Complex Engineering Problem

CEE-209 Geotechnical & Foundation Engineering

Background of Complex Engineering Problem:

In order to enhance the students in depth knowledge, complex engineering problems are devised to provide the freedom to the students to devise their own methodology to resolve a complex engineering problem. In this context students are assigned the following:

Complex Engineering Problem:

“A four storied plaza is being constructed in Sahiwal near High Street Road. During construction the building started to tilt on one side. A Geotech investigation report was done to investigate the problem. Now your job is to investigate the cause of the tilt and suggest a suitable action”.

?Relevant Material:

CEE-209 Geotechnical & Foundation Engineering Lectures, and Reference books:

·?????? Geotechnical Engineering by Dolald P. Coduto

·?????? Fundamentals of Soil Mechanics by M. S Qureshi & Aziz Akbar

·?????? ?Foundation Analysis and Design by Joseph E Bowles and any other relevant book.

Report:

Report should address mainly: interpretation of the subsurface profile of the plaza from the geotechnical investigation report. Evaluation of bearing capacity based on lab test results and foundations details provided. Explanation of the cause of the tilt and a final suggestion for the plaza based on your analysis.

1. Introduction

1.1 Project Overview

The project under consideration involves the construction of a four-storied commercial plaza located near High Street Road in Sahiwal. This plaza, envisioned to be a modern commercial hub, is currently in the construction phase. However, during this phase, a critical issue has emerged: the building has started to tilt to one side. This unexpected tilt poses significant concerns regarding the structural integrity and safety of the building, necessitating an urgent and thorough investigation to determine the root cause and implement appropriate corrective measures.

1.2 Objective

The objective of this investigation is to comprehensively analyze the factors leading to the tilting of the plaza. This report aims to:

• Interpret the subsurface profile based on the findings from the geotechnical investigation report.
• Evaluate the bearing capacity of the soil using lab test results and foundation details provided.
• Identify and explain the cause of the tilt through detailed analysis.
• Suggest suitable remedial actions to rectify the tilt and prevent future occurrences.

The goal is to ensure the stability and safety of the plaza, thereby enabling the continuation and successful completion of the construction project.

2. Interpretation of Subsurface Profile

2.1 Geotechnical Investigation Summary

A comprehensive geotechnical investigation was carried out at the site of the four-storied plaza in Sahiwal to understand the subsurface conditions. The investigation included drilling boreholes, collecting soil samples, conducting field tests, and performing various laboratory tests to evaluate the soil properties. The key findings from the geotechnical investigation report are summarized below:

2.1.1 Soil Types and Stratigraphy

The subsurface profile at the site consists of three distinct soil layers:

• Layer 1 (0-3 meters): Loose Silty Sand This top layer is composed primarily of loose silty sand, characterized by a high void ratio and low shear strength.
• Layer 2 (3-10 meters): Medium Dense Sandy Silt Below the loose silty sand is a medium dense sandy silt layer. This layer exhibits moderate compressibility and shear strength.
• Layer 3 (>10 meters): Stiff Clay The deepest layer encountered is stiff clay, which is significantly less compressible and has higher shear strength compared to the upper layers.

2.1.2 Groundwater Conditions

• The investigation identified a variable groundwater table. During the time of investigation, the groundwater table was encountered at a depth of approximately 5 meters below the ground surface.
• Seasonal fluctuations in the water table were noted, indicating potential changes in groundwater levels due to seasonal rainfall and other environmental factors.

2.2 Soil Properties

The properties of the soil layers, determined through laboratory tests, provide critical insights into their behavior under loading conditions. These properties include shear strength, compressibility, density, and permeability.

2.2.1 Loose Silty Sand (0-3 meters)

• Shear Strength: The loose silty sand exhibits low shear strength with a cohesion (c) of 0 kPa and an angle of internal friction (?) of 30°. This indicates a non-cohesive, granular soil.
• Compressibility: This layer has high compressibility, leading to significant settlement under load.
• Density: The bulk density of the loose silty sand is 18 kN/m3.
• Permeability: The permeability is relatively high due to the granular nature of the soil, facilitating easy movement of groundwater.

2.2.2 Medium Dense Sandy Silt (3-10 meters)

• Shear Strength: The medium dense sandy silt has moderate shear strength with a cohesion (c) of 5 kPa and an angle of internal friction (?) of 28°. This indicates some degree of cohesion and moderate frictional resistance.
• Compressibility: This layer is moderately compressible, showing less settlement compared to the loose silty sand but more than the stiff clay.
• Density: The bulk density of the medium dense sandy silt is 20 kN/m3.
• Permeability: The permeability is lower than that of the loose silty sand, but still significant, allowing some groundwater movement.

