Compaction of soil can produce a stiff, settlement-free and less permeable mass. It is usually accomplished by mechanical means that cause the density of soil to increase. At the same time the air voids are reduced. It has been realized that the compaction of the backfill material has an important effect on the earth pressure.
Some theories introduce the idea that compaction represents a form of overconsolidation, where stresses resulting from a temporary or transient loading condition are retained following removal of this load.
2.6.1 Study of Duncan and Seed
Duncan and Seed (1986) presented an analytical procedure for the calculation of peak and residual compaction-induced stresses either in the free field or acting against vertical non-yielding structures. This procedure employs a hysteretic Ko -loading model (Fig. 2.15) to track the vertical and lateral stresses for a lift of backfill as it is placed, and as overlying lifts are subsequently placed and compacted. In their model, it is assumed that the effect of compaction could be considered as a cyclic surcharge on the backfill surface. When the surcharge is applied on the soil surface, it will increase the vertical stress and the horizontal stress. In Fig. 2.15, as the virgin loading is applied on the soil, both σv and σh increase along the Ko -line (Ko = 1-sinφ).
However, when the surcharge is removed, σv and σh would decrease along the virgin unloading path. All unloading is subject to the passive failure limiting conditions.
When virgin reloading was applied again, the increment of earth pressure is less than that induced by virgin loading.
The hysteretic model was used to the analysis. Compaction was represented by a transient, moving surficial load of finite lateral extent by directly modeling loading as an increase in vertical effective stress (Δσv). To simulate the compaction loading, a parameter of the peak virgin, compaction-induced horizontal stress increase (Δσ’h,vc,p) is defined as the horizontal effective stress which would be induced by the most critical positioning of the compactor. If the soil had been previously uncompacted (that is, the soil had no “lock-in” residual stresses due to previous compaction), Δσ’h,vc,p can be obtained by using the simple elastic analysis. The hysteretic Ko model described up to this point is a one-dimensional model. But a compactor does not cover the entire backfill surface and the real case is three-dimensional. To account for the three-dimensional effects, an “equivalent peak vertical stress” is applied to represent the compactor in the Ko-model. Compaction loading would be modeled on the basis of Δσ’h,vc,p transformed to an equivalent peak vertical stress increase (Δσ’v,e,p)
which can be calculated as
In this model the peak compaction loading was based on directly calculated lateral stress increase, rather than on the basis of a directly calculated peak vertical stress increase subsequently multiplied by Ko, Ka or some other coefficient. Seed and Duncan (1983) presented a study and recommendations for the calculation of Δσ’h,vc,p
for various situation. Seed and Duncan (1983) concluded that either in the free field, or at or near vertical, nondeflecting soil/structure interfaces, Δσ’h,vc,p resulting from surficial compaction loading can be calculated directly by simple elastic analysis. The parameter of Poisson’s ratio, ν for surficial compaction loading was chosen according to the empirically derived relationship
Seed and Duncan (1983) also pointed that based on the observation of field measurements, the loading imposed by a typical vibratory roller can be modeled as approximately two to four times the static weight of the roller. For Δσ’h,vc,p acting at a vertical, nondeflecting soil-structure interface due to concentrated surficial loading can be taken as twice the value that would be calculated at the same point by closed-form elastic solutions. Unfortunately, as the comments by Seed and Duncan, the hysteretic model is very complex. However, based on the concept, the proposed model may be incorporated in an increment analytical procedure, which can be used to evaluate the earth pressure resulting from the placement and compaction of soil layers.
2.6.2 Study of Peck and Mesri
k and Mesri (1987) presented a calculation method to evaluate the compaction-induced earth pressure. The lateral pressure profile can be determined by four conditions on σ , as illustrated in Fig. 2.16 and summarized in the following.
1. Lateral pressure resulting from the overburden of the com acted backfill, Based on the elastic analysis, Pec
h
2. Lateral pressure limited by passive f
h φ γz
σ tan2(45 /2)
3. Lateral pressure result burden plus the residual horizontal
h
ateral earth pressure increase res
compaction loading of the last backfill lift and can be determined based on the elastic solution.
4. Lateral pressure profile defined by a line which envelops the residual lateral pressures resulting from the compaction of individual backfill lifts. This line can be computed by Eq. 2.20.
Fig. 2.16 indicates that near the surface of backfill, from point a to b, the lateral pressure on the wall is subject to the passive failure condition. From b to c, the overburden and compaction-induced lateral pressure profile is determined by Eq. 2.19.
Δ h
From c the lateral pressure increases with depth according to Eq. 2.20 until point d is reached. Below d, the overburden pressure exceeds the peak increase in stress by compaction. In the lower part of the backfill, the lateral pressure is directly related to the effective overburden pressure.
2.6.3 Study of Chen
Chen (2003) reported some experiments in non-yielding retaining wall at sity to investigate influence of earth pressure due to vibra
National Chiao Tung Univer
tory compaction. Air-dry Ottawa sand was used as backfill material. Vertical and horizontal stresses in the soil mass were measured in loose and compacted sand.
Based on his test results, Chen (2003) proposed three points of view: (1) after compaction, the lateral stress measured near the top of backfill is almost identical to the passive earth pressure estimated with Rankine theory (Fig. 2.17). The compaction-influenced zone rises with rising compaction surface. Below the compaction-influenced zone, the horizontal stresses converge to the earth pressure at-rest, as indicated in Fig. 2.17 (e); (2) when total (static + dynamic) loading due to the vibratory compacting equipment exceeds the bearing capacity of foundation soils, the mechanism of vibratory compaction on soil can be described with the bearing capacity failure of foundation soils; (3) the vibratory compaction on top of the backfill transmits elastic waves through soil elements continuously. For soils below the compaction-influenced zone, soil particles are vibrated. The passive state of stress among particles is disturbed. The horizontal stresses among soil particles readjust under the application of a uniform overburden pressure and constrained lateral deformation, and eventually converge to the at-rest state of stress.
Chapter 3
EXPERIMENTAL APPARATUS
To investigate the effects of vibratory compaction on the vertical and horizontal stresses in a cohesionless soil mass, an instrumented model retaining wall facility at National Chiao Tung University (NCTU) was used. This chapter introduces the NCTU model retaining wall facilities and the vibratory compactor used to densify the loose backfill. The NCTU non-yielding retaining wall facilities consist of three components: (1) the soil bin, (2) soil pressure transducers, and (3) the data acquisition system (Chen and Fang, 2002). The details of the foregoing apparatuses are described in the following sections.