m

Hence the radius of the single pile is 2.36 m.

Therefore the diameter of this single pile is 4.72 Meters.

The length of the piles is 25 meters.

For bearing the loading total of 3 piles are required(Han et al 2019).

4.0 Determination of FOS against bearing, sliding and overturning

Total height is 5 m.

According to Rankin’s theory per length active force on the wall is= 0.5*K_a*γ*H^2

For Φ =24 degrees, the value of Ka is 0.428

Therefore the total active pressure on the surface of the wall is = (P1+P2+P3) KN

The value of P1, P2, and P3 are 3.27 KN,26.19KN, 59.26KN. Therefore the total load is 88.72 KN/m.

Let’s assume, the length of the base of this retaining wall is 3 m

Reference list

Journals

Dhatrak, A.I., Ghawde, M. and Thakare, S.W., 2018. Experimental study on Belled Wedge Pile for different loadings in cohesionless soil. In Indian Geotechnical Conference, Indian Institute of Science Bengaluru (pp. 1-7).

Han, F., Salgado, R., Prezzi, M. and Lim, J., 2019. Axial resistance of nondisplacement pile groups in sand. Journal of Geotechnical and Geoenvironmental Engineering, 145(7), p.04019027.

Islam, M.N., 2020. Small Scale Experiments to Assess the Bearing Capacity of Footings on the Sloped Surface. Eng, 1(2), pp.240-248.

Ji, C., Zhang, J.F., Zhang, Q.H., Li, M.X. and Chen, T.Q., 2018. Experimental investigation of local scour around a new pile-group foundation for offshore wind turbines in bi-directional current. China Ocean Engineering, 32(6), pp.737-745.

Lee1a, S., Im2b, J., Cho2c, G.C. and Chang, I., 2019. Laboratory triaxial test behavior of xanthan gum biopolymer-treated sands.

Magade, S.B. and Ingle, R.K., 2019. Comparative study of moments with plate and solid elements for an isolated footing under axial load. In International conference on innovation in concrete for infrastructure challanges.

Mistry, H.K. and Lombardi, D., 2020. Role of SSI on seismic performance of nuclear reactors: A case study for a UK nuclear site. Nuclear Engineering and Design, 364, p.110691.

Olumuyiwa, O.F., 2020. Engineering Site Investigation and Shallow Foundation Design in Ore Area of Ondo State, Nigeria. Materials and Geoenvironment, 67(1), pp.21-33.

Vargas, R.R. and Zavala, G.J., 2019. Influence of the Variability of Geotechnical Parameters in Cantilever Retaining Walls Design. In Geotechnical Engineering in the XXI Century: Lessons learned and future challenges (pp. 1237-1244). IOS Press.

Wei, L., Xiao-Guang, J. and Zhong-Ya, Z., 2019. Triaxial test on concrete material containing accelerators under physical sulphate attack. Construction and Building Materials, 206, pp.641-654.

Wu, M., Wang, J., Russell, A. and Cheng, Z., 2021. DEM modelling of mini-triaxial test based on one-to-one mapping of sand particles. Géotechnique, 71(8), pp.714-727.