Soil Investigations: Even though most homeowners are interested in design, materials, and beauty, there is another important process that no one pays much attention to. It is vital to be aware of the ground on which you will put the first brick before putting it in place. One of the largest investments that you will ever do is to build your dream home.
What is Soil Investigations
The study of soil, rock, and ground water conditions at a construction site to discover whether the location is appropriate to construct a safe and permanent structure is known as a site investigation. Consider it a check-up of the land. Like you would not go long distance without inspecting your car, you would not go off and construct without inspecting the soil.
There are general investigations commonly done at the sites of a new house construction and we will be examining them in this detailed guide. We will discuss
- The soil testing and borehole tests
- The SPT test,
- Trial pits,
- The geotechnical surveys,
All of the above which are explained in a simple language, so that no homeowner will fail to understand why all these steps are so necessary.
Importance of Soil Investigations
Now we shall discuss on the importance of Site Investigation before construction of a house.
When you purchase a plot to build your house, it may appear on the surface to be a perfect one; flat and dry and capable of being built. What is below is what is really significant. There can be serious structural problems in the future due to a weak soil layer, high ground water or underground rock.
Here is the reason why site investigation of a house is so important:
- Safety of the Structure- Averts such risks as cracks, tilting, or collapse.
- Right Foundation Design – It provides that the foundation equals the strength of soil.
- Cost Savings – Eliminates deep foundations, or expensive repairs.
- Waterproofing Requirements- Determines the groundwater depth of basements and septic tank.
- Environmental/Legal Compliance -Certain areas have the mandate that soil tests be done prior to approval.
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The typical kinds of site investigation of new houses
We will travel step by step on the investigations that are usually carried out before commencing construction of a new home.
1. Desk Study and Site Survey
All site investigations are initiated by desk investigation and physical survey.
Desk Study: Engineers collect background information through maps, geological survey, and neighbouring construction works. They seek flood prone regions, earth tremors and soil type prevalent in the area.
Site Reconnaissance: This is a physical walkover survey to examine slope, drainage, vegetation and adjacent structures. Why it matters to a house: In case your location is susceptible to waterlogging or close to a fault line, the design must take this into consideration.
2.Soil Investigations
Soil testing is the core of soil investigations method. Your house is built on soil and its characteristics have to be known.
Soil investigations are of two kinds, namely:
a. Trial Pits or Test Pits
b. Borehole Test
a) Trial Pits or Test Pits
Trial pits (typically 1.5 to 6 meters in depth) are one of the simplest modes of site investigation.
Purpose: Directly investigate soil layers, seepage and find weak layers in the ground.
Pros: cheap, visual confirmation is instant, sample blocks could be taken to the lab.
Limitations: Depth is restricted, inappropriate in loose or saturated soils, high-rise construction or deep foundation construction.
Trial pits come in handy particularly in those small projects that do not need deep foundations like houses and boundary walls.
Lab testing of samples is done which are applicable to a house: Trial pits may be required only in small houses with shallow foundations.
b) Borehole Test
A borehole refers to a hole that is drilled vertically into the ground, to extract soil and rock samples of varying depths. Normally, boreholes are drilled by rotary drilling rigs, augers, or by percussion drilling equipment.
Purpose of Boreholes in soil investigations method:
- Identify stratification in soil.
- Take disturbed and undisturbed samples to be tested in laboratory.
- Record groundwater levels.
- Allow access to in-situ tests (including SPT test).
In small buildings: 5 -15 m deep with a distance of 25 -50 m. On a large-scale project: Boreholes can reach a maximum depth of 30-50 m or underground rock.
3.Soil Investigations method–Standard Penetration Test (SPT Test)
One of the most widely used in-situ soil investigations tests that are carried out within boreholes is the SPT test.
Procedure of SPT Test:
- The bottom of a borehole is equipped with a split-spoon sampler.
- A 63.5 kg hammer is dropped with a height of 760 mm.
- The sampler is pushed 450 mm into the soil.
- The count of blows needed to push the sampler the final 300 mm is noted as the SPT-N value.
Importance of SPT Test:
- Gives an indication of soil resistance and relative density.
- Relates to soil strength parameters such as bearing capacity and angle of internal friction.
- Assistance in evaluating the liquidity in seismic regions.
Advantages:
- Easy, economical, and well-liked in engineering codes.
- Delivers fast field outcomes.
Limitations:
- Not so true in soft clays or silts.
- There is poor quality of disturbed samples.
In spite of these disadvantages, the SPT test has been one of the foundations of geotechnical site surveys in the world.
4.Soil Investigations method –Cone Penetration Test (CPT)–
The Cone Penetration Test (CPT) is another commonly used in-situ method. CPT is a static test (whereas SPT is one that consists of blows with a hammer).
