Settlement calculation methods

The most common approaches to estimate the settlement in a soil stratum are based on stress strain measurements during laboratory tests performed on undisturbed soil samples. Whenever possible, experience gained from prior construction under similar site conditions should be applied. If there is no earlier experience, the construction of a trial embankment for monitoring the settlement/consoUda-tion behaviour may be considered. 

Many different methods exist to calculate the settlement of the subsoil. Reference is made to the literature for such theories. 

In literature, the term Instantaneous (or immediate) settlement is often used, but generally not well defined. This settlement is sometimes also referred to with Elastic settlement. This may lead to confusion. 

Sometimes, an instantaneous settlement is measured during a consolidation test on saturated samples. This can be attributed to imperfections in the testing method, such as the presence of air bubbles. 

Generally, a settlement calculation consists of three distinct steps  

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It is assumed that local settlement of the subsoil will lead to free spans of the concrete mattress. Then, the wave impact can cause the breaking of these spans, if the ratio of Hg/D is too large for a certain span length. A calculation method is derived on the basis of an empirical formula for the maximum wave impact pressure and the theory of simply supported beams. The collapsing of small spans (less than 1 or 2 m) is not acceptable, since these will lead to too many cracks. 

Such natural services may be interrupted in other ways. If the chemical or sewage plant upstream puts something toxic into the stream that the water treatment plant of the city downstream cannot remove by standard treatments, a new water supply or a new treatment method will have to be found at increased cost. In earlier years before the toxicity of the polychlorinated biphenyls was fully appreciated, General Electric released enough of them into the Hudson River north of Albany, New York that the whole Hudson River from Hudson Falls to New York City is now a Superfund site.7 Fishermen are advised not to eat the fish that they catch. It is not always easy to calculate the cost of a fishery lost to toxic heavy metal ions or acids draining out of a mine site.8 One settlement, involving a salmon fishery in a river in Idaho, was for 60 million dollars. The Exxon Valdez oil spill in Alaska cost Exxon 3 billion dollars. The persons whose wells become contaminated by leachate from the nearby landfill will face the costs of bringing water from a distance. This was a cost that was not included when the landfill was built. The U. S. National Research Council has recommended that the U. S. Department of Commerce resume development of a method to better measure environmental costs.9... 

Pile Group A number of semi-empirical methods have been proposed to calculate the settlements of pile groups in sand (O Neill, 1983). Large deviations can be expected when the conditions at a particular site deviate from those at which the method was derived. Comparison with available test data indicates that often calculated settlement of a pile group will be too large when a modulus of elasticity that is constant with depth is used in the analysis. 

Settlement from static CPT Settlement in cohesionless soils can also be calculated from the results of static CPT results. Typically in this method the pile group is assumed to be equivalent to a raft located at the lower third point of the pile penetration depth. The soil below the equivalent shaft is then divided into a number of layers and the compression of each layer is calculated separately. The reader is referred to empirical i tionships of Skempton (1953) and Meyerhof (1959) and analytical methods of Poulos and Davis (1980). 

For the first method, it has been foimd most realistic to use the constrained deformation modulus (m) as determined from the reloading curve in the consolidation test. In the second method the sum of initial and consolidation settlements are calculated using E = 150 S and t) = 0.1. The relationship is obtained from Butler (1975) by the back-calculation of actually measured values. The consolidation settlement is then found by subtracting the initial settlement. 

Note for the first coupon payment subsequent to the settlement date, t = 1 so the exponent is just w. This procedure for calculating the present value when a bond is purchased between coupon payments is called the Street method. In the Street method, as can be seen in the expression above, coupon interest is compounded over the fractional period w. 

The estimated settlements by analytical method are equal from the point of view of practical application or higher than those calculated by the FEM method. 

There are two typical methods to obtain self-weight collapse settlement in engineering one method is to obtain self-weight collapsibUity coefficient based on indoor compression test and then calculate selfweight collapse settlement based on empirical formula (it is referred to as calculated value ) another method is to perform actual measurement of the self-weight collapse settlement in in-situ immersion test (it is referred to measured value ). 

Based on 10 in-situ immersion tests data and numerical analysis results, this paper gives a adjust method of calculated value of self-weight collapse settlement (computed collapse under overburden pressure) considering the influence of loess deposit age and discontinuous distribution of collapsible loess. 

A relatively accurate method such as the D Appolonia et al. (1970) method calculates settlements that are about equal to the average value of actual settlements, but it underestimates settlements half the time (a reliability of 50%). To ensure the calculated settlements equal or exceed the measured settlements about 90% of the time (a reliability of 90%), an adjustment factor of two shall be applied to the settlements predicted by the D Appolonia et al. method. Table 8.15 shows values of adjustment factor for 50% and 90% reliability in settlement predicted using Terzaghi and Peck (1967), D Appolonia et al. (1970), and Schertmann (1978) methods. 

Consolidation settlement is usually significant in soft to medium stiff clayey soils. Creep settlement occurs most significantly in overconsolidated clays under large sustained loads and can be estimated by using the method developed by Booker and Poulos (1976). In principal, however, long-term settlement can be included in the calculation of ultimate settlement if the design parameters of soil used in the calculation reflect the long-term behavior. 

Although the method of t-z and Q-z curves employs localized springs, the calculated settlements are usually within a reasonable range since the curves are backfitted directly from test results. Factors of nonhnear behavior of soil, complicated stress conditions around the pile, and partial corrections to the Winkler s assumption are embedded in this methodology. Besides, settlement of a pile can be estimated for complicated conditions such as varying pile geometry, different pile materials, and different soil layers. 

Vesic (1977) introduced a method to calculate settlement of a pile group in sand, which is expressed as... 

Calculating settlements with the Finite Element Method (FEM)... 

The FEM is in the first place a method to calculate deformations (horizontally and vertically). The analysis of time-dependent settlements is possible as well, although the calculation of the degree of consolidation will require the input of the horizontal and vertical permeability of the soil and the computation of groundwater flow (dissipation of excess pore-water pressures) in the FE mesh. 

Although the soils involved are mostly noncohesive, calculation of expected settlements can be performed using the method of Borland and Burbidge. This is on the basis of the values iVcpr from standard penetration tests, expressed by the following formula (Eq. [15.84]) ... 

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