Construction Technology – Geotechnical Engineering
Geotechnical engineering is the discipline that focuses with the engineering properties of earth materials (also called ‘geomaterials’). Investigation of soil and bedrock are done to determine how a proposed construction will be done. High-rise buildings, bridges and all types of construction require knowledge of the earth materials in order to be safely built and this is achieved through the efforts of geotechnical engineers. They design foundations and they also assess the risk from natural hazards to humans, property and environment.
Question 1: a) Please briefly describe the unique features of geomaterial (i.e.,
soil and rock).
Geomaterials are materials available in the environment that are basically used in construction. These include rocks and soil. The unique features of these materials enable man to make wonders since the very beginning of humanity. Among these materials, soil is the most frequently studied. The properties to be consider are, bulk density, porosity, permeability, consolidation of state, shear strength, plasticity (of clay and silt soils), and the expansion properties when absorbing water are the properties of soil relevant to geotechnical engineering.
Bulk density is the mass of the material’s particle divided by the volume they occupy. Porosity is proportional to the particle size—as the particle size increases, the porosity of the material also increases. Permeability is the material’s ability to transmit fluids. Consolidation of state is a geological process that makes soil volume decrease upon applying compression stress over a period of time. Shear strength describes the strength of materials to resist deformation caused by shear stress. Plasticity of a material refers to its non-reversible change of shape when a force is applied to it. Soil expansion properties are caused by their absorbing property that is also dependent on the particle size. These properties are always being considered in the usage of soil and rocks.
Question 1: b) What are the fundamental goals for site investigation? Please
briefly describe 4 different general approaches for site investigation and provide 2 examples for each general approach.
The fundamental goals for site investigation are, to design all ground conditions, to establish geotechnical design parameters, and assess constructability issues. All the goals must adhere to the cost efficient system.
There are four different approaches for site investigation. These are surface exploration, subsurface exploration, geophysical exploration and soil sampling. In surface exploration, the engineer observes the physical conditions of the site by taking geologic maps, doing geophysical methods, walking around the site, or by any means available. In subsurface exploration, the engineer determines the physical properties of the soils and rock through test pits, trenching, borings, and cone penetration tests. In geophysical exploration, the engineer investigates through seismic waves, electromagnets, etc. In soil sampling, the engineer tests for structural properties of the soil to be used to approximate the properties of the soil in the site.
Question 1: c) Please briefly describe the procedure of standard penetration test (SPT).
The procedures for a standard penetration test (SPT) are, utilization of 140 pounds or 63.5 kg of weight, and a slip or guide pipe for a length of 30 in for free fall. A donut hammer is being hanged in a rope-pullet system. The rope is being pulled and free fall of the donut hammer will force the drill rode that is in contact in the surface of the ground.
The number of blows required to drive the tube 300mm (final penetration of tube) into the ground is recorded as the standard penetration resistance of the ground. The blow count is the indicator of the density of the ground and is used in geotechnical calculations.
Question 2: a) what are the design requirements for foundation engineering? Which
factors are determined during foundation engineering design?
In foundation engineering, there are design requirements to be considered like the Ultimate Limit State (ULS), Serviceability Limit State (SLS), economics and constructability. ULS is considered to find the ultimate stress and to design the building that can stand the ultimate stress. SLS will lessen the structural deformation, tilting of the structures. Economics is important in any engineering projects. In geotechnical engineering, the right chooses of selecting of materials to be used. Constructability will help the workers and engineers to construct a structure easily.
The four factors to be determined during a foundation engineering design are the location, size and shape, structural details and construction details. In a location, there is a need to determine the minimum depth to avoid frost, scour. Increasing the depth will also increase the bearing capacity. The location to be used must free from adjacent structure that have defects in the construction. Size and shape depends on strength and orientation of the loads, bearing capacity and settlement, and the space. In the structural details, loads and other stress are being considered and the structure type and their respective characteristics (steel. Cast iron, concrete, masonry, etc.). Construction details discuss the information on how to build the foundation, is it necessary for soil treatment, will excavation wet or dry.
b) Please draw a general model for a foundation resisting vertical downward loading and comment on the different response occurred in shallow and deep foundation.
Fig. 1. A general model for a foundation that resists vertical downward loading.
The response on deep and shallow foundations is different. In deep foundation,
piles foundations are used and in shallow foundation, pad, strip and raft foundation are
used. Piles experience more vertical load than pad, strip and raft and piles usually uses
reinforcement to increase the resistance to load.
Question 2: c) Please list 3 types of shallow foundation and briefly describe the
difference among them.
Thee three types of shallow foundation are pad, strip and raft foundation. Pad foundation support structural column and they are provided by steel grillage to support heavier column. This foundation can be Simple Square, rectangular or square. Strip foundations are used when rows of columns are used in a structure. It is advisable to use this column because it saves money when there are continuous rows which are almost adjacent. And lastly is the raft foundation is used when the soil has low bearing capacity and the columns are close to each other.
Question 2: d) Please provide 4 examples of large-displacement pile, 2 examples of small-displacement pile, and 4 examples of replacement. Among the examples you provide, please pick one example for each category of pile (i.e., large-displacement, small-displacement, and replacement pile) and briefly describe their construction process.
