Segments and Foreign Operations - Additional Financial Information for Segments (Detail) (USD $)	3 Months Ended								12 Months Ended
	Jan. 31, 2013	Oct. 31, 2012	Jul. 31, 2012	Apr. 30, 2012	Jan. 31, 2012	Oct. 31, 2011	Jul. 31, 2011	Apr. 30, 2011	Jan. 31, 2013	Jan. 31, 2012	Jan. 31, 2011
Segment Reporting Information [Line Items]
Revenues	$ 229,742,000	$ 282,879,000	$ 289,560,000	$ 273,443,000	$ 271,924,000	$ 289,813,000	$ 289,310,000	$ 261,711,000	$ 1,075,624,000	$ 1,112,758,000	$ 999,905,000
Capital expenditures									77,503,000	70,826,000	67,203,000
Depreciation and amortization									61,397,000	56,040,000	44,837,000
Assets	812,226,000				805,836,000				812,226,000	805,836,000	816,652,000
Water Resources [Member]
Segment Reporting Information [Line Items]
Revenues									278,529,000	274,556,000	248,907,000
Capital expenditures									36,077,000	27,816,000	28,535,000
Depreciation and amortization									15,285,000	14,222,000	12,405,000
Assets	154,207,000				194,902,000				154,207,000	194,902,000	178,744,000
Inliner [Member]
Segment Reporting Information [Line Items]
Revenues									133,256,000	132,108,000	116,566,000
Capital expenditures									2,579,000	3,218,000	2,938,000
Depreciation and amortization									2,647,000	2,880,000	2,332,000
Assets	67,027,000				61,601,000				67,027,000	61,601,000	46,262,000
Heavy Civil [Member]
Segment Reporting Information [Line Items]
Revenues									278,131,000	343,760,000	353,304,000
Capital expenditures									1,088,000	5,413,000	4,302,000
Depreciation and amortization									7,096,000	7,389,000	7,095,000
Assets	114,361,000				123,475,000				114,361,000	123,475,000	187,333,000
Geoconstruction [Member]
Segment Reporting Information [Line Items]
Revenues									133,247,000	89,210,000	77,969,000
Capital expenditures									6,760,000	8,240,000	3,242,000
Depreciation and amortization									11,413,000	8,819,000	3,619,000
Assets	170,064,000				113,612,000				170,064,000	113,612,000	118,805,000
Mineral Exploration [Member]
Segment Reporting Information [Line Items]
Revenues									246,582,000	268,909,000	199,946,000
Capital expenditures									24,682,000	16,921,000	19,309,000
Depreciation and amortization									19,030,000	15,145,000	13,070,000
Assets	213,908,000				203,277,000				213,908,000	203,277,000	165,230,000
Other [Member]
Segment Reporting Information [Line Items]
Revenues									5,879,000	4,215,000	3,213,000
Capital expenditures									1,650,000	2,529,000	2,717,000
Depreciation and amortization									550,000	5,381,000	4,712,000
Assets	6,804,000				13,585,000				6,804,000	13,585,000	15,600,000
Discontinued operations [Member]
Segment Reporting Information [Line Items]
Capital expenditures									1,672,000	4,126,000	3,289,000
Assets					52,893,000					52,893,000	57,047,000
Unallocated corporate expenses [Member]
Segment Reporting Information [Line Items]
Capital expenditures									2,995,000	2,563,000	2,871,000
Depreciation and amortization									5,376,000	2,204,000	1,604,000
Assets	85,855,000				42,491,000				85,855,000	42,491,000	47,631,000
Water systems [Member]
Segment Reporting Information [Line Items]
Revenues									219,693,000	208,558,000	181,281,000
Water treatment technologies [Member]
Segment Reporting Information [Line Items]
Revenues									54,276,000	51,410,000	49,109,000
Sewer rehabilitation [Member]
Segment Reporting Information [Line Items]
Revenues									133,256,000	132,108,000	116,757,000
Water and wastewater plant construction [Member]
Segment Reporting Information [Line Items]
Revenues									129,446,000	200,513,000	237,360,000
Pipeline construction [Member]
Segment Reporting Information [Line Items]
Revenues									108,408,000	112,446,000	85,486,000
Soil stabilization [Member]
Segment Reporting Information [Line Items]
Revenues									167,711,000	115,402,000	100,685,000
Environmental and specialty drilling [Member]
Segment Reporting Information [Line Items]
Revenues									7,246,000	17,902,000	14,312,000
Exploration drilling [Member]
Segment Reporting Information [Line Items]
Revenues									249,247,000	271,442,000	201,479,000
Other products [Member]
Segment Reporting Information [Line Items]
Revenues									$ 6,341,000	$ 2,977,000	$ 13,436,000