Foundation Prep Work
ite Investigation
Site investigations should always be done before any design work, in order to obtain geotechnical information, parameters, and data about the site. Site investigation reports usually include location of the proposed structure, size of the proposed structure, site surveys, possible loads, topography, history of the site, overall stability of site, material composition and data of site (ex. Standard penetration test counts), soil and ground conditions of the site, surface drainage conditions, and moisture content. Site investigation reports should also include an appraisal section of whether there are any adjacent existing foundations, due to their effects on the proposed structure.
For shallow foundations, tests that should be done include determining maximum shear capacity for bearing capacity and determining settlements and moisture content. The structure should be investigated to a depth of 1.5 times the width of the footing area or building for shallow foundations.
For deep foundations, tests that should be done include using boreholes to determine piles’ suitability and determining bearing capacity and settlements. For deep foundations, investigation should continue down 3m beyond top of bedrock in order to determine whether the thickness of the rock layer is sufficient.
The following information should be determined for the corresponding structural activities.
Site investigations should always be done before any design work, in order to obtain geotechnical information, parameters, and data about the site. Site investigation reports usually include location of the proposed structure, size of the proposed structure, site surveys, possible loads, topography, history of the site, overall stability of site, material composition and data of site (ex. Standard penetration test counts), soil and ground conditions of the site, surface drainage conditions, and moisture content. Site investigation reports should also include an appraisal section of whether there are any adjacent existing foundations, due to their effects on the proposed structure.
For shallow foundations, tests that should be done include determining maximum shear capacity for bearing capacity and determining settlements and moisture content. The structure should be investigated to a depth of 1.5 times the width of the footing area or building for shallow foundations.
For deep foundations, tests that should be done include using boreholes to determine piles’ suitability and determining bearing capacity and settlements. For deep foundations, investigation should continue down 3m beyond top of bedrock in order to determine whether the thickness of the rock layer is sufficient.
The following information should be determined for the corresponding structural activities.
| Soil | Clay | |
|---|---|---|
| Excavation | - Permeability - Stability of excavation - Shear strength of retaining structures |
- Stability of excavation - Shear strength of retaining structures - Strength of clay and possibility of reusing material as fill |
| Shallow footings | - Shear strength of footing for bearing capacity - Loading tests for settlements |
- Shear strength of footing for bearing capacity - Consolidation for settlements - Moisture content and plasticity tests |
| Deep foundations | - Test piles for bearing capacity and settlements - Boreholes |
- Long-term test pile for bearing capacity and settlement - Boreholes - Shear strength of clay |
Contamination Investigation
Contamination of land can occur from chemical, biological, or physical pollutants from land use and pollutants migrating from adjacent land uses by water or air. They can be combustible, toxic, metallic, or chemical. These pollutants (solid, gas, liquid) can cause harm to living organisms, waters, or property.
In order to alleviate contamination, disposal or treatment of contaminated soil should be done during excavation and replacing soil with clean fill or decreasing concentrated contaminated soil by blending it with clean soil. The method of remediation depends on the characteristic of the contaminant and the target. One can remove the contamination completely, decrease contamination concentrations or risk to below the required level, or remove the pathway (the path in which contaminants undertake to reach the site).
Contamination of land can occur from chemical, biological, or physical pollutants from land use and pollutants migrating from adjacent land uses by water or air. They can be combustible, toxic, metallic, or chemical. These pollutants (solid, gas, liquid) can cause harm to living organisms, waters, or property.
In order to alleviate contamination, disposal or treatment of contaminated soil should be done during excavation and replacing soil with clean fill or decreasing concentrated contaminated soil by blending it with clean soil. The method of remediation depends on the characteristic of the contaminant and the target. One can remove the contamination completely, decrease contamination concentrations or risk to below the required level, or remove the pathway (the path in which contaminants undertake to reach the site).
