Rowell A.

Narag August 23, 2020

Assignment #1 Geotechnical Engineering 2

SITE INVESTIGATION and SELECTION OF FOUNDATION

1. Explain the difference between Disturbed and Undisturbed Samples.

Disturbed soil is defined as soil that has not been contaminated by material from other strata or
by chemical changes, but the soil structure is disturbed and the void ratio may be altered. In essence,
the soil has only been remolded during the sampling process. For example, soil obtained from driven
thick-walled samplers, such as the SPT spilt spoon sampler, or chunks of intact soil brought to the
surface in an auger bucket are considered disturbed soil.

Undisturbed samples are often defined as those samples obtained by slowly pushing thin-walled
tubes, having sharp cutting ends and tip relief, into the soil. Undisturbed soil samples are essential in
many types of foundation engineering analyses, such as the determination of allowable bearing pressure
and settlement. Many soil samples may appear to be undisturbed but they have actually been subjected
to considerable disturbance of the soil structure.

Disturbed soil structure is disturbed and there is a change in the void ratio but there is no
change in the soil constituents while Undisturbed soil structure is no disturbance and no changes in
water content, void ratio, or chemical composition.

2. What is dilatancy?

Dilatancy is the volume change observed in granular materials when they are subjected to shear
deformations. Its effect can be seen when the wet sand around the foot of a person walking on
beach appears to dry up.

3. What is the purpose of a bore log?

-An important part of the preparation of logs is to determine the geologic or man-made process
that created the soil deposit.
-Presents a list of common soil deposits encountered during subsurface exploration. Usually the
engineering geologist is most qualified to determine the type of soil deposit.

- These logs also indicate the sample locations and might include some of the laboratory test
results.

a. Used to record the change of layers’ depth

b. Used to record water level
c. Used to record the water quality in deeper levels.
4. What is the objective of site investigation?

The objectives of the site investigation phase include:

• Determining the locations and thicknesses of the soil strata

• Determining the location of the groundwater table as well as any other groundwater-
related characteristics
• Recovering soil samples
• Defining special problems and concerns
Typically, we accomplish these goals using a combination of literature searches and
onsite exploration techniques.

5. What is site reconnaissance?

The foundation engineer should visit the site and perform a field or site reconnaissance.
Often such visits will reveal obvious concerns that may not be evident from the literature
search or the logs of the exploratory borings.
The field reconnaissance would include obtaining answers to such questions as the following:

• Is there any evidence of previous development on the site?

• Is there any evidence of previous grading on the site?
• Is there any evidence of landslides or other stability problems?
• Are nearby structures performing satisfactorily?
• What are the surface drainage conditions?
• What types of soil and/or rock are exposed at the ground surface?
• Will access problems limit the types of subsurface exploration techniques that can be used?
• Might the proposed construction affect existing improvements? (E.g., a fragile old
building adjacent to the site might be damaged by vibrations from pile driving.)
• Do any offsite conditions affect the proposed development? (E.g., potential flooding,
mudflows, rock falls, etc.)

6. What is significant depth?

The investigation shall be carried out to the point at which the vertical stress due to proposed
structure is equal to or less than 10% of original effective stress at the point before the
structure is constructed.

The depth up to which the stress increment due to superimposed loads that can produce
significant settlement and shear stress.
7. How is the depth of exploration decided?

-The depth of subsurface exploration will depend on the size and loading of the proposed
foundation, the sensitivity of the proposed structure to settlements, and the stiffness and
coefficient of compressibility of the strata that will underlie the foundation.

THUMB RULE FOR DECIDING DEPTH OF EXPLORATION

a. The depth of exploration should be one and half times to two times the
estimated width (lower dimension) of the footing, single or combined, from base
level of the foundation.
b. In case of weak soils, the exploration should be continued to a depth at which
the loads can be carried by the stratum without excessive settlement or shear
failure.
c. Isolated spread footing or a raft: one and a half times the width.
d. Adjacent footings with clear spacing less than twice the width: one and half
times the length
e. Pile foundation: 10 to 30 meters, or more, or at least one and a half times the
width of the structure
f. Base of the retaining wall: one and a half times the base width or one and a half
times the exposed height of face of wall, whichever is greater.
g. For floating basement, the depth of construction should be equal to the depth of
construction.
h. In any case, the depth to which weathering process affect the soil should be
regarded as a minimum depth for exploration of sites and this should be taken as
1.5 meters. In case of black cotton soil the minimum depth of exploration is 3.5
meter.

8. List the field tests used in subsurface investigations.

 Standard Penetration Test (SPT)

 Mechanical Cone Penetration Test (CPT)
 Vane Shear Test (VST)
 Miniature Vane Test (MVT)
 Pressure meter Test (PMT)
 Other Field Test Performed in Boreholes

9. What is a detailed exploration?

- Characterize properties of specific features

- Carefully selected exploration techniques, sampling, and testing.

- A detail soil exploration program involves deep boring, field tests and laboratory tests for
determination of different properties of soils required for the design of any structure.
10. What are the factors affecting quality of a sample?

