> For the complete documentation index, see [llms.txt](https://docs.openbrim.org/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.openbrim.org/technical-background/soil-structure-interaction/lateral-soil-models/soft-clay-api-api-2007.md).

# Soft Clay (API) \[API, 2007]

API RP 2A-WSD gives the Matlock soft-clay criterion in tabulated form, Sections 6.8.2 (lateral bearing capacity) and 6.8.3 (*p-y* curves); API RP 2GEO \[API, 2014] carries the same ultimate resistance with shorter *p-y* tables, and that is the form implemented in FB-MultiPier's *Clay (API)* model (Help Manual, Sec. 13.7.12, Tables 13.7.A–B) and in OpenBrIM. The undrained shear strength, the effective unit weight, the strain at 50 % of the maximum stress and the empirical constant $$J$$ are required.

### Ultimate Resistance

$$p\_u$$ increases from 3\*\*$$c$$\*\* to 9\*\*$$c$$\*\* as the depth increases from 0 to $$X\_R$$ (Eqs. 6.8.2-1 and 6.8.2-2, expressed per unit length of pile):

$$
\tag{1}
p\_u = \Bigl(3c+\gamma' X+J\frac{cX}{D}\Bigr)D \quad for \quad 0 < X < X\_R
$$

$$
\tag{2}
p\_u = 9cD \quad for \quad X \geq X\_R
$$

$$
\tag{3}
X\_R = \frac{6D}{\frac{\gamma' D}{c}+J}
$$

where,

$$c$$ = undrained shear strength\
$$D$$ = pile diameter\
$$γ'$$ = effective unit weight\
$$J$$ = dimensionless empirical constant, 0.25 to 0.5 from field testing; 0.5 for Gulf of Mexico clays\
$$X$$ = depth below soil surface\
$$X\_R$$ = depth to the bottom of the reduced resistance zone (minimum about 2.5 diameters)

### p-y Curves

$$
\tag{4}
y\_c = 2.5\varepsilon\_cD
$$

Short-term static loading (Table 13.7.A):

| $$p/p\_u$$ | 0.00 | 0.50 | 0.72 | 1.00 | 1.00 |
| ---------- | ---- | ---- | ---- | ---- | ---- |
| $$y/y\_c$$ | 0.0  | 1.0  | 3.0  | 8.0  | ∞    |

Cyclic loading, equilibrium reached (Table 13.7.B):

| $$X > X\_R$$: $$p/p\_u$$ | 0.00 | 0.50 | 0.72 | 0.72 |
| ------------------------ | ---- | ---- | ---- | ---- |
| $$y/y\_c$$               | 0.0  | 1.0  | 3.0  | ∞    |

| $$X < X\_R$$: $$p/p\_u$$ | 0.00 | 0.50 | 0.72 | 0.72\*\*$$X/X\_R$$\*\* | 0.72\*\*$$X/X\_R$$\*\* |
| ------------------------ | ---- | ---- | ---- | ---------------------- | ---------------------- |
| $$y/y\_c$$               | 0.0  | 1.0  | 3.0  | 15.0                   | ∞                      |

where $$ε\_c$$ is the strain at one-half the maximum stress in a laboratory unconsolidated undrained compression test of undisturbed samples. The tables are the piecewise-linear form of the [Soft Clay (Matlock)](/technical-background/soil-structure-interaction/lateral-soil-models/soft-clay-matlock-matlock-1970.md) curves; the differences are the tabulated points and the user-selected $$J$$.

{% hint style="info" %}
The API RP 2A-WSD table of Section 6.8.3 has two further points, $$p/p\_u$$ = 0.23 at $$y/y\_c$$ = 0.1 and 0.33 at 0.3, which make the first branch of the curve concave. FB-MultiPier's *Clay (API)* model and OpenBrIM use the API RP 2GEO tables above, whose first branch is a straight line from the origin to ($$y\_c$$, 0.5\*\*$$p\_u$$\*\*).
{% endhint %}

***

\[API, 2007] American Petroleum Institute (2007). *Recommended Practice for Planning, Designing and Constructing Fixed Offshore Platforms — Working Stress Design*, API RP 2A-WSD, 21st ed. with errata and supplements, Sections 6.8.2–6.8.3.

\[API, 2014] American Petroleum Institute (2014). *Geotechnical and Foundation Design Considerations*, API RP 2GEO, 1st ed. with addendum. (Tables as reproduced in the FB-MultiPier Help Manual 6.0, Sec. 13.7.12.)


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