- Excavation
- Contaminated soil can be excavated to a disposal or treatment plant. The site will then be replaced with clean fill material. This method is expensive but expenses can be cut by excavating only a certain amount of excavated material down to a cut off level and then filling the site back in with clean fill.
- Blending
- Contaminated soil is mixed with clean material. This can decrease the contamination concentration, until it reaches an acceptable level. This method is more cost effective since a certain amount of soil needs only be replaced by clean material.
- Isolation
- Isolation techniques include sheeting piling, barriers, slurry trenches and jet grouting.
Foundation Types
1. Shallow Foundations
Types: Strips, Pads, Rafts
Shallow Foundations are common in construction and are inexpensive due to their lightweight properties. It is usually not required to employ heavy equipment for constructing shallow foundations.
Situations where they should be used in:
Shallow foundations are used when the strata at shallow depths is strong and has enough bearing capacity to support the above building loads, with respect to reasonable differential settlements. The strong strata must be consistent at the shallow depth, so that settlement can be relatively small.
Situations where they can be used in with ground improvement techniques:
2. Deep Foundations
Types: Piles (Click to learn the different types of piles)
Deep foundations are heavy foundation structures used to transmit heavy loads created from loading and self-weight of the superstructure into the soils of the deep stratum. They can prevent large differential settlements that will affect the strength and robustness of the building. Information that need to be included during scheme design consist of:
Situations where they should be used in:
Deep foundations are used when the strong strata is located at a depth and piles, caissons, or piers are driven or bored to the strong strata so that structural columns or walls can transmit loads directly to the rock strata. Deep foundations are also used when the soils between the surface and the rock strata has high settlement rate, such as peat and silt. Loads are transmitted and resisted by end-bearing and skin friction.
If applied building loads are more than the allowable bearing capacity, ground improvement techniques should be used to strengthen the foundation or the soil strata. One method is the use of vibro replacement stone columns. This technique increases the bearing capacity and decreases soil settlement. Crushed coarse aggregates are to be placed by vibrators into areas of soil where bearing capacity needs to be increased. Not only do the aggregate replaces the soil that carries low bearing capacity, the aggregates induce additional pressure onto the soil and the vibrator increases the density of the soil by compaction of air pores in the soil, which increases the bearing capacity.
Types: Strips, Pads, Rafts
Shallow Foundations are common in construction and are inexpensive due to their lightweight properties. It is usually not required to employ heavy equipment for constructing shallow foundations.
Situations where they should be used in:
Shallow foundations are used when the strata at shallow depths is strong and has enough bearing capacity to support the above building loads, with respect to reasonable differential settlements. The strong strata must be consistent at the shallow depth, so that settlement can be relatively small.
- Rock or dense sand or gravel that goes towards great depths
- However, it should be noted that shallow foundations should not be designed below GWT and when there are great uplift conditions. The more moisture content is in he soil, the less resistance the foundation gives to the above loads.
- Firm clays at shallow depths lying on top of soft clays that go towards great depths
- Uniform firm clays when there are no trees around the proximity of the foundation structure and around temporary works and where loads can be distributed on the surface area of the foundation
Situations where they can be used in with ground improvement techniques:
- Loose sands and soft clays at great depths
- Ground improvement should be used because towards the greater depths, groundwater level changes. Also, vibration of equipment can cause overall and differential settlement.
- Peat
- Consider the acidity of soil when implementing ground improvement techniques
- Fill
- More appropriate if the fill is well compacted which will create a higher bearing capacity
- Stiff or high density sand or clay on top of soft clay lying over compact clays at great depths
- Sloping site where sloping effects should be considered and support, such as ground improvement and temporary works, should be used to construct the foundation
- Site with high GWT where dewatering techniques should be used
2. Deep Foundations
Types: Piles (Click to learn the different types of piles)
Deep foundations are heavy foundation structures used to transmit heavy loads created from loading and self-weight of the superstructure into the soils of the deep stratum. They can prevent large differential settlements that will affect the strength and robustness of the building. Information that need to be included during scheme design consist of:
- Number of piles
- Diameter of piles
- Thickness of pile caps
Situations where they should be used in:
Deep foundations are used when the strong strata is located at a depth and piles, caissons, or piers are driven or bored to the strong strata so that structural columns or walls can transmit loads directly to the rock strata. Deep foundations are also used when the soils between the surface and the rock strata has high settlement rate, such as peat and silt. Loads are transmitted and resisted by end-bearing and skin friction.
