Secondary Compression (Creep) Settlement

The slow settlement that continues AFTER primary consolidation is complete.

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No published worked example is on file for this calculation. It is checked against a hand-worked derivation of its method.

Details, formula, and sources

ScopeThe slow settlement that continues AFTER primary consolidation is complete, and which governs the long-term movement of organic and highly plastic clays: Ss = (C-alpha/(1+ep)) H log10(t2/t1). C-alpha is the secondary compression index (the slope of void ratio versus log-time from the oedometer tail), ep the void ratio at the end of primary, H the layer thickness, t1 the time primary consolidation completes, t2 the time of interest. A 10 ft clay with C-alpha 0.02, ep 0.85, from 1 year to 50 years creeps another 2.2 in. Because it grows with the LOG of the time ratio, most accrues in the first decades but it never truly stops. C-alpha is about 0.04 (inorganic) to 0.05 (organic) times Cc (Mesri). Add it to the primary total for the long-term settlement. A design aid; the oedometer data and the geotechnical engineer of record govern.

Formula or tableC-alpha-eps = C-alpha/(1+ep); Ss_ft = C-alpha-eps H log10(t2/t1); Ss_in = 12 Ss_ft.

Source and editionThe secondary compression (creep) settlement Ss = (C-alpha/(1+ep)) H log10(t2/t1), the settlement after primary consolidation, as compiled in the Das, Holtz-Kovacs, and Mesri references, by name.

Free accessThe secondary-compression relation is standard soil-mechanics theory in every geotechnical text; no proprietary table is used, and C-alpha and ep are the user's own oedometer results.

What governsEstimate. AHJ and licensed professional govern.

API field namesc_alpha, h_ft, ep, t1_yr, t2_yr, ss_in, ss_ft, c_alpha_eps

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