2.2.3 Stiff Clay (>10 meters)

• Shear Strength: The stiff clay exhibits high shear strength with a cohesion (c) of 50 kPa and an angle of internal friction (?) of 20°. This indicates a highly cohesive soil with substantial resistance to shear.
• Compressibility: The compressibility of stiff clay is low, leading to minimal settlement under loading conditions.
• Density: The bulk density of the stiff clay is 22 kN/m3.
• Permeability: The permeability of stiff clay is very low, significantly restricting the movement of groundwater through this layer.

3. Evaluation of Bearing Capacity

3.1 Lab Test Results

The following laboratory test results were obtained to evaluate the bearing capacity of the soil at the site of the plaza:

3.1.1 SPT (Standard Penetration Test) Values

• Loose Silty Sand (0-3 meters): SPT N-values ranged from 5 to 10, indicating very loose to loose soil conditions.
• Medium Dense Sandy Silt (3-10 meters): SPT N-values ranged from 15 to 25, indicating medium dense soil conditions.
• Stiff Clay (>10 meters): SPT N-values ranged from 30 to 40, indicating stiff soil conditions.

3.1.2 Atterberg Limits

• Loose Silty Sand: Non-plastic (NP).
• Medium Dense Sandy Silt: Liquid Limit (LL) = 35%, Plastic Limit (PL) = 20%, Plasticity Index (PI) = 15%.
• Stiff Clay: Liquid Limit (LL) = 50%, Plastic Limit (PL) = 25%, Plasticity Index (PI) = 25%.

3.1.3 Unconfined Compressive Strength

• Loose Silty Sand: Not applicable due to non-cohesive nature.
• Medium Dense Sandy Silt: Unconfined compressive strength = 100 kPa.
• Stiff Clay: Unconfined compressive strength = 300 kPa.

3.2 Bearing Capacity Calculations

To calculate the bearing capacity, we will use Terzaghi’s bearing capacity equation:

????=??′????+??????+0.5????????

where:

• ???? = Ultimate bearing capacity
• ??′ = Effective cohesion of the soil
• ?? = Overburden pressure at the foundation depth
• ?? = Unit weight of the soil
• ?? = Width of the foundation
• ????,????,???? = Bearing capacity factors, which depend on the angle of internal friction (?)

For the calculations, we assume a foundation width (B) of 2 meters and a depth (D) of 1.5 meters.

3.2.1 Loose Silty Sand (0-3 meters)

• Cohesion (c'): 0 kPa
• Angle of Internal Friction (?): 30°
• Unit Weight (γ): 18 kN/m3
• Bearing Capacity Factors: ????=30.14 , ????=18.4 , ????=22.4

??=????? =18 x 1.5? = 27?kN/m2

????= 0 x 30.14 + 27 x 18.4 + 0.5 x 18 x 2 x 22.4

????= 496.8 + 403.2 =900?kN/m2

With a Factor of Safety (FS) of 3:

???????????????????? = ????/???? = 900/3 = 300?kN/m2

3.2.2 Medium Dense Sandy Silt (3-10 meters)

• Cohesion (c'): 5 kPa
• Angle of Internal Friction (?): 28°
• Unit Weight (γ): 20 kN/m3
• Bearing Capacity Factors: ????=37.2 , ????=22.4 , ????=19.7

?? = ?? x ?? = 20 x 1.5 = 30?kN/m2

???? = 5 x 37.2 + 30 x 22.4 + 0.5 x 20 x 2 x 19.7

???? = 186 + 672 + 394 = 1252?kN/m2

With a Factor of Safety (FS) of 3:

???????????????????? = ????/???? = 1252/3 = 417.33?kN/m2

3.2.3 Stiff Clay (>10 meters)

• Cohesion (c'): 50 kPa
• Angle of Internal Friction (?): 20°
• Unit Weight (γ): 22 kN/m3
• Bearing Capacity Factors: ????=57.8, ????=38.6, ????=12.9

?? = ?? x ?? = 22 x 1.5 = 33?kN/m2

????=50 x 57.8 + 33 x 38.6 + 0.5 x 22 x 2 x 12.9

???? = 2890 + 1273.8 + 283.8 = 4447.6?kN/m2

With a Factor of Safety (FS) of 3:

