Granite rock has powerful alterations at several meters of depth. The clayed sand resulting is commonly known as jabre. This “in situ” mixture of cement-stabilized soil requires a laboratory formula. Even when the test section is correctly verified, the mechanical properties of the homogeneous mixture of jabre exhibit high degrees of dispersion. The laboratory work undertaken included particle-size analysis and screening, definition of liquid and plastic limits, compressive strength, dry density and moisture content over stabilized samples, modified Proctor, California Bearing Ratio (CBR) and the determination of the workability of the hydraulically bound mixtures. The stress resistance curve was analyzed by means of a multilinear model of unconfined compressive strength (UCS). Since practical engineering only requires UCS for 7 days, in order to gain greater knowledge of the material, other UCS transformations were used at other curing times such as 7, 14 and 28 days.

The granitic alluvial soil that results from the alteration of granite rock is a type of clayed sand known as “jabre”. Jabre has high unconfined compressive strength (UCS), which is the most important property of the improved cement subgrade and the main tested property in quality control for subgrade formation. Even if cement spread rate is correctly sized in the laboratory, cement-stabilized jabre is usually associated with high dispersion of UCS values. Since unconfined compressive strength (UCS) is the most important property of the improved cement subgrade, dispersion triggered by changes in the alteration grades of jabre can be predicted using a multilinear model, therefore preventing negative UCS results. Accordingly, the main purpose is to define the mechanical properties of jabre based on its different characteristics in association with the different alteration grades of granitic rock. The specific objectives are to analyze how the main properties of the soil influence unconfined compressive strength and to define the hardening curve for the estimation of mechanical resistance after cement-stabilization at different times.

As described by García-Talegón et al. (_{2}Si_{2}O_{5} (OH)_{4}). Granite-rock alluvial deposits are thicker than others associated with metamorphic lithologies and can reach thicknesses of several meters. Thus, granite massifs are affected by orthogonal joint sets. If it advances deeper, the rocks become divided into parallel piped blocks. The fracture web allows the partial alteration of the feldspars by cold water, transforming them into pure clay. This kaolinite is washed by the water leaving a sandy residue of unaltered quartz, mica and feldspar grains. Later, rainwater washes away the sand material leaving groups of rocks in unstable equilibrium (granitic towers). When this equilibrium is broken, stone build-ups are formed in the ridges and the sands become deposited in the lower areas. A generalized fact regarding granitic rock is the presence of powerful alterations that affect the plinth in several meters of depth, leading to an important silicification on its top layer. Alteration and silicification are two separate processes. The weathering process is related to climate changes, from the wettest (clayish alteration) to more prone to aridity seasons (sandstone silicification). The thickness of the alteration layer, which can exceed 18-20m, includes 3 levels: the lowest, made up of light-gray boluses; the intermediate, which is 4-5 m deep and completely sandy; and, finally, a highly silicified level, with variations of red-ocher-white colors resulting from hydromorphic processes.

Champire et al. (

“In situ” cement mixing cannot start until the spread rate is defined. Meanwhile, a preliminary study must be adjusted through a laboratory test and, eventually, a road section test. Stabilized soil samples, according to UNE-EN 13286-51 (

Anagnostopoulos, CA (

Niu et al (

Although Wu et al (

Kariyawasam and Jayasinghe (

Experimental studies on compression tests have attracted the attention of many researchers. Kasama et al. (

Liu et al. (

Another significant aspect of UCS prediction for long curing periods is approached by Saldanha et al. (

Fernandez-Ledesma et al. (

The materials and clustering investigated are part of the cement stabilized subgrade phase in road construction, used for the A-66 road (European E-803), Caceres (N) – Aldea del Cano section, which is over 31km long and is split into two separate carriageways. In general, cement-stabilized jabre has high UCS, so these materials can be used as base layers.

