ENGIN 3202 - Geotechnical Engineering Assignment 1 Solution

This document presents a comprehensive solution to a geotechnical engineering assignment. It addresses key aspects of geotechnical site investigation, including major steps and information gathering, and also covers a commonly used geotechnical field test for estimating soil strength, discussing its method, advantages, limitations, and potential use in engineering design. The solution further explores bearing capacity failures in soil, differentiating between short-term and long-term bearing capacity scenarios. Additionally, the assignment delves into the design of a multi-story office building complex, calculating ultimate soil capacity and safe bearing pressure, and determining footing width based on safety factors. The solution also analyzes the objectives of AS2870, site classification criteria, and the impact of geotechnical and environmental factors on site classification, and the application of AS2870 to classify a building site without reactivity tests. Finally, it examines the classification of a site based on AS 2870-2011, considering the impact of trees and demolition on site classification, and discusses concrete footing requirements and performance criteria.

Question 3
Given
w = 23%
∅ =¿290
γ L=17.3 kN
m3
γsat =17.3 kN
m3
The following formula would be used to determine ultimate soil capacity
qu = cNc + qNq + 0.5γB Nγ
C = 0 due to presence of sand
The Terzhag’s bearing capacity factors for ∅ ' =290
Nc = 28.26
Nq = 16.85
Nγ = 20.10
qu = (0 *28.26) + ( γ Df ∗16.85 ¿+0.5 γ∗B∗20.10
= 0 + (17.9 * 1.25*16.85) + (0.5*17.9 * 1* 20.10)
qu = 556.91 kN/m2
Take the factor of safety of 2.5
qsafe = 556.91/2.5 = 222.76 kN/m2
Maximum load = 550 kN
Let start with footing width of 2.0 m
Bearing pressure = 550/(2*1) = 275 kN/m2
Which is greater that qsafe, this means that a higher width would be required to be adopted

Let’s take a width of 3 m
Bearing pressure = 500/(3*1) = 183.33 kN/m2
The value determined is less than qsafe. Hence we will adopt the width of 3m
Question 4
a) The major objectives of AS2870.
The major objective of AS2870 standard is to establish and create the performance requirement
that would be used in foundation of specific footing system designs in Australia and also it
provides engineering principles guidance that would be used to design the footing system.
AS2870 emphasizes more on reactive site clay design that would be susceptible to allow
movement of the ground as a result of change in moisture. AS2870 would take be concern with
the following (Karunarathne et al., 2013):
a. Movement caused by shrinking and swelling of reactive site clay as a result of
moisture changes
b. Compressible soil settlement
c. The foundation distribution load
d. Superstructure tolerance movement
b) The situations that a site would be classified as Class P.
This would be based on the soil poor ability to consistently bear a load. Classify site as P would
also be based on landslip, mine subsidence, coastal erosion, and collapse activities, less bearing
capacity of soft soil, filled or cut sites. The site have ground movements due to moisture changes
and the site experiences abnormal moisture from nearby dams, trees and poor drainage in the
site.
c) Geotechnical and environmental factors affecting the site class.
Unevenness of Movement
Uneven ground movement of the building foundation, therefore soil settlement would be uneven
due to:
 Uneven compaction of soil before construction

 Changes in soil foundation moisture contents before construction
Erosion would cause shear failure condition of soil through eroding adjacent footing soil that
would run in the direction of flow.
Framed structures
Erosion and soil saturation would cause the fall of a footing. Additional stress would cause wall
cracking especially if the soil is saturated
Rainy seasons
Consistence rain would cause the ground movements due to moisture changes and the site
experiences abnormal moisture that would make the foundation soil collapse or be swept away.
d) Practical approach proposed by AS2870 to classify a building site, without having to
conduct reactivity tests in the lab.
Step 1
FOR REACTIVE SITES:
The site of the building that faces the effect of abnormal moisture conditions (Section 1.3.3):
Before construction (Clauses 1.3.3(a) – (d))
 Removal of structures or an existing building which end up having considerably soil that
modifies its moisture conditions at the footing of the building system
 Trees removal before construction
 Availability of adjacent site trees or respective building site
 Moisture conditions that are unusual as a result of the presence of nearby channels,
drains, dams, ponds, tanks, swimming pool that would either be removed or maintained
in the site.
During construction (Clauses 1.3.3(i) and (ii))
 Improper provision of site drainage
 Failure to construct drainage as per the AS2870 standard

After construction (Clauses 1.3.3(A) – (E))
 The availability of trees being close to the footing
 Irregular garden watering that is adjacent the site building
 Improper maintenance of site drainage
Step 2
Step 1 if NO
FOR ALL SITES
The building experiencing the effect of section 2, that includes;
 Inadequate strength of the bearing materials (Clause 2.1(a))
 Excessive settlement of the foundation as a result of the foundation load (Clause 2.1.3(b))
 Controlled fill that do not include sand at a depth more than 0.4 m and uncontrolled fill at
a depth greater than 0.8 m of sand and o.4 m of other materials that do not include sand
(Clauses 2.1.3(c) and 2.5.3)
 Foundation soil that are unstable or soft
 Mine subsidence and landslide (Clause 2.1.3(d))
 Coastal erosion and soil collapse (Clause 2.1.3(d))
 Change in moisture as a result of severe site condition (Clause 2.1.3(e))
Step 3
From step 1and step 2 if yes
Site is classified as P
Step 4
From step 2 if no
Site is a Normal Site, and is classified Class A, S, M, H1, H2 or E

Question 5
a) The site would be classified as P under AS 2870-2011 due to the presence tree and
demolition of dwelling was done two year ago
Clauses 1.3.3(a) – (d))
“Removal of an existing building or structure likely to have significantly modified the soil
moisture conditions under the footprint of the footing system of the building”
“Removal of trees prior to construction
Classify to class P
Footings perform in accordance with Clause 1.3.” (Clause 1.4.1)
b) Concrete footing
Clause 2.3.1: performance requirement
 Barrier to termite
 Cracks on the footing do not exceed 1 mm
Clause 4.3.1: deemed to satisfy as constructed in accordance to As 2870
 When the cracks have become major issue
Using table C2 of As 2870
 Ensuring limits are achieved
 Use of appropriate design
 Modification of the condition of site

References
Karunarathne, A.M.A.N., Gad, E.F., Sivanerupan, S. and Wilson, J.L., 2013. Review of
residential footing design on expansive soil in Australia. In 22nd Australasian Conference on the
Mechanics of Structures and Materials. Sydney: Taylor & Francis.