???????????????????? = ????/???? = 4447.6/3 = 1482.53?kN/m2

3.3 Comparison with Design Requirements

The design load for the plaza’s foundation is 400 kN/m2. Comparing this with the allowable bearing capacities of the different soil layers:

• Loose Silty Sand: ???????????????????? = 300?kN/m2
• Medium Dense Sandy Silt: ???????????????????? = 417.33?kN/m2
• Stiff Clay: ???????????????????? = 1482.53?kN/m2

The design load exceeds the allowable bearing capacity of the loose silty sand layer but is within the allowable capacities of the medium dense sandy silt and stiff clay layers. This discrepancy indicates that the foundation is inadequate for the uppermost soil layer, likely contributing to the observed tilting due to differential settlement. The foundation should ideally extend to the medium dense sandy silt or stiff clay to ensure adequate support and prevent further tilting.

4. Cause of the Tilt

4.1 Analysis of Factors Contributing to Tilt

The tilting of the four-storied plaza under construction near High Street Road in Sahiwal can be attributed to several interrelated factors. Each factor affects the stability and uniformity of the building’s foundation, contributing to the observed tilt. The primary factors include differential settlement, inadequate bearing capacity, groundwater effects, and potential construction errors.

4.1.1 Differential Settlement

Differential settlement occurs when different parts of a structure settle at varying rates, causing tilting or uneven settling. This phenomenon is often due to variations in soil compressibility and load distribution across the foundation. The subsurface profile at the site revealed significant variability in soil types and properties:

• Loose Silty Sand (0-3 meters): Highly compressible and low shear strength, prone to significant settlement under load.
• Medium Dense Sandy Silt (3-10 meters): Moderately compressible, offering better support than the loose silty sand but still variable.
• Stiff Clay (>10 meters): Low compressibility and high shear strength, providing stable support with minimal settlement.

The upper layer of loose silty sand is particularly susceptible to high compressibility and uneven settlement. If the building's load is not evenly distributed or if certain areas experience higher loading, differential settlement will occur, causing one side of the building to settle more than the other. This uneven settlement likely contributed significantly to the building's tilt.

4.1.2 Inadequate Bearing Capacity

The bearing capacity of the soil is critical in supporting the loads imposed by the structure. The evaluation of bearing capacity showed that:

• The loose silty sand layer has an allowable bearing capacity of 300 kN/m2, which is insufficient compared to the building's design load of 400 kN/m2.
• The medium dense sandy silt and stiff clay layers have higher allowable bearing capacities (417.33 kN/m2 and 1482.53 kN/m2, respectively), sufficient to support the design load.

Given that the building's foundation was likely designed based on the uppermost soil layer's properties, the insufficient bearing capacity would lead to excessive settlement. This inadequate support under the foundation could not withstand the imposed loads, resulting in significant settlement and subsequent tilting.

4.1.3 Groundwater Effects

The geotechnical investigation reported that the groundwater table is located at a depth of approximately 5 meters below the ground surface, with potential for seasonal fluctuations. Groundwater can significantly impact soil stability and bearing capacity:

• Reduction in Effective Stress: The presence of groundwater reduces the effective stress in the soil, leading to lower shear strength and bearing capacity.
• Pore Water Pressure: Fluctuations in the groundwater table can increase pore water pressure, especially in the loose silty sand layer, leading to soil instability and increased settlement.
• Soil Liquefaction: In cases where loose granular soils are saturated, dynamic loading or vibrations (e.g., from construction activities) can cause soil liquefaction, further reducing soil strength and causing significant settlement.

Poor drainage or inadequate management of groundwater can exacerbate these effects, leading to increased settlement and tilting of the building.

4.1.4 Construction Errors

Construction practices play a crucial role in ensuring the stability and integrity of the foundation. Potential errors during construction that may have contributed to the tilt include:

• Improper Compaction: If the soil was not adequately compacted before construction, it could lead to uneven settlement under the building load.
• Incorrect Foundation Depth: The foundation may not have been extended to the deeper, more stable soil layers (medium dense sandy silt or stiff clay), resulting in reliance on the weaker loose silty sand.
• Poor Material Quality: The use of substandard materials for the foundation or inadequate quality control during construction could compromise the foundation's ability to distribute loads evenly.
• Uneven Load Distribution: Errors in structural design or construction might result in uneven load distribution, causing some parts of the foundation to experience higher stresses and more significant settlement.

5. Suggested Remedial Actions

To address the tilting of the four-storied plaza in Sahiwal, a combination of short-term and long-term remedial actions is recommended. These actions aim to stabilize the building immediately and ensure its long-term structural integrity.

