Geotechnical Structure For Basement Car Park
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The aim of this design/investigate project is to design a geotechnical structure for a basement car park. The objective of the project is to maximize the parking area available by designing a permanent retaining structure for a basement car park during construction excavation.
The various types of retaining structure will be compared and considered first by logical and engineering point of view which includes several aspect includes durability, cost, sustainability and environmental impact.
For design purposes, BS EN 1997 -1 :2004 Eurocode 7 is used to design the main retaining structure and temporary works necessary. The design considered Ultimate Limit State of a structure which takes structure stability into accounts.
Chapter 1 - Introduction
A hotel chain wishes to use land previously serving as a car park for a new accommodation block. Car parking space to service this is to be constructed beneath the new structure. Therefore, a geotechnical retaining structure needed to be design to support the soil during construction and after construction. A plan view and elevation view of the site is given and also list of requirement for this project are given too.
Design of the main retaining walls
Design of any temporary work necessary
Detail of the Project
This is an urban 'Brown Field' site. The previous use was known to be low rise domestic structures followed by conversion to a car parking space. The site is flanked on three sides by public highway and a desk study reveals services as indicated in (Figure 1.1) Site Plan. Extensive site investigation has been undertaken in the past. The interpreted geological profile is indicated in Figure 1 too. The interpreted geotechnical design parameters are summarized in Table 1.1.
The proposed scheme is shown in plan and sectional elevation in Figure 1. The new extension is to be developed on the existing car parking area and includes an extended basement as indicated.
One of the key constraints concerns the noise and vibration limits imposed. The project requirement is to keep the existing hotel buildings around the proposed structure in full operation throughout the construction period. The limits proposed by the local authority are given in Table 1.2 below.
It is also clear that the basement excavation will affect the rafted foundation to the original hotel wing. To ensure that the serviceability of this structure is assured it has been deemed necessary to limit the lateral deflections of the new basement walls at 20 mm into the excavation.
The construction site is very close to a public sewer which runs in the highway footpath near one site boundary. Therefore, this aspect had been considered in the final decision for choosing types of retaining structure.
Construction Time Rates And Costs
As in many construction projects the speed of construction influences overall cost. In this case it is essential that the new hotel is operational in the minimum possible of time. Notional construction costs and time rates are indicated in Table 3. These figures have been given for the purposes of the project. The overhead rate for possession of the site, fixed costs for establishment and use of site area are shown in the Table 3 too. The construction sequences is to be assumed that activities above and below ground can be run in parallel but above and below ground activities are sequential.
Chapter 2 - Site/Soil Profile
2.1 Soil Description and Classification
To understand the soil profile and the behavior of every single type of soil is an important step for a geotechnical engineer before starting any design works. In general, soil is kind of mineral particles formed by the weathering of rocks which weakly cemented or uncemented . The void space between the particles contain water and/or air . Weak cementation are due to carbonates or oxides precipitated between the particles or due to organic matter .
There is several type of soil. Every type of soil is classified in different categories by their particle size. The three main type of soil are Sands/Gravel, Silts, and Clays. Particle sizes in soils can vary from over 100mm to less than 0.001mm. The particle size distribution of a coarse-grained soil is to be determined by the method of sieving . The typical size of soil ranges is shown in Figure 2.1 below.figure 2.1.jpg
Basically, the terms 'clay' , 'silt' , 'sand' or 'gravel' are used to differ the sizes of soil and type of soils. Two or more size usually consists in a graded mixture of particles . For example, it is not necessarily all clay size particles are clay mineral particles because clay normally consist of particles in both the clay size and silt size ranges where clay is type of soil possessing cohesion and plasticity .
In general, a cohesive soil is said that if the particles adhere after wetting and subsequent drying and if significant force is then required to crumble the soil .
2.2 Borehole Data
A set of borehole data (Figure 2.4) is given for design purposes.Figure 2.4.jpg
From the borehole data given shows that water table on the site is 1m below the ground level. From top level to 3m below is a kind of coarse gravel. It is then followed by soft to firm grey brown slightly sandy clay (alluvium) and mixture of soft brown very silty clay down to 6m below ground level. From 6m below ground level to 7.5m, the soil is covered by loose brown clayey silt. It is then followed by loose to medium dense red brown silty clayey sand with a 4.5m depth. From 12m to 16m below ground level the soil is covered by large amount of gravel.
A simplify table of soil in the site is shown in table 2.1 below for better and clearler understanding.