Examples of large displacement pile are the pre-cast reinforced concrete piles, pre-cast pre–stressed spun concrete piles, closed-ended steel tubular piles and driven cast-in-place concrete piles. The construction process of a pre-cast reinforced concrete pile is characterize as, commonly in square section from 250 mm to 450 mm. the maximum allowable axial load is 1000 kN. The length can be increase by splicing methods, welding of steel end plates or the use of epoxy mortar with dowels.
Examples of small displacement pile are open-ended steel tubular piles and rolled steel sections as H-piles. H-piles have better drivability than concrete pile, can be driven to greater depth, and weak in deflection.
The four examples of replacement piles are continuous flight auger pile, large-diameter bored pile, barrette, and hand-dug caisson. In a C FA pile, bore are formed using a flight auger and concrete or grout pumped in through hollow stem. The diameter ranges from 300 mm to 700 mm. the length is less than 30 m.
Question 2: e) Please list 5 advantages and 5 disadvantages for large-displacement pile.
Using large-displacement piles have advantages and disadvantages. Here are the five advantages of large-displacement piles: (a) material of preformed section can be inspected before driving; (b) steel piles and driven cast-in-place concrete piles are adaptable to variable driving lengths; (c) installation is generally unaffected by groundwater condition; (d) soil disposal is not necessary and (e) driving records may be correlated with insitu tests or borehole data. The disadvantages of large-displacement piles are: (a) pile section may be damaged during driving; (b) founding soil cannot be inspected to confirm the ground conditions as interpreted from the ground investigation data; (c) ground displacement may cause movement of, or damage to, adjacent piles, structures, slopes or unity installations; (d) noise may prove unacceptable in a built-up environment; and, (e) vibration may prove unacceptable due to presence of sensitive structures, utility installations or machinery nearby.
Question 3: a) what are the potential problems during deep excavation and how to
deal with them?
During deep excavation, the engineer faces many problems including health and safety, mining strategies, layouts and support systems. Dealing with these problems is not an easy task. Geotechnical investigation is important as far as the safety of the workplace and health of the workers is concerned.
Question 3: b) Please list 4 different types of excavation support wall. Among the types you list, please pick one and briefly describe its construction process.
Support walls are necessary to ensure safety of the workplace. There are four different types of excavation support wall: soldier pile wall, tangent/secant wall, guide wall and diaphragm wall. The constructions of a soldier pile wall are, the soldier piles are installed by driving or by concreting them within pre-drilled holes, the excavation is performed from top down, installation is lagging, and the installation for struts.
Question 3: c) Please briefly describe 3 different de-watering methods.
The de-watering method are of three types, these are open sump (inside excavation), Wellpoint system (outside excavation) and bored well (outside excavation). The open sump cut a small ditch around the bottom of excavation and leads the water flow to a sump that is below the general level of excavation, then pump out the water from the sump. Wellpoint system uses a number of filter wells to collect water followed by suction pump on the ground to withdraw the water. Bored well uses submersible pump in the bottom of bored well to pump out the water.
Question 4: a) What are the landslide triggering mechanism and how to mitigate
landslide hazards? (c) Please list 5 types of slope surface protection.
One of the hazards of deep excavation is landslide. Some of the factors which trigger landslides are intense rainfall, rapid snowmelt, water-level change, volcanic eruption and earthquake shaking. To mitigate landslide hazards is to use drainage, removal (cut the slope), restraint (retaining wall) and relocation.
The five types of slope surface protection are grass, trees and shrubs, chunam, shotcrete and stone pitching.
Question 4: b) Please list 6 different types of retaining wall. Among the types of retaining wall you list, please pick one and briefly describe its construction process.
Mass concrete, reinforced concrete cantilever, modular gravity, crib, gabion and soil nailing are different types of retaining walls. Gabion walls are composed of rows and tiers of orthogonal wire cages or baskets filled with rock fragments and tied together. A batter is usually provided for walls higher than 3 m to improve stability.
Question 5: a) Please briefly describe 3 general approaches for ground improvement
and provide 2 examples for each general approach.
Ground improvements can be done by using two different approaches: improvement with or without addition material. The former can achieve densification by means of vibration for granular soils and use of drainage system for cohesive soils. The latter is can achieve densification by means of using inclusion material (i.e. rocks, stone) and adding bacteria into the soil.
B Please briefly describe the construction process of jet grouting
and list 3 advantages and 3 disadvantages.
When a ground cannot be improved by classic processes, jet grouting is used. Jet grouting is a soil global treatment. It is done by jetting a high pressure cement grout. As the jet of high pressure cement grout is applied, the ground is destructed and its properties are changed into a soil concrete with desired mechanical and physical properties. Advantages to the jet grouting method include: (i) higher compressive soil strengths in comparison compared to other ground improvement methods, (ii) more uniform soil treatment, (iii) capability to improve soil areas, and (iv) definitive quantity and scheduling. Disadvantages include: (i) sensitivity to geologic conditions, (ii) doubtful grouting in limestone and highly transmissive strata, and (iii) limited experience contractors.
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