- Weak soil stratum or loose sand underneath strong or weak stratum
- Driving piles can also help to increase the density of sand and stiffness of clay
- Uniform stiff and firm clays, with trees at the proximity of the foundation
- Even though trees can affect the shrinkage and expansion of soil, trees will not affect the foundation negatively due to piling.
- Peat
- Consider drag on piles and acidicity of soil when implementing ground improvement techniques
- Suitable for high water tables and uplift conditions
- Deep foundations do not need as much dewatering as shallow foundations
- Bored piles should be cased to prevent water damage to piles
- Temporary works should be carefully designed for the stability of the foundation
- Stiff or high density sand or clay on top of soft clay lying over compact clays at great depth
- Soft clay on top to stiff clay at great depths
- Use deep bored piles with casing
- Proposed foundation is close to adjacent building
- Use sleeves with the piles to prevent surcharge or undue stress onto the adjacent existing foundation and building superstructure
- Sloping site where temporary works design and sloping effects should be considered
If applied building loads are more than the allowable bearing capacity, ground improvement techniques should be used to strengthen the foundation or the soil strata. One method is the use of vibro replacement stone columns. This technique increases the bearing capacity and decreases soil settlement. Crushed coarse aggregates are to be placed by vibrators into areas of soil where bearing capacity needs to be increased. Not only do the aggregate replaces the soil that carries low bearing capacity, the aggregates induce additional pressure onto the soil and the vibrator increases the density of the soil by compaction of air pores in the soil, which increases the bearing capacity.
Foundation: Design
Foundation design can be done using the following two methods.
Stress can be induced by the differential settlement between new and existing foundations, between different foudnation types, and unexpected ground conditions. Maximum allowable settlement for low rise buildings should be 25mm. And the allowable settlement to distance ratio should be less than 1:300. For tall buildings, differential settlement should govern where settlement should be limited to a maximum of L/150 for differential settlement (where L is the distance between columns or walls)
Groundwater must be controlled, during and after construction. This is because the change in groundwater can change the water table of the soil and affect the moisture content.
Typical soil properties
Undrained conditions are when pore water pressures within soil are supporting applied forces in the short term. Undrained conditions are often considered for fine grained soils. However, for granular soils above the water table, pore water pressures are not accounted for due to pore water draining quickly. Drained conditions will then be used for granular soils, as well as, for conditions where pore water have drained in the long term.
Typical allowable bearing capacity
- Using Elastic Theory, loads are assigned and applied onto the soil surrounding the foundation. The soil, with its particular Young’s Modulus and Poisson’s Ratio, is calculated for its deformation, which is dependent on the soil’s stiffness, due to a given pressure over a given area.
- Using Subgrade Reaction Method, a series of springs are assigned onto the soil. These independent springs, that simulates as soil mass with different stiffness properties, function as resistance to loads applied to the superstructure or foundation elements. This method is usually used for raft foundations.
Stress can be induced by the differential settlement between new and existing foundations, between different foudnation types, and unexpected ground conditions. Maximum allowable settlement for low rise buildings should be 25mm. And the allowable settlement to distance ratio should be less than 1:300. For tall buildings, differential settlement should govern where settlement should be limited to a maximum of L/150 for differential settlement (where L is the distance between columns or walls)
Groundwater must be controlled, during and after construction. This is because the change in groundwater can change the water table of the soil and affect the moisture content.