- A steel cone is driven into the ground at a steady rate.
- The penetration resistance is electronically recorded.
- Gives real-time profile of the soil. CPT is particularly applicable in sandy soils where SPT might not be accurate.
- It however needs advanced equipment and is incapable of getting soil samples.
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5.Soil Investigations method: Plate Load Test
Plate load test is carried out at shallow depths to determine soil bearing capacity and settlement due to a certain load.
- At foundation depth, a steel plate (300 -750 mm diameter) is used.
- Road load is added gradually and the settlement is recorded.
- The load-settlement curve is used to establish ultimate bearing capacity.
- This test is commonly applied in shallow foundations, road subgrades and pavements.
6. Soil Investigations method: Pressure-meter Test
A pressure-meter is placed in pre-drilled borehole and the surrounding soil is pressed radially.
- Gives soil parameters of modulus and strength.
- Applicable in the foundation design of sands and clays.
- It is also costly, but provides very reliable data in comparison to SPT or plate load tests.
7. Geophysical Methods
There are occasions when extensive site surveys require that the drilling is impossible. Geophysical techniques offer non-invasive techniques of mapping underground conditions.
Common soil investigations techniques include:
- Seismic Refraction/Reflection- Determines the depth of rocks and the rigidity of soil.
- Electrical Resistivity -Determines groundwater, clay layer, or voids.
- Ground Penetrating Radar (GPR) -Identifies utilities that are buried, shallow soil.
- Borehole tests are usually used together with geophysical survey to compare findings.
8.Soil investigations method: Groundwater Measurement
The foundation design highly depends on groundwater. When water tables are high, soil strength can be weakened, and the pressure of the water can be high.
- Standpipe Piezometers are fitted in boreholes to gauge the water levels with a period of time.
- Tests of permeability and aquifer properties can be performed through pumping.
- This is essential information in basement construction, tunnels and underground structures.
9.Soil Investigation method: Soil Samples Lab Testing.
The field tests provide initial results and the exact soil properties are determined under laboratory tests.
Common lab tests include:
- Moisture content
- Atterberg limits (liquid and plastic limits)
- Sieve and hydro meter distribution of grain (distribution)
- Unconfined compression test.
- Triaxial shear test
- Oedometer test (to study settlement)
These measurements are used as a complement to borehole tests and SPT tests to give a complete geotechnical profile.
Issues that influence soil investigations method selection.
The engineers do not apply a single method. The selection depends on:
- Project type (residential or high-rise or bridge).
- Depth of foundation.
- Soil type (clay, sand, rock).
- Groundwater conditions.
- Budget and time constraints.
An example is that a small residential project would just require trial pits and simple plate load test and a metro rail tunnel project would involve a lot of borehole drilling, SPT tests, CPT and geophysical survey.
Current Trends in Soil Investigations.
The following are the current trends in soil investigations method.
- Currently geotechnical surveys have been made quicker and more precise with the development of new technology:
- Digital Borehole logging: Automated data logging minimizes errors.
- Remote Sensing & Drones: It is utilized in topography and geological mapping.
- Machine Learning Models: Predict soil properties with the existing data.
- 3D Ground Modeling: Integrates borehole and geophysical information to be visualized.
These innovations are turning the conventional soil investigation into more integrated and data-driven.
Soil investigations Case Study with an example:
Case Study: High-Rise in Sandy Soil.
- We can take an imaginary project: building a 40-storey shelter on a sandy soil.
- Desk Study- Check geological maps and adjacent borehole data.
- Borehole Test- Drill: Boreholes are drilled at a depth of up to 40 m deep to find stable strata.
- Test SPT -To determine relative density test at every 1.5 m.
- CPT Test – Complement SPT in order to receive continuous soil profile.
- Groundwater Surveillance – Place piezometers to determine the changes.
- Lab Tests- Conduct triaxial and consolidation tests.
- Geophysical Survey – Check the depth of bedrock by seismic refraction.
- Such a combination provides a strong foundation design, eliminating the risk of suffering such problems as undue settlement or liquefaction during an earthquake.
Conclusion:
An efficient construction project takes off with the right knowledge of what is under the surface. In geotechnical engineering, soil investigations techniques such as the basic trial pits to sophisticated geophysical methods is used to give valuable information on the characteristics of soils and rocks.
The borehole test and SPT test are some of the most popular methods that cannot be omitted when it comes to getting reliable design parameters. When used along with modern equipment and lab tests, they are the foundation of any geotechnical site survey.
In a word, appropriate soil research does not only ensure structural safety, but also is cost-efficient, avoids failures, and instills trust in project implementation. This is because no matter the scale of the investment be it in a residential house or mega infrastructure project, it is always worth investing in thorough site investigation.
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