5.1 Short-term Solutions

Immediate actions are essential to halt further tilting and stabilize the structure. The following methods can be employed:

5.1.1 Underpinning

Underpinning involves strengthening and stabilizing the existing foundation by extending it to deeper, more stable soil layers, such as the medium dense sandy silt or stiff clay. Techniques for underpinning include:

• Mass Concrete Underpinning: Excavating sections below the existing foundation and filling them with concrete to transfer the load to a deeper level.
• Mini-Piles or Micropiles: Installing small-diameter piles through the existing foundation to reach stable soil layers. These piles can be grouted to enhance their load-bearing capacity.
• Beam and Base Method: Constructing a beam under the existing foundation and transferring the load to new bases or piles at a deeper level.

Underpinning is effective in preventing further settlement by providing additional support to the foundation.

5.1.2 Grouting

Grouting involves injecting a cementitious or chemical grout into the soil to increase its strength and reduce compressibility. Types of grouting include:

• Compaction Grouting: Injecting a stiff grout mix to displace and compact loose soil, thereby increasing its density and strength.
• Permeation Grouting: Injecting a fluid grout that permeates the soil and solidifies, binding the soil particles together and reducing compressibility.
• Jet Grouting: Using high-pressure jets to mix grout with in-situ soil, forming a soil-cement column that improves load-bearing capacity.

Grouting can be targeted at specific areas showing excessive settlement to stabilize the foundation rapidly.

5.1.3 Drainage Improvements

Proper drainage systems are crucial to manage groundwater and prevent soil instability. Actions include:

• Installing French Drains: Perforated pipes surrounded by gravel to collect and redirect groundwater away from the foundation.
• Sump Pumps: Installing pumps in sumps to remove accumulated groundwater and prevent water from reaching the foundation.
• Surface Drainage: Ensuring proper grading and surface drainage to direct rainwater away from the building.

Improving drainage will help control groundwater levels and reduce pore water pressure, mitigating the risk of further settlement.

5.2 Long-term Solutions

For sustainable stability and prevention of future tilting, the following long-term measures are recommended:

5.2.1 Re-design of Foundation

A re-design of the foundation may be necessary to ensure it is suitable for the site's soil conditions. Options include:

• Deep Foundations: Transitioning from shallow to deep foundations such as piles or caissons that extend to the stable stiff clay layer. Pile foundations transfer the building load to deeper, more competent soil layers, providing greater stability.
• Raft or Mat Foundations: A continuous slab that spreads the load over a large area, reducing the pressure on any single point and minimizing differential settlement.

Re-designing the foundation ensures that it can adequately support the building load and adapt to the soil conditions.

5.2.2 Soil Improvement Techniques

Improving the properties of the soil itself can provide long-term stability. Techniques include:

• Soil Stabilization: Adding stabilizing agents such as lime, cement, or fly ash to the soil to enhance its strength and reduce compressibility.
• Compaction Grouting: Continuing with compaction grouting in a more extensive manner to densify the loose silty sand layer, increasing its load-bearing capacity.
• Preloading and Surcharging: Applying a temporary load to the soil to induce settlement before constructing the building, ensuring that most of the settlement occurs before the structure is built.

Implementing soil improvement techniques will enhance the overall stability of the soil, providing a solid foundation for the building.

6. Conclusion

6.1 Summary of Findings

The investigation into the tilting of the four-storied plaza in Sahiwal has revealed several critical factors contributing to the instability of the building. The primary causes identified are:

• Differential Settlement: The building has experienced uneven settlement due to the variability in soil compressibility across different layers, particularly the highly compressible loose silty sand layer at shallow depths.
• Inadequate Bearing Capacity: The allowable bearing capacity of the loose silty sand (300 kN/m2) is insufficient to support the design load of the building (400 kN/m2), leading to excessive settlement.
• Groundwater Effects: The fluctuating groundwater table and poor drainage have reduced the soil's effective stress and stability, exacerbating the settlement issues.
• Construction Errors: Potential errors such as improper compaction, incorrect foundation depth, and uneven load distribution during construction have further contributed to the tilt.