Initially, more than 300 samples were tested for identification and provenance. For resistance characterization purposes, more than 1.500 breaks of test pieces of cement-stabilized soil were made. On site controls were applied at more than 7.500 points, estimating density and humidity. More than 200 load plate tests were carried out following UNE 103 808 (

During the stabilization process, Portland cement was used, specifically, CEM II / B-M (P-V-L) 32.5 N according to RC-16 (

UCS was assessed on day seven together with compaction rate according to modified Proctor. The specimens were manufactured with the degree of compaction reached at work (98% modified Proctor density in this case). The value corresponds to the embankment compaction degree which is was required in highway construction. Measures were produced with a vibratory compaction hammer in fieldwork and using a manual or mechanical compactor in the laboratory, always during the workability timeframe of workability, as established in UNE-EN 13286-45 (

Classification of alluvial soil sample identification trials

group | number of samples | sieving | LL | PI | modified Proctor | |||
---|---|---|---|---|---|---|---|---|

80mm | 2.0mm | 0.08mm | d_{max} (g cm^{-3}) |
H_{opt} (%) |
||||

I | 42 | 100.0 | 50.0 | 13.5 | 33.8 | 7.0 | 2.06 | 8.2 |

II | 30 | 100.0 | 47.0 | 9.7 | 32.4 | 4.4 | 2.09 | 7.7 |

III | 26 | 100.0 | 44.5 | 9.6 | 36.7 | 10.3 | 2.08 | 7.6 |

More than 1.000 ruptures were made from the samples using a vibrating hammer on site for stabilization purposes. Another 500 specimens were mixed with cement in the laboratory using a concrete mixer. More were ruptured at different curing ages (3, 7, 14 and 28d). Other standard tests were also run, such as the 100 Modified Proctor, the 90 Atterberg limits and the 90 test grading.

The tests were carried out during quality control, alongside initial identification and definition studies and subsequent control and adjustment during execution. The stabilized soil measure was of 660 000m^{3} and the on-site stabilization surface was over 1 300 000m^{2}. Before defining the cement mixture, measures were split into 3 groups with similar characteristics, thus preventing dispersion.

Physical parameters for initial jabre identification (average parameters in last row)

# 100.0 | # 20.0 | # 2.0 | # 0.40 | # 0.08 | LL | PI | d (g cm^{-3}) |
H (%) | CBR |
---|---|---|---|---|---|---|---|---|---|

100.0 | 98.0 | 65.0 | 35.0 | 17.5 | 37.4 | 9.5 | 2.00 | 8.8 | 29.2 |

100.0 | 100.0 | 48.0 | 23.0 | 11.6 | 30.6 | 3.5 | 2.12 | 6.4 | 42.7 |

100.0 | 98.0 | 45.0 | 20.0 | 9.0 | 31.9 | 7.6 | 2.11 | 6.1 | 46.3 |

100.0 | 74.0 | 52.0 | 24.0 | 11.0 | 28.4 | 6.2 | 2.12 | 7.4 | 50.6 |

100.0 | 100.0 | 60.0 | 25.0 | 12.6 | 0.0 | 0.0 | 2.04 | 10.0 | 78.0 |

100.0 | 99.0 | 37.0 | 13.0 | 9.0 | 40.4 | 15.2 | 2.09 | 9.0 | 29.1 |

100.0 | 94.0 | 35.0 | 16.0 | 8.6 | 32.8 | 6.9 | 2,06 | 6.1 | 36.1 |

100.0 | 96.0 | 66.0 | 29.0 | 14.8 | 37.8 | 11.4 | 2.00 | 7.7 | 28.2 |

100.0 | 100.0 | 80.0 | 55.0 | 40 | 43.7 | 17.6 | 2.04 | 8.6 | 15.2 |

100.0 | 95.9 | 53.0 | 25.4 | 13.6 | 29.0 | 8.1 | 2.07 | 8.2 | 40.7 |

In general, they are predominantly sandy soils with a small fraction of gravel. Plasticity is medium-low, and they are defined as SC (clayed sands) in the USUCS classification. Their appearance is grayish with whitish veins. The specimens present a minimum aggregate content (grading above 2-mm) of 35%. Additionally, they have a fine aggregate content of between 6-35%. Jabre is an alluvial soil that is associated with the alteration grade of granitic rock. In general, it has a thickness between 1.0 – 2.0m. It consists of deposits of whitish granitic quartzite sand and a clayed fraction resulting from feldspar alteration. This implies that Liquid limit (LL) values are close to 40 and the plasticity index is close to 15. The Atterberg limits report medium – low plasticity. Attention should also be paid to its remarkably high quality, which allows it to be used for subgrades.