Factors that Affect Sample Quality. It is important to understand that using a thin wall
tube, such as a Shelby tube, or obtaining a gross recovery ratio of 100 percent would not
guarantee an undisturbed soil specimen. Many other factors can cause soil disturbance,
such as:

a. Pieces of hard gravel or shell fragments in the soil, which can cause voids to
develop along the sides of the sampling tube during the sampling process
b. Soil adjustment caused by stress relief when making a borehole
c. Disruption of the soil structure due to hammering or pushing the sampling tube
into the soil stratum
d. Tensile and torsional stresses which are produced in separating the sample from
the subsoil
e. Creation of a partial or full vacuum below the sample as it is extracted from the
subsoil
f. Expansion of gas during retrieval of the sampling tube as the confining pressure
is reduced to zero
g. Jarring or banging the sampling tube during transportation to the laboratory
h. Roughly removing the soil from the sampling tube
i. Crudely cutting the soil specimen to a specific size for a laboratory test

11. What are the various methods of site investigation?

 Indirect methods
-Boring
 Semi-direct methods
-Seismic Method
 Direct methods
-by measuring in-situ a tool/instrument behavior during interaction with soil/rock and
inference of material properties

1. Open excavation
2. Borings
3. Subsurface soundings
4. Geographical methods
5. Trial pits
12. The internal diameter of a sampler is 40mm and the external diameter is 42mm. Will you
consider the sample obtained from the sampler as disturbed or undisturbed?

𝐴 (%)= x 100
Condition, if a soil sample is considering undisturbed if
the area ratio is less than or equal to 10%
Where, 𝐷 = 42mm and 𝐷 = 40mm

𝐴 = 10.25% > 10%, therefore soil sample disturbed

13. How to select the foundation based on soil condition?

Foundations are recommended based on the different soil types which are provided below:

 Rocks
This category involves rocks, hard sound chalk, sand and gravel, sand and gravel with
little clay content, and dense silty sand.

The following types of foundations are suitable for this type of soil:

1. Strip foundation
2. Pad foundation
3. Raft foundation

 Uniform firm and stiff clay.

Case I
where the foundation is not close to vegetation or existing vegetation is not important.
Recommended foundations
1. Strip foundation
2. Pad foundation
3. Raft foundation

Case II
Where trees, hedges, and shrubs are close to foundation location, or there is a plan to
plant these tresses near the structure in the future.
Recommended foundation
1. Concrete piles supporting reinforced concrete ground beams and precast
concrete floor
2. Concrete piles carrying in situ concrete slab
3. Specially designed trench fills in certain clay soil based on the foundation
location relative to trees
4. Raft foundation
 Case III
Where trees are cut down shortly before the foundation construction begin
Recommended foundation
1. Reinforced concrete pile in previously tree root zone
2. Strip foundation
3. Raft foundation

 Soft clay
This category includes soft clay, soft silty clay, soft sandy clay, and soft silty sand.

Recommended foundations
1. Wide strip footing
2. Raft foundation
3. Pile to firmer strata below
4. For smaller projects use pier and beam foundation to firm strata

 Peat
Recommended foundation
1. Concrete piles extended to the firm soil layer below
2. For small projects, pad and beam foundation taken to firm strata below.
3. Raft foundation for the case where firm strata is not available at reasonable
depth but there is hard surface crust with 3-4m thick of suitable bearing
capacity.

14. What are the guidelines in terms of inside and outside clearance for obtaining undisturbed
sample?

For good sampling process, the inside clearance ratio should be

𝑓𝑜𝑟 𝑠𝑎𝑛𝑑𝑠, 𝑠𝑖𝑙𝑡𝑠 𝑎𝑛𝑑 𝑐𝑙𝑎𝑦𝑠 0.5%

within 0.5% to 3 % 𝑓𝑜𝑟 𝑠𝑡𝑖𝑓𝑓 𝑎𝑛𝑑 ℎ𝑎𝑟𝑑 𝑐𝑙𝑎𝑦𝑠 1.5%
𝑓𝑜𝑟 𝑠𝑡𝑖𝑓𝑓 𝑒𝑥𝑝𝑎𝑛𝑠𝑖𝑣𝑒 𝑡𝑦𝑝𝑒 𝑜𝑓 𝑐𝑙𝑎𝑦𝑠 3.0%

For obtaining good quality undisturbed samples, the ratio area should be less than or equal
to 10%

It may be high as 110% for thick wall sampler like split spoon sampler and maybe as low as
6 to 9 % for thin wall samples like Shelby tube.

Minimum inside diameter, 𝐷 = 75mm

The cutting edge angle should be within 20 degrees.
 The length (L) should be at least equal to (the intended length +100mm) for residual soil
For soft soils depending upon the sensitivity. Ratios are given in table.

sensitivity (L/𝐷)
> 30 20
5 to 30 12
<5 12

15. What is meant by inside and outside clearance? What is its use?

Cutting edge is the beveled and sharp edge at the bottom of the soil sampler. It may be
an integral part of the soil sampler or a separate cutting bit may be screwed to the
bottom of the sampler. It mainly facilitates driving of the soil sampler through the soil.

Inside Clearance
The internal diameter of the cutting edge (Dci) should be slightly less than the internal
diameter of the sampling tube (Dsi) to give inside clearance (Ci)

Inside clearance – It allows elastic expansion of sample when it enters the tube
The inside clearance must lie between 1 to 10%, for undisturbed sample it
should be between 0.5 and 3 percent.
Outside clearance – It should not be greater than the inside clearance.
Normally it lies between 0 and 2 percent. It helps in reducing the force
required to withdraw the tube.