0 - 0.2
Tarmacadam surfacing ( MADE GROUND)
0.2 - 3.0
Dark grey angular to sub-rounded coarse gravel with ash, concrete and rubble fragments (FILL)
3.0 - 6.0
Soft to firm grey brown slightly sandy CLAY with some organic matter (alluvium)
Very soft to soft brown very silty CLAY
6.0 - 7.50
Loose brown clayey SILT (alluvium)
7.50 - 12.0
Loose to medium dense red-brown silty clayey SAND with occasional sub-angular fine to medium gravel of sandstone (alluvium)
12.0 - 16.0
Loose to medium dense, becoming dense red grey silty very sandy, sub rounded GRAVEL (alluvium)
16.0 - 22.95
Reddy brown and grey green weathered (iii - iv) weak MUDSTONE
Chapter 3 Types of Retaining Structure
There are several factors that influence the difficulty of basement design and construction. These factors normally are existing problems on the site and cannot be easily changed. Engineers somehow need to go for different option when designing structure to overcome the constraints. For example, the location of the proposed structure, proposed use of the structure, groundwater, the site surrounding existing structure and services. The type of basement wall will be then selected to support soils and groundwater of the basement and also to design as economically as possible.
The walling or sheeting selected for this project is to provide temporary soil support for permanent substructure construction, or it may also serve as soil retention. The walling or sheeting will be selected after comparison in terms of cost and time, constructability and etc. Several methods include the following.
- Plate and anchor wall
- King post wall
- Contigous bored piling
- Secant piling
- Steel sheet piling
- Diaphragm walls
- Reinforced concrete cast in situ
- Reinforced concrete precast
- Soldier piling
3.1 Brief Introduction for each Options
i) Plate and Anchor Wall by underpining
The total excavation depth of basement work is typically fall in the range 8 to 12m and also the ground conditions are dry and able to support 1.5 - 2m face deep. The anchored plate method is an economical temporary wall support. Pre-grouting is to be used in granular soils where the soils were unable to stand unsupported to this modest depth . Figure 4.2.jpg
ii ) King Post Wall
King Post method is usually popular for two following factor which is cheapness of materials by using timber and economy method of boring by using power augers. This method require boring holes on wall line at 2 - 3m centre depending on soil strength, depth of excavation and surcharges loads. The hole is then placed with vertical beam and to be concreted with lean mix concrete at the base of each joist below final formation level . King post wall usually used as a temporary soil support and to be used in dry or dewatered soils. Vertical settlements of wall is one of the disadvantages where failure of vertical force transferring to the base of pile.
iii ) Contigous Bored Pile Wall
Bored pile wall is usually used as an economic and efficient method for retaining structure. This techniques is very suitable for deep basements excavation and underground structure where working space is limited. This method prevent large amount of soil excavation and also help to control ground movements. Piles are usually drills into ground by using continuous flight auger (CFA) with a certain gap distance between piles. A maximum length of piles is usually around 20m depending on ground condition. Contiguous bored pile wall is not suitable for site with high water level due to the gaps between piles.
Advantages of contiguous pile walls are :
Comparative low cost and speed of construction
Low level of sound pollution ( low level of vibration)
Pile can be drill in limited spaces
Has the ability to minimize the distance between bored pile wall and existing wall for small excavation depthcontiguous bored pile wall.jpg
iv ) Secant Piles
Disadvantages of contiguous bored pile are overcome by using secant piles where interlocking method is introduced. Secant pile walls are constructed by concreting primary (female) piles first then secondary (piles) are bored through female piles before concrete reach full strength . By this the piles forms overlapping between each other.
Advantages : a)Can be installed in hard ground (cobbles /boulders)
b)Low noise pollution
c) Better wall stiffness compare to sheet piles
v ) Sheet Pile Wall
Sheet Pile wall are made up from a group of piles that interlock each other and is driven into the soil. Most sheet pile wall nowadays is using steel sheet which fabricated in factory. The use of sheet pile for temporary soil support for basement at urban area is not that popular where noise is the main constraint. Sheet pile may be installed using hydraulic can reduce the noise pollution. Sheet pile wall can be design as cantilever wall or anchored wall depends on the basement depth and soil condition that vary.