Before defining the cement mixture, samples were taken at different depths from each of the planed diggings and extractions. The resulting measurements were classified into 3 groups that shared similar characteristics to avoid dispersion. Cement tests were performed for each group for identification and stabilization purposes.

All the mentioned quality tests were processed grouping the esplanade into 500 representative sections of similar characteristics. Thus, the correlations, laboratory tests and tests carried out on site were added for possible effects. This section describes the statistical techniques that were used to produce a model to estimate resistances from the main control parameters, which were developed in order to analyze test differences.

Thus, a multilinear adjustment model of the UCS characteristic curve for stabilized jabre was been developed. UCS was monitored as a predictive value, improving quality control optimization. Other UCS models at different curing ages (7, 14 and 28d) were also adjusted. A positive linear correlation between UCS and share density, the 2mm sieve and the filler / cement binder ratio enables UCS to be used to inter-decrease mechanical properties.

Soil identification tests and cement-stabilized ruptures allow the detection of flaws with the UCS model of jabre-stabilization. This has improved the use of jabre as a stabilized soil, controlling soil parameters “in situ”. Statistical models were estimated using IBM SPSS Statistics.

Seeking the best statistical fit, shams were re-grouped into more than 450 control shares. All of them were tested for UCS, plasticity and grading. Analysis of variance ANOVA was applied to introduce variables. If there were low levels of significance, they were removed. The variables used in the multivariate analysis of UCS were the following: 2.0 mm sieving, fine aggregate (0.08 mm), LL, PI, C, dry density over sample in g cm^{-3} (d_{sm}), moisture content over sample, rupture time in days (t), dry density over compaction shares, moisture content over shares, maximum modified Proctor density, optimum moisture modified Proctor, compaction rate over sample, compaction rate over share, differences between sample and optimum modified Proctor moistures, first and second vertical compressive modulus plate-bearing test, relation between second/first vertical compressive modulus, moisture content over share and cement binder relation, relation between fine aggregate and cement (fn/C).

The association of independent variables with the dependent variable (compression resistance) was determined using Student’s t test. The independent variables that obtained low absolute values in the the t-test were removed due to their lack of significance in the statistical adjustment.

UCS is the fundamental parameter of cement improved subgrades. Thus, the high variability in granitic eluvial soils, allows the development of a linear regression model. This UCS estimate depends on the granulometry, rate of cement spread and degree of compaction over samples. In these cases, soil stabilization with UCS ≥ 1.5 MPa (7d rupture) (

Mixtures that made with different percentages of cement were produced, with UCS at curing ages 7, 14 and 28d. Every UCS was the average value of 3 ruptured samples.

In general, average resistances higher than normal were obtained (1.5 MPa) for all the percentages of cement binder, even for the 3.0% minimum content. Moreover, the minimum setting time period of workability of cement improved soil (standard UNE-EN 13286-45) and was 180 minutes.

The following subsections describe different models calculated for UCS in cement-stabilized jabre, considering all the properties that could produce a loss of mechanical resistance. There are models for the stress curve and UCS at seven 7, 14 and 28d curing. Transformations of UCS (logarithm, exponential) were also analyzed.

To sum up, the multivariate analysis offered a generalized multilinear model for the stress curve and the cubic-root transformation for UCS at 7, 14 or 28d. After the analysis of all the independent variables in previous chapters, the 2.0 mm sieve and share density appeared as common variables in all the UCS models.