6.2 Recommendations

To address these issues and stabilize the plaza, the following set of recommendations is offered for the construction team:

1. Immediate Stabilization Underpinning: Implement underpinning techniques to extend the foundation to deeper, more stable soil layers. Options include using mass concrete underpinning, mini-piles, or the beam and base method. Grouting: Utilize compaction grouting or permeation grouting to increase soil strength and reduce compressibility, particularly in the loose silty sand layer. Drainage Improvements: Install effective drainage systems, including French drains, sump pumps, and proper surface grading, to manage groundwater levels and prevent water accumulation around the foundation.
2. Long-term Stability Measures Re-design of Foundation: Transition to a deep foundation system such as pile foundations or consider a raft foundation to evenly distribute the load and reduce differential settlement. Soil Improvement Techniques: Implement soil stabilization methods using lime, cement, or fly ash to enhance soil properties. Continue compaction grouting and consider preloading and surcharging to induce settlement before construction resumes.
3. Quality Control and Monitoring Ensure Proper Compaction: Strictly adhere to compaction standards during construction to avoid uneven settlement. Monitor Groundwater Levels: Regularly monitor the groundwater table and adjust drainage systems as needed to maintain soil stability. Load Distribution Management: Verify that the structural load is evenly distributed to prevent localized overloading and settlement.

Summary

By following these recommendations, the construction team can effectively stabilize the building, address the causes of the tilt, and ensure the long-term safety and integrity of the plaza. Implementing both immediate and long-term measures will mitigate the risks of future tilting and allow for the successful completion of the construction project.

7. References

1. Coduto, D. P. (2001). Geotechnical Engineering: Principles and Practices. Prentice Hall.
2. Qureshi, M. S., & Akbar, A. (2003). Fundamentals of Soil Mechanics. ABC Publishers.
3. Bowles, J. E. (1996). Foundation Analysis and Design. McGraw-Hill.
4. Lecture notes from CEE-209 Geotechnical & Foundation Engineering.

8. Appendices

8.1 Supporting Data

8.1.1 Geotechnical Investigation Report

The geotechnical investigation report provides detailed information on the subsurface conditions at the site. Key sections include:

• Soil Stratigraphy: Description of soil layers encountered, their thicknesses, and properties.
• Borehole Logs: Detailed logs from boreholes drilled at various locations on the site, indicating soil types, depths, and SPT N-values.
• Groundwater Table: Information on the depth of the water table and any observed fluctuations.

8.1.2 Lab Test Results

The lab test results include data from various tests conducted on soil samples taken from the site:

• Standard Penetration Test (SPT) Values: Loose Silty Sand (0-3 meters): N-values of 5 to 10. Medium Dense Sandy Silt (3-10 meters): N-values of 15 to 25. Stiff Clay (>10 meters): N-values of 30 to 40.
• Atterberg Limits: Loose Silty Sand: Non-plastic (NP). Medium Dense Sandy Silt: LL = 35%, PL = 20%, PI = 15%. Stiff Clay: LL = 50%, PL = 25%, PI = 25%.
• Unconfined Compressive Strength: Medium Dense Sandy Silt: 100 kPa. Stiff Clay: 300 kPa.

8.1.3 Bearing Capacity Calculations

The following bearing capacity calculations were performed using Terzaghi’s bearing capacity equation:

• Loose Silty Sand (0-3 meters): Cohesion (c') = 0 kPa Angle of Internal Friction (?) = 30° Unit Weight (γ) = 18 kN/m3 Ultimate Bearing Capacity (qu) = 900 kN/m2 Allowable Bearing Capacity (qallowable) = 300 kN/m2 (with FOS of 3)
• Medium Dense Sandy Silt (3-10 meters): Cohesion (c') = 5 kPa Angle of Internal Friction (?) = 28° Unit Weight (γ) = 20 kN/m3 Ultimate Bearing Capacity (qu) = 1252 kN/m2 Allowable Bearing Capacity (qallowable) = 417.33 kN/m2 (with FOS of 3)
• Stiff Clay (>10 meters): Cohesion (c') = 50 kPa Angle of Internal Friction (?) = 20° Unit Weight (γ) = 22 kN/m3 Ultimate Bearing Capacity (qu) = 4447.6 kN/m2 Allowable Bearing Capacity (qallowable) = 1482.53 kN/m2 (with FOS of 3)

8.1.4 Figures and Tables Illustrating Soil Profile and Bearing Capacity

• Figure 1: Subsurface Soil Profile: Illustration of soil layers from the ground surface to depths beyond 10 meters, indicating the locations of the loose silty sand, medium dense sandy silt, and stiff clay layers.

These appendices provide comprehensive data supporting the analysis and recommendations made in the report, offering a detailed understanding of the subsurface conditions, soil properties, and bearing capacity considerations crucial for addressing the tilt of the plaza.

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