Most man-made structures bear on soil. A soil has a finite strength and not all soil can withstand all kinds of loads/pressure. Therefore, before any structure is sited on any soil, there should be proper investigation of the soil to ensure that the design to be made for the structure should be safe, economical design (through selection of appropriate foundation type) and to reduce as much as possible difficulties in construction due to the nature of the soil. Soil investigation is one of the most important aspect of construction works especially in areas with bad soil. Through soil investigation, the properties of the soil that would bear the load would be known.
A. Purpose of soil investigation – Soil contains its particles together with other organisms and substances. The purpose of soil investigation for civil engineering is not to check the number of living organisms in the soil but to:
i. Determine the sequence, thickness and lateral extent of the soil.
ii. Determine the level of bedrock.
iii. Obtain representative samples of the soils (and rock) for identification and classification or to determine other relevant soil parameters
iv. Identify ground water conditions.
v. Determine insitu soil properties.
vi. Check the contamination level of the soil.
B. Cost of ground investigations
Many clients tend to ignore ground investigations as they plan to erect their buildings, oftentimes citing cost as reason but this should not be so and is never justified. Often, they live bitterly with the after-effect of lack of ground investigations. Generally, the cost of soil investigation depends on the location and extent of site, the nature of strata and type of project under consideration. However, they should be between 0.1 – 2 % of the project cost.
C. Processes of soil investigation
Soil investigation is usually preceded by site investigation. The first stage here is site inspection normally made on foot. The site investigation gives an idea of the nature of soil present and the suitable type of soil investigation to be adopted through. Site investigation are done in two stages. These include:
1. Preliminary investigation, and
2. Detailed investigation
Preliminary investigation: a good site investigation should take into account the following points
a. Geology of the area: past literatures can give an idea of indications of probable soil condition. In other cases, an aerial photograph can be used to determine it.
b. Reconnaissance survey: an inspection of the site and surrounding area must be made on foot. Riverbanks, existing excavations, qualms, road or railway cuttings may reveal the strata and ground water conditions. Existing structures should be examined for signs of settlement damages. Further information may be obtained from adjacent owners of buildings or local authorities. Using Nigeria as example, public utility service lines like water corporation lines, Nigeria Telecommunication (NITEL) lines, Power Holding Company of Nigeria (PHCN) lines must be examined to know whether any of them pass through the site. History of flood and natural flood routes should also be noted. Availability of construction materials, their location and impact on the cost of the project should be noted too.
d. Water quality: ground water quality and the quality of water for construction should also be checked. What impact does it have on the construction? Should it be treated especially sites that contain sulphates?
e. Topography: look at the nature of the soil. Is it sloppy? Does it need to be levelled? Do you need to step-down the structure (basement construction)?
Detailed Investigation: the actual soil investigation depends on the observed nature of soil strata and the type of project to be done. Testing of soil can be done insitu (for some tests) and laboratory tests. Whether insitu or laboratory tests, there would be need to dig trial pits or boreholes for sample tests or sample collections. Sampling points are usually scattered and the number depends on the degree of variability of the soil. care should be taken to ensure that sampling points do not interfere with potential foundation trench lines. The investigation should also be taken to adequate depth that depends on the type and size of the project but must include all strata liable to be significantly affected by the structure and its construction. Sampling depths are most times shallow but deep and critical sampling depths would be required in situations such as;
1. Where piles would be used. In this case, the sampling should be extended to considerable depth below the surface.
2. Where weak stratum is anticipated beneath a firm stratum, the sampling depth should be taken up to the weak stratum.
3. In areas of old mine workings or other underground cavities, the sampling should be taken to depth greater than normal.
D. Method of soil investigation/sample collections
I. Trial Pits
The simplest of excavations for soil investigation/sample collection is the trial pits. A trial or test pit is a hole dug in the ground that is large enough for ladder to be inserted thus permitting a close examination of the site. Prior to digging trial pits, the following should be noted:
1. The number and locations of boreholes or trial pits/test pits should be planned to enable basic geologic structure of the soil and subsurface conditions of the soil to be detected.
2. The greater the degree and variability of the ground conditions, the greater the number of trial pits. For heavy structure, trial pit requires to be about 15 – 30 m apart and should be taken down to about 1.5 times the width of the structure unless rock is encountered at lower depth. If rock is encountered, it should be penetrated at least 3 m to confirm that it is bedrock and not a large boulder was encountered unless geologic knowledge indicates otherwise.
3. Trial pits should be located away from the line of foundations to avoid weakness.
The sides of the pit must be supported unless they are slopped to safe angle or stepped (Figure 1). The trial pits is limited to 4-5 m depth. To avoid cave-in of the pit, the pit should be sloped or safe angle and/or stepped and the excavated soil should be placed at least 1 m from the edge of the pit.
Advantages of trial pit
a. The use of trial pit enables the insitu soil condition to be examined visually and thus the boundary below the strata accurately determined.
b. It is easy to obtain disturbed and undisturbed soils samples from the trial pit.
Disadvantages of trial pit
a. If the pit is excavated below the water table, some form of de-watering is necessary (see Figure 2).
b. Below the depth of 4 m, the problem of strutting and removal of excavated materials becomes increasingly more and thus increases cost rapidly. For this reason, trial pits are limited to maximum depth of 4-5 m.