The variables introduced in the model proved robust for a multilinear approach, with linear UCS (t) as a dependent variable. The independent indicators were rupture times and the variables with the highest goodness-of-fit, which were sand and fine aggregate (sieve 2 mm), share density and cement content.

The results in

Determination coefficients for UCS (t) path model of jabre

Summary model | |||
---|---|---|---|

R | R^{2} |
adjusted R^{2} |
standard error |

0.848^{a} |
0.720 | 0.711 | 0.2799 |

predictors: constant, #2.0mm (%), t (d), C (%), d_{sm} (g cm^{-3}).

The scatterplot for

Relationship between dry density d_{sm} and Unconfined Compressive Strength UCS (3, 7, 14 & 28d)

Analysis of variance for UCS (t) path model in jabre

ANOVA^{a} |
|||||
---|---|---|---|---|---|

model | squares sum | degrees of freedom | average | F | sig. |

regression | 26.739 | 4 | 6.685 | 85.313 | 0.000^{b} |

remainder | 10.421 | 133 | 0.078 | ||

Total | 37.160 | 137 |

dependent variable: UCS (t) (MPa).

predictors: constant, #2.0mm (%), t (d), C (%), d_{sm} (g cm^{-3}.)

The scatterplot in

2.0mm sieving versus Unconfined Compressive Strength UCS (7, 14 & 28d rupture time)

According to the graph, it is difficult to pair variables because of their low level of similarity. Consequently,

Multilinear regression coefficients for UCS (t) path model in jabre

coefficients^{a} |
|||||
---|---|---|---|---|---|

Model | nonstandard coefficients | standard coefficients | t | sig. | |

B | standard error | β | |||

constant | -16.031 | 1.240 | -12.930 | 0.000 | |

t (d) | 0.017 | 0.003 | 0.314 | 6.698 | 0.000 |

d_{sm} (g cm^{-3}) |
7.705 | 0.640 | 0.586 | 12.037 | 0.000 |

C (%) | 0.149 | 0.023 | 0.300 | 6.413 | 0.000 |

#2.0mm (%) | 0.039 | 0.005 | 0.391 | 8.084 | 0.000 |

dependent variable: UCS(t) (MPa).

The distribution of shares in

Cement content C versus Unconfined Compressive Strength UCS (3, 7, 14 & 28d rupture time)

With the coefficients of multicollinear regression, the accuracy of the results in the UCS in jabre cement-stabilized diagram can be explained with the Equation [1]:

where UCS (t) is unconfined compressive strength (MPa) at t rupture time (d), d_{sm} dry density over sample (g cm^{-3}), C cementitious binder percentage (over dry soil) and two-millimeter sieving (% #2.0 mm).

The significance has a validity range. UCS (t) has a range of distribution between 0.7 and 3.6 MPa. Rupture time is from 7d until 28d. Density samples are between 1.90 and 2.14 g cm^{-3}. Cement content is between 2.0 and 6.0. Finally, the 2.0 mm sieving (medium sand & filler content) is between 34.1 and 54.3 %.

The variables introduced into the model were as follows: the dependent variable was UCS transformation cubic-root; and the independent variables were the three whose indicators had obtained the best results, namely sand and fine aggregate (passing fraction two millimeters), share density and water-cement ratio.

Determination coefficients for UCS (7d) model in jabre

Model Summary | |||
---|---|---|---|

R | R^{2} |
adjusted R^{2} |
standard error |

0.834^{a} |
0.696 | 0.684 | 0.2986 |

predictors: constant, #2mm (%), d_{sm}(g cm^{-3}), fn/C.

dependent variable: ^{3}√UCS(7d).