Typical soil properties
Undrained conditions are when pore water pressures within soil are supporting applied forces in the short term. Undrained conditions are often considered for fine grained soils. However, for granular soils above the water table, pore water pressures are not accounted for due to pore water draining quickly. Drained conditions will then be used for granular soils, as well as, for conditions where pore water have drained in the long term.
Typical allowable bearing capacity
| Description | Safe bearing capacity (kN/m^3) |
|---|---|
| Limestones and hardstones | Up to 3000 |
| Stiff clays | 150-300 |
| Firm clays | 75-150 |
| Soft clays and silts | <75 |
| Compact or gravel | 600 |
| Medium dense gravel and sandy gravel | 200-600 |
| Loose gravel and sandy gravel | <200 |
| Compact sand | 300+ |
| Medium dense sand | 100-300 |
| Loose sand | <100 |
| Firm organic | 20-40 |
Source: Structural Engineer's Pocket Book, 2nd Edition: British Standards Edition
Equations for allowable bearing for shallow foundations
Area of foundation = Applied loads/Allowable bearing pressure
Shallow foundations on undrained cohesive clay
qallowable = 2Cu (on undrained cohesive soil yf = 2.5)
Shallow foundations on drained gravel
qallowable = 10N Pad footing on dry soil (yf =3)
qallowable = 7N Strip footing on dry soil (yf =3)
qallowable = qallowable/2 when the foundation is on or below the water table
Source: Structural Engineer's Pocket Book, 2nd Edition: British Standards Edition
Equations for allowable bearing for piles
Concrete piles are made of cast insitu, prestressed, precast or reinforced concrete. Steel piles are needed when the foundation is required to have slender or lightweight piles. Even though steel piles require corrosion protection, they are good in supporting lateral forces.
Pile diameters generally range from 100mm to 2000mm and are 5m to 100m in length.
Typical pile capacity for bored piles = 300 to 1800 kN
Typical pile capacity for driven piles = 500 to 2000 kN
Typical pile capacity for minipiles = 50 to 500 kN
Piles should be spaced about 3 diameters apart (between pile faces). Pile capacities increases at deeper depths. However, for piles in granular soils, negative skin friction and pile capacities peak at around 110,000 kPa when pile capacities are taken at around 20 pile diameters deep.
For bored piles in granular soils, the bearing capacity is less than that of driven piles since there is loosening effect as a result of boring.
Other considerations
Negative skin friction is the additional drag force acted on the piles by surrounding cohesive soil as its soft layer consolidates and compresses when the piles are being driven and compressed to reach a firm strata layer below. Negative skin friction is also induced by the drag force of the soil weight on the piles.
NSF used for group of piles
Qskin friction = AH/Np
A = area of pile group
H = thickness of consolidation layer of soil
Np = number of piles in group
Pile caps assist in transmitting applied loads from the superstructure into the piles below. One constant pile cap depth should be used for one project, even with different piles with different diameters, in order to reduce cost and labor.
Equations for allowable bearing for shallow foundations
Area of foundation = Applied loads/Allowable bearing pressure
Shallow foundations on undrained cohesive clay
qallowable = 2Cu (on undrained cohesive soil yf = 2.5)
Shallow foundations on drained gravel
qallowable = 10N Pad footing on dry soil (yf =3)
qallowable = 7N Strip footing on dry soil (yf =3)
qallowable = qallowable/2 when the foundation is on or below the water table
Source: Structural Engineer's Pocket Book, 2nd Edition: British Standards Edition
Equations for allowable bearing for piles
Concrete piles are made of cast insitu, prestressed, precast or reinforced concrete. Steel piles are needed when the foundation is required to have slender or lightweight piles. Even though steel piles require corrosion protection, they are good in supporting lateral forces.
Pile diameters generally range from 100mm to 2000mm and are 5m to 100m in length.
Typical pile capacity for bored piles = 300 to 1800 kN
Typical pile capacity for driven piles = 500 to 2000 kN
Typical pile capacity for minipiles = 50 to 500 kN
Piles should be spaced about 3 diameters apart (between pile faces). Pile capacities increases at deeper depths. However, for piles in granular soils, negative skin friction and pile capacities peak at around 110,000 kPa when pile capacities are taken at around 20 pile diameters deep.