Other methods include;
II. Probing using hand or mechanical auger
Augers (see Figure 3) can be hand driven or mechanical augers. Hand augers can be used to excavate boreholes to depths of around 5 m using a set of extension rods. The auger is rotated and pressed down into the soil by means of a T-handle on the upper rod. Hand augers whose diameter range from 50 mm to 200 mm can be used if the sides of the hole require no support and if the soils do not contain particles of coarse gravel and above. After digging, undisturbed samples can be obtained by driving small-diameter tubes below the bottom of the borehole. Mechanical augers are mounted on vehicle or in the form of attachments to the derrick used for percussion boring.
III. Use of drilling machine
3 types of drilling are common in sampling viz.
i. Percussion boring – here, the borehole diameter can range from 150 to 300 mm with a maximum borehole depth of 50 and 60 m. It can be used in all types of soils including ones with cobbles and boulders.
ii. Wash boring – this is commonly used as a means of advancing a borehole to enable tube samples to be taken or insitu tests to be carried below the bottom of the hole.
iii. Rotary drilling – this is more suited for rocks but it can also be used for soils. Two types are common – open drilling and core drilling.
1. The borehole location should be offset from areas in which it is known that the foundations are to be sited. They should be backfilled after use.
2. For roads, the borehole need not be closer than 300 mm centres unless vegetation indicate variation in soil conditions and need not go beyond 3 m below formation level.
3. During boring, at different layers as soil conditions changes, you take soils for laboratory analysis.
4. It is necessary to use undisturbed samples for the test because disturbed samples (normally used for classification tests) would not give true behaviour of the soil on site.
Soil Investigation Report – how to prepare soil exploration report
At the end of all soil exploration programs, the soil and/or rock specimens collected in the field are subject to visual observations and appropriate laboratory testing. After all the information has been compiled, a soil exploration report is prepared for use by the design office and for reference during future construction work. This is the final product of an exploration programme and consist of:
a. Summary of the ground conditions encountered
b. A list of the tests carried out
c. Recommendations as to possible foundation arrangement
The details and sequence of the information in the report may vary to some degree, depending on the structure under consideration and the person compiling the report. Each report should include the following items.
i. The scope of the investigation.
ii. The description of the proposed structure for which the subsoil exploration has been conducted.
iii. A description of the location of the site including structure (s) nearby, drainage conditions of the site, nature of vegetation on the site and surrounding it and any other feature(s) unique to the site.
iv. Geological setting of the site.
v. Details of field exploration – that is, the number of borings, depths of borings, types of borings and so on.
vi. General description of the subsoil conditions as determined from soil specimens and from related laboratory tests, standard penetration resistance and cone penetration resistance and so on.
vii. Water table conditions.
viii. Foundations recommendations, including the type of foundation recommended, allowable bearing pressure, and any special construction procedures should also be discussed in this portion of the report.
ix. Conclusion and limitations of the investigations.
The following graphical presentations should be attached to the report
a. Site location map
b. A plan view of the location of the borings with respect to the proposed structures and those existing nearby
c. Boring logs
d. Laboratory test results
e. Other special graphical presentations
The ground condition is covered in each trial pit and summarized in form of borehole log or Journal. The method and the equipment details used should be stated in each log. The location, ground level and diameter of the hole should be specified together with details of any casing used. The name of the client and the project should also be stated.
Example of Borehole log
Location: UNIZIK Borehole No. 1: Sheet 1 of 1
Client: UNIZIK Engineering Dept. Groundwater Level: 36.30 OD
Boring method: Auger to 14 m Scale: 1:100
Diameter: 150 mm Date: 4th January, 2022
Casing: 150 mm to 5 m
Table 1; Example of the boring log
Bearing capacity of soil for foundation design
Oftentimes, the end purpose of soil investigation is to determine the bearing capacity of the soil. This requires conducting insitu soil bearing capacity tests and laboratory soil bearing capacity tests. Insitu tests can be done with standard penetration test (SPT), cone penetration test (CPT) (see Figure 4) etc. While cone penetration tests are the most popular. Laboratory tests can be done using triaxial tests while soil parameters like cohesion and angle of internal friction are determined. The bearing capacity from same point and depth insitu are determined and compared with the one from laboratory and the average determined.
Tables CPT 1 and CPT 2 below show the results from CPT carried out within UNIZIK, Awka, Nigeria.
Table CPT 1 with coordinates (6o 141 54.3511 N; 7o 71 14.8311 E)
Table CPT 2 with coordinate (6o 141 54.8611 N; 7o 71 15.3811 E)
The tables above show two points in which CPT tests were done on a land. To calculate the bearing capacity, the cone resistance value given by C is used at each of the depth. The CPT is always taken down to the proposed depth of foundation. To determine the bearing capacity value, the modified equation introduced by Meyerhof can be used.
Safe bearing capacity, qs = 3.6 qc Rw2 (kPa) for B ˂ 1.2 m and
qs = 2.1 qc (1 + (1/B))2 Rw2, kPa for B ˃ 1.2 M
Where, Rw2 = ½ (1 + Dw2/B)
and, qc = cone friction in k/cm2, qs in kPa, Rw2 = reduction factor for GWT below the base of the foundation and Dw2 = depth of foundation measured from the footing’s base
In practical, if GWT data are not available, the results obtained from the formulas above are divided by the safety factor SF and also Rw2 with assumptions that the foundations are below GWT.
Assuming we want to determine bearing capacity at 0.5 m depth from Table CPT 1, average qc = (4 + 18 + 60 + 150)/ 4 = 58 k/cm2; assuming Dw2 = 1 m, and B = 1.2 m
Rw2 = ½ (1 + 12/1.2) = 0.92
Bearing capacity of the soil at the depth, qs = 3.6 x 58 x 0.922 = 176.73 kg/cm2