In general, higher share densities are associated with higher compressions, while higher contents of sand and filler (sieving # 2.0mm) corresponded to lower UCS (7d). The ANOVA analysis yielded the parameters shown in

Analysis of variance for UCS (7d) model in jabre

ANOVA^{a} |
|||||
---|---|---|---|---|---|

Model | squares sum | degrees of freedom | average | F | sig. |

regression | 16.289 | 3 | 5.430 | 60.914 | 0.000^{b} |

remainder | 7.131 | 80 | 0.089 | ||

total | 23.420 | 83 |

dependent variable UCS(7d).

predictors: constant, #2.0mm (%), d_{sm} (g cm^{-3}), #0.08mm/C.

t-test results are shown in

Linear regression coefficients for UCS (7d) estimate model in jabre

Model | no standard coefficients | standard coefficients | t | sig. | |
---|---|---|---|---|---|

B | standard error | β | |||

constant | -4.111 | 1.739 | -2.364 | 0.020 | |

d_{sm} (g cm^{-3}) |
2.777 | 0.892 | 0.199 | 3.112 | 0.003 |

#0.08mm (%)/C | 0.171 | 0.013 | 0.853 | 13.002 | 0.000 |

#2.0mm (%) | -0.016 | 0.006 | -0.171 | -2.525 | 0.014 |

dependent variable: ^{3}√UCS (7d).

Under these circumstances, the UCS time cluster (7d) is represented by the following Equation [2]:

where d_{sm} is sample density (g cm^{-3}), UCS (7d) compressive strength at 7d (MPa), # 0.08 mm/C relate fine aggregate (% # 0.08mm) and cementitious binder (over dry soil), #2.0 UNE two millimeters sieving.

The model has a validity range of UCS (7d) and it has an interval scale between 0.7 and 3.6 MPa. Densities are between 2.20 and 1.80 g cm^{-3}. The filler/cement binder ratio is between 5.1 and 1.2. Finally, 2mm sieving (medium sand and filler content) is between 54.3 and 34.1 (%).

Determination coefficients for UCS (14d) model in jabre

Summary model | |||
---|---|---|---|

R | R^{2} |
adjusted R^{2} |
standard error |

0.991^{a} |
0.982 | 0.977 | 0.0280 |

Predictors: constant, #2.0mm (%), C (%), d_{sm} (g cm^{-3}).

Linear regression coefficients for UCS (14d) estimate model in jabre

UCS (14d)^{a} |
|||||
---|---|---|---|---|---|

Model | no standards coefficients | standards coefficients | t | sig. | |

B | standard error | β | |||

constant | -7.665 | 0.622 | -12.298 | 0.000 | |

d_{sm} (g cm^{-3}) |
3.937 | 0.325 | 0.527 | 12.097 | 0.000 |

C (%) | 0.073 | 0.009 | 0.344 | 7.938 | 0.000 |

#2.0mm (%) | 0.019 | 0.001 | 0.702 | 14.723 | 0.000 |

Dependent variable: ^{3}√UCS (14d).

This implies that the expression of the adjustment is complete [3]:

The model has a validity range. UCS (14 d) has an interval scale between 0.9 and 2.7 MPa. Density samples are between 2.01 and 2.08 g cm^{-3}. Cement content is between 2.0 and 5.0. Finally, 2.0 mm sieving (medium sand and filler content) is between 34.1 and 50.0 %.

As shown in

Determination coefficients for UCS (28d) model in jabre

Summary model | |||
---|---|---|---|

R | R^{2} |
adjusted R^{2} |
standard error |

0.845^{a} |
0.714 | 0.693 | 0.0579 |

predictors: constant, #2.0mm (%), C (%), d_{sm} (g cm^{-3}).

Linear regression coefficients for UCS (28d) estimate the jabre model

Model | no standard coefficients | standard coefficients | t | sig. | |
---|---|---|---|---|---|

B | standard error | β | |||

constant | -2.927 | 0.456 | -6.425 | 0.000 | |

d_{sm} (g cm^{-3}) |
1.934 | 0.233 | 0.714 | 8.285 | 0.000 |

C (%) | 0.037 | 0.009 | 0.380 | 4.298 | 0.000 |

#2.0 mm (%) | 0.005 | 0.002 | 0.207 | 2.303 | 0.027 |

dependent variable: ^{3}√ UCS (28 d).

