Geotechnical Engineering Design Process: From Ground Investigation to Construction

Geotechnical engineering goes beyond a few basic tests to predict the ground behaviour. The thing is, the uncertainty in geotechnical work is high because engineers can develop their understanding only by sampling as little as 0.000015% of the soil under the site. It generates a large coefficient of variation for the properties of the soils.

In doing so, ground investigation is not about finding the correct answer. It focuses on a range of possible answers, where each offers different associated risks. That is why, with all the help of various tools, they are just the instrument to provide engineers data. Based on it, engineers will have to make decisions by themselves, called engineering judgement.

To avoid this misconception, you need to understand how the geotechnical engineering design process is actually working. This article will provide you with various insights that will help you to learn, step-by-step, how ground investigation workflow works.

How is the geotechnical engineering design process?

The process goes through site investigation and soil testing to final design and construction support. Geotech engineering starts with finding relatable information and data about the subsurface within the construction site. Then, it ends at the post-construction stage after the structure and surrounding ground have reached long-term stability.

The geotechnical process takes a long time because soil and rock move water and adjust to pressure very slowly. They don’t just crush instantly; instead, it takes months or even years after water leaves the soil pores. To have a better grasp of the situation, engineers need to go through various steps, including:

Site investigation and soil testing

Understanding what lies beneath a development site is crucial in any construction process because 80% of their issues can be found during this process. Professionals need to assess ground conditions and subsurface investigations. Therefore, they can determine the ground conditions and subsurface structure of a site from the very start of the project.

The process itself involves three steps: desk-based studies, intrusive ground investigation, and laboratory testing and monitoring. The key purpose is to find if the subsurface is actually suitable for the proposed development. To give you more insight about the processes, here is some explanation about them:

Desk-based studies

Based on British Standard BS5930 and BS10175, the first stage of geotechnical engineering should always be a desk study. This is essential to build hypotheses and plan how the site investigation will be held. Here is the information that you will find in the report:

• All relevant site details: Site name, boundaries, usage, proposed usage and details of the walkover survey.
• Historical appraisal: The historical usage of the site based on structures, ordnance information, and the surrounding areas.
• Environmental appraisal: It involves various aspects like flooding, hydrology, pollution history, landfill use, contamination sources, and hydrogeology.
• Geological appraisal: It includes factors such as artificial ground, superficial strata, bedrock, mining, geomorphology, natural ground subsidence, and others.
• Conceptual ground model: This is related to the potential hazards and contamination sources from the ground.

Intrusive site investigation

After gathering all the necessary information from desk study, the next process involves using that information for excavating, drilling, or sampling ground materials below the surface. Engineers are looking for information for soil strata, rock formations, groundwater levels, and

Technique	Description	Best Used For
Trial Pits	Machine-excavated open trenches (typically 3–5m deep) allowing direct visual examination of shallow soil layers and foundations.	Shallow foundations, rapid bulk sampling, identifying buried structures.
Boreholes (Boring/Drilling)	Deep vertical holes advanced using cable percussion, rotary drilling, or window sampling rigs.	Deep foundation design (piling), bedrock profiling, deep soil sampling.
Cone Penetration Testing (CPT)	Hydraulically pushing an instrumented steel cone into the ground to measure real-time tip resistance and friction without recovering soil.	Fast, continuous geotechnical profiling in clays, sands, and soft soils.
Dynamic Probing	Driving a heavy steel cone into the ground with a drop hammer to count blow frequency per depth interval.	Assessing soil density, relative strength, and shallow bearing capacity.

Lab testing

The last part of the ground investigation involves laboratory analysis. The sample that you have received from the site will be sealed and delivered into accredited testing facilities. The testing is split into two primary streams, geotechnical testing and environmental testing.

• Geotechnical testing: Determining how subsurface reacts under structural loads, water saturation, and excavation (e.g., classification, strength and load, settlement and water, and rock mechanics).
• Environmental testing: Detecting hazardous substances in soil and groundwater, comparing concentrations against strict regulatory threshold guidelines (e.g., checking for heavy metals, hydrocarbons, asbestos screening, aggressive ground chemistry, etc).

Conceptual design

This part is where geotechnical engineers transform the raw data from the testing and investigation into actionable data for structure engineers. They analyse the data and evaluate the tools to fulfil these key objectives:

1. Translating testing data into design parameters: The lab and field test results will be converted into governing engineering values, such as soil friction angles, cohesion, and stiffness moduli, across distinct geological layers.
2. Evaluating foundation and earthwork feasibility: Deciding whether to use shallow/deep foundations, deciding how the retaining structures will be, and which ground improvements are required. The concepts will be based on the technical feasibility, cost, constructability, and site constraints.
3. Managing identified site risks: From the testing, you will uncover the hazards within the construction site. Therefore, engineers will manage how to solve these issues.
4. Defining scope for detailed design: The result from the investigation and testing will narrow down the project. It is essential to set the boundary conditions and calculations needed for the final structural design phase.