For bored piles in granular soils, the bearing capacity is less than that of driven piles since there is loosening effect as a result of boring.
Other considerations
Negative skin friction is the additional drag force acted on the piles by surrounding cohesive soil as its soft layer consolidates and compresses when the piles are being driven and compressed to reach a firm strata layer below. Negative skin friction is also induced by the drag force of the soil weight on the piles.
NSF used for group of piles
Qskin friction = AH/Np
A = area of pile group
H = thickness of consolidation layer of soil
Np = number of piles in group
Pile caps assist in transmitting applied loads from the superstructure into the piles below. One constant pile cap depth should be used for one project, even with different piles with different diameters, in order to reduce cost and labor.
| Pile diameter (mm) | 300 | 350 | 400 | 450 | 500 | 550 | 600 | 750 |
|---|---|---|---|---|---|---|---|---|
| Pile cap depth (mm) | 700 | 800 | 900 | 1000 | 1100 | 1200 | 1400 | 1600 |
Foundation Ground Conditions
The values and properties indicated in the table below can also be used instead of the bearing capaicty equations above. These values are given only as estimated values for preliminary analysis, for its respective soil materials. The values should only be used for scheme design calculations.
| Soil Material | SPT, N Value | Undrained shear strength, C (kN/m^2) | Assumed Bearing Capacity, qallow |
|---|---|---|---|
| Sand | N | 10N or 10N/2 if submerged | |
| Stiff Clay | C | 5.14 C/3 | |
| Rock | N | 10N or assumed 3000 kPa with FOS = 3 |
Foundation Types of Failure
Foundation can fail due to several cases.
- Overall and/or differential settlement are caused by excessive ground movement and deformation and can induce stresses between strips of foundation. These stresses can induce additional loads onto adjacent strips. Settlement can also lead to cracking and increasing groundwater table under the foundation. Settlement can also be separated into two parts, short-term and long-term. Short-term settlement accounts for foundation loads and soil mass. Long-term settlement accounts for time-dependent settlement effects and effects due to time dependent hydration of soil and loss of water pressure within the soil. To mitigate settlement effects, movement or connection joints can be provided between foundations portions, in order to reduce stress during movement or loading.
- When trees are located near the foundation, foundation failure is prone to occur due to movement of tree roots and the shrinkage and expansion of soils near tree roots due to changes in water level. Movement of roots are caused by the growth of trees. The shrinkage and expansion of soils are caused by trees absorbing water through their roots (shrinkage) and by rainwater absorbed by soil (expansion). Insulation and coatings should be used to control changes in moisture level.
- For shallow foundations, when the structural loads are high, this will induce cracking. Moreover, when there are concentrated loads, this will induce punching shear failure, as well as, cracking.
- The bearing capacity of the foundation is not sufficient to support the applied loads on the structure. This will create shear failure between strips of foundations and between the foundation and the surrounding soil.
- Soil creep can occur when there are changes in water levels or moisture content of the soil, due to adjacent vegetation absorbing the soils’ water. Insulation and coatings should be used to control changes in moisture level.
- Foundation failure can occur when the pile depth do not reach all the way below expansive soils that are affected by moisture content changes. Foundation settlement and heave can occur, since the pile cannot support the loads above.
- Flooding can occur, which affects groundwater movement. Insulation and coatings should be used to control changes in moisture level.
- Corrosion in steel can occur in steel reinforcement in reinforced concrete piles and in steel piles. This is due to oxidation and degradation when the piles are in contact with groundwater level, which consist of high levels of reactive ions.
- Soil dissolution due to contact with limestone can occur, mostly in karst terrains. Carbonic acid is formed, which creates cavities within the strata. Sometimes, the roof of the voids can collapse and form sinkholes. These holes do not provide any support to the foundation.