Therefore, the adjustment for UCS with 28 d rupture is as follows [4]:

The model has a validity range. UCS (28d) has an interval scale between 1.3 and 3.2 MPa. The density samples are between 1.98 and 2.10 g cm^{-3}. The cement content is between 2.0 and 5.5. Finally, 2.0mm sieving (medium sand and filler content) is between 37.4-50.6 %.

The results of the different models were grouped into the significance matrix shown in

Jabre models (characteristic curve, 7, 14 & 28d) significance matrix

Student’s t-coefficients (t) | |||||
---|---|---|---|---|---|

d_{sm} (g cm^{-3}) |
C (%) | #0.08mm/C | #2.0mm(%) | t(d) | |

characteristic curve (d) | 12.037 | 6,413 | ns | 8.084 | 6.698 |

7 d | 3.112 | ns | 13.002 | -2.525 | --- |

14 d | 12.097 | 7.938 | ns | 14.723 | --- |

28 d | 8.285 | 4.298 | ns | 2.303 | --- |

ns: nonsignificant (Student’s t-value below that of the other variables).

Student’s t-coefficients show that the highest correlation is for the filler-cementitious binder ratio, with a positive trend. With the same percentage of cement, higher UCS is yielded by higher contents of fine aggregate. UCS is also proportional to the density of the sample, especially at 14d. The only association with a negative trend was screening 2 UNE sieve (2mm) at 7d. Therefore, medium and fine sands yield lower resistance. This is probably due to its medium plasticity, although this is less significant. The t-statistic value for the 2.0 mm sieving at 14d is also indicated; at this age UCS is more associated with the sand and filler medium content than with sample density. The cement content is more important at 14d than at 28d, while at 7d is not significant. According to this trend, the filler/cement binder ratio is only significant at the first curing age (7d), not at the rest (14, 28d).

Experimental results demonstrate that compressive strength is greatly influenced by the jabre conditions such as the 2.0 mm sieving (medium sand & filler content), with a validity range between 34.1 and 54.3 %. The density samples for jabre slightly increase between 1.90 and 2.14 g cm^{-3} with a significant increase of UCS and with a distribution range of 0.7 and 3.6 MPa. For the same granular material, UCS proves to be more influenced by cement content at 28d than at 7d rupture time. Therefore, the mechanical properties of the cement and jabre mixture should be assessed for UCS adjustment (28d).

Chuang et al. (^{th} day. With the addition of ultrafine silica fume, the unconfined compressive strength increases by close to 6.5% compared with seawater alone at the 90^{th} day. On this subject, further studies considering cement-stabilized jabre mixed with seawater and possibly ultrafine silica fume contents are necessary. Rahmi et al. (

This paper shows how the soil developed from granitic rock can be greatly influenced by numerous variables. Therefore, the compressive strength of “in situ” cement-stabilized soils is not only affected by cement content or share density variables. The study has developed a procedure to identify the main characteristics of alluvial soils originated from granite weathered rocks and their influence at different times on compressive strength loosening when stabilized with cement.

The main conclusions drawn from this study are as follows:

Jabre has evolved from granite, which means that there are many variations in its mechanical behavior that are difficult to control based on general specifications. It is necessary to adjust its resistance to simple compression versus sample density, the filler mineral-cement binder ratio and, finally, the fine sand aggregate.

In general, the resistances obtained were higher than stabilized soil standard limits (1.5 MPa) for all percentages of the cement binder, even for the 3% minimum percentage.

As a result, the sampling frame of the hardening curve model of cement-stabilized jabre was generated. This implies that the most significant variables are dry density over sample, cement content percentage and two millimeters sieving.

The share density and the UCS have an empirical relationship. That density variations affect UCS has been shown on several occasions. Higher density values correspond with higher UCS. These variables are strongly associated.

The experimental results demonstrate that UCS is significantly influenced by the jabre conditions of the 2.0mm sieving. UCS shows a bigger influence of cement content at 28d than at the 7d rupture time. Therefore, for UCS (28d) adjustment, the mechanical properties of the cement-jabre mixture should be assessed.