Advantages : a) High resistance to driving stresses.
b) Sheet can be reuse
c) Easy to install
Disadvantages : a) Sheet pile can hardly be use as permanent structure.
b) Installation of piles are hard where soil contain boulders and cobblers
c) Noise pollution (High vibration)
vi ) Diaphragm walls
Diaphragm walls are reinforced concrete wall constructed in slurry supported by machine digging a trench in panels of certain length. This slurry can be bentonite slurry where has thixotropic properties . The wall is first constructed in short panels length, by installing reinforced cages and concreting, then later intermediate panels are excavated to complete the whole wall. There is 3 type of diaphragm walls in use in industry, which is cast in-situ diaphragm wall, precast reinforced diaphragm wall and post-tensioned diaphragm wall.
Advantages : a) Allow effective transfer of vertical load from the building to subsoil
b) Minimum noise and vibration disturbance
c) Allow construction on limited site area.
3.2 Comparison of each option
A table (table 3.2) of matrix below is to compare the advantages and disadvantages of several retaining structure. This comparison results will shows the most suitable retaining wall to design and construct for this project.
Durability is not usually a problem for a temporary wall depending on the soil condition. But when wall is to be design as a permanent structure, the wall should satisfy the durability requirement where wall should reach design life. For example, durability requirement for concrete wall depends on the design life, cement content, water cement ratio, cover of reinforcement and also quality of workmanship.
Rigidity means a structure property that does not bend under an applied force in vertical or horizontal load . Different type of retaining walls could sustain different loading. Some walls are good in resisting vertical loading and does not bend but some walls can only sustain horizontal force. For example, reinforced diaphragm wall is much more rigid than a sheet pile wall. Reinforced concrete diaphragm wall can be design as a permanent structure that carry load from superstructure above and does not bend in any way. Comparing 6 types of retaining structure above, Diaphragm walls, contiguous pile wall and secant pile walls are three best on rigidity.
Constructability on Site
Constructability of a structure means a structure to be constructed on site easily from start to finish by fulfilling client's requirement. Constructability also means ease of construction. A constructability review must be done before starting any construction process to prevent error, construction delay or cost overrun . The space to construct the proposed structure is limited. Figure 1.1 shows that a existing sewer pipe line is 1m beside the proposed structure on the right and also a existing structure 2m far from the proposed building on the left. One of the project requirement is to minimize the sound of construction in urban area. Choice of excavation is limited to prevent any damage to the existing properties. Trench excavation is ideal for this project. Therefore, constructing diaphragm wall is the best solution where diaphragm wall can be constructed in limited space by using trench excavation, low noise produce and machinery is not big.
The soil profile is needed to take into consideration when designing retaining structure. Some structure's construction is hardly to process when the soil contains cobbles or boulders. Most of the structure is suitable to be constructs on this project site because of the clayey soil from ground level to a minimum depth of 12m. Only when initial design for retaining structure with required depth over 12m into gravel layer, retaining structure like diaphragm wall is not that suitable due to the stability of wall.
Ground water on site are mainly from rainfall or groundwater flow through soil from rivers and seas . By reducing the groundwater within the excavation depth and structure depth by dewatering process will increase the strength of soil as the pore water pressure is reduced. Groundwater control is crucial to prevent any leakage of water into the basement car park or cause instability of structure, for example, ground heave. Retaining wall usually acts as a groundwater cut off. Several alternative ways of groundwater cut-off are:
To lower the groundwater by temporary dewatering process where ground movement is to be considered.
Temporary sump pumping is to be done is ground is sufficiently impermeable
Excavation is to be done under water and so permanent wall is to be construct under water by tremie concreting techniques.
Diaphragm wall serves as a good water barrier compare to sheet pile or secant piles where sheet pile might corrode and water will flow through the gap between secant pile.
Depth of wall
Designer needs to consider the depth of a retaining structure can be construct. Some structure is cheaper to be built in deeper depth compare to shallow depth. Table 3.1 shows the different type of retaining structure that can be construct up to the maximum height of wall in order for the structure to stay stable and safe.
Storage of Materials
Proper storage of raw materials is very important for a construction. Raw materials like reinforcement cages, cement, sand, and etc. needed to be on site on time to prevent any delay of construction. For example, steel sheet and reinforcement cages both are made in large size and needed large space to store up. Therefore, it is worth to consider this problem when choosing a retaining structure.
Environmental impact is one designer to be consider when structure is construct in urban area. The choice of wall can affect the environmental during construction, during in use and demolition . Three main causes to environmental impacts:-
Noise and vibration when boring pile. (sheet piling)
Number of vehicle used.
Use of sustainable materials (Guidewall construction for diaphragm wall)
Effects on groundwater around the wall.
End of life
Ease of removal
Ability of material to be reused
3.3 Final Decision
Diaphragm wall is to be used for my design project.
