Precompression of Soft Soils By Surcharge Preloading – Some Common Pitfalls And Misunderstood Fundamentals

Dato’ Ir. Dr. Ramli Mohamad | April 2008

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Introduction

It is quite common for designers of embankments on soft ground to specify surcharge preloading to compensate or eliminate post-construction settlements. Properly designed and executed, the method can be a powerful and economical way to build high embankments on soft ground. It certainly represents a much cheaper alternative to solutions that involve constructing a rigid foundation such as a piled slab or stone columns beneath the embankment. However, there have been many cases where post-construction settlements have continued after completion of surcharge preloaded embankments and this has led to some erosion of confidence in the method.

In essence, the method is simple. To build an embankment to a final height, Hf that would not settle or would settle very little after it has been built, the embankment is first built to a height Hs + Hf that is higher than the desired final height and is left to settle for a period Tp under the load intensity pf_+ ps due to this extra height of fill. At the end of the preloading period Tp, the surcharge fill of height Hs is removed, causing the soft soil to be unloaded, resulting in elimination or a huge reduction in post-construction settlements under the final height of embankment Hf. Most or all of the primary settlement and some of the secondary settlement that would have occurred under the final embankment height alone is forced to take place under the surcharge loading and in addition, the soil beneath the embankment becomes over consolidated or stiffer. Figure 1 illustrates the key elements of the concept of surcharge preloading to compensate for primary and secondary settlements. Usually, the aim is to eliminate 100% of primary consolidation settlement and enough secondary settlement such that the residual settlement is within acceptable performance limits. The residual settlement for a given length of time after construction can be estimated as the remaining secondary settlement that occurs during the required time after the eliminated equivalent time of secondary compression has elapsed.

Figure 1. Concept of surcharge preloading to compensate for primary consolidation settlement and secondary compression.

Common Pitfalls

However, the method has not always been applied successfully. The author has had the opportunity to examine some of the cases where the method of surcharge preloading has failed to arrest post-construction settlements. Some highway embankments have kept settling and have required regular topping up to maintain their design levels and of course such topping up have caused more severe settlements. The author finds that usually the reason for the ineffectiveness of the method is not in the method itself, but in the improper application of the method caused by a lack of understanding of the fundamentals associated with the method. Among the common mistakes made by designers when applying the method of surcharge preloading are:

  • Inadequate site investigation and laboratory testing. A thorough site investigation and laboratory testing programme should be done to obtain the relevant engineering parameters for use in the design. In particular, the subsurface profile should be determined as accurately as possible, together with the geological history of the site. The engineering properties of importance are the maximum past pressures ovc’, compression and recompression indices Cc and Ccr, the coefficient of consolidation cv, the secondary compression index Ca and the undrained shear strength su together with an estimate of the relationship between su strength and effective overburden stress before applying the method of surcharge preloading. These parameters usually vary with depth within a soil layer and it is important to plot the data against depth to provide a clear picture of their variability within the soils beneath the embankment that will be built.
  • Inadequate surcharge or no removal of surcharge. This is a rather common flaw. It must be borne in mind that one of the critical parameters in the design is surcharge ratio, i.e., the ratio of the surcharge load, ps to the final load, pf. The surcharge load ps represents the amount of surcharge that is removed at the end of the preloading period. Often, the settlement that takes place during preloading is not taken into account by designers. This settlement can be substantial and may even exceed the initial height of the surcharge. It is thus important that in estimating the values of ps and pf, the settlements are taken into account. There have been cases where at the end of the preloading period, the settlements that have occurred have brought the embankment level to the desired finished level and the designer or the resident engineer then accepts that there is no surcharge to be removed. This is equivalent to having a value of ps = 0, which will certainly make the embankment equivalent to one that has not been surcharge preloaded. There must be an adequate amount of surcharge that is removed at the end of the preloading period for the method to be
  • Inadequate stability during construction. The construction of an embankment on soft soil too rapidly can cause the embankment to collapse through failure of the base. Most soft soils will gain in strength as it is loaded provided the excess pore pressures that develop due to the application of the external load are allowed to dissipate, i.e., as the effective stresses applied to the soft soils increase or as the void ratio of the soft soils decrease. It may be necessary to provide basal georeinforcement to ensure stability during construction for total heights (inclusive of surcharge) that exceed the safe height that the soft soil can support on its own. Once an embankment can be built on soft ground, it will usually remain stable thereafter if no additional loads are placed on it, because the shear strength of the soft soils underneath can only gain in strength as time goes
  • Designing for compensation of primary settlement only. Secondary settlements can be large and it is possible to design a surcharge preloading scheme to eliminate all of the primary settlement and much of the secondary settlement that would occur under the final load, but many designers seem to prefer to ignore secondary settlements. Secondary compression is a strange thing—it requires primary consolidation to occur first under a given load, but the load intensity does not appear in the The magnitude of secondary compression depends on time elapsed after completion of primary consolidation, the void ratio and the thickness of the soil layer at that point in time, but not on the intensity of the applied load. The drainage path during secondary compression also does not appear in the computations.
  • Removing surcharge too early. For a set of given conditions, the optimum time for removal of surcharge should be determined based on the desired post-construction performance of the Removing the surcharge too early will result in the soft soils not achieving sufficient settlement to compensate for the primary and secondary settlements that would occur under the final load.
  • Inadequate instrumentation, monitoring and back analysis during construction. In designing and constructing embankments on soft ground using the method of surcharge preloading, it is very important to adequately and continuously measure and interpret data related to settlements, lateral movements, ground heave and pore pressures within the soft soils beneath the embankment. The purpose is to measure settlements and other ground movements, to assess stability during construction, to verify the parameters that are used in the design, to assess the effectiveness of vertical drains if these are used, to make adjustments in loading rates and help the designer determine if the surcharge levels need to be varied, and to help the designer determine the most appropriate point in time to remove the surcharge.

Fundamental Parameters

The success of a surcharge preloading scheme depends heavily on the proper acquisition, interpretation, usage and control of several key parameters. The engineering parameters that are of importance and how they affect a surcharge preloading scheme need to be understood for achieving a good and effective design. These are briefly discussed below:

  • yb: Bulk density, a simple property that is often assumed, can be the cause of fairly significant problems. The parameter is required for the calculation of in-situ effective vertical stress, pf0 at the centre of a soft soil layer before application of any load. The parameter pf0 in turn appears in the computations for surcharge preloading and hence it is important that this parameter is obtained as accurately as possible using measurements of densities of undisturbed samples rather than simply assuming a
  • Cc,, Ca: These two parameters are necessary for the computation of primary consolidation and secondary compression settlements. These parameters are usually obtained from laboratory consolidation tests on high quality undisturbed samples. Correlations are also available in the literature, but if they are to be used, it is best to trust the ones that have been developed based on local
  • Cc/Ca: This ratio is important in determining how much secondary settlement can be eliminated for a given scheme. The higher the ratio, the more secondary settlement can be eliminated in a given preloading period. Put in another way, for a given preloading period, it is possible to estimate the equivalent time of secondary compression that will be Or one can set the equivalent time of secondary consolidation that one wishes to eliminate, e.g., 10 years or 50 years or 100 years, and then use the ratio to determine other parameters for the scheme such as the surcharge level and preloading period. Both Cc and Ca can be obtained from laboratory consolidation tests on high quality undisturbed samples. The author’s experience with soft clays along the west coast of peninsular Malaysia indicates that this ratio is typically in the range from about 30 to over 100, hence it is best to obtain this ratio from consolidation tests on high quality samples obtained from the actual site.
  • Uf+s: Usually, the aim of a design would be to completely eliminate primary consolidation settlement under the final load pf and an equivalent number of years of secondary compression. This is done by preloading to a predetermined degree of consolidation under surcharge + final load, Uf+s. For a given set of other parameters, the higher this value is, the more settlement is eliminated under the final In most cases, designers try to aim for Uf+s = 90%, but it does not need to be always so. If time is limited and stability can be enhanced by using basal georeinforcement, it may be possible to apply the surcharge preload to achieve a degree of consolidation Uf+s less than 90%.
  • ps/pf: The surcharge ratio is the parameter that the designer has full control For a given set of other parameters, this ratio determines the duration of preloading that is needed to eliminate a specified amount of post-construction settlement. During construction as well, this ratio can be varied depending on the observations of the site instrumentation data and the back analyses of those data. It is emphasized again that this ratio must be calculated by taking into account the settlements that actually occur on site under the surcharge loading. This ratio should not be zero, as that would represent the case of no surcharge preloading. Surcharge must be removed for the method to work.
  • Tp: The preloading duration, Tp is usually controlled by the construction programme. Typical preloading durations range from 3 months to 12 months or more. Obviously, the longer the available time, the better it is, since the surcharge ratio can be kept lower or a greater amount of secondary compression can be eliminated. With the use of closely spaced prefabricated vertical drains, the preloading duration can be relatively short even for deep layers of soft ground. However, care should be taken in the selection, detailing and installation of prefabricated vertical drains if very large settlements are expected. If the preloading duration is limited due to a tight construction programme, the designer may be forced to use a high surcharge ratio which may in turn force him to incorporate measures such as counterweight berms, basal georeinforcement or other means to ensure stability during construction.
  • su/ov0’: The relationship between undrained shear strength and effective vertical stress can be obtained from both field tests such as vane shear tests or from laboratory tests such as consolidated- undrained triaxial tests. It is important to have this relationship for the soft soils that are being loaded by the embankment, as it will help the designer stage the construction to take advantage of the gain in strength of the soft soils as the embankment is built. It is also important to note that the potential slip surface in a base failure extends well beyond the edge of the embankment and the gain in strength outside the zone loaded by the main embankment body is not the same as directly beneath the embankment and this has to be taken into account in the stability. Based on the analysis of field vane shear data for soft clays along the west coast of peninsular Malaysia, the author has found that the relationship su/ov0’ = 0.259OCR0.78 gives a reasonable approximation of the undrained shear strength (Ramli Mohamad, 1992).
  • cv and ch: The coefficient of consolidation determines the rate of consolidation and hence the time required to achieve a specified degree of consolidation. If prefabricated vertical drains are used, the coefficient of horizontal consolidation, ch Usually, the parameter is used to determine the spacing of the prefabricated vertical drains to achieve a desired time for a specified degree of consolidation. What is of main interest is the time required for a specified degree of consolidation under the surcharge loading. The coefficient of consolidation is determined from laboratory consolidation tests but ch can also be estimated directly in the field through pore pressure dissipation tests using appropriately-tipped cone penetration equipment. In the absence of ch data, it is customary to estimate ch as a multiple of cv, typically 2 to 4. Based on author’s experience, typical values of cv from laboratory tests on soft clays along the west coast of peninsular Malaysia range from 0.3 m2/yr to about 2 m2/yr, although it must be warned that values of cv obtained from laboratory tests are heavily affected by sample disturbance, being lower with increasing sample disturbance.

Conclusion

The fundamentals and some pitfalls associated with the method of surcharge preloading to precompress soft soils are briefly discussed. This paper is not intended to delve into the actual methods of analyses available for designing surcharge preloading schemes on soft soils, but more of an advisory note to engineers who are already familiar with the concept, but may not be fully conversant with the fundamentals associated with the method and may also not be fully aware of all the pitfalls that are associated with the method. Hence, no formula or step-by-step guide is given for the application of the method. Readers should be able to find such formulas and guides in established geotechnical publications as the method has been in existence since the nineteen seventies, e.g., Johnson (1970).

References

  1. Johnson, S.J. (1970). Precompression for improving foundation soils. J. Soil Mech. and Found. Engg., Amer. Soc. of Civ.
  2. Ramli Mohamad (1992). Engineering the North-South Expressway on soft ground. Keynote paper, Geotropika ’02. Univ. Teknologi Malaysia, Johor Bahru,

About the author

Dato’ Ir Dr Ramli Mohamad is a director of Straits Consulting Engineers Sdn Bhd, a civil engineering consulting firm and Macroworks Sdn Bhd, an enterprise management consulting firm.