Cie6003 Geotechnical Engineering And Ground Assessment Answers

This module develops the analytical capabilities of students to assess problems and propose solutions in ground engineering, including design of shallow, deep and piled foundations, slopes, retaining walls and ground improvement schemes. An intensive case study with sufficient geotechnical and environmental complexity will be used to form the base for the student coursework on this module. This coursework will focus on Shallow and Piled Foundations plus Ground Improvement techniques.Site Investigation and Site Development Proposals have been summarized and presented on a separate 2-page PDF document on Moodle for your interpretation.

The open-endedness of the scheme is deliberate and intends to develop key Learning Outcomes as listed in 3) below, and in the Module Specification online.

Carry out bearing capacity calculations for shallow foundations for the scheme, alongside calculations for deep and piled foundations, so that a comparison can be made of the relative dimensions of the shallow and piled foundation solutions.

Determine an appropriate arrangement of ground improvement techniques to deal with the need for ground raising on site above the soft alluvial deposits and hence provide recommendations on how to engineer the roads, services and drains so that they remain in a serviceable state (focus on the geotechnical processes occurring in the soft alluvial deposits and hence provide detailed calculations to support your solution to deal with any expected ground movements).

Outline the technical and practical problems likely to be experienced when implementing the solution to the ground problems at the given site.

The soil bearing capacity for shallow foundation can be calculated using the Terzaghi’s ultimate bearing capacity equation. When the unit weight and the cohesion of the soil in addition to the surcharge are considered, the equation below can be applied:

q_u= cN_cS_c + γ DN_qS_q + 0.5B_γN_γS _γ (Das, 2017)

Where:

• q_uis the ultimate bearing capacity
• D is the depth of the foundation
• B is the width of the foundation, and
• N_c, N_q, N_γ, and, S_c, S_q and S_γ are bearing capacity and shape factors respectively (Das, 2014).

In Development Area 1, a pad footing is used to support loads of up to 800 KN. In addition to that, a strip foundation supports a perimeter wall load of up to 70 KN/m.

For square footing:

Q_u = 1.3cN_c + γ DN_q + 0.4 γ B_γN_γ

Taking D= 2.10m; B= 3m and the angle of friction φ =20° N_c =17.69, N_q= 7.44 and N_γ= 3.64.

Also, assume a unit weight of 20KN/m³ for clay soil.

Therefore, q_u = (1.3×35×17.69) + (20×2.1×7.44) + (0.4×20×3.64)

= 804.895 + 312.480 + 29.120

=1146.495 KN/m²

Pile foundation

According to Skempton, for a deep foundation, N_c = 9. Therefore;

Q_u= αcA_f + 9CA_b.

For a pile of diameter B and embedded depth D

A_b= B² and A_f =

Therefore, Q_u =αBDc +2.25 B²c

With C_u=35KN/m² and using a concrete pile of B= 400mm and D= 10 meters. Taking the adhesive factor α =0.60

Q_u =(0.60×0.4×10×35) + (2.25× 0.4² × 35)

=84+ 39.58

=123.58 KN/m²

Task 2.2

The engineering properties of a site should be improved in cases where the ground quality is not up to standard. This can be done by reducing the volume of voids in the soil, reducing pore water pressure and using stronger materials. The main methods of achieving this are consolidation and compaction. Consolidation is the improvement of the soil bearing capacity of a soil mass by the application of load over a given period.

Alluvial soil deposits have a large of amount of water that heavily reduces its soil bearing capacity. Therefore, to increase its bearing capacity, consolidation is used. If the ground quality is not improved, settlement tend to happen as the soil mass is consolidated as the result of the load of the structure.

Stress increment because of the new structure will cause a reduction in the void ratios. As a result, the height of the soil layer reduces. For a laterally confined soil

?H= m_vH₀?P

Where:

• m_v is the volume of compressibility
• H₀is the initial thickness of the soil
• ?P is the change in pressure.

By definition, the compression index Cc is:

Cc= (Hirkane et al., 2014)

Therefore;

?e =Cc log₁₀

But = = =

Therefore, consolidation settlement Sc is:

Sc = H₀ log₁₀ 

Task 2.3

Geotechnical engineers are faced with numerous challenges when attempting to implement the various ground improvement solution. Some of these challenges include:

• Unpredictable weather. When trying to improve the soil quality through the expulsion of water, raining conditions may reverse the process hence nullifying all the work (Hirkane et al., 2014). Additionally, swelling of clay due to moisture may interfere with the works.
• Improvement, which entails the importation of soils with better engineering properties, incurs high costs that increases the cost of construction.
• In addition, incidences of uneven settlement after consolidation and compaction are common.

References

Das, B. M. (2017). Shallow foundations: bearing capacity and settlement. CRC press.

Hirkane, S. P., Gore, N. G., & Salunke, P. J. (2014). Ground Improvement Techniques. International Journal of Inventive Engineering and Sciences, 2, 11-13.