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Chapter 4 Diaphragm Wall
4.1 Preplanning and Design
For designing purposes and construction of diaphragm walls, a number of item require to be considered in preplanning and design of diaphragm wall.
The sequence of excavation from ground level to the basement walls is to be well planned to minimize rig movement and to avoid changing places and moving of pipework from panel to panel of panel excavation. Soil dump truck, slurry removal vehicle, cranes and concrete mixing trucks, and to allow curing of concrete in completed panels are all parts of construction and excavation sequence that needed to be well planned.
Guide Trench Construction
The successful of trench excavation for diaphragm wall depends on the temporary guide wall. The guide walls must be design and construct to be robust to avoid any movement due to extreme loads from excavation rig service cranes or placement of reinforcement cages and reaction from stop end jacking systems. In some construction, reuseable precast concrete guide wall had been used and be interlocked each other by bolted to ensure the same standard of rigidity as in-situ cast concrete wall .
Diaphragm wall is to be constructed by a panel trench excavation first. The panel length typically will vary from a minimum of one grab bite (trench excavation machine grab width) to a multiple of grab bites which will extend to 7m. A grab bites vary between 2.3 and 2.8m depending on machine used. The panel length include two stop ends for the primary panels (Stop ends will be discussed on following pages). Secondary panels are those panels dug between two concreted panels. The panel length is limited to a certain length, and therefore panel volume, this is to ensure that sufficient concrete can be fill up the whole panel within concreting period in a day. This takes maximum daily working hours and concrete supply into account. Panel size more depends on designer and contractor decision.
Wall-Slab Construction Joints
Joints between basement floors slabs and wall is to be design carefully because the joints can transmit vertical shear and bending moment which could cause instability of structure and basement. Bend out bars and Threaded-end couplers are both used in the joints.
The depth of diaphragm wall has led to the size of reinforcement cages. These cages are usually fix off-site and delivered to site when is needed. The maximum length of cages is restricted because of transportation of long and large cages.
Slot for Tremie Tubes
A tremie tube is used to ensure concrete is placed in correct position and that no separation of aggregate occurs during concreting pouring concrete from top to bottom of walls. Therefore, reinforcement cage is to be designed to allow sufficient access for tremie tubes. For some construction of diaphragm with large panels, two tremie is to be used to maintain the concreting rate of 60 to 80m3 per hour.
4.2 Construction Sequence
End Of Construction
Top Down Basement Construction
Diaphragm Wall Construction
4.3 Work to be Done
4.3.1 Site Preparation
Basement and retaining wall construction methods involve a high degree of mechanization. A clear working space give maximum mobility for machinery, materials and workers, hence optimize the working speeds increasing construction period. Therefore, several steps are needed to be done before any construction work progress. For example, temporary road should be provided to achieve a rapid tempo of construction in wet or dry weather.
Local authorities are highly concern on the traffic management especially in urban area. Construction in central of urban area could cause serious traffic congestion due to slow moving construction vehicles and parts of road are occupied by machinery. A slightly highway direction will be changed on A marked in figure 4.2 below to provide access for construction vehicle into the site. On road marked A in figure 4.2, the road is to be assumed that is a typical single lane carriageway. Therefore, the road length will be slightly reduced nearby the construction site. An alternative road for vehicle towards junction is to be proposed to local authorities to prevent any traffic congestion. Clear barricades and road sign will be provided along the road closure.
Location of Underground Services
Site preparation in urban area includes tracing and clear marking of existing services includes underground telephone, power cables, water and sewer pipe, gas pipe, etc.
Underground services is to be assumed to present in any circumstances. Trench excavation is more likely to encounter underground service in the face of excavation parallel to the line of excavation . Many serious accidents have been caused by men or machines when underground services are struck, penetrated or during excavation. Electric shock may result from striking electricity cables during excavation.
On this project, it is clearly shown on figure 1.1 that a 1200mm diameter concrete sewer pipeline is just 1m away from the side of proposed structure. A sewer pipeline bursting could cause contamination of the ground and odour smell to nearby citizens. Many sewer pipes are under high pressure too.
As a solution, first, a confirmation of sewer pipe location is to be done on site. Once the records are obtained, it will be kept on site and be accessible to workers. Furthermore, construction of diaphragm wall uses trench excavation techniques, which highly reduce the chance of striking the sewer pipe.
Any other services includes telecommunication cables, gas pipe and electric pipe which are not shown in figure 1.1 given will be examine on site before excavation.