Foundation design

The third step in the geotechnical engineering design process is creating the detailed calculation and engineering phase. The purpose is to convert raw ground parameters and structural loads into safe, constructible, and cost-effective foundation systems. This process involves determining:

Ultimate Limit State (ULS)

The goal of this task is to ensure the ground beneath the structure won’t collapse or shear under maximum design loads. It uses these parameters:

• Bearing capacity calculation: This is to find the ultimate ground resistance by accounting for soil friction angle, cohesion, surcharge pressure, and foundation shape or depth.
• Deep foundation resistance: It calculates skin friction along the shaft and end-bearing capacity at the pile tip.
• Overturning and sliding: Ensures shallow foundations or retaining bases resist horizontal loads and overturning moments caused by wind, quakes, or lateral soil pressure.

Serviceability Limit State (SLS)

Different from ULS, this process is to find the limits of structural movement to acceptable levels. It is because a foundation can be safe against collapse but still fail functionally if it sinks too much. That is why engineers need to find the following:

• Total settlement: Finding the immediate settlement in sands and long-term consolidation settlement in saturated clays over decades.
• Differential settlement: Ensuring different parts of the foundation are settled evenly, preventing structural cracking, beam distortion, or tilting.

Engineering analysis and simulation

This is where you will rely on geotechnical software for analysis. You will apply tools like finite element modelling, load simulation, and soil-structure interaction analysis to predict how the subsurface behaves under real-world conditions. It is usually involved in finding what will happen to the structure when they add various loads, groundwater changes, and seismic activity.

Code compliance and safety factors

One of the most important processes in geotechnical engineering is ensuring the construction design and field operations comply with local and international standards. In the UK, they are based on British Standards (BS) and International Organisation for Standardisation (ISO) as mandatory, such as the following:

1. BS EN 1997 (Eurocode 7 – Geotechnical Design): The primary code that governs geotechnical design.
2. BS EN ISO 14688 Series: International standard for the identification and classification of soil.
3. BS EN ISO 14689 Series: International standard for the identification and classification of rock.
4. BS EN ISO 22475/22476 Series: Governs sampling methods, groundwater measurements, and field testing.
5. BS EN ISO 17892 Series: Standardise laboratory testing procedures for soil.

Moreover, there are complementary BS because Eurocode 7 only provides high-level principles. For the how-to, these BS serve as Non-Contradictory Complementary Information (NCCI):

1. BS 5930: For ground investigations.
2. BS 8004: For foundations.
3. BS 8002: For earth retaining structures.
4. BS 8006: For strengthening or reinforcing soils.
5. BS 6031: For earthworks.
6. BS 8081: For ground anchorages.

Final design and construction support

The final process in geotechnical engineering is finalising the design and supporting the construction process. The process is into two separate phases; they are

Final design phase (detailed engineering)

It involves producing the final blueprints, specifications, and safety sign-offs for construction and regulatory approval. Therefore, this phase provides:

• Detailed numerical and analytical calculations: The data that shows complex soil-structure interaction under static, dynamic, and environmental loads.
• Geotechnical Design Reports (GDR): Formal report detailing design perimeters, ULS and SLS, and safety factors relevant to standards.
• Construction drawings and schedules: Structural drawings with details on foundation rebar schedules, pile layouts, and other tasks.
• Technical specifications: A guideline for material requirements for concrete mixes, grout strengths, geotextiles, and compaction tolerances.

Construction support phase (observational method)

The geotechnical engineering design process ends with observing the construction process. Geotech engineers have to observe and ensure the actual ground encountered matches the design assumptions and any activities are safe. These are the activities that they have to oversee:

• Site inspections and validation: Watch over the open foundation excavations, verifying formation strata against borehole logs and signing off on pile toe levels.
• Instrumentation and monitoring: Track the real-time performance by using tools like inclinometers, piezometers, and settlement markers to prevent unexpected movements.
• The observational method: Modify the design dynamically during construction, adjusting to performance data.
• Requests for information (RFIs) and unforeseen ground conditions: Responding to contractor RFIs, issuing redesigns for localised unexpected ground conditions, and reviewing contractor submittals.

FAQs

Why is geotechnical engineering inherently uncertain? Because it has a high coefficient of variation across soil properties and requires engineering judgement to find the optimal result.

What are the primary intrusive ground investigation methods? They are trial pits, boreholes, CPT, and dynamic probing.

What is the difference between geotechnical and environmental lab testing? Geotechnical testing is about the physical properties or behaviour of soil materials, while environmental testing is for checking hazardous contaminants.

What is the difference between disturbed and undisturbed soil samples? The difference relies on the soil’s natural structure, density, void ratio, and moisture content that are preserved during extraction.