7.6.3 Settlement of filled areas

7.6.3.1 General

COMMENTARY ON 7.6.3.1

Settlement of embankments and filled areas can occur as a result of any or all of the following forms of load which are divided into "internal" and "external" forms of loading in the following sections:

• settlement of ground below the fill;
• self weight settlement of the fill itself,
• settlement due to load changes as a consequence of change in groundwater conditions (e.g. inundation settlement);
• settlement of the fill due to loads placed upon the fill (e.g. by structures).

Additional movement can occur due to seasonal changes in moisture content of cohesive soils. This is not a form of settlement (since the movement is recoverable), however the additional movement can be incorrectly interpreted as being part of the settlement.

There are likely to be significant differences in the forms of settlement affecting different earthworks formed of fill as a result of the morphology of the fill body itself, i.e. differences between embankments, fill platforms (extensive areas of fill placed largely above adjacent ground level) and infilled hollows (such as old quarries).

Some deformation of the fill, of the foundation materials or of both can occur and the behaviour of the materials involved should be studied at the site investigation stage to determine their settlement characteristics.

The acceptable degree of settlement depends on the type of function the embankment is required to serve, e.g. to carry a highway or railway or for building developments. In some cases, the major part of the settlement should be induced before the filled area is required to be used. In the case of embankments this may often be achieved by completing the fill early in the contract and topping up as necessary during the completion stage, or by surcharging the fill by increasing the height to accelerate the settlement, the excess material being removed before completion. In the case of filled areas the problem is often more difficult to resolve (partly due to the far greater risk of groundwater inundation settlement risk) and the approach to management of the earthworks should be carefully considered.

7.6.3.2 Settlement of fill due to internal loading

The designer should consider that: where a significant thickness of fill is placed over a wide area the load due to the fill can often result in substantial settlement, which is a factor that should be considered in the design of all earthworks. In many cases the finished surface of the earthwork can have adequate bearing capacity, but the designer should consider that settlement due to causes not connected with the weight of the infrastructure or structures placed on those earthworks (e.g. low-rise buildings, pipelines, pavement) can have a serious effect on those features. A variety of potential causes of settlement of the fill that should be considered can occur and can be described as "internal loading" effects, which can be categorized as:

• a) settlement of underlying ground due to the weight of the fill;
• b) settlement of the engineered fill due to self-weight;
• c) movement in the engineered fill due to post-construction changes in ground-water level or downward percolation of water;
• d) movement in the engineered fill due to seasonal changes in moisture content or pore pressures.

Settlement of the underlying ground is an important issue for earthworks, given the large footprint of the body of fill and, as is traditional, should be calculated by standard methods described in soil mechanics texts. In the case of embankments, consideration should be given to the trapezoidal shape of the fill body and the designer should consider the potential extent of the settlement bowl which might extend beyond the footprint of the earthworks; it is often the case for structures constructed on fill that the load from the fill can have a greater influence on the underlying ground than the load from the structure.

The SHW [1] has been developed and should be used for infrastructure embankments which are generally of limited height (i.e. up to 15 m), where the trapezoidal shape of the body of fill and nature of construction enables dissipation of a large proportion of excess porewater pressure during construction provided the specification is followed. Consequently, it should be borne in mind that for these embankments self-weight settlement is not usually a major controlling factor in determining the design other than at locations where differential settlement is a concern (e.g. at the approach to an underbridge).

For conventional infrastructure embankments, it is normal practice to adopt an experience based approach of assuming 0,5% to 1% of embankment height as self-weight settlement (an approximation typical of clay fills compacted wet of optimum moisture content). In addition it may be necessary to include a monitored hold period once the earthworks reach full height (or are surcharged at a higher level) and prior to completion of any sensitive elements of the infrastructure upon the earthworks. Where the magnitude of the settlement needs to be calculated, this may be done based on the equations presented by Trenter [35]:

Total self-weight settlement = 0,5(bulk unit weight × H²)/D

where

• H is embankment height
• D is the constrained modulus, which can be approximated as 1/m_v or can be based on values obtained from large diameter oedometer testing or site monitoring as summarized by Trenter [35].

If an accurate calculation of self weight settlement is required then there are a number of factors that should be considered regarding the nature of the fill and compaction conditions, as presented by Charles and Skinner [36]. It should be noted that this is particularly appropriate for large bodies of deep fill that are intended to support structures, or if the adequacy of compaction is in doubt, or if cohesive fill dry of optimum moisture content and with high air voids is used.

Where the earthworks are to be undertaken for a purpose other than a typical infrastructure embankment the designer should consider the issue of self weight settlement in greater detail. Settlement of the body of fill itself can be a significant issue for large areas of deep fill placed to enable the construction of buildings, and research by the BRE should be referred to [37].

Collapse compression on inundation with water is a hazard for engineered fill with a high air-voids content, particularly where an excavation is backfilled below the likely long term groundwater level; for these cases the designer should evaluate the likelihood of occurrence and establish whether particular measures are required within the design. Collapse on inundation is a particular hazard for structures built on fill and the specification of the fill material properties and compaction of the fill should be designed to eliminate collapse potential.

Ground movements within the fill due to a rise in overall water table within the fill are also a concern for earthworks built in areas prone to flooding; for all earthworks, designers should consider whether there are areas where concentrated infiltration of surface water can be expected, and whether this could be sufficient to generate ground movements. In many earthworks situations water pipelines will be present and leaks from these pipes commonly result in ground movements, which is difficult to fully mitigate against; however, the designer may include measures to ensure that settlement has completed prior to installation of the pipe runs, and consider whether there are any locations where a leaking pipe could lead to a major failure.

It is important to note that simply setting a compaction requirement that field dry density is at least 95% of the maximum dry density obtained from the standard Proctor (2,5 kg) compaction test will not necessarily eliminate collapse potential in many fills and consequently this should not be assumed to be an adequate compaction specification. Where there is a strict requirement to limit settlement within the body of the fill the designer should assess whether general fill in accordance with the SHW [1] is adequate, and if not then either use selected granular fill, or adopt a more stringent specification requirements as described at BRE Digest 427 [37] (see 7.6.4 and 8.4).

The designer should give consideration to the potential influence on the earthworks (or infrastructure/structures on those earthworks) of ground movements associated with seasonal changes in moisture content. The main concerns are where these seasonal changes are accentuated by vegetation resulting in significant shrinkage of clay fills (see 7.10.1 and 11.7.3), and this may necessitate restrictions on where high-plasticity cohesive fill can be placed within embankments.

7.6.3.3 Settlement of fill due to external influences

COMMENTARY ON 7.6.3.3

Settlement of the fill due to imposed loads can be as a result of:

• dead load - i.e., structures;
• live load - i.e., traffic; and
• collapse of underlying ground (see 7.2.2).

The designer should assess the fill material and compaction requirements to limit settlement to an acceptable magnitude (both settlement of the fill and due to imposed load), which might require greater compactive effort than SHW [1].

Fill which is to form a foundation for buildings should undergo particularly strict quality control, and the specification should be prepared with this in mind (see Clause 8). From a technical standpoint, a period of monitoring following completion of filling and prior to construction of structures is prudent.

The designer should consider whether a surcharge due to traffic or similar live loads needs to be considered in settlement assessments since loading of this type is normally transient and of short duration. Traffic loads are normally local to the surface of the earthwork and usually contribute only a small proportion of the total earthwork load thus they may generally be ignored in settlement assessments unless there is a particular reason to take account of short term transient loads.