Building located around the site are needed to be protected from damage and dirt-staining. Cleaning and maintaining existing building in the end of construction can be costly. In figure 1.1 shows that there is existing 3 storey with raft foundation building 2m away from basement wall. Before any excavation start, careful inspection is to be done to the existing building to determine whether there are any existing cracks due to settlement or any damage on external wall of building. Cracks and damage is to be recorded down and photographed as a proof to prevent any claims from property owner.
The most common overhead obstruction is high tension electricity cables nearby the site. Most construction vehicle are high. There is a danger when tall vehicle pass by those overhead cables and cause unwanted accident. Therefore, a clearance is to be done between the overhead cables and ground. For example, a typical "goal post" protection will be erect along the entry to the site. Figure 4.3 below shows typical "goal post" protection. figure 4.3.jpg
It is important to taking care about public safety. Any pedestrian is not allowed to enter construction site. A warning sign is to be displayed around the boundary site and barriers is to be set up along perimeter of construction.
4.3.2 Diaphragm Wall Construction
Construction of diaphragm wall uses trench excavation supported by slurry. The slurry is typically bentonite and water.
Diaphragm walls are constructed in the following steps :
1) Pretrenching to remove obstruction
2) Guidewall construction
3) Trench excavation (panel excavation)
4) Endstop placement
5) Panel desanding
6) Reinforcement cages placement
7) Concreting work (Tremie technique)
8) End Stop removal
9) Excavation of Intermediate Panel
10) Reinforcement cages placement
11) Concreting work for remaining panels (Tremie technique)
12) End of Diaphragm wall construction
Pretrenching to remove obstruction
Pretrenching is a process to remove soil by open excavation to a certain depth, typically 1-2 m depth for guidewall construction. It is also a purpose for removing shallow obstruction and provide stable support for the guidewall.
Guide wall is to be constructed after pre-trenching process. There are several purpose of constructing guidewall, these include:
To prevent the collapsing of soil near trench excavation surface.
As a template for wall excavation and panel layout
To provide a temporary supports for reinforcement cage. (by holding down the cage during concreting work)
To provide support for end-stop joint. (restrain end-stop)
To support Tremie Pipe
To provide a reference elevation for inserting props, slabs, etc.
For this project, the guidewall is to be constructed with reinforce concrete and be made from grade M20 grade reinforced concrete. The distance between both guidewall will be thickness of diaphragm wall plus a tolerance of 50mm. The dimension of guidewall (one side) will be 300mm(w) x 1000mm(d).
Trench Excavation (Panel excavation)
Construction of diaphragm wall uses trench excavation method which produce a vertical strip in soil that can collapse easily. Special excavation machinery are used to excavate the soil. Several type of machinery is used in construction field nowadays. These machinery can be cable hug or Kelly mounted and the digging mechanics can be cable or hydraulic operated. figure 4.7 2.pdf
The excavation is to be excavated in "panels". The panel length varies typically from a minimum of one grab bite (trench excavation machine grab width) to a multiple of grab bites which will extend to 7m. A grab bites vary between 2.3 and 3m depending on machine used. figure 4.8.jpg
The trench excavated is to be supported by bentonite slurry. Bentonite is basically clay of montmorillonite group, and when added with water it forms an impervious slurry with large viscosity. The slurry will produce large lateral pressure to retain the vertical soil. In case of granular soils, the bentonite slurry will penetrates into the sides under positive pressure and forms jelly. When bentonite slurry is fills in impervious clay, it will not penetrate into the soil but form a layer of thin film to gives strength supporting vertical soils. The bentonite slurry is to be placed continuously into the trench throughout excavation.
For this project, Kelly Grab is to be used for excavation. The depth and width of excavation will be discussed in following chapters. The panel length and bentonite slurry density is to be designed and results will be shown in following chapter too.
End stop Placement
Endstops are placed in both panel fronts to provide the concrete at each vertical edge of panels with a predetermined shape. The shape of stop ends can be a pipe or special keyway end stops. End stop can be place in to be permanent or temporary.
For this project, a temporary cylinder end stop is to be used. The end stops will be removed by vertical extraction shortly after the concrete has been poured. Somehow, a delay of few hours is allowed in order to enable the concrete to gain some early strength and able to stay vertical. The timing and removal of end stops will be judge by the site contractor and to be carefully observed. If end stops is extract out before the concrete is stable (gained sufficient strength to stay vertical), there is a risk that the concrete will slump.
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