SEGMENT AND GEOGRAPHICAL INFORMATION - Statement of Operations (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 29, 2023	Sep. 29, 2023	Jun. 30, 2023	Mar. 31, 2023	Dec. 30, 2022	Sep. 30, 2022	Jul. 01, 2022	Apr. 01, 2022	Dec. 29, 2023	Dec. 30, 2022	Dec. 31, 2021
Segment reconciliation
Contract revenue	$ 155,677	$ 132,738	$ 119,077	$ 102,603	$ 113,256	$ 121,399	$ 102,645	$ 91,838	$ 510,095	$ 429,138	$ 353,755
Depreciation and amortization									16,431	17,489	17,146
Interest expense, net									9,413	5,328	3,869
Segment profit (loss) before income tax expense									14,591	(11,452)	(12,404)
Income tax (benefit) expense	1,953	713	243	756	2,584	(1,526)	(1,673)	(2,389)	3,665	(3,004)	(3,987)
Net Income (Loss)	$ 8,031	$ 1,566	$ 397	$ 932	$ (425)	$ 76	$ (4,326)	$ (3,773)	10,926	(8,448)	(8,417)
Energy
Segment reconciliation
Contract revenue									426,976	357,460	286,384
Depreciation and amortization									15,176	16,507	16,156
Engineering and Consulting
Segment reconciliation
Contract revenue									83,119	71,678	67,371
Depreciation and amortization									1,255	982	990
Operating Segments | Energy
Segment reconciliation
Interest expense, net									6	11	8
Segment profit (loss) before income tax expense									16,921	(9,544)	(4,808)
Income tax (benefit) expense									4,250	(2,504)	(1,546)
Net Income (Loss)									12,671	(7,041)	(3,263)
Operating Segments | Engineering and Consulting
Segment reconciliation
Segment profit (loss) before income tax expense									10,270	10,896	9,135
Income tax (benefit) expense									2,580	2,858	2,936
Net Income (Loss)									7,690	8,037	6,198
Corporate, Non-Segment
Segment reconciliation
Interest expense, net									9,407	5,317	3,861
Segment profit (loss) before income tax expense									(12,600)	(12,804)	(16,731)
Income tax (benefit) expense									(3,165)	(3,358)	(5,377)
Net Income (Loss)									$ (9,435)	$ (9,444)	$ (11,352)