7.6.4 Selection of material properties for earthworks fill

An important activity for every earthworks project is the selection of material properties for the fill; this is considered to be a design activity regardless of whether it is undertaken by a contractor or a consultant. The material properties should be chosen to ensure that the engineering design assumptions are satisfied as well as addressing construction practicalities.

The material properties for earthworks fill should be selected to ensure that:

• the material can be trafficked, placed and compacted during construction of the earthworks;
• the earthworks will be stable during and after construction;
• excessive settlement or heave will not take place.

For the majority of fill materials the acceptable material properties should be related to limits applied to either moisture content, MCV or shear strength e.g. see Table 6/1. It is strongly recommended that only one of these properties is used for a particular acceptability limit.

For most coarse soils the upper and lower acceptability limits should be selected by reference to a particular ratio of dry density to the maximum dry density. The values are determined from dry density/moisture content relationship tests, which are illustrated in general terms in Figure 9. The most commonly adopted criteria are 95% of the maximum dry density determined from the 2,5 kg light dynamic compaction test or 90% of the maximum dry density determined from the vibrating hammer test for bulk earthworks fill. A higher value up to 100% of the maximum dry density is required for fill that will support structures where settlement is more critical.

It is recommended that the air voids content at the proposed lower acceptability limit is checked to ensure that excessive air voids will not remain within the fill at the chosen compaction ratio; however, an air void content less than 10% may not be feasible with some uniformly graded coarse soils.

It is important to note that the maximum dry density and optimum moisture content are not fundamental soil properties and the values are dependent on the compactive effort imparted to the material.

For fine soils the upper acceptability limit (see Figure 10) e.g. minimum MCV, should be chosen in relation to the requirements for placement of the fill, stability of slopes, and settlement of the fill due to internal loading (see 7.6.3). These requirements may vary for different end uses of the earthworks, which will determine the fill properties of greatest importance, e.g. permeability for a flood bund, or in-situ density for structural fill. The lower acceptability limit (minimum moisture content, maximum MCV or maximum shear strength) should be selected to reduce the air voids in the material to a value that will restrict the potential for excessive movement after compaction. A maximum of 10% air voids for bulk earthworks fill and 5% air voids for earthworks fill that is to support structures are commonly specified values. Research at TRL led to the development of the compaction requirements of Table 6/4 of the SHW [1] which are intended to achieve these values for the relevant classes of fill provided that the moisture content of the material is appropriate. Further guidance on the degree of compaction achieved using the methods specified in Table 6/4 is provided in HA44/91.

When there are specific requirements to limit the internal settlement for large bodies of fill that will carry structures (as described at 7.6.3) then the approach of selection of design parameters beyond that which would normally be considered under the SHW [1] approach may be developed. One methodology that may be used is proposed in BRE Digest 427 [37], whereby:

• the moisture content upper and lower acceptability limits of the fill are chosen based on OMC from both the standard Proctor (2,5 kg rammer) and the modified Proctor (4,5 kg rammer) compaction tests (i.e. relatively dry material for fine soils), see Trenter [35] for further details;
• and the method of compaction is selected to ensure heavy compaction is delivered (which is likely to be in excess of the SHW standard methods); and
• the earthworks are monitored to ensure a high in-situ density and low air voids are achieved.

It should be noted that a fine soil that is at Point A on Figure 10 will not benefit from further compaction and the strength could be reduced due to the generation of excess porewater pressure if further compactive effort is applied. Excess porewater pressures weaken the fill layers affected, which limits the effectiveness of compaction of subsequent layers of fill on fill; therefore a pause of a few days should be accommodated to allow dissipation prior to recommencing earthworks. By contrast the dry density of a soil at Point B should increase if additional compactive effort is applied.

NOTE Fills with a significant proportion of coarse particles represent a problem for determination of acceptability criteria, as these soils often prove inappropriate for either laboratory testing or in-situ density testing. BS 1377-4:1990 sets an upper limit of 10% of particles coarser than 37,5 mm and 30% coarser than 20 mm above which standard laboratory compaction tests are not applicable since the fill is classified as being "Grading Zone X". However, if the Zone X criteria are strictly applied, then many UK materials used as fill are classified as untestable by virtue of a relatively low granular content (e.g. well-graded glacial till). This is actually detrimental to the management of the earthworks project. Trenter [35] provides methods for adjustment of the results to allow for the influence of coarse fraction.

An experience based approach is recommended for these coarse soils to determine the most appropriate method for testing and management of the fill. For gap graded or well graded fills (granular or cohesive) the earthworks engineer may judge that there is a matrix of testable material that will strongly influence the performance of the fill. In many cases it may be appropriate to remove coarse particles to facilitate laboratory testing and base the acceptability criteria on the finer fraction of material.

Acceptability criteria based on moisture content may be used for very coarse granular fills, such as Class 1C and Class 6B of the SHW [1].

Compaction using Method 5 of SHW Table 6/4 may provide a general approach but performance should be reviewed on site. The construction and analysis of trial embankments should be used to provide definitive site and source specific guidance for compaction of very coarse fills.

The above is a limited summary only; designers of earthworks should have an awareness of the various issues that might influence the fill material that they will utilize.

COMMENTARY ON 7.6.4

It is useful for the earthworks engineer to have an understanding of both the underlying principles of fill material behaviour and the development history of earthwork engineering. The latter is important since earthworks is not a well defined science, and to resolve certain practical difficulties the standard approaches draw upon previous work. Of particular importance in the development of the subject is the testing undertaken by the Transport Research Laboratory to develop the method specification that is included with the SHW [1], details of the TRL research were recorded by Parsons [38]. Field trials by the Building Research Establishment showed the importance of control of air voids content of fill materials incorporated in earthworks for future building development (e.g. BRE Digest 427 [37], Charles et al [39]).

Informative descriptions of the history and principles that underlie earthworks are included in a number of published documents including:

• HA44/91 [17] and HA70/94 [18];
• Trenter and Charles [40] re building on earthworks;
• Reeves et al [41];
• Trenter [35].

These documents provide guidance on the selection of appropriate parameters for earthworks materials (and limited comment on the selection of suitable tests for the practical control of the construction of earthworks).

Fine soils and weak argillaceous rocks that are placed dry in a relatively loose condition are prone to collapse on subsequent wetting (Charles and Watts [42]). It is particularly important that the air voids content of these materials is restricted to prevent collapse settlement. Where possible it is advisable to avoid use of such fills in situations where inundation by floodwater or groundwater is likely.