Clarifying the Meaning of "Soft" Soil
Aug 8, 2017 News Archive
A few days ago, while finalizing our forthcoming paper for the 2017 International Jack-Up Platform Conference, Jack Templeton (of SAGE USA) brought to our attention the fact that we referred to soft soil in a way that seemed to be different than what geotechnical engineers consider the standard definition for soft soil. In previous work pertaining to jackups going on location here at 3DENT, we have mentioned that the terms “soft” and “stiff” soils are very much arbitrary. In the past, we have defined soil stiffness in terms of spudcan penetration at full preload and have used the following distinctions when classifying the types of soils in our GoL analysis:
- Very Stiff Soils – Soils that produce penetrations of 1-2 feet past the spudcan tip.
- Stiff Soils – Soils that produce penetrations that do not engage the full bearing area of the spudcan.
- Intermediate Soils – Soils that produce penetrations that engage the full bearing area, but do not exceed 10ft.
- Soft Soils – Soils that produce penetrations in the 10-30ft range.
- Very Soft Soils – Soils that produce penetrations greater than 30ft.
Upon further conversation with Jack Templeton, he produced the official soil classification table, based on SPT N-Value and compressive strength, as shown below.
SPT is shorthand for Standard Penetration Test and a soil’s SPT N-Value refers to the sum of number of blows that it takes for a hammer, at a standard weight and standard falling distance, to penetrate a standard size cylinder into a sample soil 18 inches (hits recorded every 6 inches). To note, the soil’s compressive strength is equal to twice the soil’s shear strength.
An observation to point out is that the official soil classification actually has one termed “Hard” and it is stiffer than “Very Stiff.” It is also noted that during the going on location operations, the peak bearing pressures are usually 35%-40% of the maximum bearing pressures at full preload, and the penetrations are usually in the 1 to 5ft range (past the spudcan tip).
In an effort to compare soft and stiff soils using the official definition vs 3DENT’s previously set definitions, penetration curves for a generic spudcan were developed. The spudcan used is the one used in our GoL papers and is shown in Figure 1 below. Assuming a maximum bearing pressure of just over 9ksf (typical for a modern jackup), the maximum preload reaction is set as 16,000 kips.
Figure 1. Representative Spudcan
Table 1. Representative Spudcan Properties
Figure 2. Leg Penetration Curves for Soft and Stiff Soils
From these plots, it can be seen that that the penetration range for stiff soils is 3-5ft (i.e., less than the height from the bottom of the spudcan to full bearing). For soft soils, the penetration range is 35ft to 87ft, or 74% to 184% of the effective spudcan diameter). Therefore, the previously used scale is in agreement with the standard scale for stiff soils, but soft soils have penetrations that are much greater than 30ft.
From the above, the penetration ranges at full preload reaction are seen to be 3-5ft for stiff soils and 35-87ft for soft soils. If these penetrations were used with the 3DENT soil stiffness scale, the soft soil would be labeled as very soft whereas the stiff soil would be labeled as very stiff or stiff, depending on whether or not the spudcan had a tip or not.
More work will need to be done to more confidently label soils that are analyzed in spudcan-soil interaction for GoL modeling, but hopefully in time, industry can have a set standard that all engineers, rig personal and operators can agree on and have confidence in.