What is negative skin friction on piles and pile group?
Negative skin friction is a type of downward shear drag that affects piles or groups of piles. It occurs when the surrounding soil moves downward relative to the piles. This movement is most likely to happen when a pile is driven into a compressible soil layer that can consolidate. In fact, it has been observed that even a small relative movement of 10 mm between the pile and the soil can result in full negative skin friction.
The duration of negative skin friction on piles is estimated to be approximately two years. During this time, the soft soils consolidate to a degree of around 90%. The article explores various aspects of negative skin friction on both individual piles and groups of piles.
Factors that cause negative skin friction on piles and pile group
When loose cohesionless soil is placed over a layer of soft soil or peat, the fill material will experience settlement due to the compressible nature of the underlying soil. The settlement may be further exacerbated if the fill material is deposited before the consolidation of the underlying soil is complete.
To add to the issue, if the groundwater level is lowered, the effective stress on the soil will increase, causing consolidation to occur at a faster rate. This consolidation process may cause settlement of the fill material and increase the friction force acting on any piles that may have been installed.
It is important to consider these factors when undertaking any construction activities that involve the placement of fill material over soft or compressible soils. Adequate measures such as preloading and surcharging can be taken to mitigate the effects of settlement and ensure the stability and longevity of any structures built on such soils.
Effect of negative skin friction on piles and pile groups
When piles or pile groups are used in compressible soils, negative skin friction can cause uneven settlement. This is because the pile’s capacity is influenced by both point resistance and shaft adhesion. Therefore, negative skin friction must be taken into account as a settlement issue.
However, in bearing piles, negative skin friction can be a problem for the pile’s capacity rather than settlement. Bearing piles experience little to no settlement, which means negative skin friction is not a settlement issue for them. Instead, it affects the pile’s capacity.
Negative skin friction in single pile
The negative skin friction in single piles can be computed using the following expressions:
Negative skin friction of piles in cohesive soil
Where:
: negative skin friction
: perimeter of the pile
: pile length in compressible soil as illustrated in Fig. 2.
: unit adhesion and can be computed using equation 2
Where:
: adhesion factor
: Undrain Cohesion of the compressible layer
Negative skin friction of piles in cohesion less Soils
Where: K: lateral earth pressure coefficient
: unit weight of soil
: angle of friction between pile and soil, which may vary from 1/2
to 2/3
.
Negative skin friction on pile groups
The context provided is related to the calculation of negative skin friction in pile groups. According to the given information, the value of negative skin friction can be determined by using either equation 4 or equation 5. The greater value obtained from these two equations would be considered as the value of negative skin friction in pile groups.
It is important to note that negative skin friction is a phenomenon that occurs when a pile is embedded in soil that is compressible and undergoes settlement. The weight of the soil above the pile causes it to sink, and as a result, friction is developed between the pile and the soil. This friction can be negative in nature and can reduce the capacity of the pile to support a load.
To calculate the negative skin friction, engineers use various equations and models. Equation 4 and Equation 5 are two such equations that can be used to determine the value of negative skin friction. However, it is essential to consider which equation provides a greater value, as this would be the one used to determine the negative skin friction in pile groups.
In summary, the context provided relates to the calculation of negative skin friction in pile groups. It mentions two equations, equation 4 and equation 5, that can be used to determine the value of negative skin friction. The greater of these two values would be considered as the negative skin friction in pile groups.
The given context is a mathematical formula or equation that involves two variables: “n” and “Pg”. The equation relates the number of piles in a group to the perimeter of that group. The specific formula or relationship between these variables is not provided, so it is not possible to rewrite the equation in any further detail.
However, it is important to note that the equation assumes that there is some sort of connection between the number of piles and the perimeter of a group. It is possible that this equation is used in a variety of contexts, such as in construction or landscaping, where the number of piles needed to support a structure or to create a boundary may depend on the perimeter of the area to be enclosed.
Without additional information or context, it is difficult to provide further explanation or interpretation of this equation.
: unit weight of the soil within the pile group up to a depth
Ag: area of pile group with in the perimeter Pg
Fig.1: Negative skin friction on piles and group pile
The effect of negative skin friction on the factor of safety with respect to the ultimate load capacity of a pile or a pile group
The impact of negative skin friction on the safety factor related to the ultimate load carrying capacity of piles or pile groups is a significant consideration in construction projects. To address this concern, engineers introduce a factor of safety into their calculations. Specifically, they determine the ultimate load capacity of a pile or group of piles by multiplying the sum of the working load and negative skin friction load by the factor of safety.
Negative skin friction can impose an unwanted and substantial downward drag on a pile. To counteract this effect, engineers can employ a protective sleeve or coating for the section of the pile that is in contact with the settling soil. This approach is illustrated in Figure 2.
Overall, the presence of negative skin friction represents a potential obstacle to achieving the desired level of load carrying capacity for piles or pile groups. However, engineers can address this challenge by incorporating appropriate measures such as a protective sleeve or coating into their design and construction plans. Ultimately, taking these steps can help ensure that structures built on piles are safe, stable, and durable over the long term.
Fig.2: pile sleeves used to decline negative skin friction